M1078A1 Constant power (SOLVED)

[HEMRWL]

Hello all,
I have an M1078A1 LMTV in our shop that is stumping me. When it came in, the driver stated that while driving it had a loss of power and when they went to shut it off, the switch on the dash would not work, so they used the power shut off switch below the steering wheel next to the diagnostic plug. I've been told using that itself could cause some issues. When the work order came to me, I went out to turn on the rotary battery switch and the truck immediately had power (dash switch was in off position and remote switches under cab were normal). I tried to start it, but it would just crank and never fire up. My theory is, because of that constant power, the fuel injectors and computers aren't engaging correctly. Here is a list of things I've looked at:

Batteries - Hooked up correctly, all have good charge
Rotary Battery switch - Functions correctly
Bus bars and relay in battery switch box - Working as they should
LBCD - Did the functions check from the Neihaus website, all good
ECM - Hooked up diagnostics computer, a few codes about "over voltage" but its reading the ECM so I assume its not burnt up
PDM in dash - All relays and CB are good.

The PDM in the dash is where it gets weird. I checked the two posts TL41 and TL42 for proper voltage and they're all reading 22v. One should be 12v and the other should be 24v. When I unhook the hot cables from the dash and read them individually they read what is expected. But as soon as they are hooked up, the 24v seems to be back feeding in to the 12v side (which is probably where I am getting the "over voltage" fault). The K1 and K2 relays are both working as they should and I have swapped out a known good PDM with the one in the truck. I am losing my patience with this truck, every time I think I have an "A-HA" moment, it quickly shuts me down.

Anyone have any ideas as to where else I should check or recheck? Much appreciated.

 

[Ronmar]

Well it sounds like something is mis-wired or crossed over somewhere. In order to feed 24 back onto the 12v side you would have to basically short the 12-24v battery and thats not going to work…

the switch under the dash controlls the remote disconnect. It shouldnt cause any issues as it is the same as opening the manual disconnect. 12/24 goes from batteries to manual disconnect, then remote disconnect then thru the LBCD diodes and on to the power panel 12/24 battery terminal bolts. From there it goes thru K1&K2 and back to the 12/24 ignition terminal bolts.

ground goes from the battery to the starter housing via a large cable. From there the main truck chassis ground goes to the drivers frame rail via a strap. The alternator is grounded to the passenger frame rail via a strap and there is a strap from front pax frame up to the cab.

22v does not equal charged batteries, so something is up. Battery wiring has proven to be problematic for people to get their brains around, and the 4 battery series parallel setup, besides being grossly oversized for the alternator can also hide battery issues.

if it has 4 batts, pull all 4 out, individually charge and load test them and pick the best 2, or use 2 other batteries. Just about any 2 auto batteries will work as the engine really does not need all that much crank current.

i would then hand over hand, or use a meter on the 12v and 24v cables to make sure they go only to their specified disconnect terminals and the batt terminals on the lbcd. I would also use this opportunity to throughly examine the connections at the LBCD as they are known to have issues because they are out in the weather. Its main function is to provide polarity protection , so the batt terminal is connected to the load terminal via a large diode, one pair for 12v and the other pair for 24v. The alternator 12 and 24v are connected to the batt terminals on the lbcd…

connect 2 batteries in series(+ on one to - on the other) with a jumper cable. The unconnected - terminal on the first batt is ground, connect the ground cable to the starter there. Positively ID the 12v cable, connect it to where the jumper ties the 2 batteries together. Connect the 24v cable to the unused + terminal on the second battery.

 

[HEMRWL]

Thanks for the reply Ronmar. I will definitely try all that and get back to you. Do you know if the LBCD could still be bad even if it passes all the diode and resistance checks? Would I damage anything if I bypassed the LBCD?

 

[Wingnut13]

I’ve had a similar problem with the voltage issue you describe when the ground was hooked up incorrectly on my A1.

 

[GeneralDisorder]

I’ve had a similar problem with the voltage issue you describe when the ground was hooked up incorrectly on my A1.

Please enlighten as to how the ground was incorrectly connected. I've been helping a member with similar problems.

 

[Ronmar]

Thanks for the reply Ronmar. I will definitely try all that and get back to you. Do you know if the LBCD could still be bad even if it passes all the diode and resistance checks? Would I damage anything if I bypassed the LBCD?

The LBCD has their bandaid for matching 240AH of batteries to a 100A(combined output) alternator. It performs 3 functions.
1. It has diodes for reverse polarity protection between the batt and load terminals, so current can only flow one way to the power panel.
2. It has a sensor circuit that monitors RPM from the AC terminal on the alternator regulator, and it monitors system voltage. If it does not see rated voltage above 1500 RPM it sends the signal to the remote disconnect to disconnect the batteries from a possibly overloaded alternator to protect the alt from that monster battery bank. If it still sees low voltage it sends a signal to light the charging system fault light in the dash.
3. It has a capacitor bank that is there to absorb the energy released when you disconnect an alternator at full excitation from its main load(batteries) until the regulator can reduce the field To match the remaining load.

Yes you can bypass it, just make sure your polarities are are correct. I would also disconnect its cannon plug so it doesnt get a wild hair and open the remote disconnect with the alt under load while the caps are out of circuit…

 

[Wingnut13]

Please enlighten as to how the ground was incorrectly connected. I've been helping a member with similar problems.

If I recall correctly, the main battery ground in the battery box was hooked up to the negative terminal of the 24v battery. It should be at the negative terminal on the 12v battery.

This gave 22VDC at the power panel in the cab on the 24v side and I don’t recall what the 12v side was reading.

Hopefully this helps

 

[HEMRWL]

Thank you all for the replies! Definitely gives me plenty to look at and get me out of this dead end thought process. Unfortunately the truck is parked outside I have no means at the moment to get it in to my bay. Once the weather clears, I'll get back at it and keep everyone updated with the results.

 

[HEMRWL]

All, sorry for the delayed response to this thread (if anyone is still following it). The constant power issue has been solved.

Long story short, behind the drivers dash panel, J913 was plugged in to P912. (Those should not be plugged in together and are used for applications this truck does not have).

The longer version of it is, this vehicle was losing power while driving it. I did not know this, but before it came to me, other mechanics had tried to troubleshoot the issue, saw two connectors that were not connected and then connected them. This information was of course lost in translation before coming to me.

The original issue (loss of power) was caused by a bad LBCD.

Thanks again for all of your guys input and suggestions.

 

[coachgeo]

Thanks for update. As they say..... "It's most always something stupid" Go enjoy her neighbor