I have a '97 Blazer that occassionally will shift in and out of O/D at cruising speeds. Actually the shifting is noticed on the tach, seeing the needle bounce between 2k-2.5k. However, most times, on return trip, will not do at all-runs strong with virtually no need for down shifting on hills (interstate driving). MPG around 18 when shifting, 22+ when not.

If I lightly put my foot on the brake pedal when acting up, will downshift and stay there, which I understand is causing torque converter to lock up.

My real question, is this a mechanical or an electrical (computer) problem? I would think if mechanical, would happen all the time.

Appreciate all responses, love this Blazer, but with 200k don't want to replace stuff that is not necessary.

Travelin Man

 

It really sounds like your torque converter is unlocking on you rather that the trans downshifting to 3rd. Very different symptoms. If the torque converter is unlocking, you really need to determine if it is doing so at the command of the PCM or on its own. To do much of the diagnosis without guessing at the problem, you really need to get a capable scan tool, one that can see the TCC solenoid state so that you can monitor it while you drive.

The readings from the TPS & MAP sensors combine to tell the PCM how much load is on the engine so it can decide what to do with the transmission. You could test the TPS sensor for signs of drop outs. The output should be ~0.5V at closed throttle and ~4.5V at wide open throttle. The transition between these throttle positions should be smooth with no drop outs or spikes. I have posted MAP sensor voltages in other threads; a search for "MAP+altitude+voltage" with an author of "swartlkk" into the advanced search window should get you those readings.

 

i have a 2002 extreme blazer and it was doing the same
if its the converter the next time it does it put your foot slightly on the brake
if its the converter it should stop instantly

 

I have tried the light pedal pressure and it works as you described. However, the 'unlocking' does not happen on a consistent basis. I would think if the converter was bad, it would be always bad. As such, wonder if it is a computer issue more so than converter. Haven't found or had the availability to a scanner when this happens. Never notice in city driving and only when on 4 lane but never on a consistent basis.

 

Do you have a different tire and/or wheel size other than what came stock on your truck? If so, the difference in size could be putting your rpm/speed right in the ballpark of the OD shift point or converter lock/unlock when you're at cruising speed on the highway.

 

If there are no driveability issues other than this,

is your speedometer working accurately, have cruise control? does it work right?
* vss

Ever have brakelight flicker? {also cruise disengage}

* Brake sw {do a wiggle test on the pedal arm while someone watches lights}

If I was throwing one part at the problem, it would be the TPS. Would also check connections between it and PCM