Fog light failure...

will_95gt · Dec 17, 2007

Ok I have a gt with a cobra front end. It took me a while to get lights to fill the holes in my cobra front bumper and now that I finally have them installed they don't work :-\.

The lights are brand new and I ordered each one separately so its very unlikely that both lights are bad so I'm thinking its the car. Another clue is that when the fog light switch is in the on position the small light on the switch (by the coin holder) doesn't turn on.

So, how should I go about diagnosing the problem? Where is the fuse for the fog lights located? Its not under the dash according to this diagram.

Thanks,

-Will

DarkStallion095 · Dec 17, 2007

Well I think I know the answer to your problem

Check your fuses. When mine went out it also took out my horn, but then I replaced the burnt fuse in the under-hood fuse box. I think its a number 10. Let us know what gives

Blackened302 · Dec 17, 2007

not sure about the fuse, but i'm pretty sure the relay is under the hood. that may have gone bad, too. do you have test light or volt meter?

HISSIN50 · Dec 18, 2007

Good replies.

There's a fog light fuse under the hood (as DS095 noted). And like Paul said, the relay is under there as well. Underhood fuses.

Remember that in OEM form, you need the headlights to be on for the fogs to work.

The LED in the switch can burn out but the switch will work (this happened to me after a drink sweated onto the switch).

I like the idea of using your test light at the fogs. You might have a bad ground pathway to each light, which would keep either from working (I bring it up since it sounds like you had the front end apart). When you use your test light, clip the ground lead to an independent ground (instead of using the OEM one) to see if the test light illuminates.

If nothing at each light, you can go back to the relay or you can go to the switch. I'd check the input to the relay (positive relay coil terminal). If no-go, you have to go to your switch. Look for input voltage going into the switch. YOu might just want to start with this test instead (It takes a few mins to remove the console guts).

Good luck.

will_95gt · Dec 19, 2007

HISSIN50 said:
Good replies.

There's a fog light fuse under the hood (as DS095 noted). And like Paul said, the relay is under there as well. Underhood fuses.

Remember that in OEM form, you need the headlights to be on for the fogs to work.

The LED in the switch can burn out but the switch will work (this happened to me after a drink sweated onto the switch).

I like the idea of using your test light at the fogs. You might have a bad ground pathway to each light, which would keep either from working (I bring it up since it sounds like you had the front end apart). When you use your test light, clip the ground lead to an independent ground (instead of using the OEM one) to see if the test light illuminates.

If nothing at each light, you can go back to the relay or you can go to the switch. I'd check the input to the relay (positive relay coil terminal). If no-go, you have to go to your switch. Look for input voltage going into the switch. YOu might just want to start with this test instead (It takes a few mins to remove the console guts).

Good luck.

Ok, So if i'm using a test light and I connect the ground to something else (which can be the body of the car?) than how do i connect the other pos/neg wires of the test light? Would I have to splice open the wire that goes to the fog light?

And as far as testing the relay input, is this something I would just pop open the plastic and touch a multimeter to? If so, what would i be looking for in terms of results?

Thanks for all the good responses guys!

-Will

HISSIN50 · Dec 19, 2007

will_95gt said:
Ok, So if i'm using a test light and I connect the ground to something else (which can be the body of the car?) than how do i connect the other pos/neg wires of the test light? Would I have to splice open the wire that goes to the fog light?

And as far as testing the relay input, is this something I would just pop open the plastic and touch a multimeter to? If so, what would i be looking for in terms of results?

Thanks for all the good responses guys!

-Will

To ground a test light, choose a bolt head or screw or shiny metal under the car. The fastener needs to go into something solid under the car (which most do).

Then you use the 'pen' end (with the pointy tip) to test the positive side of a given fog light connector. Just unplug a fog light and put the probe into each terminal. One of them should illuminate the test light.

I'm thinking that testing the fog light relay might be a little tough. I'd do the above test. Then if it fails, then test for 12 volts into and out of the fog light switch. If you have 12 volts leaving the fog light switch, have a friend turn on the fog lights while you put your hand on the fog light relay. You should feel it click. f you have 12 volts leaving the switch but the relay doesnt click, we can go into testing the relay (you should use a DMM for this. You have to lift up the relay just enough to expose the terminals but keep the relay connected. Then you can get your meter probe onto the terminals for testing).

toyman · Dec 19, 2007

Just a reminder as HISSIN50 noted, you MUST have your headlight switch on and on low beam if the headlights are on.

will_95gt · Dec 20, 2007

thanks guys!

I'll try to make some headway on this tomorrow and get back to ya.

will_95gt · Dec 20, 2007

ooook....

Here's the update:

Today the drivers side fog light magically started working . But anyways, I checked the garage and found a test light. I played around with it for fifteen minutes only to finally read the print clearly inscribed on the back which read "90-140 Volts"--So I went to radio shack. Radio Shack didn't have a test light so I just grabbed a multimeter.

Ok, so just to verify that the multimeter was working and such I unplugged the working (drivers side) fog light and touched the pos/neg ends of the pins on my multimeter into the female end of the fog light cable. It takes a little moving around to get it to read properly but after a few adjustments (each adjustment resulting in a small spike in voltage on the meter) it eventually read about 13 volts on the multimeter. (the multimeter might be reading a touch higher than it should--it said my 1.5 volt battery was a little over 2 volts.)

