ATX Thunk

[Gloffer]

My 93 ATX just started exhibiting a strange behavior.

After running down the freeway for an hour or so in hot weather and exiting the freeway, there's a "thunk" in the transmission about a second after coming to a stop. When leaving from the stop, the engine will rev a little before engaging the transmission.

This is not limited to freeway driving and occurs when driving around town after everything has had a chance to warm up. This problem does not occur until everthing is good and hot. Also the problem seems to be eliminated by dis-engaging the OD before coming to a stop.

My SHO is approaching 150,000 miles and has never had any tranny work done, except for fluid flushes/filter changes every year or so. I just had the tranny flushed and added a bottle of Lucas ATX conditioner as part of the fluid exchange. I also replaced the ATX mount, both engine mounts, and put in aluminum SFBs. This work had no effect and I still have the "thunk" as before.

Should I start saving my pennies for a new ATX or is there something that can be done by dropping the ATX pan?

 

[Mr Anonymous]

If I'm not mistaken, external ATX coolers weren't standard until '94, so if you don't have an external cooler, I'd recommend adding one, especially in your part of the country.

If you add the cooler and get another flush, the problem may resolve itself by virtue of the cooler fluid temp. You could very possibly be boiling the fluid which could cause problems. Next time it happens, pull over and check the fluid and see if you observe any foam (or bubbles in the fluid) on the dipstick. If so, boiling fluid is likely.

Of course if you do have the ecternal cooler, you should probably take it to a real transmission shop to have the system flushed and they can check the flow rate in and out of the cooler to make sure there's no restriction.

P.S. When you get it flushed, consider adding the Mobil-1 synthetic ATF which has a slighly higher boiling point than regular ATF, and could help extend the life of your tranny even further.

 

[sho 'nuff]

Have you tried to pull any codes on it yet? That would be the first thing to do. Some of this behavior in the transmission sounds like a VSS(Vehicle Speed Sensor) is going out.