Throttle position sensor or trans problem?

[KiwiNick Lurker Find all posts View Profile Close KiwiNick]

Hi all,
Hoping someone might be able to help me with a couple of problems I’m having. We brought our 2001 TZ Prado 3.4 litre petrol auto 12 months ago was running great at the time but have had a couple of things start to happen which I haven’t been able to sort.

Open road travelling down long decent, if I use the brake to keep it under 100kms the engine braking will kick in (which is fine) but when I accelerate once out of the decent the engine will still be holding the lower gear it won’t change up. To get it back into drive I manually change it by taking the over drive off then putting the overdrive back on.

Also about once a month the car will just cut out/stall, when creeping forward (in drive but foot not on the accelerator) at an intersection.
It also seems to be lacking power or holding the higher gears for longer than it should on uphill climbs so the revs drop right off rather than automatically changing down. Once again you can manual change to solve this but I would like to get it working like it used to.

Over the last 12 months/15,000kms I have done the fuel filter, flushed the transmission, cleaned the MAS, changed spark plugs. The guys at the garage and transmission specialists have scanned it and only once out of 4 scans has it registered a fault code (P0120 – throttle position sensor circuit). Vehicle has done 160,000kms. Wondering if the next thing to look at is replacing the TPS or if anyone has any other ideas. Thanks

[glen_ep Addicted PP Member Find all posts View Profile Close glen_ep]

Perhaps the first place to start is to inspect electrical plugs for all the sensors - unplug them, check for corrosion, clean and reseat them. There's 2 sensors on the throttle body (TPS and IAC) and several on the automatic transmission (either buy a workshop manual or go through all the plugs you can find).

If it continues, second step may be to test sensors or replace them - second hand ones will be sufficient for a test - I wouldn't waste money for genuine sensors until you've found the culprit.

[KiwiNick Lurker Find all posts View Profile Close KiwiNick]

T

Perhaps the first place to start is to inspect electrical plugs for all the sensors - unplug them, check for corrosion, clean and reseat them. There's 2 sensors on the throttle body (TPS and IAC) and several on the automatic transmission (either buy a workshop manual or go through all the plugs you can find).

If it continues, second step may be to test sensors or replace them - second hand ones will be sufficient for a test - I wouldn't waste money for genuine sensors until you've found the culprit.

Thanks glen_ep, good point about the sensor plugs, I have checked and cleaned most of them, but will get my hands on a manual so I can make sure I get all of them. Will report back after the weekend. Cheers, Nick

[MrX Member Find all posts View Profile Close MrX]

What about vacuum hoses? Not as familiar with Prados as I am with other cars but pretty sure on some cars perished rubber hoses can manifest in weird transmission symptoms.

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

If the transmission behaves ok apart from the scenario of going downhill then that would
clear the throttle position sensor, the MAP sensor and RPM inputs all these would be used
to determine the actual engine load and therefore required gear.

Down hill change scenario is activated by the brake pedal, but in my Prado the amount of braking
applied and or the steepness of the hill seem to come into it. ie light braking doesn't cause a shift
down but heavier braking does, could possibly be the amount of time the brake is pressed for.

Operating the throttle clears it but on some occasions like towing this can take awhile and I as you
manual shift then.

So possibly something funny with the brake switch?

The occasional stalling is probably not related to the transmission issue, though then again maybe
torque convertor?

Leigh

[Steve M Addicted PP Member Find all posts View Profile Close Steve M]

Ive never heard of the 90 series having brake assist ?? Only the 120 upwards. Maybe there is issues here as well. Neither of my 90 grande's had this feature ......Cheers Steve

[williade Avid PP Poster! Find all posts View Profile Close williade]

Ive never heard of the 90 series having brake assist ?? Only the 120 upwards. Maybe there is issues here as well. Neither of my 90 grande's had this feature ......Cheers Steve

Ditto Steve. Neither my 90 series nor 80 series downshift when braking. My 06 Corolla was the first car I've driven with that feature. I figured Toyota transmissions back then simply didn't have brake assist.

[croozza Avid PP Poster! Find all posts View Profile Close croozza]

Give the TPS a good clean (with MAF cleaner), and while your at it clean the inside of the throttle body (with carby clean). Do a filter change on the trans also. There is no brake assist on the 90. When you hit the brakes, does the tacho drop and then rebound back up to what It was before, if so you will have a vac leak

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

Guys you lost me with the brake assist, if your referring to what I wrote,
in an electronic transmission the gear shift down when going down hill is triggered
by the application of the brakes, ie brake switch is an input to the ECU.

If the 90 doesn't have an ECU controlled transmission then forget it I'm just
talking rubbish

[Steve M Addicted PP Member Find all posts View Profile Close Steve M]

The 90's are still cable controlled Leigh, not like the 120's which are computer controlled. Even my wifes buz box yaris has the down hill assist and works on how you described eg steepness, brake pedal pressure etc.....Cheers Steve

[croozza Avid PP Poster! Find all posts View Profile Close croozza]

Guys you lost me with the brake assist, if your referring to what I wrote,
in an electronic transmission the gear shift down when going down hill is triggered
by the application of the brakes, ie brake switch is an input to the ECU.

If the 90 doesn't have an ECU controlled transmission then forget it I'm just
talking rubbish

The trans is controlled by its own computer. Brake assist does not exist on the 90, what your referring to is normal kickdown, when you slow down, the trans will downshift at a set speed.

[Jon Avid PP Poster! Find all posts View Profile Close Jon]

Had a similar problem when I had a 90 series.
After many trips back to Toyota and re-setting of fault codes, many requests and finally threats, a call to the gearbox supplier revealed that there was a fault with the transmission sensor. This was replaced and the problem fixed.

[KiwiNick Lurker Find all posts View Profile Close KiwiNick]

Had a similar problem when I had a 90 series.
After many trips back to Toyota and re-setting of fault codes, many requests and finally threats, a call to the gearbox supplier revealed that there was a fault with the transmission sensor. This was replaced and the problem fixed.

Thanks Jon, were the fault codes transmission related or something else. What exactly was your 90 series doing?

[Jon Avid PP Poster! Find all posts View Profile Close Jon]

Toyota did not say what the fault codes were.
The problem with mine was the transmission would stay in top gear and would not kick down, very dangerous when overtaking. Had to change manually or come to a complete stop to reset.
I finally had to ask the dealer manager if he would feel comfortable riding in a lift that I reset every day (I work in the lift industry) as that was all they were doing to my car. Also had to remind him of 'duty of care' in not fixing a problem which could result in an accident.

[KiwiNick Lurker Find all posts View Profile Close KiwiNick]

Give the TPS a good clean (with MAF cleaner), and while your at it clean the inside of the throttle body (with carby clean). Do a filter change on the trans also. There is no brake assist on the 90. When you hit the brakes, does the tacho drop and then rebound back up to what It was before, if so you will have a vac leak

Hi Croozza, thanks for the advice. Tacho doesn't drop then rebound when I hit the brakes. Throttle body cleaned as the same time as the MAS around 6 months ago. Trans filter changed when the trans was flushed less than 6 months ago. Where too next?