potentiomer replacement question on 88 300 sel - PeachParts Mercedes-Benz Forum

ericzacha · #1 10-05-2006, 08:25 PM

Hey all,
My CEL came on about 4 months ago and the California diagnostic code was 4, which is the air flow meter (sometimes called mass air meter, air flow sensor among others). My car has a flutter at idle and occasionaly has a surging idle (rpm from 600 to 10,000) but has otherwise been running well. I read quite a bit on this site about the issue and even got the local indies opinion. It seemed that most people were in agreement that the thing that goes bad on the meter was the potentiomer. Some were even replacing the pot before the CEL light came on in an effort to get rid of the flutter and surging. I gave it a while, but the surging seemed to be getting worse so. . .
I bought a potentiometer and decided to replace it today. It seemed like a simple enough procedure. I marked the position, took off the pot, replaced it with the new one and tested it with a multimeter. I moved the pot up and down until my reading with the DC voltmeter on pins #1 and #2 was between .65 and .75. It was swinging between those two readings and as it did so it surged. For those who have replaced it themselves, is it supposed to vary in the voltage or were you getting a solid number that wasn't really changing much? The surging and the flutter that I can feel at idle haven't changed. I reset the computer. We'll see whether the CEL comes back on.
So, all in all, kind of a disappointing day. I was hoping to take care of the CEL and get a silky smooth idle all in one shot. If the CEL comes back on, i'm not sure if I should then replace the Air Mass Meter in it's entirety or not. Is there anything else that can cause a #4 diagnostic code on a California car? Any thoughts, suggestions or criticisms would be appreciated.

wbain5280 · #2 10-05-2006, 11:39 PM

Check out this writeup:

http://www.sg-motorsports.com/AFS%20Pot%20write-up.htm

ericzacha · #3 10-06-2006, 12:14 AM

Yeah, I actually used that article as I did the installation. Good article, I think everything went fairly smoothly. I just went out and took her for another drive. I think she might have a little more power at low speed now though it could be my imagination. I figure one of four things happened.

1. I somehow got a bad potentiomer, or installed it wrong. I find this unlikely as I got 0.7 volts without too much trouble.

2. I put the pot in correctly but another part of the air mass meter is bad. If this is true then the CEL will come back on shortly.

3. The CEL light was coming on for a different reason and the pot was never bad. If the CEL comes back on, I'm not sure how I will differentiate between problem #2 and #3. I guess it just depends on how specific the California diagnostic codes are. Error code four is supposed to be the air mass meter, but maybe there are other things in that circuit that I am not aware of.

4. I fixed the pot, which was causing the CEL to come on, but the flutter/surge problem is being caused by something else. If this is the case the CEL will not return, but the flutter/surge will remain. When dealing with 18 year old cars, Occam's razor doesn't apply anymore. Everything can be broken.

Anyway, I'm feeling better now. I learned a few things about my car, and it's not running any worse than when I started. With my skill level that is something. If anyone has any other thoughts I'd love to hear them. The 300 SEL is my daily driver so it won't take long to find out if the CEL comes back, I'll keep you posted.

dpetryk · #4 10-07-2006, 07:51 AM

Have you checked the oxygen sensor?