Transmission will not go in gear when cold 1991 GMC Sierra 2500 4wd

1990 GMC Sierra

Automatic transmission won’t go into gear from full stop when cold. If I manually shift it isn’t an issue. Not original owner. Don’t know it’s history before I owned it.

My opinion without looking at it would be an internal issue if the fluid level is full and looks clean. If the fluid looks dirty or dark, a rebuild is in your future.

Some sound opinions found on the net:

Transmission will not shift when cold

sounds to me like when it’s cold the fluid is thicker of course and is not pumping thru as it should. Might want to at least clean the filter, maybe it’s clogged up and won’t let fluid thru until it gets warmed up or something? Source: Swims350

Transmission will not go in gear when cold

This is a classic symptom of internal hydraulic leakage inside the transmission. As a transmission gets older, the various seals inside of it that keep fluid confined in specific channels tend to become hardened and no longer pliable enough to seal tightly against the shafts, servos, and other components that they are attached to. Often, in warmer weather they may seal enough to allow the transmission to operate somewhat normally. But when colder weather comes this tends to cause them to shrink more and become less pliable because of the lower temperature. After the engine runs while and the transmission fluid warms up, the internal seals and O-rings expand slightly and become slightly more able to do their job.

Repairs Needed

The repair needed is a transmission rebuild: removing the transmission from the vehicle, disassembling it completely, and installing all new seals, O-rings, and gaskets. Of course this is pretty expensive because of the amount of labor involved….

As a temporary measure, what often helps is to take the vehicle in to a shop to have the transmission flushed; this involves using a machine to push all of the old worn out transmission fluid out and replace it with new fluid. Transmission fluid contains additives that help to keep seals soft and pliable, so replacing the old worn fluid often helps somewhat. Also, adding one of the various transmission stop-leak or seal conditioner products available at any auto parts store to the fluid sometimes is beneficial. These products work by softening rubber components and causing them to swell up slightly; this improves sealing ability, especially when cold. Keep in mind though that this is not a fix; it only buys some time until transmission overhaul is needed. Source: Auto Mechanic: Steve