91 Mustang Transmission slips

We have a 91 Mustang automatic transmission that slips out of 1st into neutral only when it is cold. After warming up no issues. Any ideas? A mechanic told us that no metal shaving are present in the fluid.

The no metal shavings are a very good sign. Whats happening is from the way you describe it, the forward clutch assy is leaking, most likely from a hardened clutch piston seal. With the age of the vehicle, the rubber clutch piston seals inside an automatic transmission will harden. When they harden, they will not hold pressure which means the clutch assy will slip or in severe cases, will not apply at all. Cold weather also makes the hardened seals leak more, as the transmission warms up, the rubber seals soften and will tend to seal better. This is why you dont have any problems after the trans warms up. What you can do to help stop this problem is to pour in a can of Berrymans B-12 Chemtool in the trans. This stuff will soften the seals helping to restore hydraulic integrity to the clutch and it will help keep the transmission and valve body clean. Pour in one can and see if it helps.

Here is what you are looking for. I get it at Wal-Mart, Pep Boys, and Advance Auto Parts.


I seldom support any fix in a can. They just don’t work, but if it goes into a transmission and transman suggest it, I support it.

Im glad you brought that up Joe. I did forget to mention that in no way should this be used as a permanent fix. The only permanent fix for this condition is an overhaul of the transmission. I do know of people who I have recommended this to that have used it on a monthly basis and have actually put as many as 50 to 60k miles on their worn out transmissions. I have actually used it on my own truck (A Dodge Ram Dually) for several months until I wasnt too busy to rebuild the transmission. When I finally got into the trans, everything was immaculate. It had over 150k at the time with services every 25k. I put about 25k extra miles on the trans using the Berrymans.


I’m not a fan of a FixinaCan either but I’ve also used Berryman probably a dozen times over the years to aid in squeezing some more life out of an automatic trans; usually units that belonged to used car dealers.
Amazingly enough, in a few cases the trans was still functioning fine a couple of years later.