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Old 11-16-2019, 05:17 PM   #1
Join Date: Apr 2016
Posts: 29
Allison mt643 help

Hello, I am wondering what the function of this airline is on my 1995 allison mt643. I've been told by others it's a vacuum sensor hose and that these transmissions decide when to shift based on the rpms of the engine. This makes sense as before I cut this line it went to a tee/junction. The other hose coming from this junction went to the accelerator. Here is my issue though, I am putting this transmission into a bus with hydraulic brakes and no longer have the cummins air compressor as I removed it to make room for the gear driven vacuum pump I need for said hydraulic brakes. I am concerned about the transmission shifting correctly and am looking for advice.

This engine/tranny was a rear engine setup out of a 1995 bluebird, now a front engine setup in a 53 chevy.
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Old 11-16-2019, 08:56 PM   #2
Join Date: Apr 2016
Posts: 29
Well no replies yet, I've been trying to figure this out online though. I'm wondering if this is a modulator? I'm not too familiar with this kind of stuff but from what I can tell this basically affects the shift points and what speeds I am going at would affect this as well. As I said before this air line was also previously connected to the accelerator on the engine and I have replaced that air compressor with a gear driven vacuum pump. Would I be able to replace an air modulator with a vacuum one? Or maybe an electronic one? If so how and would the increase in rpms affect the amount of vacuum produced from the gear driven pump, i.e driving faster would make more vacuum pressure too? Would an increase in vacuum pressure/pump rpms be enough to let the transmission know how fast the bus is moving?

I guess a simpler way to ask this question would be how to replace an air modulator( if that is what I'm looking at) with something else

Any advice would be awesome I am wayyy in over my head here
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Old 11-16-2019, 09:24 PM   #3
Bus Nut
Join Date: Feb 2015
Location: Cincinnati, Ohio
Posts: 933
Year: 2000
Coachwork: Blue Bird
Chassis: International
Engine: TE 444
Rated Cap: 12
Diesels do not generate vacuum, shift modulator is mechanical on that transmission, not sure what that line is for
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Old 11-16-2019, 09:40 PM   #4
Bus Nut
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Join Date: Jul 2015
Location: Montana/Texas
Posts: 549
Year: 1998
Coachwork: Crown by Carpenter
Chassis: International 3800
Engine: DT 466e/MT 643!
Rated Cap: 16
The air line may have been for an air actuated accelerator? Not sure if that would have been tied into the transmission as a mechanical throttle position sensor...
Hopefully Cadillac Kid will chime in... I bet he'd know...

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