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Discussion Starter · #1 ·
New member here, but long time lurker as I have inherited an 86 318 with a 90 P218G.

I have been trying to track down electrical gremlins and wanted a little advice. I have a schematic and have been reading through it, but have been a bit overwhelmed. My F1 20 amp fuse blows immediately while turning the switch to the run position. The tractor still starts and runs, but dies if I engage the transmission or the PTO (I jumped the seat switch as well for trouble shooting purposes). The battery light also stays on and there is a connector on the voltage regulator that seems to be burnt/melted/in bad shape.

I think the first place to start is to look for a short for the F1 fuse. If Anyone has any other advice, please let me know.
 

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Welcome to the forum. You might have a few issues going on but as a simple test, I would disconnect the 3 pin connector from the voltage regulator. If there are any bare wires on the connector or harness, temporarily tape them up so they cannot ground against the engine or frame. Replace the 20 A fuse and see if the fuse still blows when you turn on the key. It's possible your voltage regulator or stator has a short.

Let us know what you find. If that does not solve the issue, include your serial number and we can provide more advice. As you have probably noticed on your schematic, Deere made wiring changes on machines after various serial numbers, especially 475000.

If more troubleshooting is needed, you can also temporarily replace the 20A fuse with a 12V 20A circuit breaker so you don't waste fuses.
 

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Discussion Starter · #3 ·
Welcome to the forum. You might have a few issues going on but as a simple test, I would disconnect the 3 pin connector from the voltage regulator. If there are any bare wires on the connector or harness, temporarily tape them up so they cannot ground against the engine or frame. Replace the 20 A fuse and see if the fuse still blows when you turn on the key. It's possible your voltage regulator or stator has a short.

Let us know what you find. If that does not solve the issue, include your serial number and we can provide more advice. As you have probably noticed on your schematic, Deere made wiring changes on machines after various serial numbers, especially 475000.

If more troubleshooting is needed, you can also temporarily replace the 20A fuse with a 12V 20A circuit breaker so you don't waste fuses.
thanks for the advice. I start with that this evening. The serial is 370348. Also, should I unplug at the connector (blue) or directly from the voltage regulator (red)
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Unplug the 3 pin connector directly at the voltage regulator. (The one you circled in red) You mentioned that connector is melted and in bad shape. Heat is a sign of overcurrent and likely indicates a short circuit has occurred. Coupled with your battery light being on indicates that possibly there is a fault there. Your fault could be somewhere else but this is a good place to start and if your battery has a good charge on it now, the engine will run.

The blue connector is the engine harness. If you disconnect it, you will be removing voltage from the ignition coil and the engine will not run.
 

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Discussion Starter · #5 ·
Unplug the 3 pin connector directly at the voltage regulator. (The one you circled in red) You mentioned that connector is melted and in bad shape. Heat is a sign of overcurrent and likely indicates a short circuit has occurred. Coupled with your battery light being on indicates that possibly there is a fault there. Your fault could be somewhere else but this is a good place to start and if your battery has a good charge on it now, the engine will run.

The blue connector is the engine harness. If you disconnect it, you will be removing voltage from the ignition coil and the engine will not run.
disconnected the 3 pins, turned the key and the fuse blew right away without turning over the engine.
 

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OK, sounds like a dead short somewhere in the line fed by the 20A fuse.
Sometimes the wiring going to the headlights or tail lights shorts out to the frame so make sure your headlight switch is off. That would be the easiest short to identify and fix.
If you have any non factory electrical loads like extra lights, radio, etc., make sure they are powered off.
If the lights or extra loads are not where the short is, look closely at the wires leaving the ignition switch and PTO switch and all other wiring looking for an obvious short to ground. Make sure the terminals on the ignition switch have not failed and are shorting together too.

Then I suggest you disconnect the harnesses at the TDCM module. Make sure the headlight switch is still off. Then try the key again. If the fuse does not blow, let us know. Most likely we will need to troubleshoot the TDCM.

If the fuse blows again, disconnect the battery and refer to your schematic and start following the wire that leaves the 20A fuse and look for a short. You can use a meter set to ohms to check the wiring for continuity to ground.
 

