Hi all,

I had to drive a few hundred kilometers these past days, and I've had a new and frightening issue after the first couple hours : while on cruise control on the highway, an upward slope came and the car shifted down to maintain speed. It caused some kind of tremor for maybe half a second before the 4th kicked in -- I wasn't paying much attention and wondered whether it was some rumble caused by the road itself.
However a few minutes later, the ABS/ASC fault flashed, cruise disengaged, and the speedometer suddenly dropped to zero -- while the car was still running. While waiting for next resting area to stop and analyze, I plugged the odb reader (it was on the passenger seat !) and found a transmission fault code, which I can't remember, but mentioning a CAN error.

Makes sense actually, I'm guessing the car speed is provided by the gearbox itself, connection error causing the speed to be zero (and the less amusing thing here is that the assisted steering was acting as if the speed was zero, so VERY sensitive, which is not cool at 130 km/h).

I'm afraid I panicked a bit and simply erased the code without noting it down. Stopped and restarted the engine, no further issue until I arrived a few hours later, but from time to time there was the same tremor and delay when shifting gears, especially when accelerating after a toll.
The return trip had the same tremors, no further fault code, and I just lost a few years of life expectancy due to the stress. (it was a 1500 km round trip, essentially highway).

I just checked the gearbox oil, which is reasonably red and clean, fortunately. I did change it over the summer and this was the first long trip since that time, so I'm wondering if there is a relation. Maybe wrong oil level ? I will check this in the coming days.

Or is it the first symptom of something more serious ? Is there a way to investigate and identify a potential issue without removing the transmission and looking inside?

 

It could be bad (or contaminated) right rear wheel sensor. It seems that this sensor affects many things - in addition to ABS/Trac Control, also the Cruise Control, Speedometer, Power Steering and Transmission Shifting. Have a look at it as it may just need cleaning and checking the gap (~0.75 mm). Also unplug its connector and see the condition of the pins. If everything looks good, replace it.

 

Oh well. I wouldn't have thought of this one.
I did change the bearing on that wheel recently, so I might have damaged it somehow. I'll check it out, thanks for the hint.

 

+1 on Mr. Stojanovic's comment. Those sensors are magnets and will pick up anything metallic that passes by. I had one that wasn't providing a reading at all; fortunately it was the right front. Pulled it out and it was covered with metal shavings. Wiped that stuff off and it started working again. But it does appear that I will need new rotors and pads soon ...

 

either rear wheel speed sensor will throw a trans failsafe and ABS light. right one should be responsible for speedo, left one definitely isn’t because i drove without one for a while and the speedo worked fine

 

So I removed both sensors. The left one was relatively clean, but the right one was covered in grease and whatnot.
Removed, I put the gearbox in neutral to rotate the wheel and check the rotor, and the same dashboard errors came up (abs/asc), while the speedo was "vibrating" at 0, exactly as it did on the highway. Might have been the culprit then.

Meanwhile I noticed another error from the transmission module, p0711. Will analyze that one, perhaps not related.

In any case, thank you all for the valuable input, again!

 

Greasy wheel speed sensor tends to lead to outgoing wheel bearing. Unless it is fwd car and the cw joint boot is failed.

Oh, you have replaced it. Is sensor ring clean?

 

Yes, I replaced it recently. I suppose the grease is from that time...hopefully not the bearing going already. I'll check it again in a few weeks maybe.
The ring was clean.

 

I noticed yesterday that the P0711 was back... apparently, I'm in for a harness replacement.
I erased the code and it didn't appear again on the way back, but it was a short trip. I'm guessing if I take the car for a longer drive, it will be there at some point.

For a variety of reasons, I'm not too eager to change that harness. Doesn't seem like a complicated task, but the part itself isn't cheap (nearly 200 €, and since I am out of a job currently...*), and it will require another oil change procedure --even though I should be able to reuse the current oil--, which is a pain since I can't lift the car more than 30 cm or so.

Is it possible to keep it that way for a little while without breaking something eventually? Ideally, i'd like to wait until the weather is better, so maybe a couple months.

*it wasn't the best of times to tell the boss he's a jackass, but I couldn't keep it to myself anymore

 

I guess you refer to the internal Valve Body harness. Code P0711 appears to mean "Transmission Fluid Temperature Sensor Circuit Range/Performance". This could mean the temp sensor (inside the gearbox) is on the way out or there is a bad connection along the wire to the TCU. It could be the internal connector to the sensor or the relevant pin in the round external connector. If you have not already done it, perhaps you can disconnect the round connector and inspect/clean the pins.

 

Ah, right. Worth trying this before opening up the gearbox again. I'll check it out. Thx!

 

Hi all,

Been a while but I haven't used the car much.
I couldn't try to disconnect and clean the gearbox connector : I don't see how to reach it without removing the exhaust pipe, and I really don't want to do that. Looks fairly clean anyway.

I've been having an odd behavior from the transmission over the past couple of months I'd say : the gears are changing not quite at the right moment, sometimes pretty rough (for example 4th to 5th under high load), and there is something definitely not right overall but it's difficult to really describe. Shifts are maybe a tiny bit too long. Mostly, they don't happen exactly when I expect.

I checked the transmission fluid and added a bit, but since I can't properly check the temperature, I'm still not sure the level is right. Now, I'm concerned that the issue is more serious but I can't see what it would be : a cracked A-drum does not cause these symptoms, as I understand.
Code P0711 never came back, although the longest I've driven has been perhaps half an hour.