Still tracing down a few mysterious electrical and perhaps other problems with my 83 4x4 pickup, maybe someone can help with this one? Start the car and the battery and brake lights stay on. I replaced the alternator and had it checked, it's outputting ok, then I was told the voltage (IC) regulator was bad, so I replaced that, and the light still stays on. If I rev the engine quick above 2000 RPM both lights go off and stay off, but if I let it idle as normal without revving they'll stay on the whole time I'm idling the truck. Truck idles horribly and won't stay running too, but I'm not sure if that's the same problem...
The battery light is on because the system thinks the voltage isn't high enough to be charging the battery. The truck may be running rough because the reference signal the truck uses to provide correct timing is floating all over the place. One possibility of causing both is losing the ground between the engine block and the chassis.
Check the ground wire from the battery, and make sure the both the small and large wires make good contact. One should go to the engine block and the other to the chassis. Even if they look good, run an additional ground strap from the firewall to the engine, and connect several places on the engine (intake manifold, block, etc.)
If you have a digital voltmeter, you may be able to measure several volts between various parts of the engine and the battery ground post. If this is the case, your problems are likely a bad ground strap somewhere. If in doubt, add more.
The symptoms you described are strangely similar to an 88 cadillac I had, that measured nearly 5 volts difference between the ground post of the battery and the engine block while running (very roughly at low rpm, but it smoothed out some at higher speeds). I've also seen this symptom on a Tacoma when someone removed the ground strap between the firewall and the engine during some "maintenance" and forgot to put it back on....
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