Montana 3940 HST won't start - Electrical

[nhaines]

Well I just purchased a used Montana 3940. I just got done replacing all the fluids and filters and went to check the headlights. I flipped the head lights on and the entire tractor shut off and won't restart. The engine won't even crank over. After I removed the dash I discovered that Mice have been up there and made a mess of the wiring harness. I am just trying to get it restarted at this point. I have checked all the fuses, purchased a new starter relay, verified that the PTO is off and still the engine won't crank. Does anyone have any suggestion on what to do next? Would anyone happen to have the wiring diagrams so that I can start checking voltages. Thanks in advance for your help.

 

[k0ua]

Most modern CUT tractors will have a main fuse of 60 to 70 amps. It will be inline and usually taped up near the starter. If this main fuse is blown, nothing works. See if you can find yours.

 

[pmsmechanic]

I had this symptom on a Case combine. You could start the motor and go combining but as soon as you turned the lights on everything died and there was no restarting till the next morning. Turned out to be a major wire supplying power to the cab went through a bulkhead fitting. The fitting was plastic and had a bolt on each side where the wires were bolted to. The mice had enjoyed the sheltered area and had peed all over the connections and they were corroded. There was enough juice going through to run the combine but when the lights were turned on the connection heated up from the resistance and no electricity could pass through till everything was cooled off.

It sure sounds like your situation so here is what I would do. Follow the positive from the battery to the starter. Then from the starter power usually goes to the dash area. I'm thinking that somewhere there is major corrosion on a connection. In a situation like this I don't spend a bunch of time looking for the problem. Just turn your lights on and start wiggling wires. Once the lights come on try and make them go off again by wiggling wires in the same area. You'll soon find the problem.

 

[nhaines]

I checked for the fuse and found what appeared to be a relay of some sort taped right to the wiring harness just as you suggested. I unplugged it but I am not sure if it worked yet as I have removed the entire wiring harness. I am going to attempt to ohm out every wire and try to find the short. It is going to be a mess though as quite a few wires are chewed up.

 

[k0ua]

How many terminals were on this "relay"? Any relay would have a minimum or 4 contacts. I bet you found the main fuse(2 contacts) Did you ohm it out? Was it a dead short (therefore good) or was it open (therefore bad, ruptured, blown)? I feel for you, with the chewed up harness.. That can be a handfull for someone who is even familiar with that particular harness. Nasty rodents.

 

[nhaines]

There were only 2 terminals so i would agree that it is a fuse. It was wierd that it had the electrical symbol for contact and not the fuse. I am going to ohm it out tonight and I will let you know what I find out. Looking at the parts diagrams I cannot find a replacement part number for this fuse.

 

[k0ua]

There were only 2 terminals so i would agree that it is a fuse. It was wierd that it had the electrical symbol for contact and not the fuse. I am going to ohm it out tonight and I will let you know what I find out. Looking at the parts diagrams I cannot find a replacement part number for this fuse.

Some users have reported getting them at Audio stores and even wal mart. Seems the high power audio amplifier crowd use these. I don't know this for sure, but it has been reported here. Worth a look.

 

[mddorange]

:welcome:
To the forum from Alabama. My daughter lives in New Albany.

 

[nhaines]

Ok I just made it to the barn to check this fuse. I pulled it off and its not a fuse its a diode! I am assuming it is for the charging circuit between the alternator and battery. My meter did not have the ability to test a diode but i will get one today at work and test it tonight. Here is the pictures of it.

 

[k0ua]

I doubt that diode has anything to do with rectifier charging voltage.. That is done inside your alternator with what is known as the diode trio. Your alternator is 3 phase AC and is rectified inside of the alternator case. Modern tractors have a lot of these diodes around the wiring harness often for relay coil "spike" or back EMF suppression (EMF stands for Electro Motive Force). When you remove a voltage from a relay coil the built up magnetic field collapses and a reverse polarity pulse or spike comes out of the coil into the electrical system. These suppression diodes are biased to short out this pulse. These diodes can also be used for other purposes.

If you would like fun filled experiment, take any 120 volt transformer primary winding (it is an inductor or coil of wire just like a relay coil) and touch a 9 volt battery to its primary winding. now put your hand across the primary winding, and remove the battery while keeping your hand on the primary winding. It will knock the snot out of you!:shocked:

The little 9 volt battery causes a magnetic field to form in the primary coil, and when you remove the battery this magnetic field collapses, and causes a large surge of voltage to come out of the coil.

In a pinch you could test the diode with a tail lamp and a battery and some wire. First test the tail lamp by hooking it up to the battery and make sure it lights. Then place the diode in question in series with the battery and the lamp, either direction. The lamp will either light or it will not. Now reverse the polarity of the diode in the circuit keeping all other connections the same. If the lamp did not illuminate in the first diode test, and when you reveresed the diode it still does not light, then the diode is open. If the lamp illuminated on the first test and still lights on the second test where you reverse the diode, then the diode is shorted and bad. If the lamp illuminates on the first test and does not on the second test, then the diode is "good". This is what diodes do, they allow current to pass in one direction and not in the other. Good luck