Ground problem?

[Erik Anderson]

I have a 75 GMC K25. I'm currently restoring the truck and when it comes to electrical problems I'm not too good. This site has helped me solve one wiring mystery so far so I figured I would try again. I have several things not working correctly in the cab of the truck which I am assuming means I'm missing a ground. Fuel gauge reads way over full, amp gauge has no reading, and my wipers don't work. I'm sure there is more not working correctly but this is what I have noticed so far. I believe I put all the grounds I took off back on but looking for some kind of insight from someone who knows more than me.

 

[Snoots]

Check near the top of the drivers kick panel. There should be a 6-legged 'spider' with several black wires connected.
Take it off, clean EVERY CONNECTION and reattach it to the kick panel with a dab of copper anti-sieze.

That's the dash and 'electrical options' ground point.

 

[75gmck25]

AFAIK, on a '75 K25 you will not have that "spider" ground piece, and the instrument panel will be grounded to a bolt on the top of the emergency brake mechanism. Some of your problems sound like they could be related to that ground.

The ammeter is a shunt-based wiring setup, and probably not worth fixing. There are two places under the hood where it connects to the wiring loom, and each has a fuse holder and low amperage fuse. Basically, it measures the voltage drop between those two points (using the wiring as a shunt) and converts it to an amperage reading. It wasn't that accurate when new, and gets really bad as the wiring gets old. I bought a voltage gauge for a newer truck and found instructions on the internet to convert the instrument cluster to use a voltage gauge. The orange color of the gauge needle is a little brighter than my other gauges, but it physically fit the cluster without any problem.

I'm not sure the wipers use that instrument panel ground wire. If its not the fuse, its more likely to be a problem with the wiper motor under the hood.

For the fuel gauge, do you have single or dual tanks? The primary tank on a '75 is the passenger side and the sending unit wire runs from the dash through the firewall, across the firewall behind the engine, and then down the passenger side frame rail to the primary tank. If you have dual tanks there will be a second wiring harness and the dash switch, but that harness runs across between the tanks then up the driver's side frame rail to the switch in the cab.

 

[Erik Anderson]

Thanks for the info, so I think I know where those 2 fuse holders are you mentioned for the ammeter and I know 1 of them the fuse is blown I just haven't replaced it yet because I'm not sure of the size so that must be my issue with that. I'll have to do some tinkering with the wipers it sounds like. As for the gas gauge it used to have dual tanks but the passenger was removed years ago due to the tank having a hole and the bed rotted and was replaced with a single tank bed off of an '80 something square. The ground is ohming good at the sending unit so do you think I have an issue with the power wire?

 

[Grit dog]

Just want to say the wealth of knowledge is nothing short of amazing here as are the contributions....

 

[Super-Dave]

FWIW - I got tired of chasing ground issues with multiple ground locations. Many were in difficult to reach locations without removing other parts to get to the connections. So, I finally moved every ground to a Grounding Block that was placed in a more convenient location and grounded the block to the firewall. I labeled each wire on the Ground Block and now if I have any ground issues, I can easily get to every wire in one location.

 

[Bextreme04]

I would highly recommend not using copper ant seize as a corrosion inhibitor on an electrical connection. You want to make sure it is super clean and bare metal for a metal to metal connection. Then use a high quality dialectric grease like Dow Corning 4 (DC4) on and around the electrical connections. It will keep air and water out of the connection and will not allow anything to short. It's obviously not as critical to use a non-conductive grease on the grounds, but is generally a good habit to get into with electrical connections in general. DC4 is commonly used in aviation applications and I've seen it come out of 40 year old aircraft connectors still looking soft and not broken down.

 

[75gmck25]

For the OP - if you only have one tank, and it is the driver’s side, then the way they “fixed” the wiring might be more convoluted than you would expect. All single tank trucks in 1975 would have had only the passenger side tank.

Do you still have the dash switch and the dual tank fuel solenoid under the truck, or did they remove everything ? It makes a difference in how you would fix the wiring , since the original dash switch also switched the gauge wiring feed.

Gauge line to the sending unit on the passenger side is tan, and I think it’s tan with white stripe for the drivers side tank.

 

[Erik Anderson]

The dash switch is still in place and I'm not sure about the solenoid I will have to check but I had a mechanic at work print me out the original GM wiring diagram so I'll just have to spend some time under there tracing wiring back.