Need some electrical help m108

[mikey]

I have an M108 that has sat for a few years. I wanted to get her running again this spring, but she's giving me fits.

When I parked her, service lights, low air buzzer and pretty much all electrical items worked.

With the power switch off, or on, I get no service lights. When I turn the power switch on, the gauges all jump, voltage looks good, but there is no low air buzzer. When I press the start button, I get nothing. Not even the click you normally get with low batteries. No draw from the voltage gauge, nothing

Here's what I've tried:

Replaced the ground cable in the battery box because it looked frayed at the end. Sanded the mount point on the frame to ensure a good connection
Replaced the end of the thin cable on the positive side, that goes to the wiring harness, because that looked frayed also
Removed the cover on the fuse box on the firewall (goes the the alternator) and sanded all contact points and blew it out with compressed air
Tried multiple sets of matching, freshly charged batteries, even a brand new set
Tried to bypass the "fuse box" on the firewall, but my harness ends are 2 point male and 3 point female, so that won't work.

Here's what I plan to try tomorrow:

Remove all connections at the starter, clean them up and reconnect

I'm not confident that the starter or starter relay have anything to do with this because the lack of service lights and the lack of low air buzzer.

Any suggestions and help would be greatly appreciated.

Here is a video, but it pretty much covers everything I've already explained.

20220525_183829.mp4

drive.google.com

Thanks!

 

[NDT]

Suggest you look for concealed corrosion between the battery cable strands and the crimped on lugs. This will cause the condition you describe.

 

[mikey]

Suggest you look for concealed corrosion between the battery cable strands and the crimped on lugs. This will cause the condition you describe.

Thanks for the reply. The neg cable is brand new. The pos cable to the wiring harness was cut about 2" and has a brand new crimped lug. The strands that are in the lug were not exposed before cutting and stripping it. That leaves just the pos cable that goes to the starter. I actually don't mind replacing that cable to rule that out as a possibility. But then we are back to "would the cable going to the starter affect service lights and low horn buzzer"? Regardless, I'm willing to replace it.

Thanks.

 

[NDT]

What about the jumper between the batteries?

 

[mikey]

What about the jumper between the batteries?

That dogbone is also new. Doing some reading and I have to add the magnetic starter relay to the list of things to check.

 

[G744]

Fusebox on the firewall?

DG

 

[mikey]

Fusebox on the firewall?

DG

Looks like a fusebox or relay, but not sure what it is to be honest. I have 3 m35's but none of them have this.

 

[ToddJK]

I would definitely replace all the battery cables and the starter wires if they seem old and frayed or show any signs of corrosion. You're gonna want a voltage meter and then look up the trouble shooting in the TM's for your truck. It will walk you through the process. That's how I figured out my temp gauge and my horn, both had to have wires replaced. The temp gauge wires looked okay, until I moved them, the pos literally broke right off, so that wire could have had numerous breaks inside the sleeve.

 

[mikey]

OK, I just learned that the box on the firewall is the voltage regulator for the generator. I can rule that out as part of the problem.

 

[G744]

A tip: If you unplug the Bendix connector (the one with wires), all power to the dashboard is disconnected.

DG

 

[mikey]

A tip: If you unplug the Bendix connector (the one with wires), all power to the dashboard is disconnected.

DG

But the gauges all work fine. I'm thinking this may be multiple issues compounding the situation. I'm going to dig in deeper with the wiring schematic. Thanks for your help thus far.

 

[HDN]

OK, I just learned that the box on the firewall is the voltage regulator for the generator. I can rule that out as part of the problem.

I was about to say that. All my dad's vehicles have a separate voltage regulator that looks like that. Is yours missing the cover? I guess it doesn't surprise me seeing a '50s era gasser Deuce with a voltage regulator separate from the generator/alternator.

I was told it's a lost art tuning those old regulators. I know there are solid state versions available for 6-volt systems (my dad put one in his 1942 Ford GPW, looks just like the old one on the outside), so they probably exist for 24-volt systems too in case you ever need to replace it.

EDIT: I just remembered that actual M-series 24-volt solid state voltage regulators do exist. They look like a big green or black brick. I'm not sure what they were designed for or if they were meant to replace the older ones like yours, but I just remembered my dad put one in his '53 Dodge M37. The existing wiring harness like you have there screwed right into it.

 

[GopherHill]

Definitely need to have a cover on the voltage regulator if it doesn't have one. Does that have a foot pedal starter or a relay to engage the starter? A dash 10 manual would be helpful. You can download on on this site.
Hope you get it going.

 

[mikey]

I was about to say that. All my dad's vehicles have a separate voltage regulator that looks like that. Is yours missing the cover? I guess it doesn't surprise me seeing a '50s era gasser Deuce with a voltage regulator separate from the generator/alternator.

I was told it's a lost art tuning those old regulators. I know there are solid state versions available for 6-volt systems (my dad put one in his 1942 Ford GPW, looks just like the old one on the outside), so they probably exist for 24-volt systems too in case you ever need to replace it.

EDIT: I just remembered that actual M-series 24-volt solid state voltage regulators do exist. They look like a big green or black brick. I'm not sure what they were designed for or if they were meant to replace the older ones like yours, but I just remembered my dad put one in his '53 Dodge M37. The existing wiring harness like you have there screwed right into it.

I took the cover off to clean the contacts and blow it out. I doubt that is the issue. Today I bypassed the starter button and can eliminate that as an issue. Tomorrow I'm building a test light to test voltage from the starter to the the magnetic starter relay. I'm thinking the magnetic starter relay may be the issue, but we shall see tomorrow.

 

[mikey]

Definitely need to have a cover on the voltage regulator if it doesn't have one. Does that have a foot pedal starter or a relay to engage the starter? A dash 10 manual would be helpful. You can download on on this site.
Hope you get it going.

It's got the normal push button start of an m35a2 and it has the magnetic starter relay on the side of the block. That's what I'm leaning towards now and I'll test that tomorrow.

 

[mikey]

For anyone who may come across this thread, and it's not limited to the M108, but all M35's, the issue was the magnetic starter relay on the passenger side of the block. I was able to jump the starter button and get power at the top of the magnetic starter relay. I then jumped the magnetic starter relay, directly to the starter solenoid and she tried to start. I replaced the magnetic starter relay and now she starts and runs fine.