Noob with an electrical question

Special Ed

New member
7
2
3
Location
Wied, TX
Howdy all. I am new to the group and vehicle.
I’m the curator of an Aviation museum in South Central Texas, and we acquired an M998 from the state to use as a museum courier for local trips to town
When we picked up the truck, there were no batteries in it. The facility jumped it ( they had the correct cables and 24 volt system). It started and ran great, although the “wait” light doesn’t illuminate. We waited 30 sec or so before starting, and it fired right up without issues. Everything worked (except the wait light)
We got it back to the museum... put a pair of loaner batteries in and drove it around, sTill no issues.
we now swapped the batteries with a pair of new 825 CCA but now, for some reason the gauge cluster isn’t working. None of them or their associated dash lights.
is there a fuse someplace I can check? The truck still starts and drives fine, just no gauges.
Thanks in advance for being patient during what will undoubtedly be a lengthy lessening curve. I downloaded all the TMs and can’t find a fuse or CB.
Jim
www.whereolddogsfly.org
 

Coug

Well-known member
852
401
63
Location
Olympia/WA
Howdy all. I am new to the group and vehicle.
I’m the curator of an Aviation museum in South Central Texas, and we acquired an M998 from the state to use as a museum courier for local trips to town
When we picked up the truck, there were no batteries in it. The facility jumped it ( they had the correct cables and 24 volt system). It started and ran great, although the “wait” light doesn’t illuminate. We waited 30 sec or so before starting, and it fired right up without issues. Everything worked (except the wait light)
We got it back to the museum... put a pair of loaner batteries in and drove it around, sTill no issues.
we now swapped the batteries with a pair of new 825 CCA but now, for some reason the gauge cluster isn’t working. None of them or their associated dash lights.
is there a fuse someplace I can check? The truck still starts and drives fine, just no gauges.
Thanks in advance for being patient during what will undoubtedly be a lengthy lessening curve. I downloaded all the TMs and can’t find a fuse or CB.
Jim
www.whereolddogsfly.org
There are something like 2 self resetting breakers, and zero fuses.

If the dash isn't working, then likely you have an issue with an electrical ground connection.
It's common with these trucks to have bad connections. You have to basically just keep checking every connection until you find the bad spot.

Hopefully soon one of the more experienced guys will chime in with the actual locations to check, or more useful advice than this.
 

papakb

Well-known member
1,681
230
63
Location
San Jose, Ca
Do some researching on "supplemental grounding harness". When everything is clean, tight and working as designed the trucks work as designed. After umpteen years of marginal maintenance and servicing stuff loosens up and electrical connections corrode. There's a body ground connection on the firewall just in front of the heater controls that causes more problems that it should. Pull the instrument cluster out of the way and look at that connection. In my truck instead of the bolt AMG used I put a 1" long 1/4-20 setscrew with star washers and nuts on both sides of the body then put the electrical connections on top of the nuts. This way the ground stud maintains good electrical bonding with the body itself and the harness wiring stays firmly attached to help eliminate issues like yours. It also never hurts to put some anti-oxidant electrical grease on your connectors to exclude moisture from getting into them.

You can also see where the 2 circuit breakers live to the left side of the picture.
 

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Special Ed

New member
7
2
3
Location
Wied, TX
Do some researching on "supplemental grounding harness". When everything is clean, tight and working as designed the trucks work as designed. After umpteen years of marginal maintenance and servicing stuff loosens up and electrical connections corrode. There's a body ground connection on the firewall just in front of the heater controls that causes more problems that it should. Pull the instrument cluster out of the way and look at that connection. In my truck instead of the bolt AMG used I put a 1" long 1/4-20 setscrew with star washers and nuts on both sides of the body then put the electrical connections on top of the nuts. This way the ground stud maintains good electrical bonding with the body itself and the harness wiring stays firmly attached to help eliminate issues like yours. It also never hurts to put some anti-oxidant electrical grease on your connectors to exclude moisture from getting into them.

You can also see where the 2 circuit breakers live to the left side of the picture.
Thank you very much. I will check and clean all the contacts tomorrow. What circuits do the 2 breakers protect? It seems like the maintenance manual says they are 15 amp, but not much more about them.
 

papakb

Well-known member
1,681
230
63
Location
San Jose, Ca
The schematics for the electrical systems are located in the back of the TM 9 2320-280-20-3 manual and show that both circuit breakers get their power from the Start-Run switch in the Run position. CB1 powers the heater circuits and CB2 powers everything else.
 

TOBASH

Father, Surgeon, Cantankerous Grouch
Steel Soldiers Supporter
1,444
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63
Location
Brooklyn, NY
Welcome!

I just did what PapaKB recommended for the dash a few months ago, and the gauges work perfectly now.

Wires on mine were placed incorrectly, and many were corroded.

I like the idea of dielectric grease, but others like milcommguy don't.

Open and clean and sand all connections, and then ensure your wire polarity is correct, (mine had + wires routed -, and just using a Voltmeter helped me sort out that issue). All wires are labeled with little electric crimps that have embossed numbers and letters on them. If you don't see the alphanumerics, then clean them. Then refer to the TM's (technical manuals).

NEVER WORK ON ELECTRICS WITHOUT DISCONNECTING THE BATTERIES, OR YOU WILL BLOW OUT YOUR PCB and the vehicle won't start.

And just to be a ball breaker...

nopics
 
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