transmission shifting too high rpm

Covers engines and transmissions
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62goose@gmail.com
Posts: 1
Joined: Mon Oct 28, 2019 3:39 pm

transmission shifting too high rpm

Post 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!
Dan'L
2001 Zanzibar
330 Cat, Allison 3000MH
ProCycle
Posts: 46
Joined: Sat Oct 03, 2020 1:28 pm

Re: transmission shifting too high rpm

Post by ProCycle »

Sorry I can't be of any help but I'll be watching this thread in hopes of learning something.
Jeff
1997 Ivory Edition 3740 Cat 3126 Allison
1Lotosrggp
Posts: 30
Joined: Sun Oct 27, 2019 6:08 pm

Re: transmission shifting too high rpm

Post by 1Lotosrggp »

If the PacBrake acts like it's engaged, physically check it. Have you lubricated it?
Tom
97 Sahara 3550
3126 CAT 300hp
stuplich@ymail.com
Posts: 1080
Joined: Mon Oct 28, 2019 8:51 am

Re: transmission shifting too high rpm

Post 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
Mel
'96 Sahara 3530, mine since '01
250 hp 3126 Cat, MD3060 Allison
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