I've had my truck for about 5 years now, and all of a sudden it has developed an electrical problem which makes it almost impossible to drive at night.

The main problem is when I turn on the headlights (actually the problem starts with just the parking lights on, as well) the voltage goes all loopy. At idle, barely enough power is produced to keep the headlights illuminated (they are a yellow brown color, very dim, you can't even see the instrument lights.) However, when the engine is revved up, more than 15 volts are produced when at cruise rpm's. Furthermore, the lights flare noticably while the engine RPM stays steady. This problem has remained constant through two alternators, two known good batteries, and three voltage regulators (all three tried with both alternators.) A side effect of this is the engine power changes dramatically with the lights on or off. When off, the engine accelerates fine, sounds fine. However, when the lights are on, it accelerates more slowly, and the exhaust sounds more 'blatty.' If you switch the lights on and off while accelerating, you can feel the power surge in the seat of your pants every time the lights are switched off, and you can feel the power drag when you turn the lights on, and you can hear the difference in the exhaust note.

My current thought are: bad headlight or ignition switch? Poor ground? Wrong wire connection somewhere? Any input appreciated.

 

check the head-light dimmer switch on the floor they rot out all the time could be a bad connection causing a high resistance.

 

Since it is affecting the engine, I would suspect a bad connection somewhere. It sounds like it is starving the ignition system of voltage, as well as the lights. You said something about 15 volts, so I assume you have a voltmeter or tester? If so, where are you getting these wild readings?

If you are getting fluctuating voltages at the battery itself, I would suspect a wiring problem in the alternator/voltage regulator harness, or possibly the ground connection from the battery to the engine block.

Does the starter turn the engine over well? If so, then that would be tell-tale that your battery, and all your large connections are good. And If it starts good, and the wild voltage readings are coming from inside the cab, then I would suspect a bad connection at point #3 in the diagram below. There is a small wire there that feeds the whole electrical system of the truck.

Pinpoint where the voltage is going crazy, and then write back.

 

Yes, I have a multimeter. I went outside and got some more readings. Truck starts and runs fine with lights off.

When the lights are off, the voltage is 12.97 at the battery, 13.1V at the starter solenoid, and 13.2V at the battery. Cables are new, and the fuseable link that powers the rest of the truck has been replaced (tan.) At cruise rpm, the voltage is 14.3V at the battery, with similar readings at the other two locations (I won't mention this again: battery, solenoid and alternator readings are within .2V.) Coil voltage is 8.7V.

With the lights on, the voltage is 12.6V at idle at the battery, at cruise it is 15.5V. At the coil, voltage is 12.4 at cruise.

 

OK, I think I have found the problem. We are supposed to have two tropical storms here in the next two days, so I want my lifted truck to be driveable if necessary. So I went outside and swapped the light switch with one from my parts truck. Turned on the lights, and the same thing happened: Voltage way to high when at cruise RPM. On a huntch, I turned on the vent fan, and the voltage went up as well.

That got me thinking, and I hooked up a jumper cable between the body and the battery. Eureka! The headlights got brighter, and the voltage went down to normal. So I have a bad connection between the frame and body and battery. This was making the voltage regulator think there was less voltage than the alternator was actually producing, fooling it into producing too much voltage. Also, the flaring of the lights must have been when more and less connection was made in the ground circuit as the truck bounced down the road. So I'll get two groundstraps and connect the frame and body seperately to the battery.