AT545 VACUUM MODULATOR, ALLISON MT640 SERIES AS WELL.
"MODULADOR DE VACIO PARA ALLISON"
COMES WITH HOSE AND ORING PRE INSTALLED.
1970 AND UP
51-0075
WHITE
ADJ 12.5-18 EDA 1.7
3015412
3030028
3039092
3040680
6838115
23048272
D2HZ-7A377A
D2TZ-7A377A
D2TZ-7A377B
MT640 VACUUM MODULATOR
MT643 VACUUM MODULATOR
I thought I’d give you some feedback on how the Vacuum Modulator worked as you were most helpful answering my questions. I received the missing elbow tubes at the weekend and actually got around to fitting it today. Firstly, I’m still unsure what the large and small canister modulators look like. Having removed my original unit, the diameter of the area holding the diaphragm is larger, but with a much wider crimp so the actual operating part of the diaphragm may be no different in size, the canister part is pretty much of similar proportions. Any how I changed to the new one from yourselves and with the adjustment screw backed off a 1/2 turn it works perfectly. It shifts slightly earlier on light throttle which I needed, but more importantly now holds the gear, so I can apply some throttle to accelerate or climb a grade without the unnecessary down shifts, which were also rather abrupt, as the speeds were not ideally matched. Where I notice it most is in 4th gear as I can now stay in that gear on an incline and can run in 4th at much lower speeds, as you can actually apply some throttle with out a down shift taking place, this should also increase my fuel mileage considerably. It also shifts to second and first much more smoothly as I come to a halt, seems it was doing that before at slightly too higher speed. In short the unit works really well and can certainly be recommended to any one with an Allison Transmission. I got my truck running really well and the transmission, although working much better with the engine running at peak performance, was still the weak link in refinement, the new vacuum modulator was the missing piece and now the truck is likely performing better than when new. Having adjustment makes a lot of sense to me over the “one size fits all” of the original unit which was used on all engine sizes without a thought to the different power/vacuum characteristics of the power units, now I can use what power and torque the 427 provides and so the truck feels much more powerful and refined.