1995 E300D idle control: how does it work?
 

Hi,

Can somebody give me a run down on how idle control works on the OM606.910?

A couple of years ago I spent some money with a dealership to try a rectify a highish idle issue.

When car gets to approx 90-95C on the gauge my idle goes from about 600rpm to 850rpm and when the car temp drops back to 85C so do the revs, back to 600.

They never did find the cause and I needed to get the car back on the road. I can see where they did some fault finding:-

At the main wiring harness plug connection (behind the battery) they cut a wire out of the plug from both sides, essentially bypassing it from the plug all together; possibly to do some current testing on the circuit. It is still bypassed from the plug but soldered together with a hand written label saying 'idle circuit'.

I guess there are two questions I'm asking here:-
-A run down of how the idle works and
-What testing could I possibly do to hone in on the problem

As always, many thanks.

ahh my longest head scratcher with my car. I fully understand how it works now and where and how to troubleshoot it.

The idle speed is called ELR and takes inputs from

1 circuit of the 4 pin coolant sensor on the head and the rpm sensor on the bell housing - the regulation is done to the 2 wire servo on the back of the pump. It modulates the rack position for a very smooth idle under all loads and temperatures. It provides fault codes that can be read from pin 4 of the diagnosis socket with a flashing LED tester with a running engine, it only has about 6 fault codes.

It is vital that the idle speed be set correctly on the pump with the idle speed 2 wire clip disconnected from the pump, the idle speed should be about 580 rpm with no ELR activity, connecting it back should make the engine idle correct at about 650 rpm very smoothly. The ELR will then maintain this rpm at idle speed whether you shift in D, turn the AC on or pull the steering to full lock.

The idle speed should not raise itself (unless there is an updated ELR system by MB to help spin the water pump and fan faster at high temperature idle speed). My car does not do that.

The ground connections are shared common by the sensors and are buried in the wiring harness, this can lead to silly readings by a sensor due to another sensor shorting out

Now to my problem and remedy, I was getting codes for the coolant sensor and short circuit to the ELR actuator servo, and my engine was idling at 1000 rpm. The cause was swapped around ground connections on the ELR module plug. I found this out when testing the wire continuity - it was there alright, just in the wrong pins

Zulfiqar, thanks for your detailed reply.

Are you able to tell me more about the diagnostic socket; how to pull the LED flashes and what each code translates to?

Also, is it just a 3volt LED or do I need to buy one of those diagnostic readers that someone is building and selling on ebay?

The bottom of this page has info on building an LED tester that should work with your car - K6JRF Auto Page

Sixto
87 300D

Yea! Thanks for this post! My E300 has literally been falling apart this past week and today, well this after noon, I was backing up to my garage to go over my suspension nightmare or, at least look before class tonight and, my "pre-glow" warning buzzer started sounding only, really high and distorted (hurt my ears more than normal) and after it went off it jumped to 1100 RPM!!! Thank god I had my foot hovering and, able to slam them on before I went through the garage door!! No matter what I did (tap the peddal incase linkage jammed? though I wasn't even on the gas when this happened. shift into and out of a gear) it persistently stayed at the high rpm. Shut it down, restarted it and right back to 580rpm.???

Battery cable you say, ey? Connerm, I take it you've had this issue pop-up before?? What were your symptoms?

Sometimes running a new very large diameter wire from the alternator to the battery clears up weird electrical issues that are hard to track down. Also, running a new fat ground wire from the alt. to the body or frame can fix things. Replacing that wire on the 95 couldn't be easier. Just a thought.