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| Infamous Nichrome Alternator Wire [message #365955] |
Sat, 07 August 2021 19:19  |
Greg C.
Messages: 224
Registered: October 2019
Location: Knoxville, TN
Karma: 0
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Senior Member |
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I am deep into my new dash project. I have spent the last couple of days neatening up the factory wiring harness at the fire wall, cutting some of the old connectors, and crimping on new connectors for the new switches and gauges.
I've been reading several posts from Bruce Hislop about the nichrome wire/resistance wire. I actually PM'd him, but he did not respond. While my wiring harness is totally accessible, I would like to address the nichrome wire issue. Bruce had written at some point about installing a 10 ohm 25 watt power resistor in series with a non-resistor type wire to replace the nichrome wire.
His posts enlightened me quite a bit about this subject. I now understand that the GEN light receives power from both wires going to it, unless one wire is carrying less voltage than the other, which causes the light to light either dimly or brightly, dependent upon the voltage difference between the two wires. This would indicate either battery or alternator voltage dropping below normal levels.
One thing I am not clear on is the exact purpose of the resistance wire. Is it to protect the GEN light bulb from burnout if there is no/low voltage on one wire?
He had also posted that the APC cable may not prevent the resistance wire from overheat, thus the 10 ohm resistor in lieu of the APC cable.
Bruce also posted that his coach was wired so that the resistance wire is connected to the RUN circuit of the switch, and not the ACCY circuit, which seems to be the factory norm. It is conceivable that leaving the switch in the ACCY position for some time could cause the wire to overheat. I understand the concept, but don't know exactly where to tap into the RUN circuits for the resistance wire connection.
So my questions are: do I understand this correctly? What IS the best way to approach this problem?
And if I change the resistance wire to the RUN circuit, where is the best place to cut/splice/tap?
Thank you for your time.
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
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| [GMCnet] Re: Infamous Nichrome Alternator Wire [message #365956 is a reply to message #365955] |
Sat, 07 August 2021 20:08  |
Emery Stora
Messages: 959
Registered: January 2011
Karma: 4
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Senior Member |
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Greg
The purpose of the nichrome wire is to “excite” the alternator so it will start producing current.
I believe that I was the first one to have reported the problem with the nichrome resistance wire. The problem can only occur if there is a problem with the built in diodes (or the voltage regulator) in the alternator. The alternator can then push high voltage AC current back through the nichrome wire. This causes it to heat up and burn the insulation and short out many things that are in the wire bundle.
When this happened to me I could not shut off the lights or the engine and I had smoke coming up from the dash cover by the passenger side windshield. I pulled my battery cable to cut off all voltage. When I removed my dash cover (which took a few hours of work) I discovered that I had 20 wires fused together and there were various shorts. I spliced all of the wires and I removed the nichrome wire from the bundle and ran it in its own insulating sleeve.
This happened back in 2001. When I reported it to the GMC net and gave my theory that it was the alternator, Gene Fisher looked at the problem and decided that the circuit needed a diode to prevent this from happening. I installed an early version of his APC (alternator protection cable) and then when he made additional changes I installed his modified version. I felt this was an easy fix and it didn't take any time for me to do it. Just unplug the alternator wire and hook his APC in series with that wire.
Since that occurred to me, several others then had the same problem so it can easily happen with a bad alternator. I am sure that there were some before me but they probably had a fire which destroyed their GMC before anyone could determine the cause.
Besides using the APC, or a resistor one could also just simply put a fuse in series with the nichrome wire that would burn out if excessive current went backwards through that wire.
I really recommend Gene Fisher's fix because of its simplicity and its low cost and easy installation. It basically is a diode put into series with the lead from the alternator that allows current to flow from the battery to the alternator to "excite" the field of the alternator so it will start producing a charge. The diode also prevents current from going up the nichrome wire toward the battery which prevents the heating problem if there is an alternator failure which produces the back current.
I disagree that the APC may not prevent the resistance wire from overheating. I also disagree that leaving the ignition switch in the ACCY position could cause the wire to overheat. The overheating is caused by voltage from the alternator if you have components in the alternator fail and has nothing to do with the source of voltage from your ignition switch.
Emery Stora
emerystora@mac.com
> On Aug 7, 2021, at 6:19 PM, Greg Crawford wrote:
>
> I am deep into my new dash project. I have spent the last couple of days neatening up the factory wiring harness at the fire wall, cutting some of the
> old connectors, and crimping on new connectors for the new switches and gauges.
