1999 Chevy C3500 Transmission Will Not Shift Into Overdrive While Towing

Question:

Hello I have a 99 chevy c3500 6.5 diesel It has a trans cooler I am hauling a 18000 lb rv and after hauling for a short time it will no longer shift into overdrive so I stop and let it cool down and so on what can I do

asked by: James T

Expert Reply:

Overdrive is not usually something that you would need to use when towing. Also if your truck has a tow/haul button that you press when towing then it likely shuts off overdrive. If this is something new the truck started doing then there are some things to look at.

The first thing I recommend doing is checking the transmission fluid level for your 1999 Chevy C3500. If the fluid is low then it might not have enough cooling power. If the fluid is full and looks good then the cooler you have many not be enough.

For optimum cooling you can install an aftermarket cooler to work in line with the cooler you already have. Cooler # D13503 is a popular option and for your truck I recommend adding fitting # D13018 to avoid having to cut and flare lines.

The fluid will go from the transmission, to the factory cooler, then out to the new cooler, and then back to the transmission.

Another option is a cooler that has its own fan, # D12733. Since it has its own fan it will be cooling the fluid all the time and not have to rely on vehicle movement. This also means the remote cooler can be installed anywhere there is space and does not have to be right behind the grille.

It is also a good idea to have the transmission flushed and a new filter put in. If you continue to have problems then it could be something more serious that will require repair or replacement.