Random electrical failure; car not starting

[Thud]

My 1990 325iC (126,000 mi) has started randomly deciding not to start. What happens is that sometimes (becoming more frequent) - but not always - I'll turn the key to the 2nd (pre-ignition) position and ALL electrical power will completely cease, and the car - obviously - won't start. The only way I've found to work around this issue is to disconnect the battery, wait a few minutes, reconnect the battery, and try it again (sometimes takes 3 or 4 repetitions of this procedure). It happened once when I stalled it in reverse too, but otherwise never happens once the car is started.

When I bought the car, the "original" (I assume) stereo (which had newer security features incl. code, blinking light, beep) was faulty, so they replaced it with an older (1 or 2 yrs) model which had older security features (incl. code); I'm wondering if this adversely affected the "car-side" security features of the original stereo, and is somehow resulting in faulty activation of a kill switch. Any other ideas?

[jase007]

I would suggest getting a new main relay and a fuel pump relay as well as a few spare (orange) relays and replace the main relay. Put the other relays in the glove box for insurance.

The main relay is uasually the culprit on a no start. Make sure you have juice to the engine bay as well (positive post on firewall under plastic cover on passenger side). Also, if you have juice to the main relay (back-probe the thickest wire going to the female side of the plug) then you know the relay is bad (with key in position #2).

You can locate the main realy from your owner's manual (I think). If you don't have the Bentley manual BUY IT NOW! from our very nice board host. You should also have a multimeter (or at the least a test light). I keep a test light, spare relays and fuses in every vehicle (except track ones ) that I own. Cheap insurance and very helpful for diagnostics.

Sorry for the shouting.

Jason

[Thud]

That's an idea. I'll try that.
Have Bentley's (wouldn't even think of NOT having it), as well as Chilton's and Haynes' (both pale next to Bentley manual).
Also have a multimeter; good time to put it to use.
Thanks for the reply Jason.

[Jared at Pelican Parts]

I agree here, sounds like it may be the main relay. I replaced mine after similar problems, had no problems.