'99 t and c: automatic headlights always working

Last Edited By Krjb Donovan
Last Updated: Mar 11, 2014 07:45 PM GMT


QUESTION: Hello, I am unable to manually turn the lights on. When it gets dark, the light work automatically. Even when I turn the switch off, the auto lights stay on. When I shut the vehicle off, the lights go off in one minute(as programmed). I haven't seen any schematics that show where the photo sensor is located or how it is wired into the lighting system. I have checked the light switch for continuity, the switch appears to working as designed.

ANSWER: Hi Keith, The body computer module controls the relays for the headlamps. The module 'senses' when you turn on the headlamp switch to the manual position because that action grounds the wire at the bcm. If the switch is working you should see continuity between pin 3 and pin 2 of the 8-pin socket at the switch, after removing the 8-pin plug. So test for that response. The wire from pin 2 goes through several connections in the fuse block to a ground point. The parking lights use the same pathway only in that case the connection is between pin 8 and pin 2 of the switch. So if the park lights works the ground path is OK. The grounding sensing point of the bcm for the headlamps is on pin 13 of the bcm (which is located on the rear side of the underdash fuse box). Let me know what you find and we'll go from there.


---------- FOLLOW-UP ----------

QUESTION: Hello Roland, Pin 2 does have continuity to ground, so this is good. But I see pin 13 of the BCU connected to pin 3 of the light switch and not ground as you indicated before. I also noticed that pin 3, 7 and 8 have almost 12 volts on them. This seems weird to me.


Hi Keith, You perhaps misunderstood me about pin 13 of the bcu. Indeed that light green/orange wire connects bcm pin 13 to pin 3 of the 8-pin plug at the switch and then when you move the switch to the headlamps 'on' position that grounds the light green/orange wire and the bcm is set up to notice that and should turn on the headlamps in response. So see if the voltage at pin 3 goes to 0 when you turn the switch to manual headlamp position. Similarly pin 8 at switch shows 12V in the off position and that should drop to 0 when you turn the switch to park position. And finally pin 7 shows 12V in the off position and it should drop to 0 when you turn the switch to automatic. Because you say it is always in automatic mode, I would suspect that wire black/yellow on pin 7 to show 0V at all times, which means either that the wire is shorted to ground or that the switch is faulty and has an internal short to ground of that pin. See if those in fact occur or not. I suspect that the pin 3 response will be absent which suggests that its internal contact is always 'open'. It could be that pin has just a bad connection inside the switch which you might remedy by spraying electrical contact cleaned inside the switch thru a crack or hole in the body. But if pin 7 is always grounded (and the wire itself isn't shorted to ground) then a new switch would be needed to solve the problem. If yo don't find the above to be the case, then the bcm is faulty. What is not obvious is that the headlamp switch only indirectly sends power to the head/park/auto/fog lamps and it does so by grounding one of three ground sensing pins on the bcm which then in turn activates the appropriate lamp relays. The fog lamps are activated by the grounding of the fog lamp relay actuation coil directly (without bcm intervention). Please 'rate' my answer (see below). Thanks, Roland


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