Question: Where is the dash and radio grounded?

Joe Weir

Joe Weir

Member
Feb 13, 2013
743
9
18
Was there a particular spot in the Coaches that was used as a ground for the dash components? With a plastic dash, a fiberglass "firewall", and lots
of single wire power connections, my shade tree car skilz are challenged.

There must be a ground strap somewhere. Anyone remember off the top of their head?

`

--
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen
guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
 
> My 77 Palm Beach has a multiway 1/4" quick-connect behind the back of the radio. It has room for about 8 connections if I recall.
>
> Not sure if its OEM or something added.

And that would supposedly go back to the engine/chassis eventually...?

--
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen
guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
 
The main ground for all dash components powered by the engine alternator and battery is the metal plate that all of the instruments are mounted in.
On the back of that plate (lower right corner IIRC) there is one wire that goes from there through the firewall and over to the aluminum plate under
the passenger side hood that the isolator is mounted on. There is a second wire on that under hood plate that goes direct from there to the negative
battery cable and post. A common failure is for the wire on the back of the dash plate to oxidize or come loose.

The engine (NOT the body or frame) is the master ground for ALL devices powered by the Engine alternator and and battery. Never attach anything to
the body or the the frame.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Thanks, Ken,

Wires could have been disturbed at either end while the dash was apart, or cleaning up under the hood. Good to know about that aluminum plate.

--
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen
guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
 
There originally were ground straps which grounded the aluminum body to the
engine cradle and the engine cradle to the transmission at the chain drive
area. I assume the trans grounds to the engine at the engine/trans
interface. If you haven’t checked to see if those grounds exist still and
are in good condition it would be a good idea to do so as this will ensure
any body grounds which exist for the coach 12v systems to allow proper
electrical function.

Sully
Bellevue wa.

On Tue, Apr 21, 2020 at 12:25 PM Joe Weir via Gmclist <

> Thanks, Ken,
>
> Wires could have been disturbed at either end while the dash was apart, or
> cleaning up under the hood. Good to know about that aluminum plate.
>
> --
> 76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator,
> brakes, Sully airbags, fuel lines, seats, adult beverage center... those
> Coachmen
> guys were really thinking about us second hand owners by including that
> beverage center...
> Columbia, SC.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
The ground strap between the engine (actually the rear of the trans) and the frame is only used when charging the house batteries from the engine
driven alternator, when using the boost switch, and using the cruise control unit which is mounted to the body but powered by the engine electrical
system.

Usually if that jumper or the other two braided jumpers between the body and the frame are going bad you will first notice it as the cruise control is
failing or intermittent.

Those braided jumpers have absolutely nothing to do with anything engine electrical powered including all outside lighting and any inside lighting or
accessory from the inside step forward in the coach.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
I'd like to add to KenB's post that the Engine-Frame and Body-Frame ground straps are important connections between the Engine, Frame and body
grounds. They should be checked for integrity as part of your regular maintenance routine.

They bond the engine-frame-body grounds together so there is no Potential Difference (voltage difference) between any of these grounds. Otherwise
current would flow through other metal connections between them (ie. through the steering column to the shifter cable to the transmission & engine).

They carry additional currents not mentioned by Ken:
- House loads which are operating while the engine alternator is running (roof fans, lights, water pump, DC Fridge, inverter)
- If you have a Combiner, current flows through the ground straps to Engine battery while on shore power. (converter is grounded to Body)
- The Door switch grounds through the body to operate the key buzzer and lights (if connected)
- From the wiring diagram ('77 in my case) All the rear lights (Clearance, Stop, Backup) are grounded at a point called "Main Rear Ground" which it
shows as a Ground Symbol. Unfortunately GM did not use different ground symbols for Engine, Frame or Body grounds, so I am assuming this ground is
either Frame or Body ground convenient to rear lights wiring harness.

Of course there could be electrical equipment added or changed over the years which has been grounded to a convenient place, but not necessarily the
correct place.

Above all, the main ground cable from the Chassis Battery negative terminal must be connected to the ENGINE BLOCK.

I noticed on the '77 chassis wiring diagram, it shows the Aux (House) battery grounded to the frame, while the '77 livingarea (house) 12VDC diagram
shows it grounded to the body. On my '77 Palm Beach the House battery is grounded to an OEM stud on the body beside the battery. So I assume the
Chassis diagram is incorrect in this case.

--
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
 
Bruce you are correct about the GMC schematic saying "frame". Frame in that case is the Aluminum body of the coach and not the the steel frame the
body rests on.

I did not cover anything in my original posting because house or body grounding has to do with items in the "house" powered portion of the coach. He
wanted the dash which is engine side grounding.

I gave a seminar about 10 years ago on GMC grounding. It is around here somewhere. I probably ought to find it and dust it off. In that seminar I
explained that there are 3 ground systems. One for everything powered by the engine and it's battery, One powered by the House system and it's
battery, and the 120 volt AC "safety" system that should carry no current. There are a few crossovers between the systems and that is where those
two (or three) braided jumpers come in. Obviously those jumpers need to be in great shape for those crossover function (only) to work plus the Cruise
control which was grounded in the wrong place when installed by GM. I have fixed a few of them by simply changing where they are grounded.

The three jumpers are located:
1. On the back of the transmission.
2. Across the body mount up front of the right front wheel.
3. If you have a factory installed Onan, in the left rear generator cabinet behind the Onan on the floor is a stud mounted in the body.

HTH

Ken B
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Great information.

In my case, the steel dash panel had no ground wires attached. I found a 5" strip of wire with nothing on one end, and a a crimped wire connector on
the other with 3 other wires, right behind the radio. Crimp fitting looks newish.

May have pulled that dash ground loose fitting the radio. it was probably hanging by a thread anyway. Lights were spotty, then didn't work after
the great dash upgrade. Then finally hooked up radio and it was dead. Even a drunk squirrel gets a clue eventually.

Tracing back from the aluminum panel I found two (2) additional ground wires coming through the firewall not connected to anything under the dash.
One looks factory, one PO. Though they could both be PO if done far enough back. So, I'll wager the dash ground has been a mystery before.

Its not just an RV, its an adventure...wrapped in a mystery, engulfed in bankruptcy...

--
76 Birchaven - "Wicked Mistress" - New engine, trans, alum radiator, brakes, Sully airbags, fuel lines, seats, adult beverage center... those Coachmen
guys were really thinking about us second hand owners by including that beverage center...
Columbia, SC.
 
I assume that you are going to use one of those wires you found and attach it to the metal dash plate. That wire has been a problem for many others
in the past. Where ever you attach it to the plate make sure that connection is very clean and tight. The aluminum plate tends to oxidize over time
with the connection still looking clean and tight. I have also seen the headlight switch mounted to the aluminum dash plate do the same thing. When
it fails the overhead lights do not work by rotating the headlight switch.

Ken B.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
You must log in or register to reply here.
GMCMI Fall 2026 Rally
Applied GMC

Share this page

Share this page
Top Bottom
Back