Trans problem


#1

I have a 98 gmc k1500, 5.7, auto.It shifts in and out of overdrive constantly.If you depress the gas slightly and accelerate it will go in to overdrive and stay. Any ideas?


#2

I think that we first have to define our terms.

Overdrive is the highest gear that a transmission has, which causes the engine to turn more slowly at high speeds in order to save gasoline and to reduce engine wear. However, I have heard people erroneously refer to their “passing gear” (a kick-down to third gear in a four speed transmission, or a kick-down to second gear in a three speed transmission) as “overdrive”.

Based on your description, you might be telling us that the transmission “kicks-down” a gear when you accelerate–as it should–and this is not overdrive. But, if the transmission subsequently fails to upshift to the next highest gear once you have attained a steady speed, that is a problem.

Or, if your transmission upshifts to its highest gear, rather than kicking down to a lower gear when you accelerate, that could indicate a problem. But, until you clarify the exact scenario for us, it is not likely that you will get an answer that is helpful.


#3

once the transmission is in fourth gear, the converter clutch goes in and out if you are trying to maintain speed. If you accelerate slightly the converter will lock up and stay locked up.


#4

What it is doing is what is called ‘hunting’.

Do you have any codes stored in the PCM?

At what speeds/rpms does this occur?


#5

I would have a trans shop scan it to make sure its not falling in and out of lockup which is a common problem inthese transmissions. Also, see if there are any codes stored. Post back with the results.

transman


#6

it does it at around 40-55 mph. Going to dealer on 5-14 to get diagnosis.


#7

I agree that it sounds more like the converter is locking and unlocking. The shift from 3rd to OD is a large gap, but the shift from OD to lockup is only a few hundred rpms. It might be a sensor sending incorrect information, a bad or sticking TCC solenoid (torque converter clutch solenoid), etc.