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Last week I thought I noticed something odd with my tachometer.
I also noticed the same thing this morning. My issues only occur when the car and transmission are cold, not just 'not at operating temp.', but when it and the fluids are a degree or two above freezing, left overnight.
It's a 722.6 transmission, 220cdi. ATF change only 8 or so months ago, maybe 8000miles since.
Basically, at the end of my street there's a set of traffic lights at the intersection, and the car is cold, fluids around 2-3 degreesC or lower. When I am coasting up to the lights at c.20mph, slight downhill gradient, with VERY little throttle, kinda the slightest bit of throttle, but then also maybe none, I've noticed the tachometer is bouncing, around 1000-1500rpm. When this is happening and I back off the throttle completely, or increase throttle, the issue ceases. And only this morning, did I hear, a VERY faint rubbing noise, hard to explain, I only heard this one time this morning. It was not a metallic noise, very low pitch, almost unnoticeable, could be unrelated...
The whole bouncing is not really all that noticeable through the drivetrain into the cabin. No issues with acceleration/decceleration/idle whatsoever. Gear changes are still very smooth and quick, same in comfort and sport, with decent and heavy throttle there are also zero issues. When the car is warmed up, even the slightest, also no issues.
Only noticed this occur twice in the last fortnight.
I thought a very late 2006, 722.6 transmission, even the 722.6 in general, was quite a bullet-proof unit. I've not heard or read of anything similar before, and searching the net isn't particularly fruitful.
"And only this morning, did I hear, a VERY faint rubbing noise, hard to explain, I only heard this one time this morning. It was not a metallic noise, very low pitch, almost unnoticeable, could be unrelated... "
I remembered when coolant got into my car's transmission fluid through the defective VALEO radiator. I would intermittently hear a very similar noise coming from underneath the car as I'm driving. My mechanic and I opened the coolant reservoir and we found white stuff which looks like mayonnaise (mix of ATF with coolant). We flushed the system and replaced the radiator and all is well now.
When the noise would appear though, I did not experience the RPM fluctuations you are experiencing. Plus, by 2006, all valeo radiators were phased out for the proper BEHR ones, but let's use the process of elimination and start with the easy stuff:
Check to see if there are white stuff in your coolant reservoir and whether you have a valeo radiator or a BEHR one.
Again this could be totally unrelated. I'm just sharing my experience based on what I read in your post.
Thankyou guys for your responses. Apologies I couldn't answer sooner, it's been a hectic weekend.
Coolant is perfectly fine, no issues there. Appreciate the suggestion however.
I was running a little high on ATF when I checked with the dipstick a few months ago, having changed the fluid back in Easter. However I took a hundred mL or so out, and it's at the perfect level now. I suppose there is no harm checking tomorrow once again, after my 30mile commute it will be hot enough.
Rodney, have you any further ideas on any potential throttle issues? What could cause this; if it were a throttle issue? I do appreciate the fuel delivery on a diesel is quite different to the more commonly found petrols in the US.
The electronic throttle pedal and the throttle body can get out of sync. This might help:
1. Make sure everything in the car is turned off and closed.
2. Turn the key to position 2 so all the dash lights are illuminated.
3. Press the accelerator fully to the floor, depressing the "kick-down" button.
4. Hold the pedal for 5 seconds and continue to hold it down.
5. Turn the ignition switch back one click to position 1.
6. Release the accelerator pedal, but leave the key in position 1.
7. Sit quietly and wait for about two minutes - do NOT open or turn anything on.
8. Listen for a faint "click" or "ping" noise form the dash.
9. Start the engine and drive the car for at least five minutes.
Hi Rodney, thanks for that reply. I do that procedure somewhat often, maybe quarterly, just to reset the **** points. I have done it once more on my drive back this afternoon. Obviously though, the car was still fairly warm, so I experienced no issues; not unusual.
I did find this morning however, that if I forced the transmission to stay in 2nd, so it was revving higher at 20mph, then I wasn't having the issues. It's kinda like its jumping to 3rd, realising its not viable and holding back 2nd. Obviously it isn't locked up, so it's able to slip.....
I remembered when coolant got into my car's transmission fluid through the defective VALEO radiator. I would intermittently hear a very similar noise coming from underneath the car as I'm driving. My mechanic and I opened the coolant reservoir and we found white stuff which looks like mayonnaise (mix of ATF with coolant).
Did you see this white stuff in chunks? Thick like mayonnaise floating around in the reservior?
I seem to recall some problems relating to a the torque converter lockup that were due to either a bad solenoid or bad contact on the conductor plate. This should have stored codes, thought. I'd recommend a proper diagnosis with SDS or at least iCarsoft.
Did you see this white stuff in chunks? Thick like mayonnaise floating around in the reservior?
