Runaway starter on M936? Advice Needed..

[goldneagle]

After installing my rebuilt starter and charging the batteries on my M936 I got the truck started with no issues. I tried it several times without any issues.

Today i went to start up the wrecker to see if the batteries were still holding the charge. When I tried to crank the truck I could tell the batteries were very low. So I turned off the start switch and then the Battery Switch. I noticed the starter was still trying to turn. I tried giggling both switch on and off with no effect. Had to run to the other side of the truck and got the the batteries. Ended up forcing the ground battery clamp off the terminal. Got burns on both hands for my trouble!

Now the question is what went wrong? How do I proceed from here? Obviously I need to get batteries. Do I just hook them up and see if the starter is quiet with the switches off? Please advise....

 

[m16ty]

The contacts on the solenoid stuck. Sometimes you can free it up by tapping the solenoid with a hammer but it's probably time for a new one.

This happens when the contacts get too hot and they weld together.

 

[Motorcar]

Isn't that one of the functions of the PCB, to prevent this from happening?

 

[1 Patriot-of-many]

Isn't that one of the functions of the PCB, to prevent this from happening?

No, I think that's to prevent the starter from engaging when the engine is running.

 

[74M35A2]

Yes, most likely the solenoid bus bar inside stuck. This is a copper bar just under the solenoid cap that connects the battery B+ cable to the starter motor when the starter is engaged. The point of the starter being just re-man'ed is interesting. A lot of starter reman parts are now aftermarket or clone, all from China. Where I work, we are an original equipment supplier for starters both automotive and medium/heavy duty, and see our parts being cloned, right down to the casting marks. Most of the time, the extra $ is not put into these cloned parts, such as plating, surface knurling, etc....

A properly designed solenoid bus bar and mating contacts inside the solenoid (which you can't normally see) have knurled surfaces, in addition to a very strong solenoid return spring, as to prevent exactly this. A knurled surface greatly reduces contact welding. At this point, the chance of it happening again is very high now that it has happened once. Also, now that the starter was run continuously on a low battery, it is likely that the armature commutator (where the brushes ride) has overheated and been damaged. Starters really only like fully charged batteries (12.8V each or 1.26 sp gravity), as to keep the commutator moving quickly to stay cool. It shortens the length of time the high current is passing through each commutator segment. We are testing our heavy duty starters as I write this, and we are always exceeding at least 50,000 engine start cycles on applications like this. You definitely should have made it more than a few starts!

Even if you can get it to work, it is not trustworthy at all anymore. Try to get your money back. At this point, see if you can get even a used name brand, non-reman'ed starter if need be to keep costs down, such as Remy, Leece, Bosch, Denso, or Mitsubishi. They all manufacture and supply HD starters, and they are all interchangeable due to the standard SAE 3 bolt mount. Just match it up by number of pinion teeth, heavy duty (Cummins 250) usually has 11 larger teeth, and medium duty (Cummins 8.3/LDT-465) 12 smaller teeth. You may or may not need to drill out the mounting bolt holes on the starter nose cone, as there are 2 different sizes in the field. US uses 12V for most on-road applications, and 24V for most off-road applications (MCI bus does use 24V on-road in US). The rest of the world uses 24V for all MD/HD applications, regardless of on/off road.

Absolute minimum is to replace your solenoid with a name brand (not white box) one from the company who makes the starter you have now, if you feel that the starter motor itself still has significant performance.

 

[goldneagle]

OK update: Went outside today and whacked the solenoid twice with a hammer as I was instructed by the tech at the starter shop. I then tested the circuit by touching the negative battery lead to the battery terminal to see if it would spark. No spark so no load. So the solenoid retracted off the buss bar (plate). Tested the 2 batteries in the truck. One was 12.6 volts and the other 10.6 volts. So bad battery. Installed 2 Military batteries that i just reconditioned. One was 12.6 volts and the other 12.5 volts. Tried to start the truck. Cranked right up. Shut it off and checked to make sure starter was off. Wire connection to battery was cool. No spark when I disconnected the ground. So for now we are OK. I will retest the starter daily to see if it behaves itself. Otherwise it goes back to the starter shop for TLC. I am going to send my bad original US made solenoid to be rebuilt if cheap enough. Less than $6 Priority Mail Padded Flat Rate Envelope.