>
>
> I've been reading several posts from Bruce Hislop about the nichrome wire/resistance wire. I actually PM'd him, but he did not respond. While my
> wiring harness is totally accessible, I would like to address the nichrome wire issue. Bruce had written at some point about installing a 10 ohm 25
> watt power resistor in series with a non-resistor type wire to replace the nichrome wire.
>
> His posts enlightened me quite a bit about this subject. I now understand that the GEN light receives power from both wires going to it, unless one
> wire is carrying less voltage than the other, which causes the light to light either dimly or brightly, dependent upon the voltage difference between
> the two wires. This would indicate either battery or alternator voltage dropping below normal levels.
>
> One thing I am not clear on is the exact purpose of the resistance wire. Is it to protect the GEN light bulb from burnout if there is no/low voltage
> on one wire?
>
> He had also posted that the APC cable may not prevent the resistance wire from overheat, thus the 10 ohm resistor in lieu of the APC cable.
>
> Bruce also posted that his coach was wired so that the resistance wire is connected to the RUN circuit of the switch, and not the ACCY circuit, which
> seems to be the factory norm. It is conceivable that leaving the switch in the ACCY position for some time could cause the wire to overheat. I
> understand the concept, but don't know exactly where to tap into the RUN circuits for the resistance wire connection.
>
> So my questions are: do I understand this correctly? What IS the best way to approach this problem?
>
> And if I change the resistance wire to the RUN circuit, where is the best place to cut/splice/tap?
>
> Thank you for your time.
>
>
> --
> Greg Crawford
> KM4ZCR
> Knoxville, TN
>
> "Ruby Sue"
> 1977 Royale
> Rear Bath
> 403 Engine
> American Eagle Wheels
> Early Version Alex Sirum Quad bags
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
_______________________________________________
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| [GMCnet] Re: Infamous Nichrome Alternator Wire [message #365959 is a reply to message #365956] |
Sat, 07 August 2021 21:27 |
Emery Stora
Messages: 959
Registered: January 2011
Karma: 4
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Senior Member |
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Gregg,
I agree with Emery completely. I well remember when he had his problem. I
immediately replaced my nichrome wire with the fixed resistor and stranded
wire. Later, after Gene came up with the APC (Alternator Protective
Cable), I installed it also, because the 150+ vac a failing alternator can
produce could also overheat the fixed resistor. I have that
resistor mounted in an A/C duct for cooling and it's isolated from
flammable material -- but no use taking a chance. During the 20 years the
APC has been in use, I've NEVER heard of a problem in a GMC with it
installed. There have been a FEW APC failures, causing non-excitation, but
not one, to my knowledge, has allowed overheating. Use the APC & stop
worrying, IMHO (with 61 years of electronics "tinkering" behind me).
Ken H.
On Sat, Aug 7, 2021 at 9:08 PM Emery Stora via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Greg
> The purpose of the nichrome wire is to “excite” the alternator so it will
> start producing current.
>
> I believe that I was the first one to have reported the problem with the
> nichrome resistance wire. The problem can only occur if there is a problem
> with the built in diodes (or the voltage regulator) in the alternator. The
> alternator can then push high voltage AC current back through the nichrome
> wire. This causes it to heat up and burn the insulation and short out many
> things that are in the wire bundle.
>
> When this happened to me I could not shut off the lights or the engine and
> I had smoke coming up from the dash cover by the passenger side
> windshield. I pulled my battery cable to cut off all voltage. When I
> removed my dash cover (which took a few hours of work) I discovered that I
> had 20 wires fused together and there were various shorts. I spliced all
> of the wires and I removed the nichrome wire from the bundle and ran it in
> its own insulating sleeve.
>
> This happened back in 2001. When I reported it to the GMC net and gave my
> theory that it was the alternator, Gene Fisher looked at the problem and
> decided that the circuit needed a diode to prevent this from happening. I
> installed an early version of his APC (alternator protection cable) and
> then when he made additional changes I installed his modified version. I
> felt this was an easy fix and it didn't take any time for me to do it.
> Just unplug the alternator wire and hook his APC in series with that wire.
>
> Since that occurred to me, several others then had the same problem so it
> can easily happen with a bad alternator. I am sure that there were some
> before me but they probably had a fire which destroyed their GMC before
> anyone could determine the cause.
>
> Besides using the APC, or a resistor one could also just simply put a fuse
> in series with the nichrome wire that would burn out if excessive current
> went backwards through that wire.
