Help! 2002 Toyota Highlander not downshifting

[flecklc]

The CEL and VSC lights came on last week. In the past it was me not screwing the gas cap on tight or O2 sensors - two of which have been replaced. I noticed a few days ago it is not always downshifting at higher speeds - 40+ mph (it's an V6 automatic AWD limited) Doesn't seem to matter if the car is hot or cold. The car has 145K miles and I've had all the preventive maintainance done. Transmission fluid was changed but probably around 100K. Any thoughts? I've googled and come up with:
throttle valve cable adjustment
kickdown cable
transmission fluid & filter
transmisson soleinoid
input/output speed sensor
knock sensor??
P.S. taking it to Pep Boys to get the error code for the CEL tomorrow...

 

[uclabachelor]

Check the throttle position sensor.

 

[LTC8K6]

The VSC light always comes on with a CEL, I believe.

 

[flecklc]

Wouldn't I get a code if it was the throttle position sensor? I only got one code yesterday - P0442 which is a "small evap leak"...

 

[LTC8K6]

Sounds like it's a gas cap issue...

 

[flecklc]

It's a bad knock sensor that's keeping it from shifting.getting it (and the ther one) replaced
next week. $700 for both sensors and labor...

 

[slag]

I had no idea an engine knock sensor could stop a transmission from down shifting. What is the logic there?

 

[flecklc]

I have no idea, that is what my mechanic told me. He was recommended by a friend and is on Angie's List sooo, I think I have to trust him. I googled "knock sensor" and "shifting" and found some replies:

Ok..
Just got back from the transmission shop. There had been a check engine light on for a knock sensor for some time now. My original mechanic said it had nothing to do with the transmission. The transmission shop looked up in some database the issue about fourth gear and the knock sensor and determined that it was related. The knock sensor was preventing the car from damaging itself by not shifting the car into the highest gear. There were two sensors that needed to be replaced, a sub harness, gasket and two intake gaskets. Parts 807.37 and labor 479.37 including the diagnostic tests they ran.

Hmm... it is possible. Detonation (knock) is always worst when the engine is under heavy load, and holding the transmission in a lower gear would relieve the potential for knock (which is not only damaging, it increases NOx emission). So if the ECU knows it can't tell if the engine is knocking, it just might tell the transmission to stay in a lower gear. I haven't heard about it before but I wouldn't be surprised.

 

[flecklc]

Also found this (the highlander is basically an RX-330):

P0330 is knock sensor code, but RX-330 computer will disallow trans to shift past 3rd
gear when this code is active. There is NOT a transmission problem here, it is strictly a
safety to keep motor from high stress/torque situation. Fix the O2 sensors and/or the knock sensors and that will fix it.

 

[LTC8K6]

You should have noticed the higher revs and lower fuel economy of driving around in 3rd gear all the time.

 

[LTC8K6]

You should have a lot more power available once your knock sensors are working. The ECU goes into defensive mode without the knock sensors. It reduces power.

 

[flecklc]

Well, I'm only about six miles from work and most roads are 40mph or less so I don't get great milage to begin with. I haven't noticed it being worse than usual. Power seems to be OK but then again I'm not usually driving at high speeds. It gets fixed this Friday - they had to order the parts. I'll let you know the outcome...

 

[flecklc]

My car is fixed - two knock sensors and and cables (harnesses?). $876. CEL and VSC lights are now off and it runs great...