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NEWS | April 7, 2022

M1078A1 FMTV: Culprit for Cab Power Loss

FMTV
Courtesy photo
 
Dear Editor,

My unit has had some issues with our M1078A1 FMTVs losing all cab power. The culprit is the battery disconnect relay assembly going bad.
 
Battery disconnect relay assembly (cover not shown)
Battery disconnect relay assembly (cover not shown)
 
It’s a common problem that we believe is caused by improperly shutting down the vehicle with the maintenance disconnect switch. We’ve only experienced the issue with M1078A1s, not our newer M1078A1P2s.

The cost of a new battery disconnect assembly, NSN 5945-01-501-8715, is $447.90, not to mention the downtime of the vehicle. We’ve discovered an easy solution and fixed several FMTVs for less than $3. That’s because it’s usually just a short in a relay wire, not the entire assembly that’s bad.

It’s easy to check and fix. We make sure that only a qualified mechanic troubleshoots the wiring.

Here’s the process qualified mechanics can use to troubleshoot and repair the problem:
 
  • Get a voltmeter, 16AWG wire, fully insulated male slip-on rings, insulated rings, wire crimps w/cutters and a flat head screwdriver.
Required tools to replace relay wires
Required tools to replace relay wires
 
  • If you don’t have cab power, locate the battery disconnect relay assembly and remove the cover from the assembly.
Battery disconnect relay assembly location
Battery disconnect relay assembly location
 
  • Use a voltmeter to check the voltage output of the left and right relay wires. The relay on the left should read 24V-28V and the relay on the right should read 12V-14V.
Test left relay for 24V-28V and right for 12V-14V range
Test left relay for 24V-28V and right for 12V-14V range
 
  • If either relay doesn’t have the prescribed voltage, then there’s usually a short in the wire. We’ve found that the left relay (24V) is most often the one that’s shorted out.
New relay wires (4 total)
New relay wires (4 total)
 
  • Remove the old relay wires and cut the 16 AWG wire to the desired length and then fabricate your new relay wires. Replace all four (4) wires. Before you put the new wires on, clean all of the surface areas.
Then start your truck and check the voltage output of the assembly for correct volts. If the left relay shows 24V-28V and the right shows 12V-14V, your truck should have cab power and be fully mission capable again.

First Sergeant Kegan Brown
Rocky Mount, NC
Editor’s note: Thanks for the great tip, First Sergeant!

Maintenance leaders, it’s a good idea to go over proper FMTV shutdown procedures for the maintenance disconnect switch in TM 9-2320-392-10-1 (Oct 17) too.
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