>
> I really recommend Gene Fisher's fix because of its simplicity and its low
> cost and easy installation. It basically is a diode put into series with
> the lead from the alternator that allows current to flow from the battery
> to the alternator to "excite" the field of the alternator so it will start
> producing a charge. The diode also prevents current from going up the
> nichrome wire toward the battery which prevents the heating problem if
> there is an alternator failure which produces the back current.
>
> I disagree that the APC may not prevent the resistance wire from
> overheating. I also disagree that leaving the ignition switch in the ACCY
> position could cause the wire to overheat. The overheating is caused by
> voltage from the alternator if you have components in the alternator fail
> and has nothing to do with the source of voltage from your ignition switch.
>
> Emery Stora
> emerystora@mac.com
>
>
>
>> On Aug 7, 2021, at 6:19 PM, Greg Crawford
> wrote:
>>
>> I am deep into my new dash project. I have spent the last couple of days
> neatening up the factory wiring harness at the fire wall, cutting some of
> the
>> old connectors, and crimping on new connectors for the new switches and
> gauges.
>>
>>
>> I've been reading several posts from Bruce Hislop about the nichrome
> wire/resistance wire. I actually PM'd him, but he did not respond. While my
>> wiring harness is totally accessible, I would like to address the
> nichrome wire issue. Bruce had written at some point about installing a 10
> ohm 25
>> watt power resistor in series with a non-resistor type wire to replace
> the nichrome wire.
>>
>> His posts enlightened me quite a bit about this subject. I now
> understand that the GEN light receives power from both wires going to it,
> unless one
>> wire is carrying less voltage than the other, which causes the light to
> light either dimly or brightly, dependent upon the voltage difference
> between
>> the two wires. This would indicate either battery or alternator voltage
> dropping below normal levels.
>>
>> One thing I am not clear on is the exact purpose of the resistance wire.
> Is it to protect the GEN light bulb from burnout if there is no/low voltage
>> on one wire?
>>
>> He had also posted that the APC cable may not prevent the resistance
> wire from overheat, thus the 10 ohm resistor in lieu of the APC cable.
>>
>> Bruce also posted that his coach was wired so that the resistance wire
> is connected to the RUN circuit of the switch, and not the ACCY circuit,
> which
>> seems to be the factory norm. It is conceivable that leaving the switch
> in the ACCY position for some time could cause the wire to overheat. I
>> understand the concept, but don't know exactly where to tap into the RUN
> circuits for the resistance wire connection.
>>
>> So my questions are: do I understand this correctly? What IS the best
> way to approach this problem?
>>
>> And if I change the resistance wire to the RUN circuit, where is the
> best place to cut/splice/tap?
>>
>> Thank you for your time.
>>
>>
>> --
>> Greg Crawford
>> KM4ZCR
>> Knoxville, TN
>>
>> "Ruby Sue"
>> 1977 Royale
>> Rear Bath
>> 403 Engine
>> American Eagle Wheels
>> Early Version Alex Sirum Quad bags
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
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| Re: Infamous Nichrome Alternator Wire [message #365960 is a reply to message #365955] |
Sat, 07 August 2021 21:45 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Senior Member |
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I didn't see Greg's PM until this morning... then the Honey-do list got in the way of finishing my reply to his PM.
I have posted it here for all to read.
Hi Greg,
Sorry for the delay in replying here. I typically only Log-in here if I see something I want to post about, so I didn't see your private message until now.
The Nichrome Wire provides the exciter current to the alternator field winding to get it generating power after engine startup. Once the alternator is running, it provides its own power for this. It is made from nichrome to provide 10 ohms of resistance to limit the current to the field winding. The only time there should be current flow in this wire is when the alternator is not producing power. This occurs during start-up and for some unknown reason, when the ignition key is in the ACCessory (ACCY) position. BUT it can also happen when the alternator fails.
When current is flowing in the Nichrome Wire, heat is produced. This is not normally an issue since its only for a few seconds. But can be an issue when current flows for a long period of time such as when the ignition switch is left in the ACCY position or the alternator fails.
The Alternator Protection Cable (APC) contains a diode that provides protection to the exciter wire if the internal voltage of the alternator exceeds the chassis battery voltage. This can happen if the Isolator fails (as well as several other modes of failure) My issue with this fix is it prevents the GEN light from coming ON during this type of failure. It also does not prevent heating of the cable when the key is left in the ACCY position for a long period of time.
