compression braking

[dantisocial]

I read in another thread that compression braking causes auto transmission failures (the thread was referring to th400), but when someone asked why it was treated like a dumb question. I'm not afraid to ask dumb questions, so... why? I have never heard that compression braking on an automatic transmission causes any issues. Is it heat buildup, impact-loading, or what?

[SPONSOR]

Membership in the largest desert racing community has its advantages - Participate in Forum Discussions - Send and Receive Private Messages - Maintain Public Photo Albums - Access to Groups - User Profile in our Social Network - Increased Access to more Sub Forums - Reduced Online Advertisements Join our community today

[Brock_Beeson]

diesels do it all the time.. but theyre made for it.. one worry is a pressure spike

[philofab]

If the transmission does not have a rear pump (most modern ones do not) it can starve for oil. This is the same reason you can't flat tow an automatic.

[dantisocial]

Are you talking about the physical location of the pump in the transmission, or where it is pumping the fluid to?

[glamiskid395]

quickly explain what this is? shifting down at high speeds? or?

[07FJRog]

Sounds like BS to me, I have used this method for slowing down on many trucks and now my MH also, I have never had a tranny fail.

[dantisocial]

To explain what I'm talking about: Using the engine to slow the vehicle down by either downshifting, or staying in a lower gear.
The reason I brought this up is that I have done this over and over while towing on mountain roads pulling trailers in order to avoid having to ride the brakes. I have never had a transmission (th350, th400, 4l60e) fail while doing this, and have gotten 100k+ miles out of them, too. I realize this isn't the same as punishing one off-road, so I was trying to understand why so many people say that this will destroy an auto transmission in a hurry off-road. I was trying to figure out if compression braking loads them differently off-road than while towing and driving on the street, or if it's just a myth.

[Ol' Curmudgeon]

I believe Petey was referring to using a TT TH400 for braking by downshifting, and sending r's skyward w/ no rear pump. But what do I know? I'm a buggy dork.

[FABRICATOR]

Compression braking is not usually a problem. Downshifting at too high of speed with a closed throttle is a problem. Transmissions are not designed for that. The transmission doesn't mind going from 3rd to 2nd at 65MPH with WOT because the engine is trying to speed up anyway. It's a completely different story with the throttle closed and one hell of a difference of inertia.