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Author Topic: rockwood motorhome, won't downshift  (Read 2575 times)


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rockwood motorhome, won't downshift
« on: September 08, 2010, 07:16:54 PM »
Hi all, im new to the site.  I have a 87 rockwood classic gillig bus chasis(40ft).  It has a 3208t& allison mt643.

The issue is when you climb hills, it wont downshift.. it stays in 4th.. so, im goin about 20mph..   I have changed the fluid& filter and the external spin-on filter.  It has a stone-bennett shifter.



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Re: rockwood motorhome, won't downshift
« Reply #1 on: September 09, 2010, 03:55:14 PM »
Go over to the MAK bus conversion board, theres a bunch of heads that know that combo well.>>>Dan
38' American Tradition 38TT/330 turbo Cummins
Jeep liberty 4 down
72 VW Baja 4 down
Cedar City, Utah
USAF vet. 59-63
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Mark R.

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  • 1986 Executive Industries 1967 Ford Fairlane,toad
Re: rockwood motorhome, won't downshift
« Reply #2 on: September 09, 2010, 07:00:10 PM »
When was the air modulator last changed? normal recommended replacement is like 12 months (pretty bad), look for small air line going to passenger side of trany. it will connect to a small round cylinder (1"x2" aprox) mounted in side of trany., that is the modulator. There is a diaphragm inside that fails, the air line to it is tee'd into the air throttle line, so when you step on the gas peddle air pressure is applied to the piston inside the modulator cylinder, this pushes on a rod in transmission that tells the trany to downshift if the regulator mounted on the trany that detects speed agrees a down shift is needed. The modulator is held in place with one bolt and is sealed with a o'ring that will come with the new modulator, no adjustments at all. Ok to test remove air line and retainer bolt, blow in air fitting connection on modulator if air comes out other end (trany side of cylinder) it means it is bad. Good luck with a good trany. its rated for continuous duty at 72,000 lbs.
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