Ah, the joys of excess computing power.....

The cruise control circuitry is grounded through the stop lamp filaments. When the lamps are on, 12V (nominal) is applied to the ground of the control circuit and since there is no longer a voltage differential, it switches off.

Very slick, if you ask me, and nearly failproof as the CC won't work if the stop lamps aren't good. Only problem is occasional problems from bulbs with too high a resistance in the filament causing erratic operation of the CC.

blowing bulbs means either the bulbs aren't strong enough or you have an overvoltage problem -- I've never seen an automotive lamp fail in normal 13-14V service when the correct type was used. Makes me think the specification is wrong for the lamps, this isn't rocket science!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!