When accelerating easily the transmission shifts from 3 to 4th ok around 50mph but when I get to about 55 mph and let up on the gas it drops back to 3rd. Then while driving steady around 55 the transmission randomly goes between 3 and 4 for no apparent reason. If I accelerate just a little it goes into 4th but when I ease off the gas just a little it shifts back to 3rd. The engine RPM's are around 1800 at 55 in 3rd then drop to around 1450 when in 4th.

The engine and transmission are 14 years old (2000 Park Ave. Ultra) but both had 0 miles on them when I installed it and I didn't separate them. I'm using only an auxiliary transmission cooler mounted under one of the deck vents, no lines to the radiator and after driving about 20 miles the trans temp is around 150°F. After 100 miles I changed both the transmission oil and filter.

I don't know if this is related and it doesn't happen while moving, but on a couple of occasions while sitting at a stop light the voltage drops to about 12v and there's noticeable drop in engine rpm. This lasts for only a few seconds and everything comes back up it's own without me doing anything. I've got 2 grounds from the battery, one to the frame another to the transmission. I have a braided ground from the transmission to the engine block and a ground from the frame to the transmission. All the grounds are heavy cables or braided straps.

Any suggestions or ideas what might be causing the shift issue?

Thanks

Sounds like it might be in the tune..Have had a similar issue that was in the tune.

Agreed the tune is where I would start by setting up the shift MPH codes then go from there

------------------


DARN Cars now open with
Over 30 years wiring experience between cars and trade as an avionics technician in both Canadian Air Force and civilian aviation.
Over 25 years experience building and modifying cars.
Over 10 years of full Fiero engine swaps and harnesses building and still going.

Modulator or governor could cause shift problems.
Park Ave. is ~1000 lb heavier than Fiero... if you didn't recal the governor then that could affect shifting.

Oil Temp is a bit too low... Oil loves 170-225°F to work. Radiator heat exchanger Heats AT oil when temp is cool/cold.
If you have TCC working then driving on a highway oil temp can/will drop because in your case, Main oil heater is only the TC is unlocked driving in the city.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

I'm with Fieroflyer and MstangsBware.
Your 3-4 upshift and 4-3 downshift tables are set too close.
-Greg

Need more info (scan data).

I just don't see why anyone would set up a tune that would command a 4-3 downshift at light throttle @ 55mph. Makes no sense. I would really like to see some scan data, specifically "commanded transmission gear", "TCC slip RPM", "VSS", and a few other things so I could determine what is going on.

4T65 or 4T65E?

If you have E then you need scan info to see.
My note for Shift problems is for Non-E units.

Oil Temp is still valid for all AT types.

Need info. I have a few friends in grand Rapids that are experts.

Thank you all for responding.

It's the 4T65E out of the Park Ave. that the engine was in. Is the app Torque Pro capable of scanning to get the information needed? If not I'll see if i can borrow HP Tuner from a coworker. The ecm was programmed by Timm Bacher, I sent him a link to this thread for his advice.

It is but i may have to help you add a few formulas to scan what we need to see.

quote Originally posted by theogre: 4T65 or 4T65E?

GM never made a non-electronic 4T65 transmission. You might be thinking of the 4T60 which was just a renamed version of the older 440-T4, both of which are non-electronically shifted (not to be confused with the 4T60E which was electronically shifted).

quote Originally posted by Darth Fiero: One thing worth mentioning is the fact that the torque converter clutch (TCC) will disengage with zero throttle at any speed if it is already engaged. EXAMPLE: you are cruising along at a steady 55mph and applying a low or moderate amount of throttle to maintain that speed. The PCM should have the TCC locked up unless throttle position is too high or too low to warrant TCC engagement. If the TCC is engaged and you let off the gas (basically allow the car to coast), the PCM will disengage the TCC. And if you jump right back on the gas from zero throttle, the now unlocked converter will allow the engine to rev some until the converter clutch re-engages (which may take a second or two). This might be what you have going on making you think the transmission is downshifting into 3rd gear when it is actually still in 4th (and the only thing really happening is the PCM is commanding the TCC to disengage and re-engage as a response to your changing throttle positions, which can be normal under certain circumstances). You can check to see if it is really downshifting into 3rd by pulling the shifter down into Drive/3 position at 55 mph. This will force a 4-3 downshift almost immediately. If this manually-initiated downshift feels much different than what you have going on when the shifter is in the OD (4th) position, then perhaps all you have going on is just the NORMAL TCC unlock event at zero/light throttle. -ryan

Thanks Ryan. I'm so used to driving only stick shifts (last 15 years ) that driving a car with an automatic will take some getting used to.

I'll try down shifting manually tomorrow if the weather is nice enough for a drive. There may not be anything wrong, i just needed to get used to driving the auto.

I'm also going to get a thermostatic bypass valve for the transmission cooler to allow the trans fluid to get up to operating temperature.

Got a mechanic to drive the car yesterday and he said there's nothing wrong with how it shifts.

Thanks everybody for your input.