musicaljoe

Hi again everyone, hope you’re all good. I hope someone can help figure out a really annoying problem I’ve got with my SL55.

so.... in the process of repairing one fault on my car, another one suddenly started happening. We (that’s me and my very knowledgeable AMG specialist mechanic!) don’t *think* we caused the fault by e.g. knocking against a connector... but aren’t so arrogant as to *totally* discount that, so best in mind thatvatvthe tone, we were working in the area of the alternator.

So, what’s the fault? Well, the car is always revving higher than it should. So, despite the idle speed being set at 550, it idles closer to 900rpm.

its most apparent when slowing down, because it is somehow “confused” as to what rpm it is doing, a downshift often comes with a “jolt”, because the revs are higher than the car *thinks* they are when it releases the clutch, if you see what I mean.

We have *almost completely* ruled out an air leak. Yes, I know what you’re going to say, but we have replaced, checked, or reseated absolutely every part, hose and connection that might have a leak, and testing various things (e.g. spraying in various locations to see if rpm changes) says “no air leak”.

The suspicion is that it’s something electronic.

now the weird thing is that at idle, the speed sensor 2 is showing the regs that I *should* be at, but actually the car is revving higher, which the OTHER sensors show correctly.

”Ah, faulty speed sensor” you say. “Replace the transmission plate”

yep, did that. *No change*

so....

How could both the OLD sensor AND the NEW one on the new plate BOTH be showing the wrong rpm speed, but actually show what the car SHOULD be at, whereas the other sensors are showing the CORRECT rpm, but that rpm is too high... in the example below, I should be idling at 550, but the car is actually at 900rpm (obvious from the pitch of the noise btw - Indo know what 550 and 900rpm sound like!)

does anyone have a clue as to what’s going on? And where we start checking or replacing things? My mechanic, very trustworthy and knowledgeable, is scratching his head!

any help gratefully appreciated...

cheers
Joe

bobterry99

I would look at values associated with the accelerator pedal. I'd be particularly interested in knowing if the engine management controller recognized that the throttle was in its idle position. I'd also look at the potentiometer voltages and see if they are in spec, and I'd look to see if the pedal stop position was learned.

musicaljoe

Thank you. As it goes, I did replace the pedal (was one of the first things I ever did to the car), but obviously that doesn’t account for (eg) the connection from pedal to throttle/ecu or stop position. I don’t suppose you know (or can link to) what the voltages should be do you?

bobterry99

I can't be certain I would have the correct voltages without knowing your VIN. At any rate, the primary status I would be interested in is whether or not the engine controller is running its idle routine. This is a software status, I believe, and there may be contributing factors beyond the position of the accelerator pedal. You would find it in the "Actual Values" for the engine controller.