Electrical Overheating M35A2

[mchagala]

I’m reaching out for help troubleshooting an electrical problem with my 1971 AMGEN M35A2. I’ve had the same aftermarket battery cutoff switch melt twice and I think it’s a symptom of a larger problem.

The cutoff switch is drilled into the floor under the passenger seat. It connects/disconnects the battery cable between the battery negative terminal and grounding to the frame. It has metal internals, but a plastic housing.

First instance of failure: A few minutes into a drive, I smelled smoke and pulled over. I discovered the cutoff switch caught fire, melted and fell apart. I figured the cutoff switch was old and corrosion caused heat to build up. I bought the same switch again (24v compatible) and installed it. I also replaced all battery cables (0 gauge) on both the negative side, and positive side all the way to the starter. Batteries were low so I charged them to 100%. I then turned on the cutoff and accessory switch, the low pressure warning buzz came on, and I pressed the engine start button: nothing, just a single click. I was confused at this point because with full batteries and a new cutoff switch, things should be working. Because a solenoid ‘click’ is the classic sign of not enough juice going to the starter, I assumed there must be a loose or bad connector or wire on or around the starter, so I removed and cleaned all wires, bolts and connectors between the solenoid, starter and magnetic relay. Hit the start button and the engine fired up. Must have been a bad connection somewhere, or so I thought.

Second instance of failure: A few weeks later; a few minutes into a drive, I smelled smoke and pulled over. Same as before, the plastic housing of the cutoff switch melted and fell apart. This time I was in a grocery store parking lot and didn’t have time to wait for a new switch. I removed the melted switch and bolted the 2 ends of its wires together, thus bypassing the switch. I charged the batteries. With the switch bypassed, batteries full, and all connectors still clean from just a few weeks prior – I pressed the engine start button and got only a click. I tapped the solenoid, starter and relay with a hammer in case something was stuck, but it did not help.

What could be causing such a large electrical draw while I’m driving that it’s melting the cutoff switch? My only guess is the starter is continuing to run after the engine has started, and that heavy draw heats the electrical system until something fails – in this case the cheap plastic around the cutoff switch.

Trying to recreate this intermittent failure with diagnostic tools attached would be nearly impossible; I’ve driven this truck a hundred times without this failure happening. Before I start replacing parts in a blind attempt to solve the problem, does anyone know with any level of confidence what’s happening? Is the starter solenoid getting stuck and needs to be rebuilt? Is it the magnetic relay not turning off? Something else?

 

[cattlerepairman]

I agree with your thinking.
Starter solenoid sticking or starter sticking on are my top contenders. The good news, your Deuce is now a Hybrid with the starter motor powering it down the highway. The bad news is that your switch melts and your truck eventually catches fire.

I am not considering silly reasons, such as a vastly undersized cutoff switch. It has to survive the full starting current for at least 15 seconds or so.

Look into replacing the starter with a gear reduction starter (see posts about that), if you go that route.

 

[Gypsyman]

I would trace out your battery cables and look for a wire that has insulation that is rubbed through and causing a short when the truck bounces down the road. The only way to know if it’s caused by truck movement would be to let it sit and idle (or high idle) for an extended period and see if the problem occurs without the truck moving.

 

[G744]

I've found those particular switches are junk, with just the same result in regular uses.

First off, any of them with a 'key' just never have the low contact resistance necessary to handle a Diesel starter current.

Ditch the 'security' idea and get one of these: Cole-Hersee battery disconnect



These are brutally able, and will last a lifetime. Make sure to get the marine one with a flat bottom insulator, not anything molded.

They are on eBay and Amazon.

As far as security goes, use a chain and padlock. Better yet, make a hinged lockout on the floor to keep the transfer in neutral.

G744

 

[mchagala]

I agree with your thinking.
Starter solenoid sticking or starter sticking on are my top contenders. The good news, your Deuce is now a Hybrid with the starter motor powering it down the highway. The bad news is that your switch melts and your truck eventually catches fire.

I am not considering silly reasons, such as a vastly undersized cutoff switch. It has to survive the full starting current for at least 15 seconds or so.

Look into replacing the starter with a gear reduction starter (see posts about that), if you go that route.

Haha, yes, I suppose it is a hybrid now! I'll expect my EV tax rebate! My truck starts ridiculously fast - I never have to hold the start button down longer than a second, but I will take your advice regardless and get a proper switch, and the gear reduction starter.