M1078A1 won't crank, no power to cab

[jamboly]

I got called in to try to help a small local volunteer fire dept get their recently acquired M1078A1 working. Seems someone replaced the batteries but had the two inner batteries placed in reversed, so power cables were hooked up all wrong. Got that fixed. Now have proper voltages from master battery cut off switch to some sort of dual solenoid assembly but there is no power to any of the electronics inside of the truck. Have turned on manual battery disconnect switch on lower left side of instrument panel and the manual battery disconnect switch that drive the solenoid assembly. Nothing. Anyone have recommendations as to where to go from here.
Note, at one time I was able to find a SS thread on how hard it is to get the manuals for the A1, but can't seem to find it now. The A0 manual is useless for electrical on the A1.

 

[TomTime]

I've been looking for the manuals for the A1's also. All I have found was the Electrical Schematics for it.
Sorry I couldn't be more help.

 

[coachgeo]

I got called in to try to help a small local volunteer fire dept get their recently acquired M1078A1 working. Seems someone replaced the batteries but had the two inner batteries placed in reversed, so power cables were hooked up all wrong. Got that fixed. Now have proper voltages from master battery cut off switch to some sort of dual solenoid assembly but there is no power to any of the electronics inside of the truck. Have turned on manual battery disconnect switch on lower left side of instrument panel and the manual battery disconnect switch that drive the solenoid assembly. Nothing. Anyone have recommendations as to where to go from here.
Note, at one time I was able to find a SS thread on how hard it is to get the manuals for the A1, but can't seem to find it now. The A0 manual is useless for electrical on the A1.

if batteries are wired wrong the A1 version of the reverse polarity box (I forget its name/acronym) will shut things off..... which is what it is meant to do. It is behind spare tire. Not sure if there is a reset on it.. or if you have to disconnect all lines from it then put them back on orrrrr?? orr.... you still do not have it wired right and the unit is still doing its job and shut things down.

also the solenoid pack you refer too.... that pack is main power disconnects... one of the items that triggers them to disconnect is the switch you refereed too.... the other one that can activate the solenoid to disconnect the box I referred to above. So something tells me your still not wired right so that box is doing its job.... orrr.... a solenoid is bad in there and keeping it shut off when all is fine.

with that in mind... is there not also a manual switch outside by those solenoid? I seem to recall two.. one in cab... and one outside.

Anyway; above scenarios have been discussed in here. Search for information on the solenoids. maybe some key words to search to find threads on this would be: solenoids, disconnect, main, "no power", "battery box"... some magic combination of these

best of luck. Hopefully this will get you started till folk way smarter than I am pop in to fill in more details.

 

[BERZERKER888]

JamBoly…. Welcome…
there are plenty of smart electrics gurus on this forum…coachego and Ronmar are the first 2 that come to mind… also scoot over to the FB groups related to LMTv/FMTVs… more smarties there as well…

 

[coachgeo]

JamBoly…. Welcome…
there are plenty of smart electrics gurus on this forum…coachego and Ronmar are the first 2 that come to mind… also scoot over to the FB groups related to LMTv/FMTVs… more smarties there as well…

me not smart.... I try to repeat what smart folk said earlier that I stuffed into my head, but it usually comes out wrong...... just hope it points folk in a direction that is useful.

on plenty occasions folk learn from "my wrongs" instead of making same wrong themselves. Less painful to them if I do it first lol

 

[Ronmar]

There is another remote disconnect sw over by the remote start switches in the passenger wheel well. The remote disconnects in cab and passenger wheelwell and the LBCD(new fangled polarity box) control those solenoids, but the LBCD only does it based on alternator load.

there is a manual battery switch under the box on the rear side of the battery box. So battery power goes from batts to manual disconnect, to solenoids thru LBCD diodes and on to the power panel in the cab.

The schematic TomTime linked to is for the early A1(pre-LBCD) and is not much different from the A0. Cool one though, I have not seen it before so will add it to my collection

there is one here for the later version with LBCD and remote disconnects, that you may find usefull.

https://www.steelsoldiers.com/attachments/fmtva1-vehicle-sn-18-550-thru-99999-pdf.642250/

edit: I just discovered the schematic pack Tomtime linked to has the later A1 with LBCD and remote switches in the back half…

 

[jamboly]

Thanks, the schematics are exactly what I needed. We gave up on the getting the truck working when I couldn't figure out where the power was coming from to activate the solenoids. Per the schematics it looks like a wire labeled TL551A that connects to the 24V output lug on the master battery disconnect switch is what provides the power. The solenoid assembly had been removed and the guy from the fire station put it back in when I was there. I do not remember seeing that wire hooked up when he put it back together. That will be the first thing I check when we go back to the station this week.

 

[jamboly]

Got the truck running last night. At first couldn't find the TL551A wire hooked up to the output of the battery disconnect switch. Someone hooked it up to the +24V side of the batteries (which means it stays hot all the time). Truck still didn't have power until we found the battery disconnect switch under passenger seat was in wrong position. All is well now. Thanks everyone for the input.

 

[Ronmar]

Pretty sure you are Not the first person to get tripped up by all the disconnect switches