W126 idle speed control unit
 

I check the idle control unit which was located under the dash board today because the car was still idling around 900rpm after I replaced the idle speed actuator.
To my surprise, I noticed that the control unit had not been connected all this time! The plug was dangling near the box but not connected.
I connect the cable back onto the control unit, but the idle speed hasn't changed a bit.
I was shock to realize that the car can still run with the unit not connected!
Did the previous owner by-pass the system . Is this possible?
The car is very heavy on gas and idle rough.
Please help me with some advise.
I appreciate if Steve can also help me in this matter.:confused:

The early idle control systems would go wide open when unpowered. You should get at least 1500rpms disconnected maybe over 2000.

A cheap fix that I have seen more than once is to cram a broom handle into the idle air hose and drill a 4mm hole through it. This would give a slightly high idle, warm, but a low idle cold (as the same volume of airflow produces different speed at different temps). BTW, broom handles are only one off many types of restrictions people use to shut down the airflow when the electronics fail.

Idle speed
 

Steve's right;the default idle speed on these units is 1500.Based on your stated speed of 900,someone may have already installed a restriction,because the car should be idling around 600 RPM.

Peter,
Auto Zentral Ltd.

thank you Steve for your reply! thank you Peter also!
where can I locate the restriction (broom handle) if it is put in?
where precisely is the idle hose.
is it easy to reverse it back.
will I damage the hose or idle speed actuator?
should I replace the control unit before trying to do the reverse modification?
pls bear with me agian.
thank you

Remove the idle valve. It is at the front, top of motor, barely protruding from an installed aircleaner. Remove the aircleaner and the valve. Look in the hoses both directions and into both ends of the valve. You should be able to see the slide from one direction. The restriction will close off everything except a whole smaller than a quarter inch diameter (a quarter inch would probably run 2000rpm, an eigth inch hole will run too slow at about 500rpm.) The restriction will undoubtably be here as its so easy to get to.

thanks again Steve!
manage to locate the plastic restriction in the the valve (top one).
I will try to adjust the restrictor opening smaller to bring the rpm down to about 650 untill I replace the control unit later.
I hope this is a sane thing to do!
Thank you!

unfortunately every change will involve a compromise. Lowering your warm idle to 650 will probably cause it to stall when cold as the cold idle will be under 400rpm.

I would remove the restriction and plug in the ECU and see what happens. If you get the uncontrolled max idle then remove the ECU and have an electronics tech resolder the PC board. Try again. If it don't work get back with us and there are instructions for diagnosing the electronic control.

That valve should be made out of metal and have an electrical connection. I had a plastic manual unit installed temporarily once that really was a pain to deal with. The idle speed was set to run about 550 with air on and in gear. Take it out of gear and turn off the air and it was more like 900-1000. Not a whole lot of fun to put into reverse at that speed.