perrys4

When I let off the gas I get a pretty abrupt downshift. What would cause this? The tranny was rebuilt and installed professionally and only has 15k on it. I notice it most when the revs climb under light acceleration and you klet off the throttle before it gets to fourth. I also notice that if I am in th gas the shifts are smooth.
I am just curious if the tranny mounts might be the culprit and it might not be a shifting issue at all. Any ideas's

dr bob

Ron--

There are two issues here. One is the timing of the shifts and the other is the firmness of shifts relative to load on the engine. Like all things auto trans, they are related somehow.

The firmness of the shift under load is managed by the modulator. On the 928 it's pretty easily adjustable. It reads engine load via manifold pressure (vacuum). More vacuum means lower load generally. The timing of the shift is primarily a function of the throttle valve, managed relative to th ethrottle position by a cable appropriately called the TV cable. That cable can be easily adjusted at the throttle relay mechanism on the side of the intake manifold.

I have mine set at the original modulator position, and have the TV cable one turn loose from where it was from the factory. It shifts up early and down later. I had the TV cable tighter for a long time, and while I enjoyed the quicker performance, too often I found myself in the same spot you are now-- feathering the throttle to let the car shift up earlier. I also tired of the too-frequent downshifts in SoCal freeway traffic. I have this car with gobs of low end torque, yet it wanted to downshift on throttled lane changes. When I caused an upshift it was a clunk-shift. So I took out the two turns I had put in, plus one more loose to see what the effect would be. Shift clunk has diminished when I feather the pedal, but I suspect that it could really use another turn loose.

You can adjust the cable easily. On the throttle relay bracket, there are two cables that pull on bellcrank and two that are pulled. The outer cable that's pulled is the one you will adjust. On my car there's a small wire spring retainer clip that holds the cable side of the balljoint onto the ball. It's easy to have it go sproing! and off into the abyss, so I use a hemostat to grab and hold it securely. There's a locknut that hold the adjustment to the cable, 8mm IIRC, and it may be easier to loosen the locknut while the coupling has engaged the ball, before you remove the clip. Once the nut is loose and the clip is off, it's a simple task to make the cable tighter or looser by thredaing the coupler on or off the cable. Try a turn at a time to see where you want the car to shift. Be sure to put the clip back on securely, or there's a risk the coupler will fall off and leave you stuck in a higher gear.

The throttle valve modulates the transmission control pressure. At full throttle, there's a solenoid valve that further lowers control pressure to force a downshift. Solenoid is energized by a relay, which is actuated by a switch under the throttle pedal. If you want to take out an extra few turns on the cable but still want the performance of early downshifts undeer load, you can add another switch or button or both, in parallel with that switch under the pedal. There are many references to a "parallel kickdown switch" here, and this is what it's for.

perrys4

I will mess with it. Sounds exactly like what I am experiencing. To many shifts in traffic. I have been keeping it in 3rd while in heavy traffic to keep it from making the last upshift. Greg and his shop guy drove it Monday and said it was shifting hard and they could loosen the cable but I would lose kick down. Thanks for clarifying things for me.

Joe Dyer

I vote for a vacuum leak. If it shifts OK under more throttle, and is hard under light throtlle, then vacuum line is leaking slightly. I ran a new line on mine and shifts were much better. Adjust the modulator for personal preference.

Steve Cattaneo

The Bowden (TV) cable controls kick down, up and down shifts timing only, and does not influence shift feel in an A28 4 speed transmission. Shift feel is controlled by modulator pressure ;( vacuum induced, as Dr Bob said) which controls working pressure. If the problem were vacuum, ALL shifts would feel harsh, in your case.


Your problem could be, valve body relayed, broken springs, struck command valves and or problem with the B2 piston circuit. Not an easy fix.

perrys4

Mechanic adjusted modulator but it did nothing. He said it wouldnt for the tpye of shift I was explaining. Where is the vacuum line and where does it go?

IcemanG17

If there is no vacuum at the modulator then adjusting it won't help...& zero vacuum makes the tranny think you are in a full throttle (zero vac) scenario hence the hard shift....if you have a busted vac line or a leak upstream...then you'll have hard shifts all the time.....it shouldn't hard to lift the car and hook up the mityvac to the vac line to see whats going on...however if greg checked it I'm sure its fine