Home » Public Forums » GMCnet » Low voltage on startup (jumps up after revving the engine)
Low voltage on startup [message #256097] |
Tue, 22 July 2014 10:01 |
xplorid
Messages: 213 Registered: February 2012 Location: Boise
Karma: 0
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Hi all - just replaced the alternator for the 2nd time in 2 years, with a continuing "problem" that I guess needs to be addressed: my coach starts and runs well enough, but the voltmeter reads under 12 volts on startup, and needs the engine to rev up before the volts jump up to around 14. Once it jumps up it stays there, until the next start of course.
Cannot find a specific reference to this in prior posts, but now I know lots about the nichrome resistance wire problem and the need for APC (I'll be in contact, Mr. Fisher, and I will be in Yachats mid August).
So any help advice is welcome.
OBTW, I do have some other electrical issues, mostly around gauges fluctuating with electricity, as in the gas gauge goes down when I touch the brakes or turn on the lights, and goes up when accelerating, but I assume there is short or shorts elsewhere that are unrelated to the startup voltage thing. Just mentioning here in case there is some possible connection. More on the shorting stuff later...
Jeff
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: Low voltage on startup [message #256102 is a reply to message #256097] |
Tue, 22 July 2014 10:43 |
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Matt Colie
Messages: 8547 Registered: March 2007 Location: S.E. Michigan
Karma: 7
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xplorid wrote on Tue, 22 July 2014 11:01Hi all - just replaced the alternator for the 2nd time in 2 years, with a continuing "problem" that I guess needs to be addressed: my coach starts and runs well enough, but the voltmeter reads under 12 volts on startup, and needs the engine to rev up before the volts jump up to around 14. Once it jumps up it stays there, until the next start of course.
Cannot find a specific reference to this in prior posts, but now I know lots about the nichrome resistance wire problem and the need for APC (I'll be in contact, Mr. Fisher, and I will be in Yachats mid August).
So any help advice is welcome.
OBTW, I do have some other electrical issues, mostly around gauges fluctuating with electricity, as in the gas gauge goes down when I touch the brakes or turn on the lights, and goes up when accelerating, but I assume there is short or shorts elsewhere that are unrelated to the startup voltage thing. Just mentioning here in case there is some possible connection. More on the shorting stuff later...
Jeff
Jeff,
The low voltage issue is caused by a lack of excitation voltage to the alternator until it has a chance to get to a speed where it "self excites". That and all your other issues sound very much like bad connections in the instrument panel and associated areas.
Start taking things apart and and cleaning the connections.
Shorts are "Short Circuits" and that means a wrong path to ground. This does not sound like what you have.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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Re: [GMCnet] Low voltage on startup [message #256105 is a reply to message #256097] |
Tue, 22 July 2014 10:43 |
emerystora
Messages: 4442 Registered: January 2004
Karma: 13
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It might not be shorts. I may be poor grounds. I would suggest running a new ground wire from the aluminum plate where the battery isolator is mounted to the dash plate where the instruments are mounted. A number 10 wire would be good.
Emery Stora
> On Jul 22, 2014, at 9:01 AM, jeff sugheir wrote:
>
> Hi all - just replaced the alternator for the 2nd time in 2 years, with a continuing "problem" that I guess needs to be addressed: my coach starts
> and runs well enough, but the voltmeter reads under 12 volts on startup, and needs the engine to rev up before the volts jump up to around 14. Once
> it jumps up it stays there, until the next start of course.
>
> Cannot find a specific reference to this in prior posts, but now I know lots about the nichrome resistance wire problem and the need for APC (I'll be
> in contact, Mr. Fisher, and I will be in Yachats mid August).
>
> So any help advice is welcome.
>
> OBTW, I do have some other electrical issues, mostly around gauges fluctuating with electricity, as in the gas gauge goes down when I touch the brakes
> or turn on the lights, and goes up when accelerating, but I assume there is short or shorts elsewhere that are unrelated to the startup voltage thing.
> Just mentioning here in case there is some possible connection. More on the shorting stuff later...
>
> Jeff
> --
> 1973/94 GMCII in Boise 76 455 with headers & Paterson distributor,1 ton front, 4 bags back, Precision Steering Gearbox
>
>
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Re: [GMCnet] Low voltage on startup [message #256107 is a reply to message #256105] |
Tue, 22 July 2014 11:04 |
xplorid
Messages: 213 Registered: February 2012 Location: Boise
Karma: 0
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I am functionally ignorant of electrical circuitry, but like some other things related to my GMC obsession, that might not be true for much longer.
