Tom and DCT,
There may be hacks (or failures) in the harness that we have yet to discover... I still can't understand why unplugging the PTO from the harness allows the F2 (3 amp) fuse to remain intact when the PTO function is enabled, yet it gets blown if the PTO is plugged back in under those same conditions. There should be no connection between the two functions.
There may be something failing inside the TDCM -- so lets look at how the internals of that module work. I don't have the exact detail of the internal wiring of this module...just the
functional equivalent diagram that Deere provides in the manual, as shown below:
View attachment 280985
The 3 amp fuse (F2) connects directly to the X23 pin 5 via the #500 red wire, then continues on through the seat switch and harness to the X22 pin 9 via the #810 pink wire. These are the only two loads on the F2 fuse, so lets look at what they do once they enter the TDCM.
Pin 5 of connector X23 connects internally to the Battery Discharge Lamp Control block in the TDCM. This block has only two other connections shown, the ground connection on the right side going to X22 pin 10, and the lamp drive connection on the upper left side that goes to X23 pin 1. There might be some components inside this Lamp Control block that draws excessive current from the F2 source -- but it is hard to conceive of what that might be that would be conditional on the PTO coil being connected.
The other load for the F2 circuitry is the X22 pin 9 where the seat switch connects to the Time Delay integrated circuit. That block has a single output connection which goes to the base of transistor B. When transistor B is conducting, it provides a ground connection to relay G and through the diode C to the other relay which enables the ignition when its contacts are closed (connecting the X23 pin 3 -- the 20 amp fuse supply input, to the X23 pin 2 -- the ignition coil and the hour meter.) Relay G is the control for the PTO source voltage, connecting the 20 amp source at X23 pin 5 to X23 pin 4 when closed and providing a voltage source to the front PTO switch at X4 on the #760 blue wire.
The power to the relay G coil comes from either the safety switch path when both PTO switches are OFF via diode E, or once the relay contacts D are latched on, through the connection to the other relay and the X23 pin 3 input from the 20 amp fuse.
Since we cannot know for sure what is inside the two blocks labeled
Time Delay IC and
Battery Discharge Lamp Control, it is hard to envision what component failures might be at play here that could cause the symptom detailed in this thread.
Swapping out the TDCM for a known good unit would perhaps be helpful, but harness hacks can be a source of causing module failure, and it might be risky to do such a substitution. Certainly you would not want to simply start an expensive parts replacement activity here, as these modules are not returnable to the dealer and now cost about $490 each.
Chuck
