Strange problem. Onan 218 starts fine but once you engage the blades, it stalls. I disconnected the pto clutch with no change. I installed a new clutch switch and no change. I've wiggled connectors, shined connectors, and pulled apart and reinstalled connections and no change. By looking at the schematic it looks like a few of my wires go to the TDCM. Is this my problem or am I missing something? Oh and the seat switch has been deactivated for years so I don't think that is the issue either. :unknown:
JD 318 will start, PTO switch shuts it off.
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I would pull the seat pan, have someone assist you. Then follow the seat hookup, all the way back to the TDCM unit look for hacked wiring, electric tape instead of the weave covering. There may be cuts or broken set of wires causing the problem. Chuck has the docs broken into sections for certain aspects. Let us know if you have manuals TM, CTM, PC
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The ignition on the 318 is enabled for starting when the ground speed lever is in the Neutral/STOP slot AND the parking brake is set AND the PTO switches are off...these are the same conditions that allow the starter to crank the engine, and the simultaneous satisfaction of these three conditions also provides a bypass voltage to the TDCM on terminal X23, pin 7 so it can ignore the seat switch. Here are the relevant wiring excerpts:
![Text Circuit component Passive circuit component Diagram Line]()
![Text Technical drawing Diagram Drawing Design]()
![Text Technical drawing Diagram Plan Drawing]()
For the engine to remain running when the PTO is switched on (and/or the ground speed lever taken out of the stop slot, and/or the parking brake released...) the seat switch MUST be closed by the operator being present and the wiring and fuse F2 intact and providing voltage through it to the TDCM terminal X22, pin 9.
Sorry for using multiple wiring excerpts -- but posting the entire schematic here results in a drawing that is too small to see well for discussion purposes... It is unfortunate that the drawings do not each show all the connections to key switch terminal S2...but the lower of the three excerpts comes closest to showing how these fragments interconnect.
I am pretty sure that your seat switch and it being "disconnected" is the actual problem -- my recommendation is to restore your tractor wiring to its factory condition so it is easily maintained and understood by yourself from the Deere documentation -- and that would also help other members of this forum assist you directly rather than 'guess' what hacks other prior owners may have done to your tractor...
Chuck
For the engine to remain running when the PTO is switched on (and/or the ground speed lever taken out of the stop slot, and/or the parking brake released...) the seat switch MUST be closed by the operator being present and the wiring and fuse F2 intact and providing voltage through it to the TDCM terminal X22, pin 9.
Sorry for using multiple wiring excerpts -- but posting the entire schematic here results in a drawing that is too small to see well for discussion purposes... It is unfortunate that the drawings do not each show all the connections to key switch terminal S2...but the lower of the three excerpts comes closest to showing how these fragments interconnect.
I am pretty sure that your seat switch and it being "disconnected" is the actual problem -- my recommendation is to restore your tractor wiring to its factory condition so it is easily maintained and understood by yourself from the Deere documentation -- and that would also help other members of this forum assist you directly rather than 'guess' what hacks other prior owners may have done to your tractor...
Chuck
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urstry,
I have been rereading your posts and have some additional questions. You mention that you can move the ground speed lever and not have the engine die -- do you know if a prior owner has also bypassed that Neutral switch?
Internal failures of the TDCM are rather rare, and most of those are "induced" by modifications made to the factory harness by someone without access to the documentation or knowledge in how these systems work. Even if the transistor in the TDCM driving the ignition enable relay for those cases where terminal 7 has no voltage were damaged, it would still cause the ignition to be shut down for all cases where the neutral switch were open or any PTO were on or the parking brake were off (or the seat switch were open if not bypassed...)
There must be some more modifications to your tractor's harness that we have yet to discuss here that are also part of your present symptom. You don't mention if this is a new problem or one you found when you acquired the tractor... If this tractor worked as it should before this symptom, what changed when the symptom first appeared?
There will be a logical explanation and a rather direct and simple cure once we know what the underlying conditions are...so keep us informed of what you are finding.
