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Discussion Starter · #1 ·
I looked through the forum and could find no reference to the problem I am facing. I started to mow with my 318 the other day and as soon as I engaged the front PTO (deck) the deck engaged, the pilot light sputtered and the tractor died. I discovered the 20 amp fuse had blown. I replaced the fuse and the tractor started, but when I engaged the PTO the fuse blew immediately. Acts like a direct short. Problem is, the wires are all loomed as from factory and no evidence of any wear and tear. I really hate to tear into the loom as it looks pristine. I replaced the field coil, as the one on the tractor did not test very good (magnet test) and looked pretty ratty. Now, no starter too. When ignition switch is on, and PTO engaged, still pops the fuse. Just guessing looks like a direct short in the PTO engagement wires or coil or pilot light wires. I did replace the PTO switch with a new OEM switch. I checked ground on both sides of the fuse, and both sides are grounded. One about 8 ohms and the other about 400 ohms. I am guessing ground through the ICM, but don't know if that is correct. The leads on the switch test grounded too. I am looking at going through all of the lockout switches (the seat switch is jumpered.) I am not very limber these days, and looking through tri-focals makes diy a challenge. I am hoping this rings a bell with someone, and they can point me to a weak link in the PTO power chain to save me the rigors of searching it out? Any suggestions appreciated, thanks! Could the ICM be involved?
 

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Check the PTO coil is should be 3.7 to 4.0 ohms of resistance At plug on front of tractor). As a test remove PTO from the circuit, unplug it from the harness (near front of tractor and see if fuse blows when PTO switch is turned on). The wires do run along side of the engine (blue wire) check the length back to the firewall, there maybe a plug connector in there also. Could be bad/broken/chewed/melted insulation.
 

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Chancy,

Do you have the shop manual? Below is the wiring diagram from the manual -- it may help you trace things down. You will note that the only things that are powered from the 20 amp fuse is the PTO, the ignition (through the TDCM), the headlamps/tail lights and a couple of dash light indicators. My guess is the headlamp wiring may be melted onto the exhaust system since it is fairly unprotected in that area... The tail light wires are also not offered much protection under the fender pan either, so check those as well. Normally, the tractor is operated with the light switch OFF, but it often gets pulled and left ON unnoticed if your 318 only gets daytime use.

Schematic Rectangle Font Slope Parallel


Chuck
 

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Chancy.

Re-reading your posts it seems that the fuse only blows when you attempt to engage the PTO, so the lighting circuits are likely not involved here. If you have had the connections off the back of the PTO switch, be sure that they are placed back correctly (the can fit physically in several wrong configurations -- some of which would prevent engine starting and could blow the fuse...) Use the wiring diagram and the sire colors to ensure that you get it connected correctly.

Chuck
 

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The TDCM is in fact involved in providing power to the PTO switch, and it is this TDCM module that arbitrates the safety switch conditions that confirm that power should be applied to the PTO. Here is a good list of what a "normal" TDCM module has as voltages/conditions at each of its terminals:
Font Parallel Paper Paper product Document


NOTE: The TDCM can be damaged by miss-wires in the tractor harness, and these modules are expensive and non-returnable so be careful to ensure you do not hack any factory harness connections/routing. The module can be repaired by some one with good electronic technician skills as discussed at length elsewhere in this forum.

Chuck
 

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Discussion Starter · #8 ·
Chancey,

What is the year/model of your 318? If you unplug the PTO assembly from the harness at X26, does the fuse still blow??

Chuck
Thanks Chuck, I don't recall if I tried turning the switch on with the coil disconnected. I need to do that. I replaced the coil with one from a first gen 318 with only one wire (frame ground). I just adapted it to the two wire plug. The original coil looked pretty beat up and, as I recall the resistance was out of spec or questionable. I tried the magnet test and it failed. The replacement coil had good magnet test (picked up and held a wrench when hooked to 12V.) I have looked at the wires to the coil, and they look good. The wires to the pilot light look good as well. I replaced the switch just a day ago just as a shot in the dark. The old switch tested OK. I don't think switch connections or switch itself are an issue. And, on my tractor it can only be hooked up one way. There is a two pin and three pin plug. Since there is no rear PTO, the three pin plug only uses two active pins. the other (2 pin) is the safety lockout and it should not make a difference which way it goes on. I am really thinking that there is a problem in the pilot light. It has a diode in parallel with the light. As I said, the light sputtered before going dead the first time, now it just pops the fuse instantly. I am wondering if the diode was frying when it sputtered. .The problem is, the pilot light looks like a bugger to get out (without breaking something), and the wires all look pristine. I am considering cutting the hot side of the pilot light to see if that might solve the problem. I do need to try the switch with the coil unplugged though. I can't imagine I did not already do that, but can't remember. Damn oldtimers disease! The tractor started and ran for a while after the initial fuse blow, then I did something that apparently knocked out the lockout circuit. That is why I was wondering about the computer. I have a new (used) computer, I was going to hook that up and see what happened. I was just hoping somebody would have some insight into a likely place to look before spending a couple hours crawling around on the ground to trace it all down. I have four 318s three with the P218 and one with the B43. Great machines, but a bitch to work on for an old guy. Thanks all for your input, I appreciate it.
 

