Home » Public Forums » GMCnet » Chassis wiring questions
| Chassis wiring questions [message #344540] |
Tue, 25 June 2019 11:48  |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
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While it's great to have a full set of manuals complete with wiring diagrams, I've found they're woefully lacking when it comes to certain aspects. Specifically the generator and converter. They show the generator feeding directly into the chassis harness, no bother about the 120VAC to 12VDC conversion. They don't show the converter at all nor do they show the fuse panel above the converter and its associated circuits, at least not that I've found.
Looking at the wiring diagram seems to indicate that both batteries are charged when the generator is running or when plugged into shore power but I don't believe that's correct, at least it's not happening with my coach. But if I'm correct, what's the purpose of the battery diode? Unless your feeding both circuits, they both are already separate. And you can bridge them momentarily with the Battery Boost switch. And I've complicated my issue by relocating the house battery to a custom tray up front with the engine battery. While everything seems to function as it should if I'm correct that the engine battery doesn't get charged along with the house one. So again, why the diode bridge between them?
Ken H
1978 E II
1978 Eleganza II
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| Re: Chassis wiring questions [message #344548 is a reply to message #344540] |
Tue, 25 June 2019 13:49  |
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Tom Lins
Messages: 372
Registered: February 2004
Location: St Augustine, FL
Karma: 1
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kwharland wrote on Tue, 25 June 2019 12:48While it's great to have a full set of manuals complete with wiring diagrams, I've found they're woefully lacking when it comes to certain aspects. Specifically the generator and converter. They show the generator feeding directly into the chassis harness, no bother about the 120VAC to 12VDC conversion. They don't show the converter at all nor do they show the fuse panel above the converter and its associated circuits, at least not that I've found.
Looking at the wiring diagram seems to indicate that both batteries are charged when the generator is running or when plugged into shore power but I don't believe that's correct, at least it's not happening with my coach. But if I'm correct, what's the purpose of the battery diode? Unless your feeding both circuits, they both are already separate. And you can bridge them momentarily with the Battery Boost switch. And I've complicated my issue by relocating the house battery to a custom tray up front with the engine battery. While everything seems to function as it should if I'm correct that the engine battery doesn't get charged along with the house one. So again, why the diode bridge between them?
Ken H
1978 E II
If you are thinking the engine (chassis) battery gets charged by the generator you are incorrect. The chassis (engine) battery only gets charged by the alternator like JimK said. If you add a combiner to the setup then the generator can charge both sets of batteries.
Tom Lins
St Augustine, FL
77 GM Rear Twin, Dry Bath, 455, Aluminum Radiator Quad-Bag Suspension Solar Panel
Manuals on DVD
YOUTUBE Channel: GMC Dealer Training Tapes
http://www.bdub.net/tomlins/
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| Re: Chassis wiring questions [message #344550 is a reply to message #344540] |
Tue, 25 June 2019 15:26 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Stock the electrical systems are separate so as not to run down both batteries and get stranded. Stock they are both chargered by the alternator while driving through the diodes of the isolator. In an emergency you can hit the boost solenoid switch on dash to temporarily parallel them to start generator or engine. The addition of a Yandina combiner across the isolator gives the engine battery automatic charging when gen or shore power present. Normally it is OPEN but when the relay electronics detect charging voltage present it latches charging both. When no shore power no charge voltage dectected it unlatches giving 2 independent systems like stock. It's a no brainer best under $100 mod you can do.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Chassis wiring questions [message #344551 is a reply to message #344548] |
Tue, 25 June 2019 15:27 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
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No, I wasn't thinking that but I was wondering since that's not the case, what is the purpose of the battery diodes?
And why isn't there more schematic detail when it comes to the converter. Most everything else I've looked at is covered clearly and completely. To show that the generator output goes directly to the DC chassis circuits is just wrong. And it does show one leg to the house battery and one to the engine battery which we also know is incorrect.
1978 Eleganza II
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| Re: [GMCnet] Chassis wiring questions [message #344554 is a reply to message #344551] |
Tue, 25 June 2019 15:56 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Ken,
I just looked at the house 12VDC diagram (#180 on Alan's list of large
scale diagrams). I agree with you that the generator wiring connections
shown are incorrect. Perhaps they were correct when the Onan's on-board
charger was connected, but no longer. As for the engine generator wiring,
including the diodes (internal to the Isolator), that is correct. The
diodes are included to prevent either battery from inadvertently
discharging the other while allowing them to both charge when the engine is
running.
