Trane rooftop problem

I have a friend having trouble with a trane rooftop and he asked me to post here for some opinions. He ( actually his technician ) is working on a Trane YFC120A2ELA150, serial # 320102255L. The original call was for no heat. The tech arrived and found the inducer motor running weird and the heat exchanger and inducer motor extremely sooted up. Checking voltage to the motor it would vary every second or so from 240 down to as low as 90. The motor gets it power from the board so the tech replaced the board AND the motor as well since it was severely sooted. He cleaned the heat exchanger of all the soot and restarted the unit, only to have the same results. Wiring the inducer straight to the terminal block the motor runs just fine. Also, pulling a wire off a limit switch or the gas valve the motor runs fine. He has checked incoming power thinking perhaps the board was wired to a "wild leg" but it isn't. No wild leg to that building. Talking with Trane tech support hasn't helped as they say they've never heard of this problem. Ground has been checked and rechecked. Any other idea's ?
Ed
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It sounds like your "friend" needs to find a tech that can diagnose and repair his problem. It sounds like he already got an inducer motor and board that he didnt need. Maybe if he keeps replacing parts he will finally get it right. Bubba
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Thanks Bubba, you forgot to tell me to replace the thermostat as well.. ;)
Ed
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Just replace the thermostat wiring....the thermostat is OK
wrote:

Thanks Bubba, you forgot to tell me to replace the thermostat as well.. ;)
Ed
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Bubba wrote:

Even Trane's techs can't help him properly troubleshoot the problem! Changing parts before the precise cause of the problem has been identified by troubleshooting, is not professional. - udarrell
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