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Discussion Starter · #7 ·
OK, sounds like a dead short somewhere in the line fed by the 20A fuse.
Sometimes the wiring going to the headlights or tail lights shorts out to the frame so make sure your headlight switch is off. That would be the easiest short to identify and fix.
If you have any non factory electrical loads like extra lights, radio, etc., make sure they are powered off.
If the lights or extra loads are not where the short is, look closely at the wires leaving the ignition switch and PTO switch and all other wiring looking for an obvious short to ground. Make sure the terminals on the ignition switch have not failed and are shorting together too.

Then I suggest you disconnect the harnesses at the TDCM module. Make sure the headlight switch is still off. Then try the key again. If the fuse does not blow, let us know. Most likely we will need to troubleshoot the TDCM.

If the fuse blows again, disconnect the battery and refer to your schematic and start following the wire that leaves the 20A fuse and look for a short. You can use a meter set to ohms to check the wiring for continuity to ground.
okay so when I turn the key to run, there is no power to the X2 side of the F1 fuse. Same for F2. Shouldn’t I be getting 12V there for both? I pulled the connector for the ignition switch and I am getting 12V to the red and green wire as I should. Could it be the ignition switch????
 

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If you are getting power to the combined red and green terminal of the ignition connector, power is flowing from your battery and through your 25A circuit breaker as it should.

Based on your early comments, I assume power WAS flowing through the ignition switch terminal from terminal B to A (X2) and to the 20A fuse and blowing it. However, something does not add up with your latest comments. You are saying no power is flowing to ignition switch terminal A (X2) with the ignition switch turned to RUN. If your 20A fuse was blowing, it had to get power from somewhere.

If you want to test your ignition switch, disconnect the connector from the ignition switch and check for continuity between terminal B and A when you turn the key to RUN. Then check for continuity between S1 and S2 when you turn the key to START.

Just a wild question but when the 20A fuse blew, did you ever measure to see if there was 12V on the non X2 side of the fuse holder? I do not see how power would get there but I have no idea if your wiring harness is intact or hacked and let's not overlook anything.
 

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Discussion Starter · #9 ·
If you are getting power to the combined red and green terminal of the ignition connector, power is flowing from your battery and through your 25A circuit breaker as it should.

Based on your early comments, I assume power WAS flowing through the ignition switch terminal from terminal B to A (X2) and to the 20A fuse and blowing it. However, something does not add up with your latest comments. You are saying no power is flowing to ignition switch terminal A (X2) with the ignition switch turned to RUN. If your 20A fuse was blowing, it had to get power from somewhere.

If you want to test your ignition switch, disconnect the connector from the ignition switch and check for continuity between terminal B and A when you turn the key to RUN. Then check for continuity between S1 and S2 when you turn the key to START.

Just a wild question but when the 20A fuse blew, did you ever measure to see if there was 12V on the non X2 side of the fuse holder? I do not see how power would get there but I have no idea if your wiring harness is intact or hacked and let's not overlook anything.
That’s was going to be my next comment. There is 12V on the downstream side of F1…. F2 is reading 0V on both sides.

See pics below. Left side runs straight to ignition switch. Right side runs to TDCM

Motor vehicle Gadget Electrical wiring Automotive lighting Musical instrument accessory

Electrical wiring Computer hardware Electronic engineering Electrician Cable
 

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Follow the wiring on the schematic and you can see the wire goes to the #3 terminal of the TDCM and also to the PTO switch. Unplug the large connector from the TDCM and see if you still get 12V DC on that fuse holder. If you don't get voltage, trace the 410 and 415 wires and see where the voltage is coming from.
 

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Were there any other repairs that were performed before the tractor starting doing this or did it just start blowing fuses after normal operation for months or years?

You mentioned the seat switch being jumped. It does not need to be jumped in order to start the engine. For safety, I suggest you make the seat switch functional now so that the tractor does not drive away during testing or worse yet, engage the PTO and mower blades while your feet or your loved ones feet are next to or under the mower deck.

Are there any other wiring hacks to your harness like a separate power line running to the PTO switch, PTO or the ignition coil? I and others can provide advice based on a the factory harness and wiring harness. If the harness was modified, we will be spinning our wheels and be forced to guess what is wrong and that will waste a lot of time on your end chasing guesses.
 

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Discussion Starter · #12 ·
Were there any other repairs that were performed before the tractor starting doing this or did it just start blowing fuses after normal operation for months or years?

