72 GMC 2500 - electrically dead!

[Bigblock50]

Truck is a 72 GMC Sierra Grande 2500 big block. Starts, runs and drives great, over 5 years. Now get this! This morning I decided to take the truck to the local post office 4 miles away. Starter cranked fast (fresh battery) as always, engine fired up, drove four miles and shut it off. Done in the PO, I started the truck and made a turn in the parking lot. Engine died ... but not only that! Everything electrical died, even the dome light. A few checks revealed nothing so I got it towed home. Three hours later ... no luck. Battery is strong, I checked and cleaned the terminals and the ground, tried hooking up a charger, nothing. There is no voltage at the fuse box or at any normally “live” terminals. No lights, horn, radio, etc. Continuity tester shows a good pathway between the battery negative post and the frame. Somehow no current is escaping the battery.

Ideas? I’ve about run out. 55 years of messing with cars and this is a first for me.

Bill in Luray Va

[geezer#99]

Check this.


http://67-72chevytrucks.com/vboard/s...d.php?t=545962

[dmjlambert]

Yes, what geezer#99 says may be it. Do you have a fusible link type of wire between the battery positive post and the junction block on the passenger fender, and are you getting voltage to that junction block? Let's see pictures of your battery connections.

[TwoFiftyShifter]

Voltage regulator?

[gmc684x4]

It doubtfully be voltage regulator

Check all grounds battery to engine engine to cab engine to frame frame to rad support etc

check fuseable link
Check any inline fuse that may be near the battery

[gmc684x4]

Fuseable link is in the wire from the battery to the altenator

Other things to look and wiggle are butt connectors

[Bigblock50]

I think Geezer #99 has set me on a solution. It has to do with a fusible link near the battery. There is a black wire connection there that isn’t too great, some exposed conductor. Being black I took it as a ground of some sort. It’s not. One of the black wires is a fusible link connection to the fat red wire coming off the plus terminal cable. In the same vicinity is the exposed end of a former ground cable that I replaced but didn’t bother to disconnect from the bottom end.

So ... what I think happened is that the exposed fusible link connection somehow came in contact with the exposed ground cable. Result is that the fusible link grounded itself and blew instantly. I assume getting the link isn’t a problem.

If this turns out to be the problem, it serves me right! I often fully rewire my vehicles but on this one the wiring is quite sound except where the previous owner did a lot of sloppy camper wiring. So I left it alone. Fortunately I was only a few miles from home.

Thanks for all the replies! Thanks to Geezer #99 for the excellent schematics.

Bill

[geezer#99]

There is no free lunch here, ya know!
We need pics of your truck!

[dmjlambert]

Fusible link wire is available in short sections in car parts stores. It is usually grey and comes in a package hanging on a hook in the electrical section of the store. To protect your main wire, which is 12 gauge, you will want a 16 gauge fusible link wire. Use a 6 to 9 inch section of it.

[Bigblock50]

To get everything straightened out, I’ve had to tear out all the camper wiring near the battery which was a total mess. Now I’m in a position to replace the fusible link. But I’m left with a question. The wiring diagram shows the junction block as receiving just two leads, the fusible link from the plus terminal, and the blue ignition lead. However, on my truck, the blue ignition lead is traveling inside harness wrap with a yellow lead and this looks to be factory wiring. What is that yellow lead?

What’s making me just a little uneasy is that I just noticed on the glovebox door label the option for “camper wiring.” If this was dealer work ... wow. Lots of twist the wires together connections, 3M Scotchlock stuff, etc. I’d like to just hook up the blue lead to the junction block with the 14 gauge link, but I’m being cautious.

[dmjlambert]

I suggest post the wiring diagram you are looking at. I'm not familiar with a blue or yellow lead near the junction block that I'm thinking about, and I don't know what ignition wiring would have to do with the junction block. But then I am more familiar with the Chevys and not the GMCs.

[Bigblock50]

Go up to the second entry in this thread by Geezer 99. I’m using the color diagrams at that link. GMC and Chevy use identical wiring.

It looks to me like the yellow lead ends up beneath the HEI distributor, an aftermarket addition by the PO. I think the HEI units need continuous 12v to function. If so, this lead might provide that. Tomorrow I plan to trace the wire carefully under the HEI unit.

[dmjlambert]

I'm just confused. I don't see what any blue or yellow wire has anything to do with a junction block near the battery and related to a fusible link.

Here's the full color diagram
http://67-72chevytrucks.com/vboard/s...49#post7809749

And here's what I see near the fusible link. I see a 12 gauge red wire, and the 18 gauge wire with the small fuse holder inline that goes to the ammeter in the instrument cluster.


Wiring to an HEI should come from a source of switched power, using an ignition switched power leading from the cab. I don't think it would normally have anything to do with battery power near the fusible link.

[geezer#99]

Originally Posted by Bigblock50

Go up to the second entry in this thread by Geezer 99. I’m using the color diagrams at that link. GMC and Chevy use identical wiring.

It looks to me like the yellow lead ends up beneath the HEI distributor, an aftermarket addition by the PO. I think the HEI units need continuous 12v to function. If so, this lead might provide that. Tomorrow I plan to trace the wire carefully under the HEI unit.

That yellow wire could be the feed wire from the starter solenoid. It could be spliced with the resistance wire that was used to reduce voltage to the coil for normal running with a points dizzy.
When you start the truck that yellow wire provides 12 v to the coil during cranking. When the truck starts (release the key)no power comes from the solenoid and power To the coil comes from the resistance wire.
When you unwrap it you’ll see where the splice is.

