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I've been burning up weekends trying to solve this problem, and I'm about to give up and take it to a shop. But, I thought I would ask here, first, though, and see if you tractor veterans could save me some money.

I inherited a John Deere 318 that seemed to be working fine. I had to replace a battery cable because the terminal clamp was broken. Everything seemed good, and it started up and ran after the replacement fine. I engaged the front PTO without a problem and the transmission engaged just fine because I drove it around for a minute. Then I saw smoke that smelled like burning plastic.

I disengaged the battery cables, and looked for the problem. The end of the battery cable that I bought had a lot of exposed metal at the starter terminal end, and because the starter terminal is in such a tight space, the cable end with bare metal rotated into contact with the chassis when I tightened down the nut and created a short.

I next found that the two wire connector to the front PTO was melted and burned. Since that was the only obvious issue that I saw, I replaced that connector and started the tractor back up. I started fine, but when I engaged the front PTO, the 20 amp fuse burned out immediately. I thought, fine, don't need the front PTO right now (just the back), so I disconnected the front PTO clutch completely.

The tractor again started up fine after that. But, the engine died immediately when I tried to engage the transmission. The tractor was fully warmed up and running like a champ. So, I got back under the hood and looked for the next issue. I found that the 3 wire connector to the voltage regulator was melted through between two of the wire ends. So I replaced that connector. That didn’t fix the problem.

I printed off a bunch of wiring diagrams and started to try to troubleshoot the electrical system wire by wire, connection by connection. I couldn't see any more problems. It seems like the tractor is shutting off due to a malfunctioning safety switch. The former owner had by-passed the seat safety switch by just twisting and black taping the two leads going to the seat together. I replaced the tape with a wire nut to make sure the connection was good, but I couldn’t re-attach the safety switch. That did not fix the issue.

Chasing the problem around the electrical system, I found something else that I *think* might be the problem. My multimeter was showing that the oil pressure switch is closed when the engine is off. Is it supposed to be? It looks like the oil-pressure sensor is tied into the safety switch system that will shut the tractor down when either of the PTOs or the transmission is engaged. The oil pressure light stays on after the engine is started and running fine. Looking at the wiring diagram, there is a direct circuit from the PTO/Transmission switches through the oil pressure sensor. Could something have been messed up on that line when I created the short with the battery cable?

I've been wrestling with this issue for some weeks now, and I really need the tractor and back pto going so I can mow my yard and till my garden. Any help would be really appreciated!


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10,555 Posts

Welcome to the WFM forums and to owning a classic Deere garden tractor. Good to see another new member from here in the Pacific Northwest.

You may find that the shops (yes, even some Deere dealerships) know very little about these machines as many of the service technicians are of an age where they were not yet born or perhaps only in grade school when these tractors were manufactured. You can educate yourself here on this forum and through the service literature (TM1590 and CTM2) to repair and maintain your machine -- and we can help.

The oil pressure switch is not part of any safety interlocks...it is simply an indicator. Yes it is of the type that is grounded when no pressure is present, providing a return path to the dash lamp to turn it on. A failed sender will keep the lamp lit, but not adversely effect any other tractor functions.

It does sound like you have found several obvious damaged sections of the wiring harness and repairing them now is a good thing -- we just need to continue on from those to find the root cause of your present symptoms.

Here is the TM1590 manual excerpt on the ignition safety interlocks -- what is causing the engine to die when the PTO is engaged or when the ground speed lever is taken out of the neutral slot will be described below. (A separate issue is causing the F1 fuse to blow when the front POT is turned on...likely a shorted winding in the electric clutch field coil itself...often caused by a bad bearing that allows the rotating parts of the assembly to wear through the wires in the field coil.) But first, to the ignition wiring section:

You do not mention what year model your 318 is (the serial number will let you know much about the actual configuration of the wiring -- many changes were made in the long production run of this model.) If it is early enough to have the glass fuses with the plastic cylindrical holders please understand that those are often a cause for considerable unreliability. As you may notice from the above diagram -- the normally functioning seat safety switch would provide a voltage to terminal 9 of X22 when the operator is present *if* the fuse at F2 is intact *and* the fuse holder is making good connection. The seat switch bypass you describe should make that voltage at X22 pin 9 always present, but your symptoms suggest otherwise. Not seen in the above diagram is the other end of the #710 purple wire that is crimped together with the #705 purple/black wire at the key switch terminal S2. It may not be obvious, but the voltage on the #705 wire provides another means for the TDCM to enable the spark to the engine, and does so whenever the PTO switches are off and the ground speed lever is in the neutral slot. That set of switches are best seen in the starter enable diagram here:

If your tractor is below SN 600,305 then it does not have the parking brake switch. Viewing these two wiring excerpts together we see that the ignition can be operating as it should when the tractor is in neutral, yet die immediately when the ground speed lever is moved out of that notch -- IF the seat safety wiring is not providing a voltage to the X22 pin 9.

From the sounds of things you may have several severe "hacks" to your harness form a prior owner. Start by checking the things mentioned above and we can continue troubleshooting with the information you provide as feedback when you get further into this process. I will send you some helpful information that is a bit too large to attach here. Let us know the year and SN of your tractor as that is needed to provide specific recommendations.

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