Starter Relay / Switch Question

It's been a while since if been able to fully troubleshoot my MF50 diesel's starter issue. Went to start it probably a month ago and the battery was very weak, cranked over once slowly and then stopped. Went and got my battery jumper, hooked it up and before turning on the power, the starter tried, ever so slowly to turn the engine. It was as though the starter never disengaged. Glad the battery was already drained. I'm thinking switch shorted all the way. Bought a new switch (starter and solenoid are maybe 9 months old) and thought I'd try to replace the old one. Pulled the old switch out, tested it with a meter and as far as I can tell, it's good, no internal shorting that would keep the solenoid engaged.

Then.....

I do some internet searching and find it a common occurrence that on a DIESEL (regardless of make it seams - which is why I'm here and not the Massey section) when the battery is very low, the starter solenoid can remain engaged, trying - with whatever juice is left in the battery, to start net engine!

Why is this? I would think there is a spring to disengage the contacts on the solenoid when power is removed via the switch (which is why I assumed a shorted switch). Have I got it backwards on the mechanics of a starter solenoid and the springs hold the contacts closed and power from the switch opens/pulls the plunger into the springs, collapsing them and holding them and in doing so, disengaging the starter? Don't think that squares with my wiring diagram.

Why would a weak battery continue to cause the solenoid/starter to crank after the switch is removed from the start position?

Thanks!
Bill

Can't say I've ever had that happen.

And it seems kind of counter-intuitive to me- but, with low voltage sometimes things don't work as they should. I suppose the contacts, rather than snapping in hard like they should, just sort of sit there and try, giving them a chance to weld themselves together somewhat.

When I worked at a dealership in the 70's- early 80's, I would get cap/contact repair kits for some of the big starter relays. Mostly Delco-Remy, as that was mostly what AC used. Then they put Lucas starters on the 200, and I was working on those pieces of junk all the time.

I've seen it happen on several tractors gas and diesel,the solenoid is worn most of the time and a new solenoid will cure the problem most likely.Just as bad or worse if tractor starts and the starter stays engaged

Like Bob said, arcing and welding because of the low voltage/high amperage caused from a discharged battery.

Another thing that happens, on the Delco type starters with the solenoid on top that pulls in the starter drive... The spiral on the shaft that the drive slides on, if the thrust bushing is worn, broken and gone, lets the drive spiral further out on the shaft than it should. That causes the fork that pulls the drive into the flywheel, to move further than it should, resulting in the plunger being pushed into the solenoid by the starter torque, transferred through the spiral on the shaft, which holds the contacts in the solenoid closed.

With a good battery, the engine is turned through the compression stroke, then the engine turns itself as the compression rebounds, or it starts. When the load is relieved on the starter, the plunger moves out slightly and opens the contacts if the start signal had been removed.

Nothing to do with diesel or gasoline. Weak battery results in slow starter motor speed. Slow motor speed results in low back EMF and low back EMF results in abnormally high current, thus 'welding' the contacts in the closed position. A sharp wrap on solenoid with wrench or hammer will generally jar the contacts lose/open. You are thinking that low battery & higher current are at odds, but the back EMF is a larger factor than the low battery.
Notice how the house lights dim when a large electric motor starts up from a stalled position?

Hello Billy VA,

The starter side of the solenoid switch has power only in the cranking mode. Once you turn the key to run, should be no power. It is spring loaded off, electric on. Contacts can stick and keep it on though. A voltage check on the starter side will tell you if it has power with the key in the run position. You should also check for power on the ignition side of the switch. Just in case the key is still powering it in the run position, good hunting.......

Guido.

I had it happen quite often in winter when she cranks hard with weak batteries a solenoid would stick/weld closed so she kept trying to crank even when I stopped pushing the start switch. I had to quickly remove a battery cable to make it stop.

A good charged up battery often cured the problem and sometimes the solenoid was okay after it freed up. If not a new one was in order unless I could clean and buff and smooth the pitted burned contacts

John T

Steve and Jmor have got it.Essentially,weak batteries are hard on the cranking system,and contacts in the solenoid can weld themselves.Mark

Again proving the magnetic field does NOT come from the current, but from the atoms of the wire excited by the VOLTAGE.

(quoted from post at 10:19:21 02/12/17) Again proving the magnetic field does NOT come from the current, but from the atoms of the wire excited by the VOLTAGE.

Click to expand...

o, based on that line of thinking, if I submerge the wire in X*anax (depressant) the atoms should be less 'excited' and magnetic field diminished, but if submerged in R*italin (stimulant), then the more 'excited' atoms will result in a greater magnetic field. Therefore I should easily prove or dis-prove this theory with two simple experiments, right?