Yes. That's the sign that the radiator section responsible for transmission fluid cooling is punctured and leaking transmission fluid into the coolant.
When dropped the transmission fluid pan, the white stuff looked like this:
So for some reason the issue hasn't followed me home for Christmas. I've tried to simulate it again to demonstrate it to my father, but it just will not play ball. There was one moment where it had done it, but it alas only with me in the car. So my father thinks I'm imagining the issue.
Anyhow. My parking sensors were playing up, so I took the car down to a Mercedes independent specialist, to ask them how much to have the car scanned on their SDS, and whether it could determine which sensor was faulty. £25 for a scan, and I'll then see whether there are any other codes stored for anything...
However while there I asked him about my issues I described above. He actually took the words out of my mouth, and finished off what I was saying to him.
He said it was normal/nothing to worry about. Something to do with a sensor that at low speed is causing the engine to fluctuate from a 'warm' 3rd gear, to the 'actually it's cold', stay in 2nd gear, help get the catalysts up to temperature faster. That's the bouncing. Being a diesel, it takes a quite a long time for the engine to warm, hence it is quite noticeable. And as the car warms, the issue disappears.
So the suggestion is, that it's normal, changing a sensor may improve it a little bit, but it's not really necessary and not doing any harm. It's just very situational dependent, in that, the car is cold, and the road I'm driving down puts the car in the position where it doesn't know whether it should in fact change up to 3rd or whether it's too early.
I think this sounds like a feasible explanation... What do you guys think?
On a side note, he has a bunch of 60s/70s Benzes that he's breaking, some very pretty cars there, the whole site is full of Merc bits.
It may be coming from vibrations caused by looseness somewhere down the drive shaft. Various mechanics have told me that diesels go through wear items faster than petrols, with the higher vibrations, torque and all. You could replace your flexdiscs if they are the factory ones, at 10 years old it wouldn't hurt and you should notice a tad tighter and smoother shifts. But few people are in for that kind of maintenance
It hasn't, no. I think it may have been more a fluid noise.
Yes on manual transmissions they put dual mass clutches on diesels to reduce vibration through the driveline. I had noticed my flex discs had a couple surface cracks, something that is on my to-do list, just not all that urgently, since there's no vibration making it through the driveline that I can tell, and no clunky shifts.
I think that would be one of the easier jobs I'd have done on my cars. Doesn't look too difficult a job. I've got the access easily enough. Thanks for the suggestion though.
So I had the car scanned today. Within seconds I had a response to the all my issues.
The centre -right parking sensor is the faulty one, which was throwing the whole system off. I think being next to the he numberplate k have a suspicion that someone reversed into the front of my car damaging it outside the house, in the very tight parallel parking there is. I'm going to try and see if the dealer will supply a painted one, however last time I asked, they said they could only provide a sensor and a can of paint to have it painted with. Which I was not happy with. Paying £50 for a sensor and then £30 additionally for a can of paint when I only need a tiny amount. Particularly frustrating when the standard silver/black and white are all available painted at no extra cost.
As for the main concern, the transmission issue.....
Well on checking the system the report above was printed. Says the signal from transmission oil temperature sensor was implausible. Again a different guy this time, finished off my sentences when I was explaining my issue to him, so he knew what I was on about.
The code on the transmission is stored, but isn't current, so he cleared it, and said come back in a couple of days, and we will recheck it. I'm going to drive it tomorrow morning to get the tyres fitted, and it will be -6ceclius, so the issue should come back, and bring back the code if relevant, which I imagine it is.
Thankfully being the 722.6, when I change the sensor; I believe it to be part of the conductor plate(?), then no live coding is required, thankfully!!!
Itll at least be an opportunity to get the fluid changed for the second time, 9 months after the first change.
I'm just ecstatic I know which sensor is the faulty one, saves a great deal of trial and error. I find the CLK easy to parallel park, it's easy to estimate where the front is, however the rear, not being visible, means it's particularly hard to park outside the house, in a gap that's no more than 6" longer than the car! Parking is ridiculous outside my uni house it'll be a damn site easier once these sensors are back to working again!
So took the car back to have it rescanned after a few days of driving. 150miles or the abouts. It has been very cold and I've not experienced the issue after the code was cleared. It was -5C, and trying to simulate it and I can't. And the code hasn't come back. He offered to scan it again in a month or so after I've done a thousand mile or so.
Parking sensors arrived today, painted it and put it on the one star told me. That side at the front worked fine. But the LHS didn't pick up anything. As I bought two, one as a spare I experimented with the other three stitched the one and it all works fine now.
I just don't know why SDS didn't detect this last week...
It must be that it was detecting it sending out a signal as the test and not receiving it? As once the one SDS identified was changed, the parking sensors didn't beep and turn off, so the car thought it was all good.
11-10-2016, 05:15 PM
