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transmission shifting too high rpm

Posted: Fri Jun 11, 2021 1:44 pm
by 62goose@gmail.com
Transmisson won't shift into higher gears until engine rpm reaches 2400. Then if I back off the throttle a bit below 2000 rpm, it starts downshifting. Exhaust brake switch is not on (but it sort of acts like that) . Ran Error codes and got 66 00. Serial Commnications Interface (SCI) fault. Reccomends checking connection to engine computer (ECM ??) and transmission ECU Wireing harness. Questions: 1. should I disconnect battery power before disconnecting ECM or ECU? 2. What does the SCI wiring look like? is it a twisted pair, coax or just two separate wires. 3 Does the wiring harness go through the electrical compartment ahead of the drivers side front wheel? (I have had corresion problems there.) Any help appreciated!

Re: transmission shifting too high rpm

Posted: Fri Jun 11, 2021 2:28 pm
by ProCycle
Sorry I can't be of any help but I'll be watching this thread in hopes of learning something.

Re: transmission shifting too high rpm

Posted: Fri Jun 11, 2021 6:08 pm
by 1Lotosrggp
If the PacBrake acts like it's engaged, physically check it. Have you lubricated it?

Re: transmission shifting too high rpm

Posted: Sat Jun 12, 2021 1:01 pm
by stuplich@ymail.com
62goose@gmail.com wrote: Fri Jun 11, 2021 1:44 pm Transmisson won't shift into higher gears until engine rpm reaches 2400. Then if I back off the throttle a bit below 2000 rpm, it starts downshifting. Exhaust brake switch is not on (but it sort of acts like that) . Ran Error codes and got 66 00. Serial Commnications Interface (SCI) fault. Reccomends checking connection to engine computer (ECM ??) and transmission ECU Wireing harness. Questions: 1. should I disconnect battery power before disconnecting ECM or ECU? 2. What does the SCI wiring look like? is it a twisted pair, coax or just two separate wires. 3 Does the wiring harness go through the electrical compartment ahead of the drivers side front wheel? (I have had corresion problems there.) Any help appreciated!
Dan'L
Google "allison serial communications interface wiring diagram",
https://www.google.com/search?q=allison ... nt=gws-wiz