My preferred solution is to use a 12v lamp in place of the Nichrome wire. It provides the required current-limiting resistance as well as a brighter indicator that there is a problem with the alternator. You could also replace it with a 10-20 ohm power resistor, but you lose the visual indication of a failure.
Note that 1975 models and later included a diode in series with the GEN light to fix a dull GEN light glow issue caused by the voltage drop across the isolator. This also prevents the GEN light from coming ON with the failure mode mentioned above. Using a Lamp in place of the Nichrome wire addresses this issue too.
A previous owner of my moved the Nichrome wire on my coach to the "Run" position of they key switch. There is an unused terminal on the switch that can provide this. From my readings on the subject, the alternator end of the Nichrome wire can be found at the bulkhead connector behind the fuse panel. The electrical diagram labels the exciter wire as being a BROWN wire. You can pull power from an IGNition power point on the fuse panel, but be sure to run the power through a diode, then through a lamp (or resistor). The reason for the diode is to prevent power generator by the alternator from back-feeding to the ignition once the switch is turned OFF. Otherwise the engine will continue to run!!!
Here is a simplified diagram of the alternator circuit. From this diagram, you want to replace the resistance with a lamp and move the diode to the ignition switch side of the exciter wire resistance.
http://www.gmcmhphotos.com/photos/alternator-wire-protection-diode/p68185-basic-alternator-operational-circuit.html
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Infamous Nichrome Alternator Wire [message #365963 is a reply to message #365960] |
Sun, 08 August 2021 01:45 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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The two solutions mentioned above attack the potential problem in different ways. If the alternator fails (the real cause if the problem) the APC diode blocks the reverse voltage / current from flowing and overheating the nichrome wire.
Installing a resistor and disconnecting the nichrome wire does not stop the current flow, but moves the high heat generation to the installed resistor. Doing that prevents the nichrome wire from melting other wires in the bundle.
This does not happen very often, BUT if it does you sure have a lot of dash wiring work to clean it up. If you decide to do the resistor, I would also install the APC. It is easy to install and cheap. (belt and suspenders)
I only have the APC on mine because it was easy to do. .
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Infamous Nichrome Alternator Wire [message #365975 is a reply to message #365955] |
Sun, 08 August 2021 15:03 |
Greg C.
Messages: 224
Registered: October 2019
Location: Knoxville, TN
Karma: 0
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Senior Member |
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Thanks everyone, for the replies.
I think I will take Bruce's advice and install a new wire along with an indicator lamp.
I am still somewhat confused though. It was stated that the 75 and later coaches had a diode in this circuit. Mine being a 77, is the ACP necessary?
Someone, somewhere, posted that you merely had to insert a diode in the wire leaving the alternator to accomplish the same thing as the ACP.
So, on my to do list is to replace the nichrome wire with a stranded tinned copper wire, insert an indicator lamp in series in this wire where it can be seen, move the power source to this wire from ACCY to IGN power.
And put a diode somewhere. Or the APC cable. Or both.
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
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| [GMCnet] Re: Infamous Nichrome Alternator Wire [message #365977 is a reply to message #365975] |
Sun, 08 August 2021 15:17 |
Emery Stora
Messages: 959
Registered: January 2011
Karma: 4
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Senior Member |
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Don’t just take the posting on the net or anywhere on the internet as the truth.
The diode in the circuit you refer to only helps keep the light from lighting on the dash. It does nothing to prevent high voltage back flow in case your alternator/voltage regulator fails.
Why go to all the effort you are proposing when just plugin in a ACP will take care of the problem. I' used it for many years and it has worked well. A lot of other GMCers are using it also.
It appears that you are reluctant to use an ACP. It is an inexpensive solution to the problem and quick to plug in. It’ll save you time and money.
Emery Stora
emerystora@mac.com
> On Aug 8, 2021, at 2:03 PM, Greg Crawford wrote:
>
> Thanks everyone, for the replies.
>
> I think I will take Bruce's advice and install a new wire along with an indicator lamp.
>
> I am still somewhat confused though. It was stated that the 75 and later coaches had a diode in this circuit. Mine being a 77, is the ACP necessary?
>
> Someone, somewhere, posted that you merely had to insert a diode in the wire leaving the alternator to accomplish the same thing as the ACP.
>
> So, on my to do list is to replace the nichrome wire with a stranded tinned copper wire, insert an indicator lamp in series in this wire where it can
> be seen, move the power source to this wire from ACCY to IGN power.
