I have a 2001 MCSS. When driving on the highway at about 60MPH my RPMs are at about 2700ish. I can shift from (D) to (3) at that speed and the drop in RPMs is only 200. The thing that is puzzling me is when I let off of the gas completely at that speed (In overdrive) the RPMs drop to the 1100-1200 range. Its almost seems like the tranny stays in 3rd when you're in the gas, and only shifts into 4th when coasting. I don't think this is normal. Any feedback would be greatly appreciated.




Chief

 

Anybody? I know this is an old thread, but I'm curious about this issue. Would code PO472 have anything to do with this?

 

Is the code p0472 or p0742? Sounds like a converter issue. I don't have a way of looking up the code on my phone to research it.

 

Sounds like 4th might be going out. You have a 4T65E btw.

 

X2 on both of Zipper's points.

 

I dont think so... sounds about normal to me, your gear ratio in 4rd is 1.0 in 3rd and .70 in 4th... so Im not doing the exact math but beacuse the diference is exponential and you dont hit redline untill 100 mph (apx) in 3rd gear than as long as your just going (not up or down hill) the wheels can spin faster than the engine and transmission but not vice versa (according to gear ratios of course). so if changing to third sees an increase of 200 rpm then thats about right for how much the engine has to increase its speed to maintain the momentum of the wheels. If that made sense, there was a day I could crank out the formula but thoes have since passed. the real test is in acceleration, not the maintaining of speed. Any other symptoms?

 

At 70 MPH in 4th gear, O.D., the RPMs should be about 2100 to 2200 raising to 2500 on an upgrade. If when cruisin at 70 MPH your engine is at 2500 RPMs that is slippage. The cause could be anything from degradation of fluid to a bad tourc converter, bad sensor or valve in the Trans or even a bad electrical connection from the pcm to the Trans.
Best thing to do is to get a diagnostic from a Trans shop. Unlike the motor, the Trans needs to be tested to find the exact cause and solutions.

 

Wow completely over looked that he was at 2700 at 60 mph. Get the codes scanned is the first step, then do some research. Slippage is usually a easy fix with a big fix in the future or a big fix off the bat. I'd say there's a rebuild but it might not be slippage if your rpms arnt fluctuating while driving.