83 500SL Idle Speed Puzzler

[CaptnAl]

A long-standing problem with my Euro 500SL is the idle speed, which starts out fairly fast (1000 rpm) when cold, steps down to an appropriate speed after warmup, but about 20 minutes into a drive, the idle speed drops too low (about 450 rpm). The car can stall at a stop sign, even if I've shifted into neutral. The puzzling aspect comes from what I've replaced, with no change in these symptoms. New idle control valve, new electronic idle control unit, and confirmed the contact with coolant temp sender at the thermostat. The idle speed does respond to gear shift position and A/C being on.

Thoughts or experiences that might help??

[txstinger]

I had the same problem – However, mine started with a high idle & after fixing the Idle Speed Controller, it became a low idle problem.

Your controller seems to respond to all inputs (temp sensors & impulses).

What I did was adjust the fuel mixture screw on the CIS until the voltage at the Idle Speed Adjust Valve was at 5 +/- 1 volts DC. Then I fine-tuned the Controller by: Using a voltmeter capable of measuring amps.

• Start engine and obtain operating temp
• A/C off and in Park
• Stop engine and ignition switch in Off
• Remove 2-wire connector at Idle Speed Adjuster and pull 1 wire out of connector
• Re-connect the connector minus this 1 wire
• Place meter black lead on free wire & red lead into empty connector slot – completing circuit through the meter – set meter to read 1000+ milliamps (1 amp)
• Start engine and obtain operating temp – if idle goes above 900 rpm, you have a loose connection at meter leads
• A/C off and in Park
• Adjust the CIS mixture until amp reading at Adjuster valve is 1050-1200 milliamps - RPM should be about 650
• Done

Double check operation of idle at PARK – IN Gear – A/C ON

[Steve Gutman]

It's on the driver side inside the front fender and attaches to the coil. If it feels really hot, it may be shot. Mine was dying as well.