1987 diesel fire truck (mini pumper pickup) electrical issue

[Baradium]

I just picked up a 1987 brush truck from the Army for the fire department I volunteer with. This truck has just over 11,000 miles and the base fire dept was hiding it when they got new trucks because they liked it so much (they kept it on the range so it wasn't visible and it'd been left out of inventory for some reason). A senior officer figured it out and made them get rid of it so it went to surplus. It was an in service truck until just before, and was supposed to just have dead batteries from sitting.. I went to get it with that expectation, but I have had no luck getting this truck running.

It turns out the batteries weren't actually dead. In fact, essentially all of the *add on* electrical works. But NOTHING that was factory installed works (also, the add on electrical that was keyed hot doesn't work either).

The only thing that was done to the truck was the radios were removed. I'm thinking that they had the radios hooked into the primary feed for power for the truck and cut that wire, but I can't find it. The truck does have some fairly common emergency services modifications, so while the factory harness itself is intact, the connection for the main power feed wire MAY not be in the same place.

One of the big modifications is a battery master switch, which is done via ground interrupt. The batteries tie together via ground as well as hot, and then the ground wire goes through a switch before going to the engine block. This part of the system is working without issue. Another modification is the main feed battery cable goes to a little distribution block on the firewall (which is where the add on equipment gets its power) before continuing down to the starter. There are a couple wires coming off of this point that could be part of the factory harness, but without having codes to look for I'm not sure. The truck also has an ammeter installed, but it is remote calibrated so it works off of that distribution point. The alternator installation is also different, but I believe it's just because it's a much larger alternator.

I am fairly convinced that the problem is under the dash, but I can't for the life of me find any wires that look to be the cause. Everything under the hood looks to be intact with the modifications. There is a second distribution point that seems to be for a keyed source. I believe the glow plug controller is getting it's signal from this source. With the electrical draw of the glow plugs, I cannot easily just provide power to any of these points for diagnosis. I can disconnect the glow plug controller to try to power the dash, but I'm not sure how much that will help me actually find the problem.

The wiring diagrams I found seem to be gas specific, does anyone have access to a diesel one? This is a military truck, but it was civilian spec, so it is 12volt.

[GM72K10]

Awesome find. I bought an 86 very similar except that the body was removed before I got it.
Wound up replacing the engine compartment harness (easiest route)
I take it you are keeping it as is and can't get it running.
Glow Plug controller was in need of replacement among other things on mine. I still can't get the horn to work either. A few bad grounds and some shorts. Had to rewire the headlight switch wiring also.
My plan of attack would to be disconnect everything and start over .
Just my two cents. I used the stock K30 K31003 diesel wiring harness diagram, it helped enough to get mine running again.
It is quite different from the cucv wiring harness I might ad.
Good Luck.Posting pictures of your engine compartment compared to mine and my truck when I got it. You have a much better truck to start with and I paid the same price!

[gmachinz]

Yeah these 6.2's are a bit varied on the engine wiring....I'm working on diesel specific harnesses at the moment so if you need something under hood, drop me a line. Some connectors are extremely hard to find-I've recently finished one with the 6-wire round controller and fender mounted glow plug relay. There are many variations and quite often the GM diagrams don't match actual production harnesses I've discovered.

[Baradium]

Originally Posted by GM72K10

Awesome find. I bought an 86 very similar except that the body was removed before I got it.
Wound up replacing the engine compartment harness (easiest route)
I take it you are keeping it as is and can't get it running.
Glow Plug controller was in need of replacement among other things on mine. I still can't get the horn to work either. A few bad grounds and some shorts. Had to rewire the headlight switch wiring also.
My plan of attack would to be disconnect everything and start over .
Just my two cents. I used the stock K30 K31003 diesel wiring harness diagram, it helped enough to get mine running again.
It is quite different from the cucv wiring harness I might ad.
Good Luck.Posting pictures of your engine compartment compared to mine and my truck when I got it. You have a much better truck to start with and I paid the same price!

Thank you

Yeah, we are trying to keep it as is to remain an in service fire truck. We'd been watching it, with no plans to buy because the starting price was too high for us, but then it didn't sell and they relisted it with a lower starting price so we jumped on it.

The horn likely doesn't work on yours because the fancier sirens have a hands free mode that uses the horn button. Normally you have a switch that selects between siren and horn, so when they remove the siren and associated hardware the horn wire is left hanging loose.

The frustrating thing is that they had kept EVERYTHING working on this truck while it was in service, and the only thing that was done was the radios were removed. The siren etc are still installed. The truck sat for 2 months after it was dropped off, but we suspect there is a single wire somewhere that was cut and is causing all our issues. Thank you for the pictures, it looks like there were a lot of similarities. Do you have any tags showing who converted your truck? It very well could be the same company.

[Baradium]

Originally Posted by gmachinz

Yeah these 6.2's are a bit varied on the engine wiring....I'm working on diesel specific harnesses at the moment so if you need something under hood, drop me a line. Some connectors are extremely hard to find-I've recently finished one with the 6-wire round controller and fender mounted glow plug relay. There are many variations and quite often the GM diagrams don't match actual production harnesses I've discovered.

Thank you,

We have a volunteer who has a same year truck and manuals for it that he's going through, so if it has the diesel stuff it will help some.

Of course in our case, we don't exactly match the production harnesses anyway. Best case we have a factory harness but connections aren't guaranteed to be made in the same place.

I have an '85 crewcab sitting at my house, I need to pour of it some to see if it gives me any ideas, but again, we aren't sure where the problem is either.

[Baradium]

I haven't talked to the other guy who was working on it, but apparently a wire on the starter was hooked to ground, and that caused all the issues. Apparently we now need a new starter solenoid and alternator at minimum, but hopefully that is all and now we know what the issue was at least.

[GM72K10]

Originally Posted by Baradium

I haven't talked to the other guy who was working on it, but apparently a wire on the starter was hooked to ground, and that caused all the issues. Apparently we now need a new starter solenoid and alternator at minimum, but hopefully that is all and now we know what the issue was at least.

Awesome.
Hopefully that's all it is.

[Baradium]

Originally Posted by GM72K10

Awesome.
Hopefully that's all it is.

News actually gets better. The other guy is a heavy truck mechanic so he isn't used to working with glow plugs. When you first start the truck the alternator can hardly keep from discharging (on the add on ammeter) while the glow plugs are still running, but once they stop it shows a healthy charge rate which comes down normally. Voltmeter is reading in a good range as well. I think he didn't leave it running long enough for the glow plugs to stop cycling and just thought the alternator wasn't working.

The starter solenoid is because he saw some corrosion, he wants to swap it for preventative maintenance. He must've dropped the starter (I was going to do that yesterday) and that's when he figured out a cable we thought was a ground cable going to the back of the truck was actually the starter one. The cable we thought was the starter one must go back to the motor on the hose reel at the very back of the truck.

It's running great. I'm waiting on paperwork so I can title it and transfer it to the Fire Dept, but other than that I think it could be ready to go back into service within a week.