Trans Failsafe

Hello,

I guess this must be a sign that I've not been on this forum for a while....

On the way in to work, this morning, after driving for 10 mins, I could feel that the car didn't appear to be shifting properly at low speed. Then a few seconds later, the Trans Fail-safe message appears with the orange warning traction-warning light at the top of the dash. The car is stuck in 3rd gear, but I get to work and the during the drive, the engine idle appears a bit eratic.

Once in the car-park at work, I try to restart the car and it starts with a rather high idle for a few secs, which then setlles down, but is eratic and lumpy, and Trans Fail-Safe error is still there. I look under the bonnet and don't know why, but I think 'lets see if it's the MAF?' and so I unplu it and the engine dies. I turn of the ignition and re-connect the MAF, but now the engine starts and only runs for 0.5 to 1 sec. (I think the gear-lever selector switch is working OK, because for a split second the dash show the correct gear-lever position, befrore going blank and showing the Trans-Fail error)

I know I need my diagnostic kit, but being at work without it, I was wondering whether anyone may have any ideas?

Can the Trans Fail-safe error appear due to an engine problem?

Thanks

Hello, Tim,

Thank you for the reply.

I checked the battery voltage with the iginition on: 12.34v at bettery terminal in engine-bay.

then I disconnected the batteries for a minute (easy with installed battery-switch) and tried again:

Engine started-up right away and no warning lights. very smooth idle and acceleration to red-line.

Battery Voltrage now 14.3 V at engine-bay terminal.

Waited a few moments with engine running fine and then Trans Fail-Safe meaasge re-appeared and without trying to shift gear. Engine-reving was now missing beats intermittently and idle not so smooth.

Could be that after a few minutes when engine adaptation kicks-in (after having disconnected batteries) the problem re-appears.

Have been reading that MAF problems could lead to this error?

Hello, Tim,

Thank you for the additional input and information on the M62. I've just come back from a further tinker with it during my lunch break. The car was still behaving the same way:

Initially no fault (I think fault resets anyway after 20 mins when unloader realay removes power)
Initially shifting seems OK.
Then jerky shifting from N to D and from N to P, but no faults indicated.
Then immediately after, both the ASC light and Trans Fail Safe message appear.
Iratic engine idle going even very low < 500 rpm

Now, I think the clue may have been in the ASC light which I am not sure whether it is a normal companion of Trans Fail Safe?

So I did a quick reset by disconnection the batteries for 1 minute and tried again with the igition key in position II - no errors
Then, turned iginition OFF and in the engine bay, I disconnected the ASC throttle control motor unit.
Now ASC light on in key porition-II
Started engine and the idle seemed steady, smooth and normal.
Drove car around car park for 5 mins and gearbox seems to be working OK. smmoth shifts and no errors (touch wood).

So, my conclusion is now that the ASC throttle control unit may be the culprit or its potentiometer (does it have oine?)

Is this ASC ssytem used to reduce engine power during gear shifts? Otherwise, I am not sure what the connection to the gearbox is unless there is some other wiring fault?

However, the gearbox seems to be shifting smoothly again - at least during my brief test, which would prevously show the trans fail messsage.

So I am releived that it looks like not a gearbox problem (£££).

Maybe I will remove and clean the ASC control motor, and see if it helps.

Update:

Have removed ASC-throttle actator and tested it on the bench. Seems to work perfectly, so not sure what the issue could be. Unless with its car wiring?

Also removed throttle potentiometer and tested it with multimeter. Its readings seemed consistant and repeatable (4 k pot)

I guess I'll find out more when driving home this evening

Update:

I drove home with the trans failsafe message re-appearing a frew minutes on the road. Not fun to drive like that in heavy traffic. Carsoft diagnostics on the EGS showed a stored throttle data error on the CAN bus. Then 'Live Data' for the EGS highlighted the fault with the 'gas-pedal position' which (whilst idling- 0%) was intermittently showing 100% and other values in between, just as the trans failsafe error re-appeared after a reset. The rest of the gearbox readings appeared OK.

