Schaefft

Hi everyone, first time in a long time I'm signing up in an actual forum but I am getting a little desperate with the issues I currently have with my R129, hopefully there are still knowledgable people around to help!

I bought an early RHD '99 SL320 with the M112 last month, the vehicle had a non start issue which has since been resolved (ignition system fuse was pulled, only noticed after replacing the crank position sensor...).

Since then the car had major trouble starting up and actually staying running. After replacing the MAF which has naturally not solved the issue I have now discovered that the throttle body is not behaving as you would expect: Upon turning the key to the ON position the throttle body is wide open (80 degrees+). Its not stuck as it opens and closes perfectly with the key on/off. It actually actively tries to resist closing when manually closing it, its just humming along constantly and is not reacting to any throttle position reset procedure.

Obviously thats a big problem when starting the car as the air/fuel ration is completely off at start, rpm surging, immedidate triggering limp home mode. There are no codes stored in the ECU other than a current (and historic) P0507 - Idle Speed Control Implausible.

I've just installed a replacement throttle body and it has exactly the same behavior. I thought the throttle pedal position sensor could be at fault but I have no codes stored for it. Is there a way to measure its output? Its impossible to track the throttle pedal position in my scanner with the engine running before its going into limp mode. Other codes are related to steering angle sensor and such and shouldn't be related. The battery supplies decent voltage, alternator output is good after it was replaced.

It would be great to get some opinions on this. Since the M112 is used in pretty much all models at the time Im hoping for someone to have encountered this issue before. Thanks!

mahunt

Other than resetting the ECU by interrupting the battery, does your car use the same "relearn" procedure as the mid-year cars?

Something like this:

1.Turn ignition to 2nd position, but don't start car. Press and hold accelerator pedal to the floor for 10 seconds.
2. With pedal still pressed, turn key to OFF position, leave key in ignition.
3. Release pedal.
4. Wait 2 minutes before removing key.
5. Start and drive car.

I just received a "free" throttle body from a forum member that was suspected as bad. My V12 came to me with 2 different throttle bodies, one probably original from 1996 (when the car was manufactured) and one from 1999. I liked the 99 throttle body better as is was identical except it had much smaller throttle blade retainer screws (better flow).

I swapped out the 99 throttle body for the suspect free one and initially had some difficulty getting it to sync. I tried turning the key to the second position and listened for the throttle bodies to go through their adjustment (didn't help). Everything sync'd up when I did the reset procedure through. Now I have 2 matching throttle bodies and one good spare.

Schaefft

It is indeed supposed to be the procedure, although some people say that just turning on the ignition and letting it sit there for 2-3 minutes is supposed to solve the issue. Maybe I need to try 5 times before it works once but having done the above procedure and just letting it sit multiple times now, I dont think this is the issue. I dont think that a un-synchronized throttle body would behave like mine either, would it? You'd expect it to be in the closed position by default.

The battery had been disconnected over night multiple times by now as well, unfortunately doesnt make a difference.

Schaefft

One piece of info that could be useful: The throttle body does have the same behavior with the throttle pedal position switch unplugged. Plugged back in, I cannot read any acceleration values, nor does my scanner recognize that the kicktown switch is pressed. Please let me know if this is normal when the engine isnt running. I dont think it is.

mahunt

The throttle body I received free was off just a bit which resulted in a somewhat erratic idle speed gain of about 500 RPM.

Like I said, I tried the "leave the ignition on" sync procedure, but it didn't help. Only the full reset ECU procedure worked for me.

If your throttle body starts out all but fully open, that is indeed a different matter. Maybe your kickdown switch is stuck on (logically or physically).

Schaefft

Well, it does stay fully open, which is the issue as this can't be as intended. The kickdown switch is in it's unpressed position, at least the scanner tells me that it is not recognized as pressed (on) either. However, that's the case whether I press it or not. My question is whether that's normal with the engine off.

mahunt

Engine off and throttle blade 80% open is NOT normal.

Both (actually, all three now) of my throttle bodies are open just a bit from fully closed while the engine is off or even taken out of the engine.

About the only thing you could check physically is whether or not some of the connector pins are askew. I'd pull the somewhat oval connector plug and take a peek in the connection socket and look for shiny straight pins.

Schaefft

Already did, they all look good on both throttle bodies. I wonder if there might be a short in the throttle body wiring.

Schaefft

Quick update: I've opened up the ECU and noticed that one of the resistors blew up, maybe from someone jump starting the car before. I've had the ECU cloned and threw the new one in the car today. Still exactly the same issues of the throttle body being wide open with key ON, still going into limp mode straight away. Opened up the ECU again to see if there was anything wrong but nothing obvious.

I'm honestly close to admitting defeat. Unless there is a wiring issue I really cannot tell what else would have an effect on what the throttle body does. The throttle body is new, the ECU is new and it all behaves identically with the throttle position sensor disconnected so that cannot be it either. I am still only getting a idle speed control implausible fault code which does not tell me where the issue is.

mahunt

You don't indicate where you are in the world.

When repair of these complex machines goes beyond my capability, I go to the experts. I don't necessarily mean the dealership, instead independent MB techs that have their own repair shops. The folks that used to repair our cars at the dealerships now have their own businesses.

It's time to find one in your area.

Schaefft

I got one 10mins from here, however Im reluctant to have my car towed there as long as there is a chance I can diagnose the issue myself without losing a massive amount of money. These cars aren't particularly complex, especially compared to anything modern. There is only a handful of reasons that could cause this behavior although diagnosing each is becoming harder and harder as the potential error sources get smaller. Any specialist would have run through the same process of narrowing down the problem, the only advantage that they have is that they could have encountered exactly the same issues before, speeding up the process. I do admit that I should ask them whether they have experienced the problems I am witnessing though.