I repeated this process on the female end of the wire leading to the passenger side fog light and got mixed results. With a few of the adjustments the multimeter would spike to about five volts only to drop down to zero. Now, this is the same response the drivers side was giving me, only the pass side never finally reached/maintained the 12V level.

It was pretty dark out so I was having trouble tonight... Once I get some more light tomorrow morning I'll give it another shot.

HISSIN50 · Dec 20, 2007

Sounds good Will. For future reference, if you have a Harbor Freight nearby, they put test lights on sale for 99 cents all the time. And they have DMM's for under 5 dollars on sale. They both work just fine.

At parts stores and wallymart you can buy alignator clips with an uncrimped end. I crimp the end to fit over the black meter lead. Now you can clip the ground lead and only have to hold the positive lead.

It sounds like it could have been an issue with getting the probes both seated into the pins. It would be unusual to see only 5 volts - I would expect 0 volts or 12 volts. I wonder if the ground connection for your meter was loose.

When you got the 12 volt reading on the passenger side, were you able to plug the connector into the fog light and get the fog light to illuminate? You might have to wiggle the connector around to make the light illuminate. If so, you know there's a an issue with the physical mating of the connectors (and not an electrical issue).

I'd retest the driver side light to see if you can achive 12 volts. Try using a shiny bolt in the frame/chassis/metal bumper to ground the meter. You can also ohm out the ground wire in the fog light connector (a good ground will have less than 5 ohms of resistance, though with the fog light harness, you might see a little more).

will_95gt · Dec 22, 2007

HISSIN50 said:
Sounds good Will. For future reference, if you have a Harbor Freight nearby, they put test lights on sale for 99 cents all the time. And they have DMM's for under 5 dollars on sale. They both work just fine.

At parts stores and wallymart you can buy aligator clips with an uncrimped end. I crimp the end to fit over the black meter lead. Now you can clip the ground lead and only have to hold the positive lead.

It sounds like it could have been an issue with getting the probes both seated into the pins. It would be unusual to see only 5 volts - I would expect 0 volts or 12 volts. I wonder if the ground connection for your meter was loose.

When you got the 12 volt reading on the passenger side, were you able to plug the connector into the fog light and get the fog light to illuminate? You might have to wiggle the connector around to make the light illuminate. If so, you know there's a an issue with the physical mating of the connectors (and not an electrical issue).

I'd retest the driver side light to see if you can achieve 12 volts. Try using a shiny bolt in the frame/chassis/metal bumper to ground the meter. You can also ohm out the ground wire in the fog light connector (a good ground will have less than 5 ohms of resistance, though with the fog light harness, you might see a little more).

Yes, the drivers side cable--the one I could get the 12V reading on--does infact work well to illuminate the fog light once plugged in.

Thanks for the advice about the Multimeters and the alligator clips. Btw, I picked up my MM from radioshack for like 8 bucks.

Ok after using the multimeter today under the sun light (with the advantage of being able to see what i'm doing) I was able to get a clear 12/13 volt rating from each fog light cable, yes, both passenger and driver's side cable show 12V but still only the driver's side fog light actually illuminates.

This tells me that it must be the actual pass side fog light itself that is not working. Is it easy to see if the bulb is burnt out by just looking at it? If the bulb and the filament looks fine and intact is there any other test I should run before sending it back to the manufacturer?

Thanks guys,

-Will

HISSIN50 · Dec 22, 2007

Ok after using the multimeter today under the sun light (with the advantage of being able to see what i'm doing) I was able to get a clear 12/13 volt rating from each fog light cable, yes, both passenger and driver's side cable show 12V but still only the driver's side fog light actually illuminates.

To be totally redundant (take no offense. We check eachothers' work), this test was performed with the MM biased across the two fog light wires (so you were using the Fog-light's ground wire, and not a piece of metal on the car)? If so, I like your theory below.

This tells me that it must be the actual pass side fog light itself that is not working. Is it easy to see if the bulb is burnt out by just looking at it? If the bulb and the filament looks fine and intact is there any other test I should run before sending it back to the manufacturer?

You can sometimes see a bad element if it totally fell apart. Otherwise, they have fooled me before (they look good but aren't). One quick test: A bulb is a resistor. If you set your meter to read impedance (Ohms), put the meter leads on each leg of the bulb. If you read resistance (start on a higher scale if your meter isnt digital, and work your way down the scale), that would indicate the bulb is ok. If it gives a numeric reading (I test little BS interior bulbs and get like 8 ohms resistance. Your fogs will be different), jiggle the bulb in your hand and see if the reading changes (be sure the leads stay on the bulb). This will check for a loose filament.
Lastly, if the meter reads open or infinite, then that suggests the filament is broken (the circuit is open. The filament completes the circuit in our little test).

Alternatively, or in addition to (if the above test passes):

If you want to, take the bulb from the passenger side and put it on the driver's side pigtail. If it illuminates, something is still up with the connection on the passenger side (the connector isn't seating, etc) because the [originally] passenger-side bulb is viable.

Methodical · Dec 22, 2007

Maybe try some electrical contact spray on the connectors.

Fog light failure...

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