>
> And put a diode somewhere. Or the APC cable. Or both.
> --
> Greg Crawford
> KM4ZCR
> Knoxville, TN
>
> "Ruby Sue"
> 1977 Royale
> Rear Bath
> 403 Engine
> American Eagle Wheels
> Early Version Alex Sirum Quad bags
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
_______________________________________________
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| [GMCnet] Re: Infamous Nichrome Alternator Wire [message #365980 is a reply to message #365977] |
Sun, 08 August 2021 16:28 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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Senior Member |
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We provide a ACP , only difference is that the connections on our units are
soldered, not crimped.
We have seen several failures that caused issues.
On Sun, Aug 8, 2021 at 1:17 PM Emery Stora via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Don’t just take the posting on the net or anywhere on the internet as the
> truth.
> The diode in the circuit you refer to only helps keep the light from
> lighting on the dash. It does nothing to prevent high voltage back flow in
> case your alternator/voltage regulator fails.
> Why go to all the effort you are proposing when just plugin in a ACP will
> take care of the problem. I' used it for many years and it has worked
> well. A lot of other GMCers are using it also.
>
> It appears that you are reluctant to use an ACP. It is an inexpensive
> solution to the problem and quick to plug in. It’ll save you time and
> money.
>
> Emery Stora
> emerystora@mac.com
>
>
>
>> On Aug 8, 2021, at 2:03 PM, Greg Crawford
> wrote:
>>
>> Thanks everyone, for the replies.
>>
>> I think I will take Bruce's advice and install a new wire along with an
> indicator lamp.
>>
>> I am still somewhat confused though. It was stated that the 75 and later
> coaches had a diode in this circuit. Mine being a 77, is the ACP necessary?
>>
>> Someone, somewhere, posted that you merely had to insert a diode in the
> wire leaving the alternator to accomplish the same thing as the ACP.
>>
>> So, on my to do list is to replace the nichrome wire with a stranded
> tinned copper wire, insert an indicator lamp in series in this wire where
> it can
>> be seen, move the power source to this wire from ACCY to IGN power.
>>
>> And put a diode somewhere. Or the APC cable. Or both.
>> --
>> Greg Crawford
>> KM4ZCR
>> Knoxville, TN
>>
>> "Ruby Sue"
>> 1977 Royale
>> Rear Bath
>> 403 Engine
>> American Eagle Wheels
>> Early Version Alex Sirum Quad bags
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
--
Jim Kanomata ASE
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.gmcrvparts.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: Infamous Nichrome Alternator Wire [message #365986 is a reply to message #365955] |
Sun, 08 August 2021 19:02 |
Tom Katzenberger
Messages: 399
Registered: June 2019
Location: Kingsville, MD
Karma: 4
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Senior Member |
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Will the Digi Panel and/or guage be sufficient to indicate over or under charge situations?
Tom & Oki Katzenberger,
Kingsville, Maryland,
1977 23' Birchaven, 455 C.I.D., Micro Level, Howell EBL-EFI Spark Control, Macerator, York Air Compressor, 6 Wheel Disc, Quadra Bag, Onan W/Bovee Ignition
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| Re: Infamous Nichrome Alternator Wire [message #365988 is a reply to message #365955] |
Sun, 08 August 2021 19:42 |
Greg C.
Messages: 224
Registered: October 2019
Location: Knoxville, TN
Karma: 0
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Senior Member |
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I want to take the time to thank everyone who has responded to my questions. I now have a much better understanding of the purpose of the nichrome wire and associated wiring.
Emery, I'm not reluctant to purchase an APC. I like to get all the information that I can about a problem, make sure that I understand all the possible implications of various solutions, and then try to make an informed decision.
All these folks here have owned a GMC for a lot longer and a lot more miles than me, and I am very appreciative of all the time you take to answer foolish questions.
Like many (most) here, I am now into thousands of dollars and hundreds of hours on mine just trying to get everything working and safe as designed. I make mistakes frequently, and by asking questions I hope to minimize those.
Thanks to all once again!
Greg Crawford
KM4ZCR
Knoxville, TN
"Ruby Sue"
1977 Royale
Rear Bath
403 Engine
American Eagle Wheels
Early Version Alex Sirum Quad bags
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| Re: Infamous Nichrome Alternator Wire [message #366003 is a reply to message #365955] |
Mon, 09 August 2021 05:43 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Thanks Bruce. U am going to have to look at the schematic again and see what you are describing. I am always open to a better way.