After exercising the accelerator pedal a few times the readings appeared again normal from 0 to 100%. This could be seen even with the engine off and key in position-II.

After a further reset of the Trans Fail message, the car was OK agaoin and was driving normally with no errors.

Looks like I'll have to replace the throttle sensor ASAP. It makes sense, though, as I guess neither the ASC or EGS can work properly without knowing the throttle poston. Interstingly, the DME had shown no errors. But, the engine running was rough at times.

(What I had removed and cleaned earlier was actually the ASC throttle potentiometer, not the main one,)

Hello, Tim,

Yes you are correct: The gearbox does not use the secondary throttle position info.

There are two throttle position sensors (both have the same part number): one for the main throttle and one for the secondary, ASC throttle. The one read by the EGS, is the main-thottle sensor and this is the one, which tonight I discovered was giving 100% throttle reading at Idle, which should be 0%, thereby leading to the error: Trans Fail Safe and ASC light.

The ASC actuator and ASC throttle position sensor had both tested OK off the car, and I cleaned and rechecked them anyway.

Not sure, why the ASC light would come on with the Trans Fail safe message, but maybe the ASC also needs to know the main throttle position?

.... or maybe the ASC light has a dual function - also to warn about Trans Fail?

( I had thought that the ASC light only could only signify the ASC throttle actuator or it position sensor, so started looking at these items )

No errors had been stored on the ABS/ASC control unit nor the DME: Only on the EGS, which gets the data from the DME through CAN.

Hello, Reinhard,

The diagnosis showed the likely fault to be the main throttle potentiomer ('gas-pedal position' readings all over the place when looking at the EGS live info.) However, I don't know why the ASC light would also come on, so initially I checked the ASC actuator and ASC throttle potentiometer, but found them OK.

Well, I removed the main throttle potentiometer and tested it on the bench (not so easy to remove, this one). It seemed to work fine (!?). Also checked the wiring, just by looking at the 3 terminals with an Ohm meter. The readings 5k and 450k etc between the wires was constant with moving and pulling the wires and connector, so hopefully OK.

Anyway, as a test, I swapped the main-throttle potentiometer with the ASC-throttle pot to see if the problem gets fixed and I'll find out on the drive home this evening.

Interestingly, the two pots are both the same part, are located within a few cm of eachother, and have identical connectors, which could lead to crossed wirinng if not careful.

It seems to me that the ASC actuator motor moves so fast that ASC throttle control is probably an all or nothing afair and therefore its pot only has to indicate on of the two extremes. I experimented with the ASC system by alternately disconnecting its actuator and its pot: The ASC light comes on immedaitely as soon as the key is in position-II, as this is when the system is cycled once for test. But no Trans Fail Safe error in this case.

That's why I am thinking that the ASC light must either have a separate use - also for highlighting a Trans Failsafe problem, (in which case, the light is a kind of red-herring); or otherwise a Trans Failsafe mode makes the ASC to be inoperative, which might also make sense. It's probably the latter, actually.

If I can get home without the Trans Failsafe re-appearing, then I guess it must have been the main-throttle pot as indicated. Co-incidentally, I Have an MOT tomorrow morning, so I hope it will be OK.

swapped the two potentiometers around

Update:

Drove home in heavy traffic and no errors indicated! No ASC light ! No Trans FailSafe !

So, having swapped round the throtte potentiometers, I now have a potentially faulty one on the ASC throttle, which I guess is not so crucial, but I am thinking of buying a replacement.

I can't seem to find any Bosch ones and the closest is probably this Hella one on ebay:
http://www.ebay.co.uk/itm/HELL…926229592?#ht_1688wt_1237

But I've always recognised Hella more for their lights rather than manufacture of sensors?

Looks totally identical, though.

Thank you!

MOT passed on Saturday, and so far, still fine.

(No errors from ASC system, either, so might originally have been a dirty connection on the original main-throttle pot- now swapped on to the ASC throttle and apparently Ok)