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Question for the electrical Gurus: Relay to convert R Defrost Switch for light use?

beer_goggles_009

NAXJA Member
NAXJA Member
Location
Cincinnati, OH
Do any of you NAXJA electrical Gurus know what relay is needed to convert the rear defrost circuit into a non timed circuit to use for rock lights.

Whats known. Rear Defrost rely (red) has a timing feature and is located under the dash on above the drivers right knee. This relay has a timing feature that turn the relay off after an amount of time. duralast part number is 19368. I would like to replace this relay with a non-timed relay so this circuit can be used for rock lights. The relay pin pattern is the same as the relays under the hood. If you replace the relay with a fuel pump relay the switch for the defroster no longer works, its always on. (circuit was verified energized with a test light).

Can any of you electrical gurus share you knowledge on what relay would be needed to still allow the defrost switch to work but not have the timing feature. Alternatively is there any way to remove the timing feature of the relay.

Thanks,

Jim
 
Not that the wiring functions the same on the earlier XJs as it does on the later ones, but the timer on my '98 is located on the back of the instrument cluster circuit board.
My rear window defroster stopped working more than 5 years ago...
Using the factory service manual pin out schematics I found that the by grounding the relay the rear defroster would function and the timer wasn't working.
The timer closes the grounding circuit for the relay coil to energize the defrost timer relay. My solution was to cut the grounded wire going from the defroster switch to the timer and cut the wire from the timer to the relay; then run a separate wire from the switch grounded wire to the relay.
 
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It might be easier to take the switch out of the dash and figure out which terminals are on and off, then bypass the factory harness with your own wire. Run it down to a regular relay up by the lights and then wire in the power from the battery and the rest to your lights.

Then you don't have to figure out the factory stuff, which sounds complicated. You just need to run two or three wires to power and ground so the relay works the lights.
 
The rear defrost switch is probably a Momentary ON switch for clicking on the timer and not suitable for rock lights.

I agree that the factory wiring is not worth the effort to decipher and use. Get another fog lamp switch, wire it up to a standard relay, and use you own wires.
 
Just curious...... Why not just put another switch in and wire it up. Would be alot easier then trying to convert the defrost switch.
 
Thanks guys sounds like its not worth the effort and a just putting in a new electrical circuit and switch is the best way to go.

Magimerline

I was trying to convert the switch because I hate to mess with wiring and it never turns out as nice as factory stuff when I do.
 
A factory fog light switch will pop right in and look just like original.
So what I'm reading is that I can swap out the switch and just re-route the wires from the heating grid to some lamps and vuala?
 
Josh - I don't know. The wire harness for the rear defroster is probably different than your standard on/off power switch like the fog lights.

But you bring up an interesting point, especially if you're looking to put some sort of lights in the rear. Someone would have to change the switch, change the relay, then test for power at the other end of the harness.

All I was suggesting to Jim was that a factory fog light switch would fill up the hole in the dash like factory. Run wires to the lights from the dash. But if you fiddle with it, you might get the factory harness to work.

I'm about 10k miles from the '91 in my garage or I would go play with it and see.
 
I think I may just mess with it myself here sooner or later. I love the idea. I've got a set of lights for the rear and aside from figuring out how I'm going to mount up some rock lights back there, running wires is gonna be the hardest part. Not that it's all that hard, but I really don't want to run wires from the battery to a switch in the cab (undoubtedly mounted somewhere inconvenient) and bury them under carpet all the way to the back.

I'm gonna be doing that with larger gauge wires for my air pump, power inverter and possibly secondary battery, but I've been putting that off too. Haha! I like the idea of easy with rear rock lights.

Edit: Where the hell is Kastein when you need him?
 
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I got told I should look at this thread...

A factory fog light switch will pop right in and look just like original.
exactly what I was going to say. Same cutout shape in the panel, just pop a fog light switch in instead if you want non-momentary.

As for the rest - the relay's a funky one with a built in timer IIRC. I'll take a look in the FSM wiring diagrams and see if I can figure a way to swap it out for a regular one or something. Depending on where you intend to pass the wires for the rock lights out through (closer to the back? closer to the front?) it may make more sense to simply pop in a fog light switch and add your own relay under the hood than it will to try and work with the factory wiring.

EDIT: ok, grabbed my 96 FSM (your mileage may vary, this was just the one I saw first) and... randomly hit exactly the right page (8W-48-3) - first time that's ever happened.

The wiring goes like this:
* power for the defroster is through the 40A fuse in position 3 of the PDC, followed by a 25A fuse in position 18 of the fuse block under the dash. Why? Ask the Chrysler wiring guys.
* the relay has 5 pins:
- 12V in (from fuses)
- 12V out (to heater grid)
- fused ignition switch output (this runs the timer/logic board inside the relay)
- ground (if you don't know what this is, you should probably not be working on your vehicle's electrical system just yet...)
- defroster switch signal (connect to +12V to turn on, connect to ground to turn off)
* for some reason the combination flasher, the LCD illumination relay, and the door lock/unlock relays are partially covered by this schematic. Ignore it, it's not relevant.

I *think* if you replace the defroster switch with a fog light switch and wire like so:
* fog light switch pin A (indicator lamp) goes to black wires on old defroster switch socket (ground)
* fog light switch pin B (switch) goes to dark blue / white wire on old defroster switch socket (which goes to the relay 'defroster switch signal' pin)
* fog light switch pin C (switch and indicator lamp) goes to black / red wire (defroster relay 12V out)

... it will work. The only downsides are that the lights won't turn off till the timer runs out (you could use an SPDT switch, but then it wouldn't look factory) and that your rock lights will only work when ignition power is on (probably a good thing.)

If you want to disable the timer, I'm not sure how to do that yet... will have to think about it a bit more.
 
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Finally got back to read this thread...

So, will the lights stay on as long as the switch is on, but only turn off when the timer runs out?

I guess you could give the ignition switch a flip when you're done using your lights, but that would be annoying.

Is there a relay that can be swapped out and work correctly?

And thanks for the efforts, Ken.
 
Finally got back to read this thread...

So, will the lights stay on as long as the switch is on, but only turn off when the timer runs out?
correct.

I guess you could give the ignition switch a flip when you're done using your lights, but that would be annoying.

Is there a relay that can be swapped out and work correctly?
Yeah, would get annoying. I'm sure there is a relay that can be swapped out, or a way to modify the existing one, but I haven't come up with the way to modify it yet and haven't looked at what relay could be swapped in.

And thanks for the efforts, Ken.
no problem. Electrical research usually gets my interest, I love figuring this kind of stuff out.
 
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