I am consulting with a good local shop that has run out of time and ideas on this car. It is a 1991 300SL five-speed with appox. 78K miles. Generally good condition. The car came in with a variety of hard starting, poor acceleration, and other running problems, all of which have been fixed.

The following parts have been replaced:

- fuel filter and pump
- coolant temp sensor
- throttle position switch
- decel micro switch
- cold start valve
- idle air tubes
- plugs, wires, rotor, cap
- EHA
- air sensor potentiometer
- OVP relay

The remaining problems are that when the car is started hot or cold, it will idle at 1100 -1200 rpm until you press and release the throttle, at with point the idle drops to around 650 RPM. If you turn on the AC, the idle goes very low and if you turn the wheel, it almost dies.

When the fast idle occurs, the voltage to the idle control valve is around 7 V. When the idle drops, there is no voltage. You can unplug the connector and there is no difference in the idle.

The ECU was swapped for a known good unit. A functioning idle valve from another car was tried, but made no difference.

I have checked the verified the operation of the decel and throttle position switches at the ECU connector. I drove the car extensively with my meter connected to the EHA, and it is functioning perfectly.

So, any ideas why the ECU refuses to control the idle?

Thanks,

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe