Help!...Cab Electrical Problems from HELL!!!

[dyno]

I'm working on cleaning up my 1980 K20 4x4 cab wiring and I cannot figure out this problem. I have all the interior and dash torn out and all wiring is visible.

Instrument illumination, wiper switch and fuel gauge all have a related problem.

1. Wipers only work if you push wash. Tried 3 switches.

2. Wiper switch light stays on all the time with ignition on, instrument illumination lights off.

3. When Wiper switch is plugged into harness, fuel guage drops to empty, unplug it and fuel gauge comes back up. Push wash, wipers work and gauge comes back up, release and gauge goes to empty.

WITH WIPER SWITCH DISCONNECTED:

4. Rotate headlight switch to turn on instrument illumination, fuel gauge drops to empty. Turn them off and fuel gauge comes back up.

I have spent 4 nights trying to figure it out and I am stumped.


Thanks,

Dyno

[1971gmcshortbed]

I would bet money you are missing a ground somewhere.

[keith61760]

I have been sitting here reading this trying to figure out what may be going on. The only thing I can think of is a short somewhere. The only place I could think is in the dash area on the left side of the column. Thats the only area I can think where wires for those would be near each other.

[loverofchevystrucks]

I am thinking someone had been taping things. To be honest you should just rewire if your able to. Someone rewired my truck and if the widow switchs sparks kills the whole truck it a pain in the ass. Research around to find a original harness or a custom one.

[dyno]

Thanks for trying to help.

I am restoring the truck and have chopped out a pile of rat's nest wiring some moron had done previously. I eliminated a bunch of extra courtesy lighting and a after-market botched up A/C system. I may have cut something somewhere that I shouldn't have because it worked fine before.

[dyno]

Correction: I edited the post. Courtesy lights (dome light) are fine. It's the cluster lights (instrument illumination) that are part of the problem.

Originally Posted by 1971gmcshortbed

I would bet money you are missing a ground somewhere.

Question: Does the wiper switch need to be grounded by the screws that hold it to the dash? Wipers will only work if it's screwed onto the dash. The wiper illumination lamp stays on all the time when ignition is on - even when instrument illumination is off.

[soule64]

I ran into similar problems rewiring my '76. One thing you have to remember is that the wiper system uses a series of grounds to switch the motor on/off/fast. The only hot wires that run thru the switch is for the backlight and washer clutch. Also, switches for 76-77 are different internally than 78+ and are NOT compatible (you can only function off, park and fast OR slow). I found a new OEM switch on Ebay (advirtised as one 75-76, but actually for 78+) for $12 and installed it because the original one on the truck had the backlight housing broken. I could not get both the fast/slow to work no matter what combination of wires I used. I thought the motor might be bad, so I replaced it for one for 1976, but acted the same. I finally bought another new motor, this time for a '78, and it works perfect. Switches for 75-76 are not compatible with 78+ motors and vice-versa. The only wire that I have that comes in close contact with the with the guage cluster is the hot wire for the switch's backlight (gray) that taps the instrumant backlights (also gray). I spent three days with one of our mechanical engineers trying to figure it out - I have a picture of the white board we used develop a schematic, but I can't seem to find it on this computer. If I find it, I'll post it.. The switch does not ground thru the mounting screws (on mine, anyway).

[soule64]

I think this will work, but disregard the "+" signs in front of the "park, low, high" assignments, these are actually switched grounds to the motor. Also found a schematic for 77-80 with the switch called out.

[soule64]

Schematic...

[dyno]

You Rock!!! Thanks for the schematics. I will study them over the weekend and fiddle with it some more.

BTW, I found a loose black wire that was broken off at the factory butt splice connector. I tested it and it's in the instrument illumination circuit. Have no idea where it went, but will try and track it.

[dyno]

Originally Posted by 1971gmcshortbed

I would bet money you are missing a ground somewhere.

That was it!!! Found a broken off black wire under the dash, touched it to ground and EVERYTHING works correctly. The wipers work fine now also.

Thanks for your help,

Dyno

[1971gmcshortbed]

You got it man glad I could help.