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Chancy,

Thanks for the detailed story on the fuse issue -- perhaps the diode on the dash indicator lamp is indeed shorted, or the lamp itself went shorted when it burned out of old age...

The 318 Operator's Manual says to remove the hood latch bracket to access the dash lamps -- just four bolts.
Font Rectangle Slope History Soil

...then the lamp holders just twist out.
Font Parallel Terrestrial plant Pattern Rectangle


Since you have already changed out the PTO field coil for another, no need to suspect the replacement is too low a resistance if it came from a working tractor. Worth a shot testing it if it was just a shelf spare though...perhaps you already measured the resistance and know it is a good unit.

Chuck
 

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Discussion Starter · #10 ·
Chancy,

Thanks for the detailed story on the fuse issue -- perhaps the diode on the dash indicator lamp is indeed shorted, or the lamp itself went shorted when it burned out of old age...

The 318 Operator's Manual says to remove the hood latch bracket to access the dash lamps -- just four bolts.
View attachment 282782
...then the lamp holders just twist out.
View attachment 282783

Since you have already changed out the PTO field coil for another, no need to suspect the replacement is too low a resistance if it came from a working tractor. Worth a shot testing it if it was just a shelf spare though...perhaps you already measured the resistance and know it is a good unit.

Chuck
Hey Chuck, Thanks for posting the pages from the manual. I have a technical manual reprint (several hundred pages), and it doesn't have that procedure listed or those photographs. That makes it a whole lot easier. You can't tell that the bulbs just twist out by looking (tri-focals again). You just saved my day. Your manual doesn't say anything about the diode does it? The diode appears to be contained in a small length of plastic tubing between the blue and black wires. Thanks,Chip
 

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Chip,

That information is in the Operator's Manual (OM-M79598) not the Technical Manual (TM1590) so you may not have seen if if you don't have the first document...

It does make it easier to know to twist out the lamp socket. ;)

Chuck
 

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I think you have windings in the clutch shorting to each other. When this happens, the resistance goes down and the current goes up. Interesting thing is as windings in the clutch short to each other, the current increases, but the magnetic holding power stays the same, so the clutch continues to function just fine until it blows the fuse.

Here's a company that makes excellent replacements:

 

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Discussion Starter · #14 ·
Solved: For anyone else facing this problem, I solved it by wrapping the positive wire going to the PTO clutch coil. Never did find the short, must be through the loom somehow but could find no evidence of arcing anywhere. Wrapped it with plastic wire loom and placed wire ties wherever it contacted the firewall, oil cooler, or engine shroud. The technical manual says it is definitely in the 20 amp power field, not the control module. That means it is in the wires to the clutch or to the dash light. As Chuck said above, the light socket twists out(1/4 turn CCW) and there is a diode in parallel with it. I was able to remove the socket and check the bulb and the diode to determine that they were OK. Now, why do I only have 3 volts at the 20 amp fuse...tomorrows project. Good luck!
 

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I think you have windings in the clutch shorting to each other. When this happens, the resistance goes down and the current goes up. Interesting thing is as windings in the clutch short to each other, the current increases, but the magnetic holding power stays the same, so the clutch continues to function just fine until it blows the fuse.

Here's a company that makes excellent replacements:

It is a bummer they are out of stock now.
 

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Discussion Starter · #16 ·
It is a bummer they are out of stock now.
Hey Dukewags, You called it exactly right! The short version is replace the coil with a GOOD one!! For those who might need it here is the long version: I replaced the original coil with a used one from one of my parts tractors. The used coil tested OK, but in fact was ready to die. I put it on the tractor and it lasted long enough to get two lots mowed and get the City of my A**. The charge light was on the whole time during the mowing. I misinterpreted it thinking that I had jiggled the short out of contact (see reply above.) Finally that coil started blowing the 20 amp fuse too. In the process, it burnt up the rectifier as well (one of the ac leads was melted.) I bit the bullet and bought a JD replacement coil. Looked for an aftermarket, but they were out of stock. Over $400 for the JD version. After that the charge light was on constantly. Tested rectifier and it is only putting out around 8 volts DC, new one ordered. Grass is ready for another mowing, so cobbled it back together with the malfunctioning rectifier and engaged the PTO. My lumbar spine is fused due to an illness (mobility is zero.) Between that and trifocals, I got the drive belt on over the little finger on the idler pulley. Amazing how rapidly a $60 belt can vaporize when not installed properly. I did manage to scavenge a really nice hydraulic lift table from the scrap yard a few years ago, The next project is to get it going so I can use it. You just can't fix it if you can't see it. Thanks to all for your comments, hope this helps someone else. BTW, I did completely remove the wire loom and the wiring was all in good shape. The diode (in parallel) with the PTO pilot light is easily viewed and is easy to evaluate, by removing the bulb socket. Mine was visibly good. Thanks for all of your support.
 
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