86-93 240's have a headlight switch that turns the headlights off when the ignition is off, a very handy little feature if you want to avoid accidentally leaving your headlights on and draining the battery. Retrofitting this switch to an 81-85 240 is a very simple job. I do not have a 75-80 240 (older style dash) to experiment with, but I think it would be possible.
Standard disclaimer follows:
I do not claim to be an expert or to be completely correct. I take no responsibility for any silly things you may do to your car or yourself after reading this. I can not be responsible if you break something trying to emulate something I have written here.
The switch you want is from an 86-93 240. It is a plastic switch with four prongs on the black. It does not use the same connector as the older switch, but it uses the same one as the heater fan speed switch in 81-93 240's.
Wire position diagram to follow.
An additional wire must be run from a fuse that provides switched power. On 86+ 240's fuse #???????????? was used.
If your lighting setup does not use some sort of relay, now would be a good time to add one. I added one to my system similar to the later 240 wiring diagram, only the relay is located next to the hi-low relay under the hood. The installation is simple and the reduced voltage drop is significant. If you are running individual relays for the headlights (the IPD or Daniel Stern harnesses) you are fine.
The 86+ 240's have a hole in the plastic to lock the switch in place. I chose to replace the entire plastic instrument cluster insert because mine was cracked (as are many). If yours is fine then you can just drill a hole at about the 7 o-clock position for the locating tab on the switch. I did install the switch without the hole for a couple months in my car, with no problems, but I was worried about overtightening the nut on the plastic switch, so I recommend adding the hole.
The photo below shows the location of the hole. An 81-85 surround is on the right, and the 86-93 surround on the left.
Your lights will now function like the later 240's. With the key removed, the power to the headlights is cut, so you can not leave the headlights on unless the key is in the ignition. You can leave the parking lights on at any time though, so do be careful.
I see no disadvantages to using the later switch, but if you do, please let me know! The later plastic switch is in fact cheaper than the early metal one, so if it should fail, you'll still be ahead.
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