Are you guys treating my post as a singular problem, or is the alternator excitation issue separate from behind-the-dash issues (everything behind the dash has been hacked, either by Explorer in 1994, or by POs - I have removed lots of PO wires and connections from back there already).
Thanks
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: Low voltage on startup [message #256108 is a reply to message #256097] |
Tue, 22 July 2014 11:30 |
A Hamilto
Messages: 4508 Registered: April 2011
Karma: 39
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xplorid wrote on Tue, 22 July 2014 10:01Hi all - just replaced the alternator for the 2nd time in 2 years, with a continuing "problem" that I guess needs to be addressed: my coach starts and runs well enough, but the voltmeter reads under 12 volts on startup, and needs the engine to rev up before the volts jump up to around 14. Once it jumps up it stays there, until the next start of course.
Cannot find a specific reference to this in prior posts, but now I know lots about the nichrome resistance wire problem and the need for APC (I'll be in contact, Mr. Fisher, and I will be in Yachats mid August).
So any help advice is welcome.
OBTW, I do have some other electrical issues, mostly around gauges fluctuating with electricity, as in the gas gauge goes down when I touch the brakes or turn on the lights, and goes up when accelerating, but I assume there is short or shorts elsewhere that are unrelated to the startup voltage thing. Just mentioning here in case there is some possible connection. More on the shorting stuff later...
Jeff It may be that a PO has done the Jim Bounds mod ( http://www.gmccoop.com/alt__light_circuit_refit.htm ) and used a 100 ohm resistor and that is why the alternator does not want to self excite at idle.
If not, then you need the APC from Mr. ERF.
To answer your question if the dash wiring and the alt light are two different problems: Read the Jim Bounds write up above and Mr. ERF's info and you will see the two are intertwined. http://www.gmcmotorhome.info/engine.html#alternator
You only need one fix, either the APC from Mr. ERF OR the Jim Bounds mod, not both.
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Re: Low voltage on startup [message #256110 is a reply to message #256097] |
Tue, 22 July 2014 12:07 |
Bob de Kruyff
Messages: 4260 Registered: January 2004 Location: Chandler, AZ
Karma: 1
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xplorid wrote on Tue, 22 July 2014 09:01Hi all - just replaced the alternator for the 2nd time in 2 years, with a continuing "problem" that I guess needs to be addressed: my coach starts and runs well enough, but the voltmeter reads under 12 volts on startup, and needs the engine to rev up before the volts jump up to around 14. Once it jumps up it stays there, until the next start of course.
Cannot find a specific reference to this in prior posts, but now I know lots about the nichrome resistance wire problem and the need for APC (I'll be in contact, Mr. Fisher, and I will be in Yachats mid August).
So any help advice is welcome.
OBTW, I do have some other electrical issues, mostly around gauges fluctuating with electricity, as in the gas gauge goes down when I touch the brakes or turn on the lights, and goes up when accelerating, but I assume there is short or shorts elsewhere that are unrelated to the startup voltage thing. Just mentioning here in case there is some possible connection. More on the shorting stuff later...
Jeff
Have you checked the 2 wire connector on the side of the alternator to make sure it is fully seated ?
Bob de Kruyff
78 Eleganza
Chandler, AZ
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Re: Low voltage on startup [message #256112 is a reply to message #256097] |
Tue, 22 July 2014 12:46 |
xplorid
Messages: 213 Registered: February 2012 Location: Boise
Karma: 0
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Thanks for the attention. I do not recall any indications of the JimB mod or an APC in my 2 years of rooting around. I just replaced the alternator (actually 3 times because of faulty rebuilds) so I think connections at the alternator are clean and solid.
This alternator behavior is not new, done it since the day I picked up the coach in Orlando 2 years ago. Replaced the alternator the first time within the first week of ownership, and again last week. Same behavior - Dash gauge reads battery voltage on startup until the engine is revved up and voltage jumps up to alternator strength/gen light goes off.
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: [GMCnet] Low voltage on startup [message #256121 is a reply to message #256112] |
Tue, 22 July 2014 14:47 |
Ken Henderson
Messages: 8726 Registered: March 2004 Location: Americus, GA
Karma: 9
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Jeff,
Your symptom is that of a failure in the field excitation circuit.
