Idle dipping and rising when headlights are on - not Idle Air Control Valve

Question

On my 1993 VW Mk1 Cabriolet I've been trying to find an issue that causes the idle to rise and dip (to near stalling levels) when power is being drawn (in this case, when the headlights are on - there's no air con).

At first I thought this would be the Idle Stabiliser Solenoid/Valve and so replaced it, however the problem persists. The solenoid is getting power as I can feel it clicking when I rest my fingers against it and turn the lights on, Also, the engine idles stably but very low if I unplug it.

There appears to be no leaks in the vacuum hoses.

It's almost like the stabiliser solenoid increases the idle too much and so clicks off, only for the idle to drop too low and it to click on again, and so on and so on.

Would anyone know what else I should be looking for here?

The engine is a JH - 1.8lt injected (same as Mk1 GTi)

Answer 1

Check that your alternator belt is tight and in good condition. Also check that your battery is fully charged and in good health. There is a real possibility that the fault is being cause by the headlight switch, wiring or bulbs. Do the revs dip when you switch on the rear heated screen?

I personally had a headlight loom catch fire on my old Mk1 Cabby GTI. Remember that this car has no headlight relays so the switch and loom is providing all the juice for the lights. Check all the earthing points, the condition of the switch (in my old 924 - same switch - some of the plastic melted when one of the terminals was all corroded and green). When checking the condition of the loom, also don't forget to check the registration plate illumination lamps.

A common upgrade on the Mk1 is to build a relayed loom for the lights which makes them significantly brighter and places far less strain on the switch.