Erratic 7G-Tronic behaviour

[MarkManning]

Hi guys, I've read through some of the other threads about jerkiness between gears, but the intermittent fault I'm getting is altogether different.

We have a 2006 C209 CLK 320 CDI Sport coupe which we've had for just over a week.

The gearbox seems to suddenly decide that it no longer wants to change gears, the other day my wife was driving at a roughly constant 45mph at about 3k revs and it wouldn't let her manually change up, so I suggested she pull over, put it in park and back into drive (when it went into drive it gave a hard thud) she pulled off and it stayed in 1st up to 4.5k revs, didn't dare take it over the red line in case it still didn't change up. After again pulling over and this time switching the engine off and back on, the box played nice for the next couple of days.

Today, it drove perfectly all day until I tried to park outside my house, when I put it into reverse it stayed in neutral and just revved, back in drive, still neutral. After a couple of goes I put it into park and then it would go into gear, but each shift into 1st or R1 gave hard thud again.

The trans oil was changed at 40k & it's now done nearly a ton. Weirdly, the S.S. never tells you when to change trans oil after 40k, but I'm not sure that could be causing this problem anyway.

Any ideas gratefully received.

 

[brucemillar]

Mark

Have you checked the ATF level. Low levels and or poor ATF can cause all sorts of strange behaviour. May be worth dipping it or getting an indie to do it for you.

 

[MarkManning]

Hi, I want expecting the ATF to be the cause, since it's so smooth the rest of the time, but I'll take a look and post back. Do you know how often it should be changed. The service book reads once at 40k but no mention of it after that, I can't imagine it's a great idea to just continually to it up?

 

[st4]

Hi, I want expecting the ATF to be the cause, since it's so smooth the rest of the time, but I'll take a look and post back. Do you know how often it should be changed. The service book reads once at 40k but no mention of it after that, I can't imagine it's a great idea to just continually to it up?

it will need changed - the once at 40k is silly, its really every 40k mikles, but I suspect it will be an electronic component thats failed.

 

[davidjpowell]

the thud does sound worryingly like the one I experienced on my first C320 cdi, just the once. It happened while I was in Germany so not much I could do and had no other obvious symptoms.

Shortly after I was discussing some wheel corrosion with an MB Tech and mentioned it in passing. He was going to take a look at it. The fluid was burnt and they pronounced the gearbox dead. They did not diagnose more than that as it was under warranty so a straightish swap.

As others have said g/box oil is cool and if its banging around STAR might also read some codes.

 

[jaymanek]

It will most likely be the speed sensor plate... get it plugged into a star.

 

[MarkManning]

Jaymanek, it's booked in for STAR treatment tomorrow, the guy at Mercare mentioned the speed sensor plate when I booked it in over the phone.

My wife took the car today and it's just done it again on the way home (Is it what those in the know call Limp Home Mode, when it won't change gear?)

It's done nearly 60k since the ATF was changed so I'm happy to pay for that anyway if it's just a sensor on the SSP causing a fault (I take it the ATF will need changing to do the job). I'll try to get the dealer to pay for the sensor and the labour, I think I'm being reasonable there!

 

[steve333]

Jaymanek, it's booked in for STAR treatment tomorrow, the guy at Mercare mentioned the speed sensor plate when I booked it in over the phone.

My wife took the car today and it's just done it again on the way home (Is it what those in the know call Limp Home Mode, when it won't change gear?)

It's done nearly 60k since the ATF was changed so I'm happy to pay for that anyway if it's just a sensor on the SSP causing a fault (I take it the ATF will need changing to do the job). I'll try to get the dealer to pay for the sensor and the labour, I think I'm being reasonable there!

I would err on the side of caution & get the autobox fluid/filter changed(see if they can drain/refil the torque converter at the same time as mb don't do it)shouldn't set you back more than £200.

 

[MarkManning]

I've just had it diagnosed, they said that the error code was showing a fault on one of the sensors inside the electrohydraulic control unit, which is £720+VAT for the part alone.

On the plus side, he said that they've NEVER needed to change one and the MB dealer they buy parts from has only ever changed three or four, presumably under warranty. He said that in all likelihood it would be dirty oil causing the sensor to error.

On the down side, if it turns out that it DOES need that part, it will need an otherwise unnecessary second ATF change!

There was also a faulty temperature sensor, unrelated but could be causing it to use more diesel than necessary. The seller has agreed to pay for that.

I'll post back once the oil change is done and I've had a chance to see if the fault is cured.