1988 560Sl Idle Diagnosis
 

Attached are the checks I did on my 1988 560Sl idling fast. The engine was warmed up and checked for vacuum leaks of which there were none. All checks were done individually. Please give me your assessment of these tests.

I would have to say the idle switch is bad. At the idle switch connector, with the car at idle, either 1 to 2 or 2 to 3 should be 0v. I don know right off which it is, but it should be one or the other. The other will be 0v at WOT.

PaulM

I would address the switch first, test it as per the MB FSM specification.

Here are some documents you might find useful:
http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Engine/107/M117_56/073-112.pdf

http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Engine/107/M117_56/073-009.pdf

http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Engine/107/M117_56/073-152.pdf

If itis OK then my next guess would be the IACV...When you unplugged the IACV idle increased. That is expected. It should run around 1500 with the IACV disconnected.

Your IACV might be sticking or dirty. That might be preventing it from properly opening/closing to control the idle.

Another item might be vacuum leaks. Vacuum leaks can cause high idling as well..

In our 420SEL's case it was a sticky idle valve. When I took the IACV out of the car and powered it with my bench top power supply. I could vary the voltage/current and observe the valve getting stuck part way though its full range swing.

A replacement valve rectified the problem.

tomeads:

Reading voltage at the throttle switch is misleading; the switch grounds either the idle controller (ICU) at closed throttle (Pin 1), or the ignition controller (EZL) at WOT (Pin 3). Pin 2 is the ground.

Your test should be to look for continuity to ground from Pin 4 of the ICU socket with the throttle closed. (Remove ICU from socket)

Since you did not mention idle speed in RPMs, what is your view of "fast"?
And, does the idle speed change between Neutral and Drive?

Thanks for your input.

Answers to your questions:
Fast=1800RPM
Idle speed lowers when shifting to drive

Please help me with these items:
Is ICU same as Idle Control Valve(in front of intake manifold)
Where is pin 4
Where is Ignition Controller (EZL)

The ICU is what you have termed a "relay"; it is much more than a simple relay.

Pin 4 of the ICU socket is in the ICU socket!

Locate the ignition coil, and follow the low voltage wires back to the EZL.
It is a box with two 4-pin plugs, a coax plug from the crank pickup, and a vacuum hose connection.

1800 RPM is indeed too high; specification is for 600 +/- 50 in N, 550 in D

More tests on Idle Control Valve
 

Performed a voltage test on the Idle Control Valve(on top front of engine) and with the key on the voltage to the unattached plug for the valve ranged from 3.64 to .25.

I attached a 6 volt battery to the valve with the plug removed and the car not running and could hear the solenoid moving.

I attached a 6 volt battery to the valve with the plug removed and the car running and the idle improved greatly.

Is the Overload Protector Relay(inside passenger kick panel) not supplying enough voltage to the valve?

I also OHMed the Throtle Position Sensor(attached on side of throttle body) and did not get any reading.

The OVP does not provide power directly to the valve. The valve is controlled by the idle control unit located at the passenger's feet behind a panel.

This control unit takes in a variety of different sensor data and compute the proper valve position for optimum idle.

If the valve is sticky it may not be able to set the proper idle.

This was the case on our 420. I removed the valve, applied power and could SEE that, as I varied the voltage/current, the valve would open/close but it would stick somewhere in the middle.

I replaced it with a good used one that moved freely and the idle has been good ever since.