You mentioned the seat switch being jumped. It does not need to be jumped in order to start the engine. For safety, I suggest you make the seat switch functional now so that the tractor does not drive away during testing or worse yet, engage the PTO and mower blades while your feet or your loved ones feet are next to or under the mower deck.

Are there any other wiring hacks to your harness like a separate power line running to the PTO switch, PTO or the ignition coil? I and others can provide advice based on a the factory harness and wiring harness. If the harness was modified, we will be spinning our wheels and be forced to guess what is wrong and that will waste a lot of time on your end chasing guesses.
So I unplugged the TDCM and still getting power to the downstream side of the fuse. I checked the PTO switch at X3 since purple 415 shares Pin 3 with F1 and sure enough, there’s 12V there…

I have to call it quits for tonight but will have more time tomorrow evening. I inherited the tractor because my father was going to get rid of it. Before that, it was sitting for months at the shop. And I’m not sure what they did to it… the only mods to the harness seem to be the jumper wire for the neutral switch (which I returned back to the switch) and then the seat jumper. The only other thing that seems different to me is some type of relay that runs off the starter. I might just start from scratch and label every wire with tape and track them all down.
 

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It should be easy to chase wires and find out why the PTO wire is getting power.

The relay you mention could be a “Starter Improvement Relay”, a factory recommended modification to provide a 12V input to the starter solenoid. In simple terms, in times when ignition switches get old, this relay provides a full current input to the solenoid.
 

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Discussion Starter · #15 · (Edited)
It should be easy to chase wires and find out why the PTO wire is getting power.

The relay you mention could be a “Starter Improvement Relay”, a factory recommended modification to provide a 12V input to the starter solenoid. In simple terms, in times when ignition switches get old, this relay provides a full current input to the solenoid.
Update: I’m almost certain it was the ignition switch. I pulled out the ignition switch which looks like it was replaced and did a continuity check. Then looked at how it was wired and it was way wrong…

When the switch was turned to “run” it was connecting S2 with B, which explains why I was getting 12V at the PTO switch and downstream of F1.

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Not to mention, from what I’ve gathered, I think it’s a completely wrong ignition switch
 

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Good job chasing that down. I assumed when you were checking for power you still had the wires disconnected from the switch as you mentioned in post # 7 and also disconnected from the TDCM.

Yes, there are a couple different configurations of ignition switches used in Deere garden tractors. I think yours might be the wrong switch as you point out. Check the part number of your switch vs. the parts listing for your tractor at Deere.com or one of the online Deere dealer websites. Hopefully no damage occurred to the TDCM when voltage was applied but as the S2 contact is supposed to get +12V during cranking, you might be fine.

Once you get the switch sorted out, if you have other issues, there is a good list for checking the normal inputs and outputs of the TDCM.
 

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As I recall, the reason for different ignition switches is based on this - tractor engines with magneto ignition systems need an ignition switch ( or relay circuitry) that grounds the magneto in order to shut down the engine.

Engines with an ignition coil like yours use the ignition switch to apply 12V to the coil ( or via intermediate safety circuitry and then to the coil) to run. To shut off the engine, the ignition switch disconnects 12V from the coil/circuitry.

Both types of ignition switches look similar and use the same key but the terminals and internal connections are different.
 

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Discussion Starter · #18 ·
As I recall, the reason for different ignition switches is based on this - tractor engines with magneto ignition systems need an ignition switch ( or relay circuitry) that grounds the magneto in order to shut down the engine.

Engines with an ignition coil like yours use the ignition switch to apply 12V to the coil ( or via intermediate safety circuitry and then to the coil) to run. To shut off the engine, the ignition switch disconnects 12V from the coil/circuitry.

Both types of ignition switches look similar and use the same key but the terminals and internal connections are different.
update: new switch worked. Fired right up and PTO turned on and off properly. Then I shut it down and the PTO no longer turns on after the second startup. Bummer. More troubleshooting ahead
 

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Discussion Starter · #20 ·
Check the fuses again to make sure the 10A is not blown.
Does the PTO lamp come on?
fuses are not blown. I can start the tractor once and the PTO and PTO light will turn on and off properly as many times as I want.

if I shut the tractor down and restart, the PTO or PTO light will not turn on at all. If I let it sit for a few hours, it will engage again. I properly gapped my PTO clutch to .018” as well.
 
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