Analyze PT SL Symptoms?

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Hoping someone can help me with diagnosing my PT SL. Its one of the early models which has been back to Saris twice, once for the initial fix, and about a year ago when they returned it with the new epoxy-sealed moisture-proof internals. Anyway, after a long stretch of working perfectly, it started dropping data so badly I gave up, and switched the cpu to cyclo-computer mode, using a non-PT rear hub and spoke magnet. The one ride in this mode proved totally frustrating, as the cpu had me going 70+ mph at times (when I certainly wasn't) and turned a 35-mile ride into 80-something miles, and in general was a random-number generator. Today I mounted my spare cpu (I have two) and while the bike was resting against my car, I watched the cpu record speeds of 7-8 mph, and a distance of .5 miles, while the bike was obviously stationary. Then on the real ride the cpu turned itself off pretty early (I didn't bother watching it). Since I'm getting flaky results even in cyclo-computer mode and two different cpus, my own diagnoses suspects something wrong with the harness pickup, but the cpu's ability to record speed and distance while standing still really has me baffled. Any thoughts?

Although I've never experienced the exact symptoms you mention, it wouldn't hurt to make sure your hub is tight. Loose SL hubs can be the source of wacky data. Here's my post on another thread on how to tighten it:

http://cyclingforums.com/showpost.php?p=3287945&postcount=4

It also wouldn't hurt to make sure both hub and CPU batteries are good. If rain was involved before it started acting up, then remove the battery cover and let the hub "air out" for a day or two.

If all this doesn't help, then you need to find someone else with a PowerTap so you can use the process of elimination to determine whether it's a problem with the hub or the receiver/harness.

Although I've never experienced the exact symptoms you mention, it wouldn't hurt to make sure your hub is tight. Loose SL hubs can be the source of wacky data. Here's my post on another thread on how to tighten it:

http://cyclingforums.com/showpost.php?p=3287945&postcount=4

It also wouldn't hurt to make sure both hub and CPU batteries are good. If rain was involved before it started acting up, then remove the battery cover and let the hub "air out" for a day or two.

If all this doesn't help, then you need to find someone else with a PowerTap so you can use the process of elimination to determine whether it's a problem with the hub or the receiver/harness.
Uhl: Thanks. However at the moment I'm trying to understand why two separate cpu units both generate meaningless numbers in cyclocomputer mode when there isn't even a PT hub on the bike. My hunch is that if I can resolve that issue, it will turn out that the hub is fine. (Otherwise I have two unrelated problems.)

However at the moment I'm trying to understand why two separate cpu units both generate meaningless numbers in cyclocomputer mode when there isn't even a PT hub on the bike.That has me baffled as well. But as you originally mentioned, it does seem to point to a harness issue. Maybe the wire is squashed somewhere along the line.

Do you have access to a spare harness or friend's PowerTap bike to test?

Uhl: Thanks. However at the moment I'm trying to understand why two separate cpu units both generate meaningless numbers in cyclocomputer mode when there isn't even a PT hub on the bike. My hunch is that if I can resolve that issue, it will turn out that the hub is fine. (Otherwise I have two unrelated problems.)

Well you know it does weird stuff with or without the PT hub, so I think you can eliminate the hub as the cause.

You also know that it does weird stuff to two different CPUs, so one would reason that it's not the CPUs that are wigging out.

What does that leave? The harness. Have you tried the suspect wheel and CPU on a different wiring harness?