Thank You
Ken B
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Infamous Nichrome Alternator Wire [message #369699 is a reply to message #365955] |
Thu, 19 May 2022 11:53 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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The DigiPanel will squawk at you on over voltage. And, though it doesn't seem to matter on the GMC charging circuit, the use of a lamp bulb or Nicrhome wire has a slightly different response to current than a fixed resistor. Any time you replace either in a circuit, see what they're there for.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: Infamous Nichrome Alternator Wire [message #369715 is a reply to message #365955] |
Thu, 19 May 2022 21:33 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Senior Member |
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The GEN lamp circuit will only provide about 0.1 Amp of excitation current, whereas the 10.0 ohm nichrome wire will provide about 1 Amp of excitation current, so the alternator will come to life quickly.
Normally the GEN light will come ON if the alternator output is either a few volts above or below the battery current. With the Isolator in circuit on the motorhome, along with the voltage drop on the alternator output wire under heavy current flow, GM had complaints about the GEN light dimly glowing. So in later coaches they added a diode in series with the GEN light. This compensated for the voltage drop across the isolator and fixed the dim glow GEN light issue. The problem I see with this fix is if the alternator output voltage climbed above the battery voltage as would be the case when the Isolator failed OPEN, this diode would block the current back through the GEN light and the driver would be blissfully unaware of any problem. Meanwhile the high voltage from the alternator could be damaging electrical devices in the coach. They should have added a second diode in the opposite direction in order to give an indication of high voltage fault.
The same thing happens when you install a APC cable. The diode in the APC allows current to flow to the alternator when the alternator voltage is low. But it blocks current flow when the alternator voltage fails high so you are unaware of a problem. An external lamp in place of the nichrome wire will provide the desired higher excitation current and also provide a visual indication of a problem. I added two diodes (in parallel but wired back to back) in series with the lamp to compensate for the Isolator voltage drop (no dim glow). This way I have indication on this lamp of an alternator failure in both high and low voltage. If the alternator voltage goes too high, the lamp will be extremely bright and may burn out, but I should be able to see this bigger, brighter lamp during a failure.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Infamous Nichrome Alternator Wire [message #369738 is a reply to message #365955] |
Sat, 21 May 2022 21:30 |
boybach
Messages: 566
Registered: December 2020
Location: Vancouver Island
Karma: 4
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Senior Member |
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What's in an APC? What do they look like and how long are they? Can I make one myself? I have lots of wire/cable in various gauges.
Larry
Larry - Victoria BC -
1977 ex-Palm Beach "Ol' Leaky" 40,000 miles, PO said everything working but forgot the word NOT. Atwood helium fridge, water heater & furnace. SS exhaust system, Onan, Iota Converter, R134A, New fuel lines & heat exchange hoses
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| Re: Infamous Nichrome Alternator Wire [message #369739 is a reply to message #365955] |
Sun, 22 May 2022 05:10 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Senior Member |
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The Alternator Protection Cable (APC) simply inserts a diode in the Exciter wire where it plugs into the alternator. The diode will allow current to flow into the alternator when the alternator output voltage is less than the battery.
IF the alternator output voltage is higher than the battery voltage, as happens in certain types of failures, the diode prevents current from flowing from the alternator back into the Exciter wire thus preventing it from overheating. However the blocking of this current will prevent the GEN light from coming ON and thus you are blissfully unaware there is a problem. On the '77 &'78 coaches, GM added a diode in line with the GEN light to prevent a "dim glow" caused by the combination of the Isolator voltage drop and wiring harness voltage drops when the alternator was delivering higher currents. This diode also prevents the GEN light from turn ON when there is a high voltage failure. They should have used a second diode in parallel with the first but wired in the opposite polarity to maintain the high voltage failure indication.
You can make your own APC cable with a 3 Amp or higher (200 volt plus) diode wired in series in the exciter wire (pin 1 of the alternator plug). The cathode (Band end) of the diode towards the alternator.
Here is a series of photos showing the nichrome wire and some solutions.
http://www.gmcmhphotos.com/photos/g3254-10-ohm-alternator-wire-77-2f78-models.html
This photo shows the APC cable addition of a diode at the alternator:
http://www.gmcmhphotos.com/photos/10-ohm-alternator-wire-77-2f78-models/p3731-apc-cable-circuit.html
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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