Wednesday, June 13, 2007

Notes from Heisenberg's cat

We are at Pacific Power Products, the Detroit/Allison dealer in Kent (map).

We arrived late Monday afternoon, and the night shift started looking at our TPS problem. It is always a bit frustrating when we come to a big Detroit distributor, because they always insist on running the troubleshooting procedure from step 1, even if you've already done that work.

After an hour or so confirming what we already knew, to wit: the wire harness and TPS pedal all check out fine with a VOM, and there is no apparent problem with the ECM, they had us fire up the coach and demonstrate the problem. After several stall-tests in place, and a trip around the block, pretty much everyone was stumped. We could reproduce the problem with 100% certainty either driving or stall-testing in place, but only in gear, and more interestingly, only in forward gear. As a final task at the end of the night, one of the techs crawled down the hatch below our bed onto the top of the tranny, and poked and prodded at the wire harness from the ECM to the first bulkhead connector. All appeared fine, and we buttoned up to wait for the day shift.

Of course, as fate would have it, we could not make the problem re-appear for the day shift no matter what we did. Several stall tests and a road trip failed to reproduce the problem. Which prompted me to come up with the following theory to explain all the observed facts:

A small fracture in the wire or one of the connector pins, or more likely a deposit of corrosion on one of those pins, was the source of our intermittent problem. At rest, acceptable contact was being made, and things worked fine, however, under load, the engine twisted ever so slightly in its mounts, enough to cause the wire or pin to intermittently lose full contact, causing the TPS dropout symptom. In reverse gear, the twist would be in the opposite direction, thus explaining why the problem only occurred in forward gear under load. Just a theory, but it fits the observations.

So poking and prodding the harness and connectors has either abraded the corroded area or relaxed the fractured area so as to make good contact. When the day shift worked on it yesterday, we could not cause the failure to recur by wiggling the harness further. At this writing, things seem to be working fine, and we're going to leave here and keep our fingers crossed. If the problem recurs, at least we know to start looking at the harness between the ECM and the first bulkhead connector, and I have a pretty good idea which wire will need to be repaired or replaced.

We were actually finished and buttoned up yesterday afternoon, but we had run some questions up the chain back at Detroit Diesel HQ, and we decided to wait here until they got back to us this morning. Last night we hopped on the city bus and went down to Kent Station for dinner, at Duke's Chowder House. News this morning from Detroit was no additional help, so we are packing up and will be heading up to the Everett area just as soon as they're done figuring our bill.

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