HTH,
Ken Hen.
On Tue, Jun 25, 2019 at 4:28 PM Ken Harland via Gmclist <
gmclist@list.gmcnet.org> wrote:
> No, I wasn't thinking that but I was wondering since that's not the case,
> what is the purpose of the battery diodes?
>
> And why isn't there more schematic detail when it comes to the converter.
> Most everything else I've looked at is covered clearly and completely. To
> show that the generator output goes directly to the DC chassis circuits is
> just wrong. And it does show one leg to the house battery and one to the
> engine battery which we also know is incorrect.
> --
> 1978 Eleganza II
>
> _______________________________________________
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Chassis wiring questions [message #344557 is a reply to message #344540] |
Tue, 25 June 2019 16:46 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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kwharland wrote on Tue, 25 June 2019 12:48While it's great to have a full set of manuals complete with wiring diagrams, I've found they're woefully lacking when it comes to certain aspects. Specifically the generator and converter. They show the generator feeding directly into the chassis harness, no bother about the 120VAC to 12VDC conversion. They don't show the converter at all nor do they show the fuse panel above the converter and its associated circuits, at least not that I've found.
Looking at the wiring diagram seems to indicate that both batteries are charged when the generator is running or when plugged into shore power but I don't believe that's correct, at least it's not happening with my coach. But if I'm correct, what's the purpose of the battery diode? Unless your feeding both circuits, they both are already separate. And you can bridge them momentarily with the Battery Boost switch. And I've complicated my issue by relocating the house battery to a custom tray up front with the engine battery. While everything seems to function as it should if I'm correct that the engine battery doesn't get charged along with the house one. So again, why the diode bridge between them?
Ken H
1978 E II
Ken,
Now you have me being curious.
What diagram are you looking at?
What is the title of the drawing?
Do you know where you got it?
I think I just now figured out how you are confused. If I am right, it is to be expected.
In the 77-78 Chassis, there is a part marked Gen Output Diodes.
In the 77-78 Living Area the same unit is marked Battery Separation Diodes.
What has you confused is that they were still calling the main engine alternator a generator.
What you may have also missed is the living area is the only diagram that also has both the Motor Generator (Onan) and the 120VAC to 12VDC converter.
The diode are what allow the main engine to charge both the main engine battery and the house bank.
The motor generator will only supply the 120VAC to the house systems and one of those is the converter to charge the house bank. That converter only charges the house bank. There is also a little voltage regulator that will charge a generator start battery that most coaches do not have and it has usually been removed.
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: Chassis wiring questions [message #344564 is a reply to message #344540] |
Wed, 26 June 2019 07:37 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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You can bench test the isolator from center to outboard posts with meter leads reveresed then normal on Ohms (4 tests)
Should get a low reading one way and high the other.
With engine running you should see about 14 V after cold start from each outer post to the aluminum plate mount Grd on 20VDC scale. Both sides engine and house should be very close to same reading on DCV. If you probe the center alternator post you should see about 14.7-15 volts DC. There is a .7 Volt drop from center to outer battery terminals across the diodes. But the voltage regulator gets it's reference from the engine battery side (actual charge Voltage to battery) after the diode so the system is "smart" in that respect. Study the circuit and it is a bit odd with a 12V junction post on the horn relay. Check those connections and spray the 2 pin connector on the back of the alternator at the voltage regulator input before removing the alternator for remote testing
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Chassis wiring questions [message #344566 is a reply to message #344564] |
Wed, 26 June 2019 11:15 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
Karma: 0
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JohnL455 wrote on Wed, 26 June 2019 08:37You can bench test the isolator from center to outboard posts with meter leads reversed then normal on Ohms (4 tests)
Should get a low reading one way and high the other.
