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To suppliment: The idle bypass valve on the pre 85.5's is heated with a 12 volts from the same circuit that powers the fuel pump. There is no DME control other than the closing of the stage two contacts in the DME relay.
The post 85.5's have a single sided servo design. The DME pulses the valve against a return spring. The width (duration time) of the pulse dictates the closing amount.
The 944S and possibly the 968 use a true servo valve where the valve is driven both ways by the DME and not spring loaded to stay open. So the voltage/current can be turned off to the valve if the possition is acceptable for up to temp running.
This is why you have to jumper two of the pins on the test connector in the engine compartment to set the idle on the post 85.5 cars.
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Hugh - So Cal 83 944 Driver Person
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