Downshifting issue on acceleration...

[poleclimber63]

I own a 1998 SR5. Auto, A strange issue has arisen. I've experienced this 3 times so far. During regular driving, the truck shifts normally. When I went on an extended highway drive, I noticed that if I pressed the accelerator, even to the floor, the truck will not down shift. I could shift down, no problem, but it won't downshift via accelerator. When I pulled over, shut the engine off, and restarted, it's back to normal operation. The first time I experienced this, I tried manually downshifting, then I re-started, this worked but about a hour later on the highway it happened again. I have no codes. Could it be a shift solenoid or ecu fault? Any advise narrow down this would be appreciated. I'm not really interested in attempting to "shotgun" this problem as both these options will be expensive and it happens infrequently... It doesn't appear to be a tranny temp issue, as my temp gauge didn't indicate any unusual readings when this issue appeared.. This has me stumped as it has happened infrequently, and upon re-start the issue disappears. This is why I'm leaning towards an electrical/electronics problem... ie sticking shift solenoid, or ECU... could it be as simple as a kickdown cable adjustment??
Thanks! Pic for attention...

 

[phattyduck]

The cable to the trans is for line pressure control. There is no kickdown switch (electrical or mechanical) in the system, unlike many euro vehicles from 'back in the day'.

In the 'no-downshift' state, what happens if you slow to a stop? Does it stay in overdrive all the way to stopped? Or is it simply that at speed it doesn't want to downshift? Does it at least unlock the torque converter when you add throttle?

-Charlie

 

[spartacus]

Slack in kickdown cable or tps sensor could be going out.

 

[poleclimber63]

The cable to the trans is for line pressure control. There is no kickdown switch (electrical or mechanical) in the system, unlike many euro vehicles from 'back in the day'.

In the 'no-downshift' state, what happens if you slow to a stop? Does it stay in overdrive all the way to stopped? Or is it simply that at speed it doesn't want to downshift? Does it at least unlock the torque converter when you add throttle?

-Charlie

The first time I noticed this happening, I floored it and I had no downshift at all. As I was just starting up a hill, the truck started to slow and revs dropped, so I pulled over, discovered I could shift manually into lower gears, and then shut the engine off. I re-started and the vehicle drove normally, and accelerated normally. An hour or so later, it happened again. Same response. Then I entered town and several days later, on the way home, it happened again. Since arriving home, it hasn't happened again.. I can't exactly recall if it drops out of overdrive... The next time it happens, I'll pay attention this.

 

[poleclimber63]

Slack in kickdown cable or tps sensor could be going out.

I havent changed the kickdown cable adjustment... I was thinking about the TPS. Is there some way to monitor the TPS output voltage?

 

[spartacus]

Not sure but you can check the resistance.

 

[phattyduck]

Is there some way to monitor the TPS output voltage?

TPS is reported via OBD-II if you have a scanner that reads live data. It will NOT report a 0-100% value - it will be a reduced range (like 20-80% or something) of the actual reading from the TPS. You would be looking for a 'continuous' signal, not full range. A bad TPS will have dead spots that report totally different than slightly higher or lower throttle opening.

At highway speeds, the torque converter unlocking is like a small downshift. If this isn't happening, the transmission can't shift gears. Not sure why that might be happening though.

-Charlie

 

[poleclimber63]

Not sure but you can check the resistance.

TPS is reported via OBD-II if you have a scanner that reads live data. It will NOT report a 0-100% value - it will be a reduced range (like 20-80% or something) of the actual reading from the TPS. You would be looking for a 'continuous' signal, not full range. A bad TPS will have dead spots that report totally different than slightly higher or lower throttle opening.

At highway speeds, the torque converter unlocking is like a small downshift. If this isn't happening, the transmission can't shift gears. Not sure why that might be happening though.

-Charlie

I checked for codes since I returned from my trip and nothing appeared till this afternoon!! P0120 appeared while I was idling and scanning live data... I cleared the code. I set my reader up for live data, and kept an eye on it while running errands around town. Looked totally normal. on my return home, I let it idle for a while and voila!! P120 appeared again! I took it out for a drive but there was no evidence for any driveability issues. As a new Toyota TPS is $100 I think Ill just change it and keep an eye on it. I thank you both for your input!!!