Auto shifting to lower gear

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In the past 6 months have been the owner of a 2000 Prado TX Diesel brought here locally (Kenya) from the first onwer here locally prior to being sourced second hand in 2007 from Singapore and probably Japan before that.

During steady state driving the autobox will suddenly shift down one or two gears and then change back. All without any change in throttle position or effort required. During the last longer drive the electronic enigne controls went into safe mode four times in 30 minutes driving (idle only) which requires stopping the car to turn off the iginition and restarting. That seems to resets the ECU and the car reacts normally again to throttle input. The car also seems to hunt a bit at any constant speed. Not always, this is also intermittent.

The gear shift on the auto box is very smooth and it always reacts to throttle input except when it has gone into safe mode.

Turns out that the previous owner had changed the auto box for the same problem and thought it had been cured (or so they say). So it is not the gearbox itself but likely to be in the elctronics. Any ideas on where in the electronics to start looking?

patrick van oudgaarden
Naivasha, Kenya

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Re: Auto shifting to lower gear

sounds similar to an issue i had with mine, which turned out to be a faulty throttle position sensor.

if you check the ECU codes, it should give you a clue on why the ECU is going into failsafe.

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Re: Auto shifting to lower gear

Have an update on the problem - got an appointment at Toyota East Africa very quickly today (as often a case of who my car dealer knows) and had the workshop guys in with all the diagnostic gear. Not a single fault registered in the memory despite 4 times to idle only and frequent high rev excursions this morning driving into town. The throttle position came up clear and when disconnecting one or two engine sensors for trial purposes the onboard systems immedately recorded the fault. So it seems that whatever is producing the behaviour is not registering as a fault.

We took the car out for a 10 km test drive with diagnostics attached. Increasing speed but not at full throttle the revs suddenly went up beyond redline to +5000 and the car would not respond to throttle. Then 1 or 2 seconds later the revs came down to normal again. When this happens there is no change in the car's torgue, speed or accelleration. It is as if the engine just disengages from the drive train. Unfortunately the mechanic had not put the diagnostics on record yet and we couldn't reproduce it with record on. All what was produced by the subsequent recordings was that it was slow to change up from 2nd to 3rd gear - lets the revs go higher than expected.

Car is staying with Toyota and hopefully know more tomorrow.