I just bought this car today and it's in pretty tip top shape with only 150k on it. So far this is the only problem that I've noticed. The previous owner is a mechanic and added to modifications to this car. One being a secret switch that turns the fuel pump on and off to prevent theft, the other being a switch that kills the battery completely in the event that the car isn't being used for an extended period of time. Any ideas?

 

Okay.... Is the battery being charged? Losing spark, fuel, ?



Modifications do not at all sound relevant as the car wouldn't even start if they failed.

 

that battery kill switch sounds potentially suspicious.

where exactly is that installed? the battery to starter (or battery to ground) connection has to handle 100s of amps.

I wonder if its not a ground related issue. the headlights are grounded to a spot near them on the inside of the fenders, and they get their power off a 'step' relay (which switches between high and low beams), but the low beam power is then routed back to the dashboard, through the bulb-failure-sensor, and back out to the lights.

 

My 740 would die every time I switched the high beams on or off. It was a bad headlamp ground.

 

Thanks for the replies everyone! I realized yesterday that high beams were on by default. The car runs fine with the low beams but the car stalls as soon as I turn on the high beams. Did you trace the cables that plug into the bulbs to a faulty ground?

 

hmm, looking at the greenbook for my 1992 740, no step relay, the dimmer stalk switches the high/low directly.

so the highbeam specific circuit is a blue-whote wire from the dimmer switch to the fuse panel, goes to fuse 17 and 18. fuse 17 goes to a blue-white wire to the left high beam, and fuse 18 goes via a red-white wire to the right high beam. the left headlight is on pin 2 of firewall connector "B" in the left A-post, and the right high beam is on pin 4 of connector "A" on the right A-post.

both headlights are grounded to the inside of the fender nearest them.

 

My issue was due to bad grounds terminating at the fender. I went through and cleaned them all up (unscrewed it, wire brushed the connector and where it mates to the fender).

A few had corroded wires near the connector. I removed those, stripped wire until I had clean copper, and then re-attached/replaced the connector. Been working fine for about three years now.