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Old 08-19-2006, 11:48 PM
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'96 Eldorado--how to turn off the headlights?

I'm trying to find out how to disarm the automatic headlights on a '96 Cadillac Eldorado. I already have the daytime running lights disconnected, but this thing turns the headlights on when it gets dark, whether you want them on or not, if the engine's running and it's in gear.

The twilight sentinel switch is all the way to the left in the OFF position, if this has anything to do with it.

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Old 08-25-2006, 10:07 AM
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'96 Eldorado--how to turn off the headlights?

Looking through some diagrams, it appears the only way to do this to "fool" the sensor that detects how bright/dark it is into thinking it is always sunny outside.

Theory of operation: the sensor is on the dash looking up at the sky. During daylight the sensor's resistance is low (like a solid wire). Darkness makes its resistance high, so the automatic headlight module turns on the headlights.

Permanent fix: bypass the sensor with a piece of wire, which "fools" the automatic headlight module into daylight operation (no headlights). Unplug the sensor, and insert a wire where the sensor plugged in. Test, then cover with black tape.

Bypassable fix: Connect a switch across the sensor wires. When the switch is off, the automatic headlights function normally. When the switch is on, the module measures only low resistance, thinks its always daytime, and the lights stay off.

Neither modification will change the way the headlight switch works manually.

Jon P
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Old 08-25-2006, 10:58 PM
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I tried the sensor yesterday. I shined a bright flashlight on it, but the lights stayed on. I took it out, and the lights stayed on. I replaced it with the blade of my pocket knife, and the lights stayed on. I fumbled it down the defroster vent, so the lights are permanently on. At least on my Ford you can turn the stinkin' auto-light system off with a simple switch.

Maybe I'll try a chunk of 12ga copper house wire. If that fails, maybe I'll try a regular old 12ga.
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Old 08-26-2006, 03:10 AM
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Nothing seems to be working....hmmm...

Sounds like a defective lighting module....

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Old 08-26-2006, 05:40 PM
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Doc here,

OK, Never worked on a twilight System on a Caddy...But I did a little reading on the subject let me throw some speculation into the mix..

From What I read , the Twilight system is part of the BCM , and has some codes..try a code reader, see if you pull any of these up..

DTC 38 - Solar/Twilight Sensor Supply Voltage Low
DTC 39 - Solar/Twilight Sensor Supply Voltage High
DTC 40 - Solar/Twilight Sensor GND Short to Voltage
DTC B2419-Twilight Photocell Circuit Problem
DTC B2420-Twilight Delay Pot Failure

OK , from what I read, It comes from The DRL module behind the Radio..from a A1 and A2 harness..If you pull that module , the fog lamps and (maybe) the lamps failsafe "On", and The (??? Don't know why..) auto Trunk lid will fail to operate...

From the codes, I figure If the feedback is too high, (jumped) it will trip a code and go into "Failsafe" because there is no load on it..

If the feedback is too low, (disconnected) It will Failsafe because it has no loop.

If the feedback is ground, it assumes a short and failsafe because the sensor is shorted. The same applies to the photocell circuit problem and pot failure..

What it sounds to me like you have is a failed Cds Cell (photocell sensor), and there is no/too high/too low load on the loop back to the BCM..Again I don't have a schematic in front of me nor have I ever worked on one in a Caddy..

Sounds like to defeat it, you need to either TRY and bypass it, then reset the codes..OR ...Find out the exact resistance of the Cds Cell, and install a resistor (and maybe a diode) In it's place (to make it THINK it has a working "OFF" cell in place) ...OR...replace it with an OEM Cds cell, and attach it to an Instrument lamp that runs anytime the key is on, so the BCM thinks it's out of the "Twilight zone"...

Like I said, I've never worked on one before, but were It I, I would probably try these steps to see what happens..

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