Wiring Issues. 81 gmc.

[hickvillemayor]

I have a 1981 GMC serria. The truck has a 454, is 4 wheel drive and has dummy lights. I spent all day today trying to solve an issue, I hope someone might have run into before.

Everything works fine with no issues...until.... You turn the lights on. When you turn the lights on... The fuel gauge drops to empty, signals no longer work, lights in the dash go dim.

I figured it was the headlight switch itself... But changed it out and it did not make any difference.

The next thing I will try is to change the whole gauge housing out.
Anyone else got an idea?

Thank you in advance.

 

[chengny]

The next thing I will try is to change the whole gauge housing out.

Nah, don't start there. Those issues you mention are almost certainly symptoms rather than the cause.

The first diagnostic step would be to determine if there is a total system voltage drop occurring when the headlights are turned on. The 3 things you mention as acting up (gas gauge, blinkers and dash illumination) are unrelated circuits and fed by three different fuses. They all use different ground points as well. The blinkers ground to the radiator frame, the dash lights ground to the common bus block under the dash and the fuel gauge grounds to the frame back by the gas tank.

I will make the assumption that this issue only appears with the H/L switch is in the headlights position - not when its in the parking lights position. True?

First, check that it is only when the H/L switch is pulled all the way out that these symptoms appear.

If you find that to be the case - you have identified the problem as existing in the H/L circuitry.

Dig a little deeper and determine if it only happens with either the high beams or low beams; or does it happen with the dimmer switch in both positions.

Make sure you have a good healthy battery.

Check the condition of both battery cables and make sure that the connections at both ends are clean and tight.

If you find nothing wrong with the battery or cables, pull your alternator and have it tested at a local auto parts store - they normally will bench test it for free.

If you check all of the above and everything appears to be okay - but you still have a problem - you will have to determine whether there is a total system voltage drop. If you had gauges that would be as easy as turning the headlights on and observing the volt meter. If it plunged down with the headlights energized...that would be proof.

But since you have no voltmeter, you will have to test the system voltage with a VOM/multimeter. They are cheap - $20 bucks at Radio Shack will get you what you need.

If none of my suggestions help, get a multimeter and someone here will walk you through how to test system voltage.

But seriously, don't take the drastic action of tearing your dash apart as a first step.

 

[hickvillemayor]

I will make the assumption that this issue only appears with the H/L switch is in the headlights position - not when its in the parking lights position.
Very true.

These symptoms appear at anything besides pushed all the way in.

It happens on either high or low... makes no difference... unless you turn the radio stat soo that the dash lights are off... then it makes most of the issues disappear.

13 volts when not running. 15 volts when running. 12.5 volts roughly when the lights are all on and the stuff is going wonkers.

Any ideas?

Thanks.

 

[chengny]

I will make the assumption that this issue only appears with the H/L switch is in the headlights position - not when its in the parking lights position.
Very true.

These symptoms appear at anything besides pushed all the way in.


These two statements above appear to be contradictory.

The first one says you agree with me that the problem does not appear when the H/L switch is in the parking lights position (i.e. halfway out)

The next statement says that it happens unless the H/L switch is pushed all the way in. I take that to mean it can happen when the H/L switch either in the 1/2 way or fully out position.

I'm going with - it can happen if the H/L switch is in either the parking lights or the headlights position.

Since the only lights (and gas gauge) that are affected are in the instrument panel - and turning the dimmer to it's lowest intensity seems to "make most of the issues disappear", let's think about the Instrument Lamps circuit.

First I gotta ask - if dim panel lighting is the main issue and it is resolved by turning it down...what is "most of the issues" that disappear when the rheostat is turned down? The lights will be dim, but does the gas gauge return to operation and the blinkers start working?

Also when you say the blinkers are affected, does that mean the whole system (front/rear directional lamps, side marker lamps and dash indicators) or just the dash indicators?