Alternators require a small "initial excitation current" to be applied to
the field before they'll begin to produce current and "self excite".
If you check the wiring diagram, you'll see that pin 1 on the alternator,
the field excitation pin, is supplied from two sources: The GEN light and
the 10.0 Ohm resistance wire connected to the IGN buss. Since your GEN
light is working, the 10.0 Ohm resistance wire is NOT. It may be broken,
intentionally disconnected, or merely has a poor connection. Without that
wire, only the current through the bulb is exciting the field, so the
alternator must turn faster to achieve its self-excitation voltage.
If you install an APC without properly connecting the resistance wire, the
alternator may well not ever self-excite. The wire is brown with a single
white stripe. You can find it going into the left side of the fuse panel
behind the glove box. If you twist it between your fingers, you can feel
the insulation rotating on the smooth Nichrome wire inside. Tracing it to
the left side through the wire loom is a PITA. You want to just replace it
with a fixed 25 Watt 10 Ohm resistor connected between the Cruise Control
fuse and the brown 16 gauge wire which goes through the firewall connector
to the alternator. Insulate the resistor well and position it where it
can't overheat anything -- it will get hot if the ignition is left on
without the alternator producing voltage. If you put a 6A/600 VDC diode in
series with that resistor, you won't need an APC since that's what the APC
is.
HTH,
Ken H.
On Tue, Jul 22, 2014 at 1:47 PM, jeff sugheir
wrote:
> Thanks for the attention. I do not recall any indications of the JimB mod
> or an APC in my 2 years of rooting around. I just replaced the alternator
> (actually 3 times because of faulty rebuilds) so I think connections at
> the alternator are clean and solid.
>
> This alternator behavior is not new, done it since the day I picked up the
> coach in Orlando 2 years ago. Replaced the alternator the first time
> within the first week of ownership, and again last week. Same behavior -
> Dash gauge reads battery voltage on startup until the engine is revved up
> and voltage jumps up to alternator strength/gen light goes off.
> --
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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Re: [GMCnet] Low voltage on startup [message #256123 is a reply to message #256121] |
Tue, 22 July 2014 15:28 |
xplorid
Messages: 213 Registered: February 2012 Location: Boise
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All good to know. Ken, that's the answer I thought from reading everything, but was not sure. I'll try to update the thread when I get thru the work around for the nichrome for the benefit of others. In the meantime, is there real danger in driving as is?
1974/94 GMCII by Explorer
Manny 6.5 TD Al radiator
1 ton front 4 bags back
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Re: [GMCnet] Low voltage on startup [message #256124 is a reply to message #256123] |
Tue, 22 July 2014 15:34 |
Ken Henderson
Messages: 8726 Registered: March 2004 Location: Americus, GA
Karma: 9
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No, it's safer than when it was working right because in case of a runaway
alternator the bulb will probably blow before anything else is harmed, and
the Nichrome wire should not get hot to burn everything touching it.
Ken H.
On Tue, Jul 22, 2014 at 4:28 PM, jeff sugheir
wrote:
> All good to know. Ken, that's the answer I thought from reading
> everything, but was not sure. I'll try to update the thread when I get
> thru the
> work around for the nichrome for the benefit of others. In the meantime,
> is there real danger in driving as is?
> --
> 1973/94 GMCII in Boise 76 455 with headers & Paterson distributor,1 ton
> front, 4 bags back, Precision Steering Gearbox
>
>
> _______________________________________________
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> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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Re: [GMCnet] Low voltage on startup [message #256126 is a reply to message #256123] |
Tue, 22 July 2014 16:09 |
Mr ERFisher
Messages: 7117 Registered: August 2005
Karma: 2
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The apc cable is all you need for the nichrome wire
FREE WIFI @ Mickey D
On Jul 22, 2014, at 1:28 PM, jeff sugheir wrote:
> All good to know. Ken, that's the answer I thought from reading everything, but was not sure. I'll try to update the thread when I get thru the
> work around for the nichrome for the benefit of others. In the meantime, is there real danger in driving as is?
> --
> 1973/94 GMCII in Boise 76 455 with headers & Paterson distributor,1 ton front, 4 bags back, Precision Steering Gearbox
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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