Checking the removed 1st chamber throttle servo (BMW 5 Series E12)

Removal the servo drive

Remove the air filter.

Disconnect the servo connector.

Disconnect the vacuum and atmospheric valve hoses from the servo drive.

Disconnect the idle speed switch connector.

Remove the servo from the carburetor by unscrewing the three mounting nuts (to make this operation easier, you can first remove the carburetor from the engine).

Checking for leaks

Apply 12V to the servo connector pin "2" and connect the connector pin "3" to ground; at the same time, the contacts of the vacuum valve open.

Connect the tester (in ohmmeter mode) to plugs "4" and "7" of the servo drive connector.

Attach the hand vacuum pump hose to the "A" port of the servo and create a vacuum (the servo pusher is pushed in), at which the tester will show a resistance of 500-700 Ohms.

Disconnect the ground wire from the servo connector plug "3"; at the same time, the contacts of the vacuum valve close.

Disconnect the vacuum pump hose from the servo motor port "A". If the resistance value increases by more than 200 ohms within 1 minute, replace the servo motor.

Checking the stroke length and retraction duration of the servo pusher

Using a hand vacuum pump, apply a constant vacuum of approximately 250 mbar to port "A" of the servo drive.



Apply 12V to the servo connector plug "2" and connect the servo connector plug "3" to ground. The servo pusher should move in quickly (no more than 1 sec).

Disconnect the electrical wires and vacuum pump hose from the servo drive.

Measure the distance (see photo) between the end of the servo housing and the end of the idle switch (in a free position), which should be within 56.8-59.2 mm.



If the duration or length of the pushrod retraction stroke is not within specification, replace the servo.

Apply 12 V to the servo drive connector plug "2" and connect the connector plug "8" to ground. The servo drive pusher should extend for no more than 1 sec.

Measure the distance (see photo) between the end of the servo housing and the end of the idle switch (in a free position), which should be within 69.5-71.5 mm.

If the pushrod extension stroke duration or length is not within specification, replace the actuator.

Checking the idle switch

Connect the tester (in ohmmeter mode) to the plugs of the idle switch connector and measure the resistance, which when the switch is closed should be 9 Ohm, when the switch is opened - R = ∞.

Checking the vacuum and atmospheric valves

Connect the tester (in ohmmeter mode) to the plugs "2" and "3" of the servo drive connector and measure the resistance of the vacuum valve winding. Then connect the tester to the plugs "2" and "8" of the servo drive connector and measure the resistance of the atmospheric valve winding. In both cases, the resistance value should be within 35-80 Ohms.

Potentiometer check

Connect the tester (in ohmmeter mode) to the plugs "4" and "5" of the servo connector and check the total resistance of the potentiometer, which should be within 1.3-2.5 kOhm.

Connect the tester to terminals "4" and "7" of the servo connector and measure the resistance with the servo pusher extended, which should be within 0.9-1.9 kOhm.



Slowly push the servo pusher in and measure the potentiometer resistance, which should be between 0-500 ohms.

If the resistance values are not within the specified limits, replace the servo.

Installing the servo drive

The servo drive is installed in the reverse order of removal. If it is necessary to adjust the servo drive, replace the idle stop screw.