Poor accelration - Page 4 - PeachParts Mercedes-Benz Forum

lsmalley · #46 09-21-2014, 01:40 AM

Quote:

Originally Posted by ps2cho

Your idle is probably high because the potentiometer is not aligned correctly. It has a huge influence on idle speed.

potentiometer is only able to fit on the throttle body one way. The potentiometer is notched and it only rotates about 120°

lsmalley · #47 09-21-2014, 04:23 PM

Quote:

Originally Posted by ps2cho

Your idle is probably high because the potentiometer is not aligned correctly. It has a huge influence on idle speed.

So not wanting to leave anything to chance I did take your advice, but instead of removing everything I unplugged the potentiometer and connected one I had on the shelf and started my car,...idle was of course high, but I took a flat head screw driver and turned the meter towards the closed position and idle dropped down to a nice 650 rpm. I reconnected it back to the one on the car and idle was high. I performed this test several times by manually moving the meter with the flat head getting the idle to rev and tho drop then I decided to take out everything and check the positioning of the potentiometer and what I found was that everything was correctly installed....BUT, there are 2 screws holding the potentiometer to the throttle body and if you loosen them up it provides enough wiggle room for the potentiometer to be misaligned!

So I loosened them up and pushed the potentiometer up to where the screws would hold it a bit higher up on the throttle body and tightened them. Buttoned everything back up and started the car and waited for the idle to stabilize and I got a beautiful 650-700 rpm idle! Thank you PS2CHO!

liquiddog · #48 09-21-2014, 07:06 PM

Very good info

400Eric · #49 09-24-2014, 12:45 PM

Good deal! Closure! So few threads have closure. They're just left hanging, unresolved, for eternity.

Regards,
Eric

ps2cho · #50 09-25-2014, 12:20 AM

You have to adjust the potentiometer using a voltmeter so don't call it wrapped up yet -- finish properly using the info below:

Here's some more details I wrote about it a while back:

Quote:

The airflow potentiometer is a very simple, early version of a MAS. Do not confuse it with a MAS though. The 103 engine does NOT have a MAS. This part is not sold seperately so after diagnosis (mentioned below) you will have to find a used one attached to the airflow body of another unit. The main function of the airflow potentiometer is to increase fuel delivery upon load and it also affects engine idle speed if it is out of adjustment by more than .1v.

The test to determine if it is functioning or not is to probe the connection and you want to be looking for 0.70v +- 0.1v. To get this reading after replacement, the 4 screw holes are deliberately positioned and enlarged to allow for adjustment.
You can purchase the correct backprobe tool from many auto or electrical stores (Klischmann). The problem I have noted with this method is contact between pins which will trip the CEL and give you a false reading of 80v which makes no sense. In this instance, I find it easier to remove the connector backing to reveal the pins on the connector then you can simply probe them without use of any tools other than a DMM.

Replacement requires you undo the wiring connector bolts that hold it down to give a little more space, and undo (17mm) the main fuel return line to the fuel pressure regulator. ADJUSTMENT IS VERY, VERY FINE. I'm talking millimeter adjustments.

lsmalley · #51 09-25-2014, 01:41 AM

Thanks for the helpful info! Actually I was referring to the 2 screws that hold the potentiometer to the throttle body. Sorta like if you put on a hinged cabinet door slightly off the door will not shut properly. So the way I put on the potentiometer back on the throttle body it was slightly to the side that caused the potentiometer to remain slightly open even with the butterfly plate at full rest and so it caused the high idle. The actual internal workings of the potentiometer are fine because everything is back to normal.

ps2cho · #52 09-25-2014, 11:18 AM

Yes I know thats how you do the adjustment. You need to check and make sure the voltage is reading .7v +- .1v
Otherwise you are just wasting fuel if its off since it holds the throttle open or closed as you have experienced.

If by some stroke of luck you just happened to reinstall it within sheer millimeters of the correct position and its reading .7v +- .1v then that definitely would have been your day. Its incredibly difficult to get it right since even as you do the voltage check and move it, as you start tightening down the screws...even that is enough to throw it out of the spot and you gotta do it again.

liquiddog · #53 09-25-2014, 06:39 PM

The way I read it he was referring to the one on the throttle body. Not the one the airflow meter that slides.

lsmalley · #54 09-27-2014, 06:13 AM

Yes, I think I mixed up the name/terminology of the parts. I'm referring to post number 2015404245, not the other one with the plastic cover and the adjustable screws. Sorry for the confusion.

ps2cho · #55 09-29-2014, 11:18 PM

The best way to center the throttle plate is to cut up a few pieces of paper and put them in all 4 corners. It seems to center it perfectly. Just a tip.

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