Slow downshifting auto while stopping?

[white90dx]

Hey guys,

Hopefully an automatic transmission guru can chime in here on this (Doctor J!?). My alltrac is an auto (based on the V6 auto - A540H), and is lazy at downshifting while coming to a stop. Most of the time it isn't much of an issue, but during quick or panic stops, it seems to get stuck in second gear, which is enough to kill the motor. It takes maybe 1 or 2 seconds longer to downshift that I think it should. All other shifting is fine - up through the gears, torque converter lockup, etc.

I've read through the transmission repair manual, and all I can find is that maybe the shift solenoid is sticking. Another possibility is that the previous owner had Dexron III or other ATF put in there when it specifically calls for Toyota "Type T" - not sure if this could cause a problem.

Anyone have any ideas?

-Charlie

[Doctor J]

Thanks for the compliments; my transmission expertise is mostly limited to the electrical diagnosis and the teardown of failed units.
When I did the shift lag time study on my Camry, I hooked up the test lights to each solenoid wire. By comparing the time between the light on and RPM drop it is possible to estimate how long it takes for each gear to apply. In addition you can see the solenoid operation sequence.
I also found the interesting downshift sequence by watching the test lights (and computer downshift commands).
With the gas pedal released, braking slowly, the tranny was in overdrive until 20 mph, then shifted to 2nd for a second, then back to 3rd and when the car was almost stopping it got down to 2nd and 1st.
The downshift command is generated based on closed idle TP sensor input, the speed sensor in the cluster signal vs. tranny speed sensor, and the stop light switch.

[white90dx]

Testing the computer shift pattern outputs compared to the RPM is a good idea. I know the Auto transmission computer uses the Tt output on the check connector for showing what gear the transmission is in, along with torque converter lockup solenoid state. I think I'll hook that up tonight and see if it follows. I think that might be easier than using the actual solenoid wires. I guess I should check out the TPS while I'm in there - though that looked original when I took the throttle body out to clean it a few weeks back.

Is there any reason that possibly the wrong transmission fluid would cause problems? I'm a little surprised that the A540E (V6 transmission) and A540H (Alltrac auto) use different fluids. Maybe that means that I'm barking up the wrong tree with that thought?

-Charlie

[white90dx]

Update, in case anyone is interested...

I did a quick drain and refill on the transmission while I was changing the axle seal (funny how it started to leak after I changed the axle!)... Found out the transmission was probably overfilled by 0.5-1 quarts when it was exhibiting the problem. I refilled it with the correct amount of fluid (ok, filled it with too much then had to drain some out...) and the shifting is perfect! The car is much more enjoyable to drive now, it also downshifts a bit better when accellerating.

Watching the Tt signal was also pretty interesting, but the problem was intermittant enough that I couldn't catch it on the multimeter.

Cliffs notes: Fixed the problem by draining and refilling with the *correct* amount of fluid.

-Charlie

[Doctor J]

On most 4 CYLINDER MODELS, overfilling the transmission for as low as 300 cu cm causes rough shift issue.

[white90dx]

Originally Posted by Doctor J

On most 4 CYLINDER MODELS, overfilling the transmission for as low as 300 cu cm causes rough shift issue.

Speaking of which, it is important to check the fluid level correctly for your transmission... the A140 and A540 transmissions have different techniques.

-Charlie

[argojos1]

the manual says to put Dexron II auto trans fluid

[white90dx]

Originally Posted by argojos1

the manual says to put Dexron II auto trans fluid

Unless you have the A540H AWD auto (not A540E, the V6 auto), which then requires Toyota Type-T.

Also, Dexron II is not available anymore, Dexron III is the correct substitute.

-Charlie

[argojos1]

II and III are mostly the same thing