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.

 

[HDN]

Wouldn't a starter issue result in a burned-out starter? Their duty cycle when cranking the motor is only around 10 or 15 seconds plus a cooldown period. I'd think if it were the starter being stuck in the ON position it would be burned out and you wouldn't be able to start the truck.

 

[glcaines]

I think you may have two problems. The first problem is you are using a wimpy cutoff switch that can't handle the current. I bet that if you touched the cutoff switch prior to failure that it would be very hot due to the high current. I also think you may have some weak batteries that need replacing, possibly with more capacity. If your starter was staying engaged, the high RPM would have destroyed it.

 

[mchagala]

Thanks for the valuable input. Here’s what I gather:

My original theory that the starter is continuing to run after the engine has started is unlikely because the starter itself would have burned up. I get the logic.

The cutoff switch needs to be upgraded, no argument here. However, I want to be careful about assuming the cutoff switch is the problem itself:

• If the cause of this problem is a surge of electricity, a stronger cutoff switch might withstand the surge and pass the problem on to the next weakest electrical point of failure – like the alternator or something else expensive.
• If the cause of this problem is the low contact resistance of the cutoff switch, it seems more likely that the cutoff would melt while the starter is running, and not a few minutes later.
• The cutoff switch was originally installed by C & C Equipment, the shop that bobbed my truck, and I want to assume they would use an adequate cutoff switch.

Gypsyman’s suggestion to look for a short seems logical, but I’m not able to detect anything while idling.

Any additional feedback would be helpful as my electrical knowledge is elementary and I could be wrong about all my assumptions above. The only purpose I have for this truck is to get coffee Sunday mornings and take my kid to Little League – neither of which I can do while intermittently catching on fire!

 

[williamh]

I don’t think the stock alt puts out more than 65amps. The major current draw is starting. You can turn off the switch and the truck still runs. No batts ? No problem just get 10 guys to push it fast enough to spin the motor and you’re good. Melting a disconnect ?!?! I would think that you got a bad battery or an intermittent short. If the starter was still spinning , my guess you would know. Bad switch made of plastic that says rated for 500amp surge not able to handle 60 ? Yup. If there was a short or starter staying on it would show itself about .5 seconds after connecting the batts.

 

[mchagala]

I am resurrecting this post because I have new information and the problem isn’t solved.

To recap, my deuce had a battery cutoff switch on the negative battery cable. It worked fine for 10 years until the battery cutoff switch melted and fell apart while I was driving. I replaced it and the new cutoff switch also melted and fell apart a few miles later.

Based on the feedback on this post, here are the steps I have taken since then:

• To eliminate the cutoff switch being the possible source of the problem, I removed it and wired the negative battery cable directly to the frame.
• To eliminate an electrical short as being the problem, at least where there is heavy current around the batteries and starter, I replaced the negative battery cable and the positive cable all the way to the starter with appropriately gauged wire. I removed all wires from the starter, checked visually, cleaned the contact areas and reconnected.
• To eliminate bad batteries as the problem, I purchased new batteries.

After completing the steps above, today was my big day to try to start the engine again. I connected the new batteries. I turned the Accessory switch to On and the low pressure warning buzzer sounded as expected; all good so far. I clicked the starter button and only got what sounded like the starter solenoid clicking one time. I repeated several times and got the same results. My immediate reaction was the batteries must be low. I went to disconnect the batteries and found the battery terminals and battery cables extremely hot.

What else could be causing the batteries and cables to get this hot? It seems that whenever I click the start button, everything immediately gets hot and the batteries drain.

I now see the problem was never the cutoff switches, they were just the first thing to melt as heat moved down the battery cables. The problem was never the starter getting stuck in the on position because in this case the starter never turned.

 

[G744]

If possible, separate the heavy wire going into the motor and attempt to start.

If the solenoid pulls in OK, there is most likely an internal short in the motor part.

G744

 

[Mullaney]

I am resurrecting this post because I have new information and the problem isn’t solved.

To recap, my deuce had a battery cutoff switch on the negative battery cable. It worked fine for 10 years until the battery cutoff switch melted and fell apart while I was driving. I replaced it and the new cutoff switch also melted and fell apart a few miles later.

Based on the feedback on this post, here are the steps I have taken since then:

• To eliminate the cutoff switch being the possible source of the problem, I removed it and wired the negative battery cable directly to the frame.
• To eliminate an electrical short as being the problem, at least where there is heavy current around the batteries and starter, I replaced the negative battery cable and the positive cable all the way to the starter with appropriately gauged wire. I removed all wires from the starter, checked visually, cleaned the contact areas and reconnected.
• To eliminate bad batteries as the problem, I purchased new batteries.

After completing the steps above, today was my big day to try to start the engine again. I connected the new batteries. I turned the Accessory switch to On and the low pressure warning buzzer sounded as expected; all good so far. I clicked the starter button and only got what sounded like the starter solenoid clicking one time. I repeated several times and got the same results. My immediate reaction was the batteries must be low. I went to disconnect the batteries and found the battery terminals and battery cables extremely hot.

What else could be causing the batteries and cables to get this hot? It seems that whenever I click the start button, everything immediately gets hot and the batteries drain.

I now see the problem was never the cutoff switches, they were just the first thing to melt as heat moved down the battery cables. The problem was never the starter getting stuck in the on position because in this case the starter never turned.

.
Yessir. I am kinda thinking that you have a starter motor problem. Agree with @G744... Maybe remove the starter and engage the starter with a pair of batteries. Does it "freewheel" after you disengage the solenoid?

 

[HDN]

I'd say there's definitely a short somewhere that's cooking the batteries. I hope that somewhat isolates the issue. I say that having not really studied the wiring diagram. Something must've worn out to cause the problem.