Huge tranny problem in my neighborhood?

September 13, 2007 2:38 PM

[Mechanicfilter] My truck is 'hard shifting'. Any mechanically minded MeFiers not emotionally attached to some advice?

The scoop: 1996 GMC 1/2 ton automatic transmission. When the tempurature dips below 0 C (32F) the shift from 1st to 2nd gear is quite hard and noticable. This 'hard shifting' happens from 2nd to 3rd gear as well but is less noticable. No trouble in summer/warmer months. Is this a huge problem? Would it be cheaper to take it into the shop now to get fixed rather than when the tranny eventually goes? Bonus points if you could explain what the hell is happening inside my truck that is causing this.

posted by Totally Zanzibarin' Ya to travel & transportation (7 comments total)

Just to state the obvious, but have you checked the oil level?

Also, is this a front wheel drive or rear?
posted by Brockles at 2:47 PM on September 13, 2007

Oil is fine. Truck is rear wheel drive. The thing that gets me is that it is based on temperature.
posted by Totally Zanzibarin' Ya at 2:51 PM on September 13, 2007

Having the fluid and filter changed in the tranny is pretty cheap. You might also consider having them put in some synthetic ATF rated for cold weather performance. If this is new behavior than it is probably the first signs of your transmission wearing out. In the cold weather the fluid is thick and can delay the shift. Some synthetics resist this so may help.
posted by caddis at 3:08 PM on September 13, 2007

It might be an engine mount. When the transmission downshifts, the kickdown is accompanied by an abrupt change in engine rpm and torque. When it's colder out the rubber bushings in the mount are harder absorb less and transmit more of the kickdown.

One factor decreasing the confidence I have in my own diagnosis: I'm guessing the truck has a V6 or V8. These kinds of engines are typically longitudinally (3 or 4 pistons on the right side, 3 or 4 on the left side) mounted in vehicles with rear or 4 wheel drive. However, some cars/trucks do have V6s and V8s mounted in this fashion transversely (3 or 4 forward and 3 or 4 further back). My explanation there of engine orientations stink, but please the upshot is an engine mount is a lot cheaper to replace than a transmission!
posted by KevCed at 3:51 PM on September 13, 2007

Some thoughts - is the engine tuned right to run correctly when cold? If something's a little out of whack, especially if it's tripping an emissions sensor somewhere, that could be affecting the computer gadget that controls your transmission (commonly, the transmission control module, or TCM).

Also, before you put synthetic transmission fluid in there, make sure it's the right fluid (i.e. Dexron II or Dexron III). Some cars, a Jeep Cherokee, for instance, use uncommon AT fluid, and can be damaged by using an unapproved product.

If you decide to have a mechanic look at it, take it to a trusted generalist, not a transmission shop. That is, unless you'd just like to buy a new transmission...
posted by M.C. Lo-Carb! at 4:10 PM on September 13, 2007

Not so easy, especially because there was a change in mechanical equipment on the transmissions around then.

If your transmission has a vacuum modulator, check the vacuum modulator hose that goes to the intake manifold to see if there's water in it. My weird theory here: the vacuum modulator is not regulating mainline pressure when it's cold because the vacuum's blocked by an ice pellet, and you are shifting with full boost (shifting with the same abruptness as you get under wide-open throttle) until the water melts and the modulator can function.

If there is no vacuum modulator, there should be a pressure control solenoid. If the solenoid is sticking - tolerances close up slightly when it gets cold - then you will get the same effect as if there's water in the modulator's hose.

A sticking pressure regulator will do something similar.
posted by jet_silver at 9:15 PM on September 13, 2007

what is the make-up of the transfer case? i had an old cruiser that had a similar problem, got worse, then on a 4wd trip i lost all gears. this was simply due to teeth being ripped off after 25 years of 4wd action. probably not your problem, but dont write off that there could be something in there that is causing this.
posted by edtut at 5:43 PM on September 15, 2007