W202 throttle response...

[Steve_Perry]

I just wanted to ask those in the know, how does the accelerator work on W202's? are they a traditional direct link to the engine, or is it more like 'fly-by-wire' i.e. the accelerator sends a signal to the ECU which then does the necessary.

My car lost power again this morning but right at the last stage of the journey so the car's engine had been at normal operating temperature for a while. I was on a main stretch of road and hoofed it past a slow white van, pedal to the floor including kickdown and the car whooshed past as per expectations, however as soon as I lifted off the throttle and slowed up for the roundabout the car seemed to be devoid of power.

Once at work, in the car park, I slipped the car into neutral and blipped the throttle and the response was damped to say the least. The car would rev but it was like there was a large time delay. I turned the car off, removed key, switched the ignition back on and revved the car again and this time all was normal, she revved like a good'un.

Any ideas guys? I don't want to take it to the stealer for a Star Diagnosis because they're not going to find anything wrong, they didn't the last time round. I might see if I can get hold of George for his advice.

I guess I'll continue to keep an eye on it at this stage I just want a greater understanding of what actually happens when I press the 'go faster' pedal.

Thanks guys,

S.

 

[Shude]

Originally posted by Steve_Perry
I just wanted to ask those in the know, how does the accelerator work on W202's? are they a traditional direct link to the engine, or is it more like 'fly-by-wire' i.e. the accelerator sends a signal to the ECU which then does the necessary.

Definitely fly-by-wire, well if it isn't then how the heck does the speed limiter work?

 

[jimmy]

The throttle cable is connected to a variable resistor or potentiometer type thing that alters the current or signal going to the ECU.

You can clearly see it under the bonnet, passenger side just in front of the ECU box. You can 'blip' it bt hand when the bonnet is up. The end of the cable needs adjusting occasionally and that can cause a very marked delay in response from pedal movement to car movement!!

It needs to be adjusted so that the cable is not quite pulling on the arm at rest.

 

[Dieselman]

Does this apply to the petol engined cars? If so how is the throttle actually opened?

On a diesel there is no throttle so no connection required.

 

[jimmy]

Originally posted by Dieselman
Does this apply to the petol engined cars? If so how is the throttle actually opened?

On a diesel there is no throttle so no connection required.

The throttle pedal, cable and potentiometer is exactly the same on my diesel as it was on my petrol C180. The throttle on a petrol engine must be controlled by some sort of electric solenoid or motor, on diesels it adjusts the fuel pump.

The ECU must interpret the readings from the accelerator and adjust the throttle or pump accordingly.

It is worth checking the cable, a slack cable gives a very definite delay between the pedal being pressed and the cable actuating the potentiometer, obviously only from tickover as once the pedal is pressed the slack is taken up.

 

[Steve_Perry]

Originally posted by Steve_Perry
I might see if I can get hold of George for his advice.

I spoke to George on Thursday, told him all the symptoms and he is abolutely convinced it's my MAS sensor playing up (again). Trouble is MB will just claim that there is nothing wrong with the car as there are no fault codes in the ECU log as of yet

I've got 2 years from last July before the MAS goes out of warranty. Apparently MB cover most replacement parts with a 2 year warranty.

I also asked George if he thought the fault could lie with the variable potentiometer that the sends the throttle signal to the ECU, he said yes it could but his first line of attack would be the MAS.

Big thanks to George Fraser for taking the time to talk to me, I know how busy he always is. Thanks also to you guys who replied to my post.

Hmm I wonder why a MAS would intermittently fail after a full throttle condition?

S.

 

[GrahamC230K]

Re: Re: W202 throttle response...

Originally posted by Steve_Perry

Hmm I wonder why a MAS would intermittently fail after a full throttle condition?

S.

Don't know, but I think my odd kompressor-less moment the other week was related to my MAS and perhaps the ECU adjusting tolerances.

 

[Shude]

Evil, evil, evil! How can something so small cause so many problems?

 

[Steve_Perry]

Originally posted by Shude
Evil, evil, evil! How can something so small cause so many problems?

What's evil is I'm sure MB know the score and charge a fortune for them when they only charge £70 quid for the same unit on their commercial vehicles... That's evil.

George was saying he had a ML430 in recently that was "acting up" all sorts of weird symptoms, again he said there were no faults in the ECU log, all showed clear. He changed the MAS unit and the ML drove out like a different car.

Trouble is I don't want to be paying another £270 quid for yet another one, grr we'll see how it goes but if I'm gonna get approx one incident per week then I'm taking it back to MB along with a printout of this thread.

S.

 

[sym]

Steve

Can't believe its gone again !! I guess only one way to find out though.

Just remember, if MB won't pay up (sounds like they should though here . . . ) there is a way to knock £200 off, should you end up having to foot the bill.

