Turning on headlights engages 4wd

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Mar 14, 2017
Nashville TN
About a year ago I bought a pretty beat up 1988 FJ62. I have thoroughly enjoyed rehabing it and even got it painted and put on new bumpers. It took me some time to figure out that the 4WD was always engaged but after I did the solenoid replacement described in this forum, the switch works perfectly. Before all this work, I had noticed that the green 4WD indicator light always came on when the headlights were turned on. I assumed this was some strange electrical issue but couldn't possibly mean that turning on the headlights actually engaged 4WD...

Well, it does. I have verified this multiple times and I am baffled. I searched the forum but could not find any mention of such an issue.

Can anyone shed any light on this or offer any reasons as to how this could happen?
So one or both of the solenoids is behind powered by something tied into the headlight switch. I would suggest you go back to the solenoids with a multi-meter and figure out which one of the wires is powered when the lights are switched on. Work backwards from there.
If the 4WD dashboard pushbutton has a backlight (so you can see it at night - I don't recall if it does), perhaps the 4WD switch has an internal problem causing that 12+ from the backlight to feed the solenoids and turn on your 4WD. That's my thinking, at least.
Thanks for your responses. I tested the voltage at the solenoids. The upper solenoid had no voltage with the ignition on but increased to 7.5V when the headlights were turned on. Then the voltage would increase to 12V if the 4WD switch was pushed. For the lower solenoid, the voltage was 12V with the lights off but decreased to zero when the lights were turned on. Pushing in the 4WD switch made no difference in the voltage with the headlights on. It remained at zero.
Good results so far.

As I'm sure you're aware, the 7.5 volt reading with the lights on is your problem. The factory service manual has testing procedures for the components of the transfer case electronics - if you don't have the manual there are lots of links to digital versions on this site. In the mean time, as suggested above by Steve H, I'd point the finger and start my testing with the dash switch.

I notice in the wiring diagram that the backlight in the switch - which comes on with the other dash lights and it switched via the headlight stalk - is wired through the taillight control relay. Temporarily remove that relay from under the dash. Re-test the voltages at your transfer solenoids the same as you did yesterday. See if it still reads 7.5v with the lights on.

If it doesn't read 7.5, you've isolated the problem and can probably modify the switch short term to solve this. If you still get 7.5, time to dig deeper.

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