Hard downshift when cold

I have a 2004 Pontiac Montana minivan. I get a hard downshift from 2nd to 1st only when the engine is cold - especially noticeable in cold weather. After a mile or so of driving (or a few minutes of idling) is shifts fine. All upshifts are fine all the time. The tranny fluid level is fine and is in good condition. Any thoughts on what it could be?

First,"the tranny fluid is fine and in good condition " on a 2004 with how many miles and …how cold are we talking about ? If you havn’t changed it with average miles per year giving you a total of 90k, it should be now…then complain.

“The tranny fluid level is fine and is in good condition.”

How have you determined this? I suggest that if it is over 30,000 miles it is at best questionable. This is one area that the manufacturers for some reason have failed to instruct the owners of desirable maintenance.

How old is the current fluid?  (miles and months)  What does it look like and smell like? 

It is a shame that many transmissions fail because no one changed the fluid and so many owners fail to change the fluid because the car's manufacturers tend to de-emphise this important money saving maintenance.   I guess a car with less listed maintenance sell better, but don't last as long.

Could high idle speed be causing the hard downshift? Try to monitor the RPMs when the hard downshifts occur.

My first thoughts are along the lines of Rod Knox. The only normal things that could be easily checked are the idle speed (think sticky throttle body or idle air control valve), throttle position sensor, and MAF sensor. Outside of that, it would be best to get it to your best local, independent transmission shop to have a tech drive it and put it on a scanner.

We’re at 75K and I have had the fluid changed at the recommended intervals by a mechanic (not the jiffy change shops). Obviously tranny could still be the problem but I would like to test any alternatives first. The hard downshift is a quick rise in rpms just before coming to a complete stop- it revs higher and the vehicle surges a little before settling into 1st gear. As the engine warms up the surge at a stop becomes less noticeable and within 2-3 minutes is gone completely, usually for the rest of the day (unless it sits for 6-8 hours in really cold temps).

You want to clean your idle air control valve and its port into the intake. (The IAC sits at the throttle body). You also want to clean the throttle body. The man air intake snokel & MAF sensor will be coming off to clean the throttle body, so a good dose of MAF sensor cleaner is also a good idea. Those rpm surges mean the computer will keep the line pressures too high in the transmission - thus the hard shift.

Manufacturer recommendations for transmission servicing are generally terrible. About every 30K its best to have a transmission shop drop your pan and change the filter. Figure about $100.

Sounds like the problem only happens when the engine is cold and on fast idle, not an IAC issue.
Has the mechanic been using fluid specified by Pontiac, not some “universal” stuff?

So here’s the update - took the van to a different mechanic and had the tranny fluid and filter changed. Mechanic said fluid and filter were a little dirty but not too bad - also no chunks, filings, etc in the fluid or pan.
Drove it this morning and still had the same symptoms of surging when braking during the first mile or so, so I took it on an extended test drive. I found that when I am cruising at various speeds and I take my foot off of the accelerator the rpms surge (it seemed to commonly be a surge starting from 1000 up to 1500 rpms but I’m not sure if that is a pattern or not) before the vehicle begins slowing and it did not happen around a gear shift at the higer speeds. Is that consistent with cigroller’s thought of IAC valve and throttle body or are there other possibilities?

Based on your description, my best guess on what you’re noticing when you pull your foot from the gas pedal while cruising is normal. It is your torque converter clutch lockup disengaging. Its true that it is counter-intuitive. One would think that if you pull your foot from the gas pedal the rpms should drop. But if you’ve been at a steady cruising speed for a while they will surge momentarily instead. That’s what is supposed to happen.

If that guess is correct then the hard 1-2 downshift is something else. And I would still say to clean the throttle body and IAC. I’m not sure about circuitsmith’s reasoning so perhaps he’ll clarify. The idle, whether fast (cold engine) or normal (warm engine) is still controlled by the IAC valve. These can easily be more likely to get a little squirrely when cold. The same goes for the effects of any build up on the throttle body. If the idle ends up too high this is interpreted by the MAF sensor as a load that is higher than it actually is. The computer ups the line pressure in the transmission = bump shift. I could have something not quite right in there, so I’ll stand to be corrected.