Try this quick test;

Pull the fuse in the slot labeled INST LPS - that would be the lowest fuse in the far LH column and pull the H/L switch out all the way.

Now with the instrument lighting disabled, observe whether the gas gauge and directionals function normally.

I am starting to think you have a short in the IP lighting circuit to the panel ground. I would be more confident if you said only the RH directional wasn't working. Anyway pull the IP lighting fuse and see if everything else returns to normal.


BTW - did this just start happening all of a sudden, when you got the truck it was like this, it started a while ago and now it happens all the time, last week I hit a tree and it been like this ever since, etc.?

Give us some history.

 

[hickvillemayor]

Oh snap. I didn't even know there was a parking lamp setting. LOL I guess it has not worked for a long time.

Yes the blinkers start working properly, instead of both of them flashing, one brighter than the other... and the gas gauge goes back to working again. The blinkers get affected all over the truck.

I bought the truck about ummm 5 years ago. It was not very well taken care of. A lot of easy fixes had not been done.

This just started about 2 weeks ago out of the blue... I did notice for a bit.. when engaging the 4 wheel drive the e brake dummy light would come on. That was about 4 weeks ago.. then It turned into this... I took the dash apart down to the gauges and replaced the 4 burn out lights. 2 of which were the non ground dash lights. The only one that was working was the grounded one. I have not seen the ebrake light come on since, but the gas gauge and blinkers thing started... Hence why I was thinking short in the circuit board on the gauge pod.

Thanks, Kurt.

 

[chengny]

So it does appear to be a short in the instrument lighting circuit - right? With the panel lights fuse pulled, the other symptoms go away?

The gray 8 circuit is not huge and stays fairly local to the fuse block. The fuse only feeds 5 components (see the marked up wiring diagrams below) and the fault should be easy to locate.

The instrument illumination circuit:

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[hickvillemayor]

getting there...

I tried today taking that 5 amp fuse out and what do you know everything works fine, even with the dimmer turned.
I am soo happy I have it knocked down to one circuit. This also give me the ability to drive at night again, just without dash lights and have everything work.

YOU DA MAN!!

Kurtis.

 

[hickvillemayor]

Fail

Soo Today I ran the truck with no 5 amp fuse in and it still does it... But only on high beam? Soo I guess now it would be a good idea to check the high beam circuit? It shows both signals flashing but only on the dash. I checked the truck tail lights and they both don't flash even though that is what the dash shows..

Thanks.

 

[crpntr78]

May I ask a question also? Similar but different. I am having trouble with my headlights working or not working actually. I turn on the headlights but all I get are the parking lights to light up. No headlights at all. Tried flipping to brights, same result. Tail lights work along with all turn signals. Dash lights don't come on either. Dome light comes on and stays on but I think it may be the door switch. I have replaced the headlight switch. No blown fuses or burnt wires that I have found. Any other options I may be missing? Thanks.

 

[crpntr78]

Well the dome light problem was not the door switch. They still stay on constantly. The only way to get them off is by unhooking the battery. Also tried another front light wiring harness and with this harness I don't even have parking lights. So I've tried a 4 light harness and a 2 light harness and the only difference is that with the 4 light harness I get parking lights.

 

[hickvillemayor]

Check your grounds.

 

[Georgeb]

The dome light is activated by grounding the white wire. It gets ground from either jam switch or the headlight switch by twisting the knob. Check to see if anyone has cut ot spiced into that white wire, if the saitch is turned on or if the white wire is pinched or shorted to ground.

 

[crpntr78]

Okay, I will give them a thorough going over. Another question, I'm full of them, Last night while working around the exterior harness at the firewall and I noticed a pinkish colored wire that the PO had cut. I checked it for hot with key on and it was hot but faintly with the test light. It's located on the lower left corner of the exterior fuse block harness as youre looking at it from the front. Any ideas. I have a hard time reading schematics. Thanks again for any and all help.