Chuck
I have been rereading your posts and have some additional questions. You mention that you can move the ground speed lever and not have the engine die -- do you know if a prior owner has also bypassed that Neutral switch?
Internal failures of the TDCM are rather rare, and most of those are "induced" by modifications made to the factory harness by someone without access to the documentation or knowledge in how these systems work. Even if the transistor in the TDCM driving the ignition enable relay for those cases where terminal 7 has no voltage were damaged, it would still cause the ignition to be shut down for all cases where the neutral switch were open or any PTO were on or the parking brake were off (or the seat switch were open if not bypassed...)
There must be some more modifications to your tractor's harness that we have yet to discuss here that are also part of your present symptom. You don't mention if this is a new problem or one you found when you acquired the tractor... If this tractor worked as it should before this symptom, what changed when the symptom first appeared?
There will be a logical explanation and a rather direct and simple cure once we know what the underlying conditions are...so keep us informed of what you are finding.
Chuck
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Members,
It occurred to me that there are quite a few 318 and 316 Onan vintage tractor owners that may not be familiar with the way in which the TDCM is designed to operate, and just as importantly how rare it is for the module itself to fail compared to all the troubles that harness modifications can cause -- and these are not limited to defeating the seat switch although that is at the top of the list.
Attached is a quick document I put together that may be helpful in measuring voltages at the TDCM module connectors X22 and X23. The TM1590 Service Manual sections on pages 240-20-20 through 240-20-27 give some procedures for verifying the module and the harness, and the theory description on pages 240-15-4 through 240-15-7 cover the PTO and ignition interlocks pretty well. The attached is meant to augment those manual sections and to provide a straightforward "go-to, first step diagnostic" for isolating issues with the safety systems on these fine vintage machines. Let me know if any of this is in need of further explanation or expansion.
View attachment TDCM connector voltage measurements and their meanings.pdf
Chuck
It occurred to me that there are quite a few 318 and 316 Onan vintage tractor owners that may not be familiar with the way in which the TDCM is designed to operate, and just as importantly how rare it is for the module itself to fail compared to all the troubles that harness modifications can cause -- and these are not limited to defeating the seat switch although that is at the top of the list.
Attached is a quick document I put together that may be helpful in measuring voltages at the TDCM module connectors X22 and X23. The TM1590 Service Manual sections on pages 240-20-20 through 240-20-27 give some procedures for verifying the module and the harness, and the theory description on pages 240-15-4 through 240-15-7 cover the PTO and ignition interlocks pretty well. The attached is meant to augment those manual sections and to provide a straightforward "go-to, first step diagnostic" for isolating issues with the safety systems on these fine vintage machines. Let me know if any of this is in need of further explanation or expansion.
View attachment TDCM connector voltage measurements and their meanings.pdf
Chuck
Does the pto alone kill the engine or does trying to drive also shut it down.
Provided your neutral switch is still installed and working. That can be checked with a test light.
If it’s just the pto that shuts it then that switch is bad, if taking it out of neutral also kills the engine then it’s the seat switch, or it’s associated wires.
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Moe,
The bold green line of text in my post #9 on the first page of this tread is not a link to a manual page...it is a PDF document that is self contained and should open with Acrobat or any other PDF reader when you click on it. The document you see when you open it is just a list of what to look for when these symptoms are present...sort of a structured path to troubleshooting.
By the way --- your 1977 316 does not have a TDCM assembly like the later open frame Onan powered tractors, so this voltage check list will not apply in your case.
Chuck
The bold green line of text in my post #9 on the first page of this tread is not a link to a manual page...it is a PDF document that is self contained and should open with Acrobat or any other PDF reader when you click on it. The document you see when you open it is just a list of what to look for when these symptoms are present...sort of a structured path to troubleshooting.
By the way --- your 1977 316 does not have a TDCM assembly like the later open frame Onan powered tractors, so this voltage check list will not apply in your case.
Chuck
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I was going to post the lack of amperage or a voltage drop to the ignition will kill the engine….good find
-Vern
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