[Bigblock50]

My mistake, somehow I said blue for B when I meant black. Once I cleared away the amazing tangle of wires and splices and connectors, I’m left with figuring out which of the wires is the BLACK ignition wire that connects at the junction box. The fuse shown on the diagram was there, but see the attachment for what it looked like. Thick green wires Scotchlocked to a yellow wire.

Maybe the most helpful thing for me to know is, where does the BLACK harness connection wire come from? The top of the radiator on the passenger side? See second picture. This is a fairly thick black wire.

[dmjlambert]

Do you mean the thick wire bundle that is wrapped with black tape? It goes across the top of the radiator. It includes the main 12 gauge battery wire, and the black 18 gauge wire that goes to the ammeter in the instrument cluster. The wiring diagram shows it going around the back of the engine, but that is not how it is actually routed. It goes into the same wire bundle as the red wire. Perhaps your wiring harness has been modified when they did that camper wiring. Do you have an instrument cluster with gauges, such as a battery gauge (ammeter), and temperature and oil pressure gauges, or is it the simpler one with warning lights?

[Bigblock50]

Thanks for the detailed info. I’ve located the red battery lead coming out of that wrapped harness on the radiator. It had been intercepted by a variety of gimcrack splices and who knows what. Kind of Laurel and Hardy wiring. I believe I have the 18 gauge black harness wire located in the same bundle. I do have a gauge cluster so I can run a continuity test just to be sure.

If you look at one of my pictures, that same bundle of wires also contains a thicker black wire, maybe 16 or 14 gauge. That wire (cut off) ran to the junction block. I’m guessing it’s a power lead for the camper (long gone). The fuse assembly for the harness wire was, as you can see, made of green wire and had been spliced in.

I guess one thing that would help me is a few pictures (flash or in sunlight) of the correct original wiring in the junction block/fusible link area.

Thanks again.

[dmjlambert]

Well my wiring in that area is not original. I have a fuse holder similar to yours because the original wiring that area was eaten/damaged by the PO's rats (or the PO). I have spliced fresh wire onto the original wires inside the wire bundle going across top of the radiator. I used wire of the same gauge and color to splice onto the red wire, by removing an inch of insulation, twisting the wires together, soldering, and applying heat shrink tubing. The gauge of the black wire that leads to the ammeter is less critical because almost no current flows over it. I make splices with a good twisted physical connection, solder, and heat shrink tubing. I'm not a fan of crimped connections, but that may be because I'm good at soldering and I see no reason to not solder. I suggest trace/pull the wires to figure out where the bigger black wire is going and don't hook it up if it's just something left over from the camper wiring. I have only 3 wires going to the junction block: The fusible link wire from the positive battery terminal, the red wire and the black wire that has the fuse holder inline. That red wire and the black wire go into the channel across the top of the radiator, along with the passenger side headlight and parking light wires.

[Bigblock50]

Your description is clear, no picture needed. Super-helpful. I will trace the heavy gauge black wire -- I'm pretty sure it goes to an "AUX" junction box on top of the DS fender.

Honestly, I should have taken a picture of what all this wiring looked like before I stripped it out (perhaps a little too quickly). After purchase in 2016 I immediately had to fix all the rear end lighting and trailer hitch wiring which were a frightful mess, a fire ready to happen. I left the spaghetti under the hood alone because it was so far off the wiring diagrams. But when this fusible link fried (thank goodness in a parking lot) I decided that was it. Hopefully getting it back to the the simple original setup will work out. It will sure look better.

I'm attaching a photo, not really the best. I'll get a better one done soon.

Thanks for the help. I'll report how it all turns out.

Bill

[gmc684x4]

Have you thought of buying new harness from either American auto wire or M&H both do a good reproduction harnesses

This would be a great way in insure you removed all bad and worn wiring on your truck

If installed properly will help in reducing chance electrical fires under the hood

[gmc684x4]

After you get back running you will need to check for other short circuits that may be in your head light and engine harness sections or elsewhere

To insure it was not some other short that took out the fuseable link

[Bigblock50]

Gmc684 - I’ve considered the whole harness approach and may decide to do that. The only thing that held me back on this truck is the magnitude of the task given the extreme amount of meddling literally everywhere, under the dash, engine compartment, etc. I’ve done a number of harnesses and the jobs that went smoothly (though not easily) were cases of replacing worn out original stuff. This truck was worked on numerous times by Los Angeles area gas stations starting in 1972 - I have every receipt. So I know what would lie ahead. When I finish up my 58 Buick Roadmaster, I might gulp and tackle the Sierra Grande.

Meantime I just want it running so I can move it around my property and keep it on the road. Thanks for the wise input.

[Bigblock50]

DMJ - I was able to confirm the black harness wire by jumping to the ammeter. It travels with the heavier red wire over the radiator as you said. I’ll be getting a fully color coded harness diagram today.

By the way, I noticed today that the big firewall connector in front of the driver has been somewhat toasted. I don’t think it was the recent incident because I smelled nothing when I threw open the hood. But I do need to replace it, hope it’s available.

[gmc684x4]

The harness under the hood breakdown in two peices
One side is engine the other side is headlight harness whick covers your fuseable link alt lights etc

I have not seen new bulk head connectors available separately outside a new reproduction harness

I prefer the reproduction harness over the painful style where you have to run and terminate every connector
You also have to bundle it up to look neat

I am putting a complete new harness in my 72 even though its a low milage truck its still 48 years old
And relatively unmolested wiring

New or old wiring you have to insure all connections are tight loose connections can cause over heating issues

[Bigblock50]

So you’re saying you can buy two separate harnesses just for the engine compartment? I might go for that. The lights and hitch coupler wiring are ok, worked those out soon after I got the truck.