Join Date: Mar 2002
Location: Austin, TX, USA
Re: Headlights & brake applied engine dies
Are you saying that the voltage at the battery and alternator is 14 volts, but at other places it falls to 12 volts and below? If that is the case, somewhere you have a bad connection eating up 2 volts. If this sounds right, read on.
The bad connection can either be in a power or ground circuit somewhere. Power circuit connections like the fuse panel, firewall bulkhead connector, or ignition switch. There may also be a bad connection in the ground circuit - like the fender connection to the negative battery cable.
Figure out a way to extend the leads on a digital volt meter. I like to use a ong wire with alligator leads on them. In a pinch, a jumper cable will work - it's way too big, but handy and makes a good connection to the battery. Extend the negative lead of the meter and connect the other end to the positive post of the battery (be careful to not let it touch ground). You can then begin measuring the voltage at several different power points of the system. Good starting points would be IGN fuse (both sides), ignition switch, ignition feed to the distributor, and horn relay. You wouldn't need the engine to be on for this, but you would need the IGN to be live. Take your readings with and without the lights on since they are affecting your problem. Your reading should be very near 0 volts, though you may see a couple 10ths of a volt depending on the place in the circuit you test. From what you describe, it sounds like 2 volts or more is getting lost somewhere.
If the hot side doesn't reveal a problem, check the ground. Just move the jumper cable to the negative post of the battery. You can leave the meter hooked up as is, though you'll read negative voltage, or you can reverse the meter leads. Now check all the ground points in the system, like the fender, dash panel, engine block, etc. Again, you should see near 0 volts.
If during either test you see a large voltage drop (like 0.5 or more I'd say), there is a bad connection somewhere in the circuit between your meter leads. A schematic is a good tool to help you navigate your way through the various circuits.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI