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Old 12-15-2002, 03:09 PM
Posts: n/a
Idle speed too high


The idle speed of my 85 380SE is always too high.
It 's around 1500 RPM in park, 1000 RPM in drive and
the speed is always high hot and cold.

What might be the cause? I suspect it's the Idle Control Unit.
Where should I start to diagnose the problem?
I just cleaned the idle control valve.

I appreciate your help.

85 380SE
Austin, TX
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Old 06-11-2005, 04:09 PM
Registered User
Join Date: May 2005
Posts: 7

I am working on the same type of problem on my 84 380SL. I am sure they are similar engines. I cleaned my idle control valve but to no avail. I will keep you posted but look at my posting and the answere Rixk gave me.

Kevin Gallagher
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Old 06-11-2005, 04:33 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The early idle controls, such as in your car, uses an electronic controller to drive a valve closed. It is naturally open.

The first hint can be taken from what happens when the connector is removed. If the valve does nothing then its likely nothing is coming through the wires. The controller controls the valve by applying electricity to a magnet opposing the spring in the valve. It wants to vary the power to this magnet but voltage is rather hard to vary for control. The mechanism for control is called pulse width modulation. The way it works is a constant pulsing at a given frequency varies its duty cycle. Each cycle looks if drawn like a square wave. The power is on and then it is off. If the frequency were 1hz it would be on then off with a cycle of once per second. Such a square wave with a duty cycle of 50% would show 50% up and 50% down. if done at 1hz it would be 500ms on and 500ms off. If the duty cycle were 70% on, then the time would be 700ms on and 300ms off.

The frequency is fixed on this system and the control ranges from a 70% on to 70% off. So... if you are looking at a idle valve and the control signal is 70% on, then you would probably see 9.1v across the idle valve while running at 13v battery with a crude VOM that averages. With a scope you will see 70% with 13v between the wires and 30% with zero volts between the wires. Anyway a 70% on duty cycle is all there is. If the valve doesn't respond then the valve is the fault.

This form of control is why when one hits the valve with 12v and the car dies one criticizes the controller instead of the valve. 12v is a big difference from that 9v that is all it will ever see (when averaged).
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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