Thermostart use - starter stops working

[TennFire]

I am trying to figure out a symptom on a MF250 with cold weather starting. At temperatures below 35 or so, it does not want to start when cranked. I have replaced the thermostart and confirmed its getting hot. I have a new starter switch. When I turn key to run, there is also what appears to be a spring resisted position before the crank position. If I apply the key to the springloaded tension (which I understood was how the thermostart works) for 30 seconds to a minute, I frequently have no starter response (no cranking not even a clicking from the solenoid). If I let the tractor sit for a few minutes, it seems it will crank.

Is there some trick to using the thermostart?

 

[Soundguy]

Does that unit have some sort of delay timer reheat circuit that inter locks the starter?

 

[TennFire]

I have looked at available schematics as best I could and I don't know.

 

[Soundguy]

Well, if you have the schematics, what's coming off key start and key heat positions. If there is a timer and relay from heat, does it interlock the start line?

If not, and start line is continuous, then I'd look at key and solenoid wording with an o meter, and test wires to see if there was a fault that was intermittent.

 

[pmsmechanic]

Somewhere there is a loose connection. When you use the thermostart the connection heats up and adds enough resistance so that your starter can't get enough current to turn over. You let it sit for a few minutes and everything cools of enough that the starter will work again.

Next time this happens make double sure the tractor is in neutral or even better have a helper. While holding the key onto the start position have a helper tug on the main battery wires. The tractor should start cranking usually when you pull on the positive battery cable attached to the starter. Or another method that works is to try to crank and if it doesn't want to then touch your 4 connections. Both ends of the battery cables on positive and negative. Be careful you don't burn yourself as the one that's hot is the one that has resistance and is where your problem is.

 

[Soundguy]

Could be the key switch itself heating after supplying thermo start.. but I would hope they use a relay, thus letting the key only carry field current for thermal aid

 

[pmsmechanic]

The wiring diagram in my I&T manual shows the key supplying current directly to the thrmostart. There is no relay in the circuit. The electrical problem could also be in the key switch or the wires supplying power to the key switch.

 

[Soundguy]

I'm not a huge fan of key switches sourcing big current draws...

 

[Agvg]

But does it ignite the fuel? A thermostat it a small flamethrower if it works.

 

[TennFire]

Still working on this one. I replaced both the thermostart and the ignition switch over the summer hoping that replacing those two items would solve the problem. I do not recall seeing any relays at all. Since the starter solenoid never clicks while in the "thermostart position", I don't think the switch is charging the solenoid but I may put a test light on it to check that assumption. May just have to wire a few test lights into the circuit. May also look at bypassing the switch and running a new circuit for the thermostart that incorporates a relay and uses something like a horn button.

 

[Winmac]

The engine should not crank over while in the thermostart position, only when the key is moved to start should it crank over.

Some older tractors had a heat + start position but its unlikely yours dose.

 

[TennFire]

Used a test light today and probed the switch and wiring. Thermostart tab lights in the 3rd, spring loaded, setting but not the starter (which is as it should be it appears). Starter powers in the 4th setting only. Still, if the thermostart is engaged for 30+ seconds, the starter makes no sound unless the key is turned off and left off for about 2-3 minutes.

 

[Winmac]

If you had not already replaced the switch that would be my first move, next I would check if the switch supply is loosing power or just the starter terminal. Also take a look at the wiring diagram see if there are any circuit breakers in the system.

Without looking at myself I may not be much more help.

 

[Soundguy]

Agree, make sure the feed wire to the switch is not heating and loosing power after thermal aid.

 

[irvingj]

So I've been following this thread for a while, but still did not know exactly what a "thermostart" was… then I found this youtube video: https://www.youtube.com/watch?v=Wi4mV8nrfz4

Now I know! Very clever device, which looks like it should sure help to start a cold diesel, assuming it works. (Only thing I had been familiar with was glow plugs.)

Good luck, TennFire, and I'll be following the thread to see how you do.

Learn sumpin' new ev'ry day! I love this forum!