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Discussion Starter #1
Members,

I placed this on the sister site GTT and KB suggested that it go here as well:

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
 

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Chuck,

This is great. It turns the black box into an open book with basic multi meter skills. (Who me?). As you say many get replaced as it's easier to replace then test with the JD documents. I haven't had to, so it has remained a bit over my head.

Thank You.


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Discussion Starter #3
There have been some comments that the PDF document above does not stand alone -- since the wiring diagram that maps pin number to wire color is not included. So here is a wiring diagram excerpt that shows all the pins and colors...
dual PTO wiring for 318.jpg

Chuck
 
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