View Full Version : diagnosing fog lights

July 12th, 2010, 11:35
1999 XJ sport

i replaced both bulbs (wanted yellow ones anyway).

i knew the AC worked, so i swapped that relay for the fog relay and still no go.

next i was going to pull the switch and see if it's getting power.

the light on the switch doesn't light and the fogs don't light.

anyone have any other pointers?

i figure if the switch isn't getting power then there is no way the fogs are, but i'm open to suggestions


July 12th, 2010, 11:41
Are you testing the fogs with your low beam headlights on??

They will not work without the low beam headlights on.

July 12th, 2010, 11:45

July 12th, 2010, 12:25
You could try and pull the switch apart, there's a metal spring that I had to stretch out slightly to get a good contact with.

July 12th, 2010, 13:47
Checked the fuse? If the fuse isn't blown, I'd check for power at the connector for the switch, and check for continuity on the switch terminals for the lights when the switch is 'on' (I think it's blue and purple? The wire colors/functions are posted around here somewhere).

Just for fun, I've been looking at the various replacement switches while putting together some fog lights. NAPA, Autozone, Advance, and Rock Auto all sell aftermarket replacements, and you can get them on eBay as well (both aftermarket, and MOPAR) for between $34 and $45. Apparently a lot of them die...

July 12th, 2010, 17:23
X 2 on all the above .....

With the headlights turned on and Lo-Beam selected .... There should be 12v at the Violet wire from the relay in the PDC to the switch.

Activating the switch allows the 12v from the violet wire to the light blue wire which runs to the foglights .... This is the main reason the switch fails ... It gets overloaded/overheated - just like the headlight switch.

The foglight relay is not used correctly to supply power to the foglights ... Its used for its switching action when the headlights are on lo-beam.

Ideally ... once you have the gremlin sorted ... a wiring upgrade similiar to the headlight upgrade would be beneficial - if the use of foglights are a regular nescessity.

Personally ... I'm pretty sure a hydraulics expert designed most of the XJs electrical wiring ... :laugh:

July 12th, 2010, 19:24
Its probably your switch, when i pulled mine apart all of the plastic on the inside was melted, your best bet would be to try and find on at a junk yard.

July 12th, 2010, 19:51
You can also buy replacement fog light switches from Advance, Autozone, NAPA, RockAuto (all aftermarket replacements for between $32 and $40), as well as MOPAR switches from vendors, dealerships, ebay, etc for about $45 shipped. Every used switch was running the same circuit that fried yours in the first place- why bother?

July 13th, 2010, 07:20
you guys were right. the inside of the switch was all melted and i was getting about 0.6V on the light blue wire when the switch and lows were on (i was getting 12V as expected on the purple wire).

i tried lengthening that spring as one poster suggested, but it did no good. also, it looks like the "sweeper" is still making contact properly. i also get about 160Ohms across the switch when it's "on". i'm not sure why but i'm just not getting anything out of the switch. the backlighting in the switch works, but the blue light never comes on, nor do the fogs. it looks like the the blue light or the backlight is what melted the inside moving part of the switch.

i might try jumping the wires to see if the fogs work, but that's for another day

do the 3rd party switches look and work as OEM?

July 25th, 2011, 15:41
third new switch and the fogs are working.