460 Dangerous idle motor fault

[Binman]

Volvo 460 1.8Si 18BU 1995, 70,000m

My 1995 460 18BU idle motor drives the throttle wide open when the engine is switched OFF and now also when the engine is running.

It won't return to normal idle on it's own, I have to apply +/- 12v to drive the motor. The fault started random - now its permanent.

As you can imagine this is a little dangerous. I've checked/ cleaned the motor/switch and it's fine.

As a temporary fix I've removed the 4 pin connector and set an idle of 1000rpm when hot, this will be a problem cold starting in winter.

I've posted No #605 without response, see that for more detail on the tests I've done - any help would be appreciated.

Is this an ECU problem or something simple?

[3binman]

Fault appears to be connection of white wire to diode in a 2 pin connector about 5cm from the ECU on main loom to the ECU, behind panel at the driver side footwell.

Fault not obvious unless wires were tugged because there was heat shrink over the diode at the connector. A nasty piece of design because free floating solid core wires such as on the diode will fail with vibration or movement.

This fault allowed the idle control motor to advance the throttle and it never allowed it to return.

Wire was re-soldered to the diode in the connector. Idle control returned to normal.

[mgarageu]

did your car display any fault codes?

[PAZ3154]

the piece of cable with a diode attatched is actually a modification we used to fit to cure the idle problem you have experienced. obviously yours must have come loose or possibly not fitted correctly in the first place. i've never seen one come loose before.