Electrical gremlin

Hi all,
It's been a couple years since I've poked in here as the truck has suburban has been relegated to occasional use status and was also running good while doing it. Until now, which brings me back!

Something's been up for a while but it was ignorable (hah!) until now. OEM voltmeter in the dash has been reading low, I've had a long-term parasitic drain (solved with battery disconnects), and occasionally starting would be sluggish. Well this last week while doing a dump run in the summer heat this gremlin hatched into it's final form: a dead truck. At the transfer station (city dump). At closing time. :eek:

When I say dead I mean no power to the cab. After 15 minutes of grovelling on the dump floor (an automotive low point, for sure) and under the hood, with annoyed attendants pacing around as if that would help, I finally got it to fire back up. What I did was pull the two + wires off the alternator, rip off the connectors, and direct macguyver splice the two together to insure a solid new connection, while wiggling the seemingly tight battery connection. With each new attempt the lights would come back on, I'd go to start, and the starter engagement would kill everything. Which leads me to believe that the electrical load is the culprit.

The connections between the batteries and from battery to main + out is good and recently fiddled with. Ohm meter shows great battery voltage, a low resistance ground. So I'm thinking it's power towards or ground away from the cab that is the issue. That would explain the no lights total death and the persistently low cabin volt meter in the dash. And here's where i need help.

It appears the cab and everything in it is fed by the one red wire coming off the alternator. Is this true? Any idea how that wire is routed?

What is the ground path from the cabin electrics back to the frame/batteries?

Howdy
Not sure about your truck...BUT...
In the early years of GMT400 and before, the main cab feed wire went to the starter and then back up to the load center on the passenger side firewall. It is beyond my understanding why it does this. I am sure that someone has a good reason:rolleyes:
On three of my trucks I have removed this wire and ran a new heavy gauge wire from the alt to the batteries and then another wire from the batteries to the load center. Seems to work GREAT for me.
So, if it were me, I would start there.
Also install all new grounds from the batteries to the block, frame, and cab.
The main battery cables would also be suspect given the age of you truck.
Everyone's version of "fiddled with" is a little different:D
If you are going to keep the truck, I would replace all the battery cables with new, if it already hasn't been done.
If the dump is run by local municipality then it is your tax dollars paying for it so tell them people to back off or help as this your shop!
Good luck!

Thanks for the advice, I'll look for that on my starter and do the same rewiring job on mine! Mine is a '91 burb so it's the last of the C/K's, but knowing GM I bet it's the same. Fiddled with just means that I had pulled them to do something else so that they had recently been brightened and tightened and the battery contact wasn't the issue.

The factory cables have a tendency to corrode and get bad on the inside.
The big culprit is the wire that goes across the radiator.
IMHO...your problem sounds like grounding and cabling issues.

If the wire going across the radiator goes bad the instruments would still get power from the P-side battery. But yeah I could see internal corrosion being a real issue and I think I'll take this as a sign to just replace 'em all and bypass the starter. 2 quick questions:

1. I'm not an electrician. I got 0.0 - 0.2 ohms between all measured points (engine/chassis/battery poles/alternator). Can there still be corrosion and load resistance in big cables with zero ohms?

2. quick guide to the major grounding locations?

Thanks for the help folks! I dabble in a few different forums for other vehicles and I gotta say this group is one of the best out there!

Question 1- Yes.
Question 2- Grounding points- Both batteries to engine and frame, frame to body at the firewall, front clip to the frame by the passenger side battery. I am missing some, but I think those are the main ones. I also ran grounds from the batteries to the inner fenders and from the frame to the box in the back.
Grounds should be good heavy wire with bonded ends and screw to clean bare metal(I like to use copper never-seize on all connections) and then cover the connections with some sort of corrosion prevention.

Well I replaced all of it. Sure enough there was bad corrosion hidden inside the red rubber boot of the battery to starter cable. I also re-routed the power supply from the starter to the alternator (through a huge fuse, any idea what size fuse should replace the fusible link?).

I enjoyed a higher reading on my dash voltometer than I've seen for a while, and curiously the truck went to high idle for the first time in forever. But then after a day of that, it died hard again, exactly as before. Glow plugs will light, but as soon as I go to turn the engine, something about that current flow kills everything down to the cabin lights. For several hours.

Ignition?
Some relay somewhere? (why else would the power come back after a while?)

Howdy
It is possible that you have a bad battery. I would disconnect each and then load test out of the system. Batteries can short/break internally.
Did you replace/check all the grounds?
There are also ground points inside the cab that will have multiple wires going to them. These are up inside the dash. can be a real PITA to get at.
Really sounds like you have a loose or badly corroded connection someplace.


-- Replacing the fusible link with a fuse....Have lots of extra fuses on hand. Fusible link and a regular fuse will not handle a load in the same way and you will end up blowing the fuse often. I replaced my link with a thermal 60amp marine grade breaker, my GPs are on a separate circuit with their own breaker.

Ignition switch