JD5103 bypass start relay #2 problem

[tractortrash]

I am helping a friend whose JD5103 will fire with the key in the start position then dies when the key is released to the ignition/run position. This is an intermittent problem, more often occurring in cold weather. I determined that bypass start relay #2 is energized inappropriately after the starter is cranked, based on the following tests. #1 the fuel shutoff solenoid is powered on initially then loses power when the key is released from the start position. #2 replacing bypass start relay 2 with a jumper wire between terminals 30 and 87A eliminates the no start condition. #3 the engine can be started by cranking the starter then quickly switching the key off then back to run while the engine is still turning over (this resets bypass start relay 2 and re-energizes the fuel shut-off solenoid).

My JD5105 service manual describes this no-start condition (engine dies when key is released from start to run position) and describes a 3 ohm, grounded resistor on circuit 313 that is designed to draw off a small amount of current generated by the freewheeling starter motor, in order to prevent activation of bypass start relay 2. I tested the resistor and wiring on the JD5103 and it has 3.1 ohm resistance. We cleaned the terminals and reattached the resistor, reseated the 2 diode packs, and the bypass start relays 1 and 2. The no-start condition is unchanged.

I am thinking about throwing new parts at this intermittent problem (resistor, diode packs, bypass start relays), but thought I'd first ask for advice. Could a sticky pinion gear on the starter cause this problem, by keeping the starter engaged on the rotating flywheel a bit longer, even with a functional resistor in the circuit? The tractor is ~10 years old. Does anyone have experience fixing this problem?

 

[3Lfarms]

Pull the relay and see if you have continuity between 30 and 87a on the relay itself since you can get it to work by jumpering those. If not, replace the relay.

 

[tractortrash]

Pull the relay and see if you have continuity between 30 and 87a on the relay itself since you can get it to work by jumpering those. If not, replace the relay.

Thanks for your reply. The relay bench tests OK. The relay is activated during the normal start procedure, which breaks continuity across 30 and 87a to shut off the fuel solenoid. I can prevent relay #2 from shutting off the fuel solenoid by pulling bypass start relay #1, which supplies current to relay #2. That "fixes" the problem and the tractor starts and runs normally.

It appears to me that the bypass start circuit containing bothrelays is used only for diagnostic testing, using a jumper wire to run the starter motor while measuring voltages in the start circuit. The bypass start circuit cuts the fuel supply so the engine won't start while the starter is running during the test. During the normal key start procedure, bypass start relay #1 is activated and cuts power to relay #2 so the fuel solenoid remains on (powered by the IGN/run terminal of the ignition switch). Can't find any mention in the shop manual of other safety switches that might use bypass start relay #2 to cut the fuel supply. Maybe its OK to run with bypass start relay #1 removed.

I think the problem is caused by stray current generated by the freewheeling starter motor, after releasing the key from the start position, while the starter drive remains engaged in the flywheel. The shop manual says this can generate 0.75-1.5 V, which is enough to activate relay #2 and cut the fuel supply. To prevent this, a grounded 3.0 ohm resistor is present between bypass start relays #1 and #2 to draw off small amounts of current. The resistor and associated wires on this tractor check out fine with an ohm meter. I am wondering if the starter continued to spin a bit longer than normal whether the resistor can prevent activation of the bypass circuit to maintain the fuel supply. Was hoping that someone has encountered this problem before I remove the starter to examine it for wear. This tractor is 10 years old but lightly used and well maintained (wish I owned it!).