With engine running you should see about 14 V after cold start from each outer post to the aluminum plate mount Grd on 20VDC scale. Both sides engine and house should be very close to same reading on DCV. If you probe the center alternator post you should see about 14.7-15 volts DC. There is a .7 Volt drop from center to outer battery terminals across the diodes. But the voltage regulator gets it's reference from the engine battery side (actual charge Voltage to battery) after the diode so the system is "smart" in that respect. Study the circuit and it is a bit odd with a 12V junction post on the horn relay. Check those connections and spray the 2 pin connector on the back of the alternator at the voltage regulator input before removing the alternator for remote testing
Thanks John, I did test the isolator but was only looking that the diodes were in fact being diodes. Unfortunately I didn't trust my readings and intend to go back today with a better meter that actually has setting for testing diodes plus some better test leads.
I like your idea about cleaning and spraying the regulator leads as everything is showing various levels of surface corrosion and readings were varying considerably. I have already taken the leads off the isolator and will be going back today with a wire brush and contact cleaner/lube. Again, could have been my meter as well so will know more today.
1978 Eleganza II
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| Re: Chassis wiring questions [message #344571 is a reply to message #344540] |
Wed, 26 June 2019 15:45 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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If you will think of the GMC as having three independent electrical systems with a few cross connections then it will simplify most of your diagnostic procedures.
The three systems are:
1. the 12 volt engine driven system which includes anything powered by the engine driven alternator. Anything from the front seats forward and all outside lights, except the one at the entrance door, are powered by this system. This includes the diode isolator and the engine battery. This system is the same on almost every GMC regardless of model and upfitter. It's main ground is the engine block. Everything associated with this system will be on the chassis wiring diagrams.
2. the 12 volt house wiring system. Everything here is powered by the house batteries and the converter. This includes everything 12 volt inside the coach not listed in item 1 above and the outside porch light. This will vary by coach model and also upfitter (Coachman, etc). There are several different diagrams that cover inside the coach based on the model that you have. The 12 volt current carrying ground for this system is the aluminum frame or body of the coach (Not the engine or steel frame of the coach.) You need to make sure that you have the correct diagram before starting.
3. the 120 volt AC system which is powered by either shore power or an onboard generator. Most coaches had an Onan 120 volt (only) 4KW or 6KW generator, but at least one other brand was also used. The generator is a separate manual and connected to the coach using the shore power cable. The 120 volt system does not have a current carrying ground. It does have a safety ground connected to the aluminum body of the coach and the ground pin of the shore power cord. There is no connection anywhere in the coach between safety ground and the current carrying 120 volt neutral wire. That connection is done at the power source which is shore power or at the generator (usually Onan) depending on where you have the shore power cable plugged in.
So when working on the GMC you need to determine which of the three systems you are working with before starting your diagnosis.
There are several cross connections between the above 3 systems:
Between Item 1 and 2 above there is a manually operated boost relay used to supply a high current connection for temporary power to be used for main engine or Onan engine starting.
Between item 1 and 2 there is also a lower current one way
connection to allow house battery charging from the engine driven alternator.
Note: Because 12 volt systems use ground as a current conductor and item 1 and item 2 above use different ground systems, there is also a cross connection of braided straps from the back of the transmission across the rear transmission mount to the steel frame, and a second strap from the aluminum body across the front right body mount to the steel frame. There is a third strap near the Onan if you have the Onan generator option installed from the factory. This strap is not installed on some non-GMC upfitted and transmode coaches. These braided straps provide a high current 12 volt return between path the two systems and are frequently degraded or broken on these 40+ year old coaches. Nothing, other than these straps, is ever directly electrically connected to the steel frame of a GMC coach.
Between item 2 and item 3 above there is a converter. It's sole purpose is to take 120 volt AC and convert it to 12 volts DC to run 12 volt items, including house battery charging, in the house.
That's it. Good luck. HTH
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Chassis wiring questions [message #344573 is a reply to message #344540] |
Wed, 26 June 2019 18:37 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
Karma: 0
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Senior Member |
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After disconnecting all wires from the isolator, I've identified it as the problem. It's an open circuit between the center post (alternator) and the post connected to the engine battery so with the engine running that battery isn't being charged and the voltage regulator doesn't have a reference voltage so it's essentially not regulating the alternator output.
And the diode between the center post and the chassis battery is no longer a diode, just a resistor. So I'll be shopping for new isolator this evening.
