Ignition Module for John Deere Mower

 The Mower

It's a John Deere F525 front-cut zero-turn mower with a Kawasaki PA540A engine.

Symptoms

It wouldn't restart when hot twice on one mowing session. The engine would turn, but there was no hint of ignition. The mower started and went about 20 feet into the next mow, then died with no sense that it was going to restart.

Troubleshooting

I installed a new spark plug, but still no go. And no spark was visible with a spark tester. The ignition module "IM" (JD p/n AM132770) seemed like the most likely culprit. Unfortunately there's no good bench test for these. I ordered one of the BM11 types from Amazon for under $16.

I tested the IM with a component tester and VOM diode test. It showed p-n junctions in both directions - no shorts or open circuits, so that was inconclusive.

I continued testing, adding a ground jumper from the IM's mounting bolt to make sure there was a good path to ground. I was finally able to get the blower cover off (one pesky mounting hole had to pried loose from inside the cover!). I cleaned the magneto contact points for a good ground and re-gapped the coil when I remounted it. The white wire from the primary winding to the IM had good continuity, both tab contact areas were clean. Starting attempts  showed a weak orange spark with no ignition ... even with  starter fluid.

So I decided to put a scope on it ... because that's what I do (for lack of a better excuse).

The primary is the lower blue trace, 5V/division. The yellow trace is the spark plug lead (with spark plug connected), using an Hantek HT25 10,000:1 clip-on ignition probe, 0.5V/division.

I hadn't done this before, but I see a 10kV negative spike which looks reasonable. I wondered if maybe the magneto was bad. Moments later Amzn pulls up with ignition module in-hand. 10 minutes later it's installed. 

 

I hit the starter and it's a bit rough then a backfire. The second attempt starts normally enough. I back off the choke and we're off to the races. .. she's running strong. I lower the throttle and get a new scope sample.

Here you can see the positive voltage excursions for both the primary and secondary windings are much improved.

Okay, so it's back on with the blower cover, fuel pump, air cleaner, fuel tank, and fender.

It's now been a week since my last mowing attempt. I fire her up, and start to back out of the shed. No go. Forward. No go. Hmm, I reset and release the parking brake. Still no go. Crap - I must have done something to the transmission linkage. But wait ... the transmission is still disengaged from me pushing it into shed. I slide the lever back. Tink - we're back in business and the next couple of hours of mowing are uneventful.

I can't say that this is the most logical troubleshooting session I have ever had. It was easy enough to rule out the interlocks early on. The next time I will check the IM grounding and primary signal before I order the part. The interplay between the IM, coil, spark plug, and HT lead complicates things a bit. But the consensus is to swap out the IM first, assuming wiring and spark plug are in good order. For $15 and change, it's a reasonable approach.