194,000 miles - In August, had traveled about 150 on highway when check engine light came on and speedometer went to zero - got off exit, engine compartment smelled hot. Tried to move car, would not initially go in reverse or forward - let cool, cked trans stick with engine off - no fluid - added 1 qt and drove slowly home on back roads. Was able to get to a car repair shop - check engine code 720 (speed sensor). Reset and drove car for nearly two months with no problem (no trans slipping nothing). Couple days ago, driving highway, 130 miles, the RPMS went to 3000 on cruise - got off highway, and fluid had literally blown up all over inside. Was red and below car looked like 2 to 3 qts had spilled out. Had car towed to trans shop. Just called - miffed - said van drives fine, but fluid is coming out of vent - and trans fluid is "overfull" and radiator fluid is low. So - have a problem but can't get to bottom of it. He is going to drain trans fluid and add to appropriate level and add rad fluid and drive locally. All I know now is that I can't drive it on trips. Any ideas? A GM mech buddy of mine said it may be a blocked coolant hose, but feels it is trans related - all don't seem to understand why the trans isn't slipping or symptomatic. The car has been great and it's paid off - don't want another car payment so may be looking at new trans here soon

Did they even look to see if there was water in the transmission fluid? I bet there is. IN which case your radiator is shot and so is your transmission.

Would it be 'driveable' without symptoms?

The transmission cooler is usually in the radiator, if it corrodes, sometimes the trans fluid gets into the radiator, sometimes the coolant ends up in the radiator.

The trans fluid is generally checked, warm, engine running, in drive with ebrake set while idling on level ground. So you probably did overfill it somewhat too.

With an automatic... your first indication something is wrong sometimes is the car won't move, clutch packs have very little material on them so it doesn't take much slipping to ruin them. Water in the fluid, low fluid, excessive fluid (it whipps up a lot of foam that gets sucked into the pumps) and the tranny goes bye-bye in a hurry.

I've had a transmission fail once. Started it up at work parking lot, drove it out of the lot, made a turn on the road stopped a 1/4 mile down the road at a stoplight and it wouldn't move at all when the light turned green. Nothing but reverse. Vacuum modulator went bad and apparently it sucked enough of the fluid out to kill the transmission.

There isn't one failure mode... some give a warning others don't until its too late.

Water and transmission fluid don't mix well, Instead of looking like cherry koolaid, it can look more like Pepto-Bismol. Its usually easily visible.

Understood - I guess my question is - if what you are saying is true - wouldn't the tranny have seized up when driven by the mechanic? If water had been in tran?

Not nessessarily... its all a factor of how much and how long. Though it IS a possibility. I'd think the clutch packs would go south before bearings could seize.

Been driving this week - no issue - mechanic said tranny is shifting fine, no slips. There was no "pepto bismol" mixture with fluid noted. So the issue is, overheating trans with extended highway driving (over 100 miles). Mechanic mentioned he could put an external cooler on trans to prevent overheating. Problem is, I currently have no confidence in driving the van. Next step is dealership -

I'd install a transmission cooler and start using full synthetic ATF, which will significantly lower the transmission's temperature. Then, you should be fine.