I know some people on here work on trucks, well I've got a real good problem.
2002 379, pretty standard truck. The company I work for has had it for about three years, and the whole time it's been acting up to varying degrees.
The constant issue is that the battery protection system indicator is always on, dimly. Occasionally, while the truck is running, the battery protection system will kick on, start sounding it's alarm, and power off everything except the engine.
Lately, though, it has started a new trick. Occasionally, the whole instrument cluster will start freaking out, all the lights and alarms go on, all the gauges drop to zero, but it all comes back to normal if you cycle the key, even without actually killing the engine. It also sometimes kicks on all the warning lights when put in reverse, but I haven't been able to get it to do this for me.
The other day, the low air alarm started going off with 115psi in the tanks. I checked at the air pressure sensors in front of the brake pedal, and was only getting 4v there. After turning it off and on a few times doing other tests, I had 10v there and the light went out.
I'm kinda lost on this truck, since I don't have all the documentation I need on it. I did order a battery protection module for it since that's the only thing I could find that might be keeping the light on, but I don't think it'll have any effect on the other problems.

Has anybody run into this before? I'm really not used to having electrical problems beat me.

 

Pull the headliner down and check for water. You might find water leaking in through the cab roof light bolt holes, or the pass through for the wires.

Another possibility is the windshield is leaking onto the fuse panel. I've seen with my own eyes a windshield leak right onto the low air warning sensor.

Wrap yourself around the driveshaft and take a look at the plug and wires for the reverse switch on top of the tranny. Or, you could pull the floor panel around the shifter and look down, if you can't fit in over the driveshaft.

Some trucks have ecm fuses hanging down by the "hoop" slung under the trans, and get soaked with engine oil. Pull them out, spray them and the fuse holders with brake cleaner.

Another one I saw was in the big harness that goes from the engine to the cab. A rubbed wire, shorting out on a screw or bolt. Or under the driver's side door, just forward of the air tank.

And as off the wall as it sounds, check the battery cables. Clean the connections, ooh, hang on a sec. Just thought of something.

Check the cables that go to the battery protection system. I was working on a 387 on the side of the road when everything was fine except it wouldn't stay running. One of the relay connections were broken due to electrical corrosion. You might have a loose or ready to break connection in there somewhere.

Hope that helps.

 

Leaks-
I have heard rumors the washer line popped off this thing a couple years back- I didn't see any signs of corrosion, and had full battery voltage at the relays behind the dash.
I did get frustrated with not being able to test the circuit breakers in this thing's last round, so it's got all new fuses. I had the whole lower dash cover off and didn't see any corrosion on the fuse panel or air pressure switches. I'm fairly sure the ecm fuses are in the battery box, but I'll double check that.
I'm pulling it back into the shop tomorrow morning to work it over some more, since it threw a huge fit today.

I'm almost wondering if there might be some moisture somewhere that is freezing and pushing a contact apart. Doesn't that battery module have a relay in it?

 

No experience with Peterbilts here but it certainly sounds to me as though you have a bad relay or a corroded wire somewhere. Have fun and good luck. Let us know how you make out.

 

bigrigfixer got it right. either reverse switch and wiring related to switch, main harness junction, or, main harness on engine has corrosion in/at plug junctions. corrugated protective cover is very abbrasive and will wear insulation off wires, causing shorts. check harness from firewall to all sensors on engine, i bet its worn off somewhere and shorting to another circuit.

 

I spent a couple hours under the truck today, no shorts/worn spots found. I got a call last night that it had some new symptoms- The speedo and tach quit, along with the rest of the electric gauges. It starts and runs fine, but doesn't sound like it's revving over about 1500rpm, cruise does hold engine rpm though. The voltmeter doesn't move. None of the lights in the dash come on when you turn the key on, but the mirror heat light comes on. The diff lock light is on all the time with the key on. I've been told the highbeams just come on by themselves as well, but haven't been able to confirm this.

 

You could also check the speedo wiring, you might have a 4 wire sensor. If so, disconnect and swap both 2 wire connectors to it. That might solve ONE problem. Sometimes the speedo wires run up over the tranny and rub on the shift tower. Those ones are wound tight almost like a braid. The tach wires could run up over the tranny to meet up with the speedo wires, or run under the flywheel housing to the left side and up into the main harness. Usually, (if it runs under the flywheel housing) it's a loose connector.

Is the diff lock turned on? Or is it lit up either way it's switched? Check the diff lock sensor itself, sometimes they stick. Or its wiring is shorted to itself at a plastic tie strap somewhere.

Either way, sounds like you got a possessed truck. If it catches fire, roll it out of the shop and let it go.

Let me know how it goes, and I'll think on it some more.

 

Yeah, I made sure it wasn't in the shop for the night.
The diff lock is off. All the wiring on top of the trans checked out, I did check the speed sensor, and the reverse light switch, all the wiring was nice and neat.

I'll pull it back in on Monday if there's a slot and start cutting zip ties, separating harnesses and really looking deep into everything. I did check all the main runs, and it looked good, no abrasion spots, no oil soaking, I pulled the ECM fuse and cleaned the socket out, no changes.
That's the other thing that confuses me- no change if I move a harness, jiggle connectors, etc.

 

Don't feel so bad. My foreman had a headache on a particular truck, we just replaced the cab mounts. Coincidentally, a wire rubbed and shorted out in the harness above the air tank shortly afterwards. And it would only fault after the truck got washed. Took him months to find the short.

 

Do you have a wiring diagram for this thing? My boss is too cheap to spring for a real manual, and the local Peterbilt shop isn't overly helpful. They'll sell me all the parts I order, no problem, but as far as actual help-

 

No, sorry. I don't have a wiring diagram. I'm just going by memory.

 

I just thought of this, maybe check the alternator power and ground cables. The ground cable where it terminates at the block (or starter) could be loose. Same goes for the power. Should be tight on both ends. Maybe slip the alternator belt off and spin the pulley. I can't see this being the problem, but it's just one more thing to check.

 

i had the ground wire from cab to frame get corroded and cause trouble

 

How to fix any truck problem.
1. Pull truck into shop.
2. Pop radiator cap off truck.
3. Pull old truck out of shop.
4. Pull NEW truck into shop
5. Put cap back on radiator, TRUCK FIXED!!!!!

I had to have a little trucker fun!

 

Apparently, after two other people screwed with it, it's now been decided that the truck will go to another shop. So much for the parts I ordered.
Oh, the speedo and tach thing was because the untrained monkey they tell me is a co-worker was messing with it, and blew a fuse. Rather than fix it, or tell anybody, he just put the thing back together and sent it out.