Thanks to all for the responses, I'll miss that once I sell our coach.
Ken H
1978 E II
PS Still looking for some insight as to the fuse panel above the converter, is there a schematic somewhere?
1978 Eleganza II
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| Re: [GMCnet] Chassis wiring questions [message #344577 is a reply to message #344566] |
Wed, 26 June 2019 19:54 |
richshoop
Messages: 190
Registered: April 2017
Karma: 0
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A better cleaning item for electrical contacts is Scotchbrite. Wire brushes tend to embed the bristle material in the terminals and once you use a wire brush on one kind of metal, it is contaminated and will redeposit the base material on whatever you are scrubbing on next. Use the Scotchbrite and throw it away after the use!
> On June 26, 2019 at 9:15 AM Ken Harland via Gmclist wrote:
>
>
> JohnL455 wrote on Wed, 26 June 2019 08:37
>> You can bench test the isolator from center to outboard posts with meter leads reversed then normal on Ohms (4 tests)
>> Should get a low reading one way and high the other.
>> With engine running you should see about 14 V after cold start from each outer post to the aluminum plate mount Grd on 20VDC scale. Both sides
>> engine and house should be very close to same reading on DCV. If you probe the center alternator post you should see about 14.7-15 volts DC. There
>> is a .7 Volt drop from center to outer battery terminals across the diodes. But the voltage regulator gets it's reference from the engine battery
>> side (actual charge Voltage to battery) after the diode so the system is "smart" in that respect. Study the circuit and it is a bit odd with a 12V
>> junction post on the horn relay. Check those connections and spray the 2 pin connector on the back of the alternator at the voltage regulator input
>> before removing the alternator for remote testing
>
> Thanks John, I did test the isolator but was only looking that the diodes were in fact being diodes. Unfortunately I didn't trust my readings and
> intend to go back today with a better meter that actually has setting for testing diodes plus some better test leads.
>
> I like your idea about cleaning and spraying the regulator leads as everything is showing various levels of surface corrosion and readings were
> varying considerably. I have already taken the leads off the isolator and will be going back today with a wire brush and contact cleaner/lube.
> Again, could have been my meter as well so will know more today.
> --
> 1978 Eleganza II
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: [GMCnet] Chassis wiring questions [message #344578 is a reply to message #344571] |
Wed, 26 June 2019 19:51 |
richshoop
Messages: 190
Registered: April 2017
Karma: 0
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Senior Member |
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Ken is spot on about considering the electrical systems for our coaches as three different individual systems. Please keep in mind that the first two systems use one kind of wiring and the third (the 120V) uses a different kind. Be careful to use the proper wire!
Always, when adding anything 12V to the coach, to ALWAYS have a fuse at the feed end! (I know I said always twice, but it is so important)
> On June 26, 2019 at 1:45 PM Ken Burton via Gmclist wrote:
>
>
> If you will think of the GMC as having three independent electrical systems with a few cross connections then it will simplify most of your diagnostic
> procedures.
>
> The three systems are:
>
> 1. the 12 volt engine driven system which includes anything powered by the engine driven alternator. Anything from the front seats forward and all
> outside lights, except the one at the entrance door, are powered by this system. This includes the diode isolator and the engine battery. This
> system is the same on almost every GMC regardless of model and upfitter. It's main ground is the engine block. Everything associated with this
> system will be on the chassis wiring diagrams.
>
> 2. the 12 volt house wiring system. Everything here is powered by the house batteries and the converter. This includes everything 12 volt inside
> the coach not listed in item 1 above and the outside porch light. This will vary by coach model and also upfitter (Coachman, etc). There are several
> different diagrams that cover inside the coach based on the model that you have. The 12 volt current carrying ground for this system is the aluminum
> frame or body of the coach (Not the engine or steel frame of the coach.) You need to make sure that you have the correct diagram before starting.
>
> 3. the 120 volt AC system which is powered by either shore power or an onboard generator. Most coaches had an Onan 120 volt (only) 4KW or 6KW
> generator, but at least one other brand was also used. The generator is a separate manual and connected to the coach using the shore power cable.