S.

 

[Dieselman]

Steve

You had a MAS recently didn't you. All MB parts are covered by a guarentee and it should be replaced free of charge.

 

[Steve_Perry]

Originally posted by Dieselman
Steve

You had a MAS recently didn't you. All MB parts are covered by a guarentee and it should be replaced free of charge.

Dieselman,

Yep back in July. The MAS is covered by a two year warranty. Trouble is MB couldn't find anything wrong with the car and the problem so far has been intermittent, two occaisions within the space of 8 days. I'll keep tabs on the situation and if it deteriorates then I'll go back to them and ask them to test the MAS specifically.

S.

 

[Steve_Perry]

Yet another hiccup on the way in to work this morning Seems to be roughly a once a week occurence. I'm compiling notes for when I return to the stealer

S.

 

[Steve_Perry]

News Update

Today the dealership finally managed to trace the fault. The car acted up again and fortunately I was close to the dealer for them to have a look at the car whilst the fault was still active as opposed to them telling me there aren't any logged faults.

The answer???

Fault P1542 - Pedal value sensor (B71)

Basically the variable resistor that translates my accelerator pedal movements into a signal input for the ECU has an intermittent fault. *Sigh* yet more expense I'm currently waiting for the dealer to get back to me with a price for a replacement. I need to get another job quick I've had enough of this unemployed lark.

Anyone know how much a 'pedal value sensor' would be for a C240? Maybe I could replace it with 2 bakebean tins and a piece of string.

At least for the moment the problem seems to be easily fixed by switching off then back on and the car more okay than problematic but I can only see it getting worse and I'm always second guessing as to how the car will respond each time I put my foot down. Not so much fly-by-wire as randomly-limp-by-wire.

All thoughts on cost greatfully received, Nick/K. any ideas?

S.

p.s. I was kinda hoping it would be the MAS again as at least that would be covered by warranty, oh well.

p.p.s Big thanks to Mark, one of the mechanics at the dealership who was a great help. He actually let me into the service area to look at the STAR printout and also explain the full nature of the problem.

He was certainly of more help than the suit who asked me to leave the service area because I wasn't covered under insurance and who claimed all the parts people were too busy to look up a price for a 'pedal value sensor' so would get back to me with a phone call.

Talk about a total difference in attitude.

 

[GregE240]

Steve,

Glad they traced it!

<stupid question>

Was your original MAS sensor knackered? Has this been the problem all along? I'd be a bit pi$$ed if I'd shelled out £300 for nothing.

Sorry, two stupid questions actually!

ATB,

Greg

 

[Steve_Perry]

Originally posted by GregE240
Steve,

Glad they traced it!

<stupid question>

Was your original MAS sensor knackered? Has this been the problem all along? I'd be a bit pi$$ed if I'd shelled out £300 for nothing.

Sorry, two stupid questions actually!

ATB,

Greg

The original MAS was defo knackered. Whilst this problem means the car is slow to take off and revs to say 3,000rpm, the knackered MAS left the car unable to run at all, the best she could manage was 10mph tops in about 5 to 10 seconds and wouldn't rev much beyond 1,000rpm.

Similar faults but subtly different, a bit like Graham having his MAS fail then having his Kompressor Clutch fail a few weeks later. Unlucky timing.

S.

 

[GregE240]

Steve,

If you do a Google Search for Pedal Value thingy (sorry, in teleconference), there are Tech bulletins relating to 1998 CLK models with these failing.

Dunno if the W202 is covered?

Greg

 

[Steve_Perry]

Originally posted by GregE240
Steve,

If you do a Google Search for Pedal Value thingy (sorry, in teleconference), there are Tech bulletins relating to 1998 CLK models with these failing.

Dunno if the W202 is covered?

Greg

Greg, thanks for the info... To be honest I dunno, I'm sure it's worth following up, I will try to phone MB MK tomorrow and see what, if any, recalls apply to my W202.

S.

 

[Steve_Perry]

Well after me chasing today, the dealer finally came back with prices... £204 for a pedal value sensor (essentially a variable resistor, nice money if you can get it ) Total price including fitting... £288.50 that includes the cost for the fault diagnosis on Wednesday. So do you reckon an hour's labour to take off a variable resistor and to replace? They claim they do a retest too

Hmm I might buy the part and get George to fit it either on the way to or way back from Germany.

We'll see how it goes. Again, I think this a rip-off when considering the nature of the actual part itself. I know what variable resistors are, I used to work with them on a regular basis, I'd love to know the justification for the price of this one.

Have yet to hear back from MB technical with regards to whether this part is covered by a product recall.

 

[Dieselman]

Try cleaning it with switch cleaner. It could be dirty or burnt on the tracks.

Worth a £3 can of cleaner.