Chassis wiring questions

K

ken harland

New member
Dec 30, 2005
292
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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
 
Ken, both coach and engine batteries are charged from the alternator.
The alternator is connected to the center tap of the diode bridge and the
coach battery is connected to one of the ends and the engine battery is
connected to the other. This allows the alternator to charge both coach
and engine batteries with no direct connection between the two.
ronC

Ron & Linda Clark
North Plains, ORYGUN
1978 Eleganza II

On Tue, 25 Jun 2019 10:48:59 -0600 Ken Harland via Gmclist

> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

Ron & Linda Clark
North Plains, ORYGUN
1978 Eleganza II
 
Ken,
Learn to reach out to us to help.
You will not find any SOB coaches that has diagrams anywhere close to ours
and dealers like us for these old parts. SOB means Some Other Brand

On Tue, Jun 25, 2019 at 11:02 AM rallymaster--- via Gmclist <

> Ken, both coach and engine batteries are charged from the alternator.
> The alternator is connected to the center tap of the diode bridge and the
> coach battery is connected to one of the ends and the engine battery is
> connected to the other. This allows the alternator to charge both coach
> and engine batteries with no direct connection between the two.
> ronC
>
>
> Ron & Linda Clark
> North Plains, ORYGUN
> 1978 Eleganza II
>
> On Tue, 25 Jun 2019 10:48:59 -0600 Ken Harland via Gmclist

> > 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
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> >
>
>
> Ron & Linda Clark
> North Plains, ORYGUN
> 1978 Eleganza II
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
> 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

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, FI-Tech EFI
Manuals on DVD
http://www.bdub.net/tomlins/
 
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
 
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
 
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 <

> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> 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

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 - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
Thanks to both Tom and John L, while I didn't think the engine battery was charged by genset or shore power, I didn't realize that the house battery
is charged by the alternator which explains the diode's purpose.

And right now I think I've got an alternator problem because it's not charging anything! I'll have it tested tomorrow and know for certain and then I
can check the function of the diodes under alternator charging conditions.

And thanks for confirming that the schematics are in error. With these old coaches you never know what modifications have been done by previous
owners. I'm lucky in this one is fairly original in all respects...at least until I got ahold of it :)
--
1978 Eleganza II
 
> Ken, both coach and engine batteries are charged from the alternator.
> The alternator is connected to the center tap of the diode bridge and the
> coach battery is connected to one of the ends and the engine battery is
> connected to the other. This allows the alternator to charge both coach
> and engine batteries with no direct connection between the two.
> ronC
>
>
> Ron & Linda Clark
> North Plains, ORYGUN
> 1978 Eleganza II

Thanks, I've learned that from other replies as well.

> Ken,
> Learn to reach out to us to help.
> You will not find any SOB coaches that has diagrams anywhere close to ours
> and dealers like us for these old parts. SOB means Some Other Brand

Thanks Jim, I was referring to the GMC manuals, I have a full set.
--
1978 Eleganza II
 
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
 
> 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
 
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
 
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
 
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)

>
>
> 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
 
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!

>
>

> > 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
 
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
 
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
 
> 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
 
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