> The 120 volt system does not have a current carrying ground. It does have a safety ground connected to the aluminum body of the coach and the ground
> pin of the shore power cord. There is no connection anywhere in the coach between safety ground and the current carrying 120 volt neutral wire. That
> connection is done at the power source which is shore power or at the generator (usually Onan) depending on where you have the shore power cable
> plugged in.
>
> So when working on the GMC you need to determine which of the three systems you are working with before starting your diagnosis.
>
> There are several cross connections between the above 3 systems:
>
> Between Item 1 and 2 above there is a manually operated boost relay used to supply a high current connection for temporary power to be used for main
> engine or Onan engine starting.
>
> Between item 1 and 2 there is also a lower current one way
> connection to allow house battery charging from the engine driven alternator.
>
> Note: Because 12 volt systems use ground as a current conductor and item 1 and item 2 above use different ground systems, there is also a cross
> connection of braided straps from the back of the transmission across the rear transmission mount to the steel frame, and a second strap from the
> aluminum body across the front right body mount to the steel frame. There is a third strap near the Onan if you have the Onan generator option
> installed from the factory. This strap is not installed on some non-GMC upfitted and transmode coaches. These braided straps provide a high current
> 12 volt return between path the two systems and are frequently degraded or broken on these 40+ year old coaches. Nothing, other than these straps, is
> ever directly electrically connected to the steel frame of a GMC coach.
>
> Between item 2 and item 3 above there is a converter. It's sole purpose is to take 120 volt AC and convert it to 12 volts DC to run 12 volt items,
> including house battery charging, in the house.
>
> That's it. Good luck. HTH
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: Chassis wiring questions [message #344659 is a reply to message #344540] |
Sat, 29 June 2019 09:11 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Senior Member |
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Glad your Ohm meter pointed you to a failed isolator. That test showing a "fail" is a good go/ no go test. The opposite condition though rare is where it meters ok, fails under real world but you can be sure yours is shot. There are lots of slightly more robust units out there for a good price. I believe mine is a 120A rating. I would trickle charge both battery banks and load test them ahead of firing up your rig. Usually it's a bad battery that takes out the charge system. As they fail the impedance can drop and they become "electric heaters" when charge current applied, overloading the charge system. And you will have peace of mind that your batteries will get you home when needed.
Once system is opperating correctly invest in the Yandina combiner to mount next to your isolator area. One ground and 2 battery leads to the isolator B1 and B2 terminals will bring an update into the 21st century.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Chassis wiring questions [message #344664 is a reply to message #344659] |
Sat, 29 June 2019 12:32 |
kwharland
Messages: 246
Registered: November 2005
Location: Central Florida
Karma: 0
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Senior Member |
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Thanks John and I'm only readying it to be sold. And a bad battery is probably what did in the isolator, I recently replaced both and will follow your suggestion about getting a good charge on them before connecting a new isolator.
1978 Eleganza II
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| Re: Chassis wiring questions [message #344723 is a reply to message #344659] |
Mon, 01 July 2019 13:44 |
sbrenton
Messages: 6
Registered: June 2019
Karma: 0
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Junior Member |
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JohnL455 wrote on Sat, 29 June 2019 09:11
Once system is opperating correctly invest in the Yandina combiner to mount next to your isolator area. One ground and 2 battery leads to the isolator B1 and B2 terminals will bring an update into the 21st century.
I happened to find and read this thread and since I have a new (to me) GMC and I have a bad diode in the "Isolator" it's time for me to do something. Does the Yandina Combiner completely replace the diode set which was original in the GMC? Is Yandina comparable to Blue Sea?
Thanks in advance
Stephen
Stephen Brenton - 1973 Painted Desert 23ft
Round O, South Carolina. The Lowcountry.
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| Re: Chassis wiring questions [message #344730 is a reply to message #344540] |
Mon, 01 July 2019 15:27 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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Noted someplace, Alan Henderson runs a computer printing business, and does some phenomenal stuff with BIG format printers. If you weren't at the recent rally, they're available by mail/ups. If I had a GMC in the shop as a regular thing, I'd have them on the wall on hardboard of some type. We always put a blowup of the transmitter schematic on the wall. Mods in red, dated etc. For temp mods hang a piece of Saran Wrap <tm> over it and use it to write changes on
--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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