Community Forums

philipswanso · 02-21-2016, 02:16 PM

I have a 95 Bluebird short bus RV with a Cummins 5.9 and MT643. The transmission has never downshifted automatically and has no slow down going downhill. Also the exhaust brake has never had any affect. I have:
1. Replaced the clutch plate diaphram in the torque converter
2. Replaced the clutch facing in the torque converter.
3. Checked the lockup valve and spring on the valve body.
4. Replaced the air actuated powered modualator.

Still no lockup or exhaust brake. Beginning to suspect air source going to the modulator. ???? Any ideas? The parts I replaced looked perfect. I don't think they ever worked so zero wear. Thanks!!

Phil Swanson
95 Baby Bird
Wanderlodge

1ciderdog · 02-21-2016, 05:46 PM

It doesn't help you but I had a 643 in a 1993 33 foot Safari and it did not have a lock up. Was a lousy set up so I sold it after a year or so because I just didn't like that.

dbarton291 · 02-22-2016, 07:08 AM

At full throttle, does it upshift at or near engine governed speed?

philipswanso · 02-22-2016, 11:24 AM

No, it shifts into 4th gear long before governed engine speed but doesn't downshift for passing or going up a hill. It needs to be pulled down manually.

Thanks, Phil

dbarton291 · 02-22-2016, 11:45 AM

Sounds like something is causing full modulator pressure in the valve body at wide open throttle. This would cause very early upshifts and badly delayed downshifts. It probably doesn't get first gear until you are all the way at a stop. I would suggest checking the air pressure at the air modulator at idle and at wide open throttle to see what those pressures are.

philipswanso · 02-23-2016, 09:25 AM

Quote:

Originally Posted by dbarton291

Sounds like something is causing full modulator pressure in the valve body at wide open throttle. This would cause very early upshifts and badly delayed downshifts. It probably doesn't get first gear until you are all the way at a stop. I would suggest checking the air pressure at the air modulator at idle and at wide open throttle to see what those pressures are.

Thanks, I will check this out. How much pressure should I see at idle and at full throttle? Thanks, Phil Swanson

dbarton291 · 02-23-2016, 10:54 AM

If I remember correctly, max pressure is 75 psi at wide open throttle. It should be zero at idle.

Mr_D · 03-22-2016, 11:49 PM

Quote:

Originally Posted by 1ciderdog

It doesn't help you but I had a 643 in a 1993 33 foot Safari and it did not have a lock up. Was a lousy set up so I sold it after a year or so because I just didn't like that.

That's what I remember seeing someplace too, no lockup in that trans. With no lockup there is very little engine braking even with an exhaust or engine brake.

dbarton291 · 03-23-2016, 08:46 AM

It's the AT 540, 542, 543 and 545 that had no lockup clutch in the torque converter. ALL the MT series transmissions were built with a lockup clutch. If an MT was not engaging its lockup clutch in operation, it was malfunctioning in some manner.