Transmission Shifts to Overdrive too soon

chevrolet
avalanche

#1

Have a 2002 avalanche Z71. Transmission was rebuilt 33k ago. It currently shifts a little hard going from 1 to 2nd. Real problem shows up between 40 mph and 50 mph. It suddenly shifts into overdrive which drops the engine rpm waaay down and starts lugging the engine.

Took it back to the guys who did the rebuild. They drove it around and noticed that the computer reported no codes. They claimed it was the TPS and said to replace it. I did. Problem is still there.

Took it to a highly trusted mechanic. They said they think the valve body needs replacing but they don’t do transmissions. The guy they recommend drove it around and he thinks it is the torque converter.

I am wondering what causes the computer to decide when it is time to shift into overdrive and I am wondering if there is some sensor that has gone whacko and needs to be replaced instead of starting to tear the transmission apart.

Theories? Recommendations?

Thanks!


#2

Most transmissions will shift into overdrive at 40-50 MPH.

Assuming 30-31 inch tires, and a 3.73 axle ratio, and the 4L80-E transmission. In 3rd gear (1:1 ratio) at about 47 MPH, the engine should be turning about 2000 RPM, if you’re not accelerating at that point, then the transmission will shift into 4th gear (.75 :1 ratio), leaving you at about 1500 RPM at about 46 MPH, which is not going to lug the engine, and is completely normal.


#3

I was thinking the same thing, and if the engine is lugging at that point, the problem might not be with the transmission.


#4

I had an 02 Avalanche and currently own an 04 with 4:10 gears that shifts into OD at about that point. If the truck has been running OK for 33K on the rebuilt trans, it is unlikely the transmission. How many miles on the truck? Has the fuel consumption gone up recently? Any engine codes?

The trans takes into account manifold pressure (vacuum) and throttle opening (TPS), rpm and I’d guess internal trans pressures plus calibration values in the ECU to determine shift points,. Maybe a failing MAP sensor? or even a MAF sensor maybe. It would help to hook up a data-logger scanner so you can drive to the speed where the OD kicks in and see what’s going on.


#5

Mustangman mentioned

  • Intake manifold vacuum
  • TPS
  • Engine rpm
  • Internal trans pressures

To that I’d add the vehicle speed is probably part of the shift-point calculation. And the current selection of the shift lever is obviously an input.

First step to resolve this is probably for a shop to check for both engine and transmission codes, verify the fluid is at the proper level, the proper fluid, & still in good shape. Then they’d measure the internal transmission pressures. Still a no-go? Move on to the other inputs above, make sure they are all correct.


#6

Some more info to answer some of the questions asked:

  • Truck has 213k miles. Both engine and transmission rebuilt at 179k
  • Transmission and engine both showed no codes per two separate mechanics (including guys who rebuilt transmission
  • Replaced TPS at recommendation of transmission guys. Also had transmission serviced. No change in behaviour
  • I don’t keep track of gas mileage but haven’t noticed greatly increased consumption.
  • Truck shifts in low 40s mph. Goes from 1,500 rpm down to 1,100 rpm and is lugging down
  • I’ve had the truck since taking delivery in Dec 2001. This behavior is new as of two months ago.

Last night while driving the vehicle to confirm the speed and rpms where it is shifting, the transmission “chattered” around that shift point. On the way back home, it missed shifting into a higher gear (rpms went up and speed didn’t). I backed off and it recovered. At home I backed into the driveway and it didn’t engage right away when I tried to pull forward a bit.

If I didn’t have a transmission problem, I clearly do now. If there are other factors causing problems with the shifting like the MAP sensor, we will see. I will post an update when all this is sorted out.

Thanks for the advice.

Scott B


#8

OP’s statement:

I assume that includes changing the fluid…