Photo by Scott Sturkol
Dead HMMWV batteries?
Think twice before you grab that handy ground power unit (GPU) for a jump start!
The ONLY approved method to jump start a HMMWV is with a NATO slave cable, NSN 6150-01-390-7058, attached to another HMMWV.
NATO slave cable
Those commercial GPUs deliver a power surge that can be too strong for HMMWV batteries! They also haven’t been approved for light, medium, or heavy tactical vehicles.
Ground Power Unit: Not approved for HMMWVs
The surge blows the HMMWV’s electronics and damages the batteries. The same thing can happen even when you jump start from another HMMWV if you don’t follow proper slave starting procedures.
Here’s how to do it right:
Step 1:
Confirm that the dead batteries can accept a charge. Use the battery analyzer, NSN 6130-01-510-9594, from the Standard Automotive Tool Set (SATS). The battery analyzer, NSN 6625-01-466-1075, in the Forward Repair System (FRS) tool kit also works. If batteries fail this test, don’t attempt to slave start. Report the vehicle to field maintenance for repair.
Step 2:
Before connecting the slave cable, set the parking brakes in both HMMWVs. Then turn off all the vehicles’ electrical devices, accessories and switches. This prevents damage to those components if there’s a power surge.
Parking brake being set; Do it in both HMMWVs
Step 3:
Always connect a slave cable to the HMMWV being slaved first. You’ll find the slave receptacle under the HMMWV’s front passenger seat. Remove the cap and plug in the slave cable. Don’t lose the cap because it keeps dirt and debris out of the receptacle when not in use. If the cap is missing, replace it with NSN 5340-01-315-7223.
Plug the slave cable in under front passenger seat
Step 4:
Connect the other end of the slave cable to the receptacle in the slaving HMMWV.
Step 5:
Set the rotary switch of the slaving HMMWV to Run. Watch for the Wait-To-Start light to turn off, then start the vehicle. Then wait for the HMMWV’s voltage gauge to stabilize in the green zone. If it doesn’t, skip ahead to Step 10.
Step 6:
Allow the slaving HMMWV to idle a minimum of five minutes with the slave cable connected before attempting to start the other HMMWV being slaved.
Step 7:
Place the rotary switch in the slaved HMMWV to Run and watch for the Wait-To-Start light to go off. Then start the vehicle.
Step 8:
Once the slaved HMMWV is started, monitor its voltage gauge until it stabilizes in the green zone, then let it idle for at least five minutes.
Idle the slaved HMMWV for a minimum of five (5) minutes
Step 9:
Disconnect the slave cable. Always disconnect the slave cable from the slaving vehicle first; this rule applies, regardless of vehicles.
Step 10:
Monitor the HMMWV’s voltage gauge for fluctuations out of the green zone. If at any time the gauge enters the red zone, immediately shut down the HMMWV and notify maintenance personnel.
Three Big No-No's
Don't operate electrical accessories or components on the HMMWV during slaving operations.
Don't rev the HMMWV's engine; keep it at idle throughout slaving. 
Don't touch the accelerator pedal or any other device that affects engine speed throughout slaving operation.
REMEMBER!
Dead battery! Don't use the GPU!