Re: Running lights turn off when headlights turn on
OK. This problem is officially insanity in my opinion. Here is a description of what has been done and what is happening.
There is a video on Youtube of someone electrical testing a headlight switch. At around the 5 minute mark in the video linked below, he tests the parking light terminal which shows 12 volts on the first pull of the switch, then 0 volts on the full extension of the switch, which means parking lights on at first pull, parking lights off at full switch extension.
how to test wire troubleshoot GM headlight switch (Can't post links. You will have to search for the video on Youtube)
He then tests the taillight terminal which shows 12 volts at the first pull and 12 volts at full extension, which to me means the tailights turn on and act as rear parking lights at first pull of the switch, then the taillights stay on at full extension of the switch, acting as tail lights, while the front parking lights turn off.
If my headlight switch is tested by itself as he is doing, the results mimic what he got. 12 volts first extension, 0 volts full extension on parking lights, 12 volts first extension and 12 volts full extension on the taillights.
If the headlight switch is plugged into the harness, while the parking lights behave as expected, first 12 volts, then 0 volts, the tail lights show 0 volts first extension, and 0 volts full extension, instead of the 12 volts reported when the switch is tested by itself.
After proofreading all of the above step by step, I realized the next thing to do was to pull the brown wire out of the harness and stick the meter probe down into the hole the brown wire was in to see if the switch terminals still put out 12 volts. They do. 12 volts first extension, 12 volts full extension. So thinking the brown wire terminal has a bad connection to the terminal in the headlight switch, I put a jumper down into the slot where the brown wire goes. I test the other end of the jumper and it shows 12 volts. I then connect that jumper end to the brown tail light wire. 0 volts. No taillights. Where did the 12 volts from the taillight terminal go?
Losing my mind and with no other options, I thought "Why not try the 12 volts from the red wire on the headlight switch directly to the brown tail light wire?". So I do that and guess what? I get 12 volts on the brown wire at a point after where the jumper is connected, and more importantly, the taillights turn on.
So how in the world can 12 volts from the red wire on the headlight switch turn the taillights on, but 12 volts from the taillight terminal on the headlights not turn the taillights on? Isn't 12 volts, 12 volts, wherever it comes from? Why would the 12 volts on the taillight terminal disappear when it is connected to the brown taillight wire, and where would it disappear to? It doesn't seem like the taillight wire is shorted out because the taillights do work with 12 volts from the red wire on the headlight switch.
( As an aside, some readers might be thinking, "Earlier he said he had rear parking lights. Now he is saying he has no rear lights at all. He must have made a mistake". Nope. No mistake. The reason the parking lights in the rear worked before is that the previous owner had jumpered the parking light terminal to the taillight terminal. I removed that jumper which resulted in no rear lights at all. So apparently the taillights will work with 12 volts jumpered from the parking light terminal, they work with 12 volts jumpered from the red terminal on the headlight switch, but they will not work with 12 volts from the taillight terminal.)