Questions and Answers

Jetta Idle Speed Fluctuation Q. Hi Vin, Amazing site! I have a 1986 Jetta 170m, A/C, 5-speed no ABS. The idle speed has been fluctuating and getting worse day by day. Today it was so wild it stalled repeatedly. Also while idling the idle speed boost valves (2, 1 for the A/C) was clicking. I always wonder about the chicken or the egg.

Is it the I.S.B. valve or is something else causing it to click? I hope this makes some sense.

Again, Great site,
Mike

A. The idle speed boost valve is a solenoid valve controlled by an electronic control unit on the fuse/relay panel that turns the valve ON when the idle speed drops below 750 rpm, and OFF when the idle speed gets above 1050 rpm. The valve is always grounded, and is opened and closed by supplying and cutting voltage. A second boost valve is fitted to cars with A/C, and is switched ON whenever current is supplied to the compressor clutch.

Here is how you can test the valve(s). I am assuming this is a gas engine and not a diesel.

1. Disconnect the harness plug from the boost valve and check for voltage. Battery voltage should be available when the engine speed drops below 750 rpm. If voltage is correct, replace the valve.
2. If no voltage is available, check the signals to and from the control unit.
3. Using the terminal numbers on the back of the control unit on the fuse box as a guide, test the fuse box sockets.
   1. There should be no resistance between socket for terminal 31 and ground.
   2. Battery voltage should be available at socket for terminal 15 with the ignition ON.
   3. Battery voltage should be available at the socket for terminal 1 with the engine running.

4. Check the wiring between the fuse/relay box and the boost valve harness plug for breaks, or shorts, and repair as necessary.
5. If the valve (s), wiring, and circuit tests are all OK, the control unit is faulty and should be replaced.