I think I know the answer to this, but I thought I'd run it by some other smart folks before tearing stuff apart.

The fog-lights haven't worked on my 97 XK8 convertible since I bought it 6 months ago. The previous owner said he couldn't get them to work in the year he had it.

I replaced the fuse with one 20% higher rated and they came on and lasted 1-2 minutes. ( I know about over fusing being a no-no, but it was just a test)

Since they came on...The power is making it to the bulb. No open circuit.
Since the fuse didn't immediately blow...no short circuit.
Since both bulbs lit equally if there is a partial short, it must be before the bulbs (I haven't seen too many "partial" shorts so not too likely)

My hypothesis... 2 owners ago someone tried make the fogs brighter and put bulbs that are drawing too much current and blowing the fuses.

Anyone know if there are bulbs that physically fit these lights, but draw more amps than the stock ones? I can only find the OEM part number so I haven't been able to see if there's a higher current bulb available that would plug-in. I don't like tearing things apart unless I have a pretty good idea where the problem is, so ...What says the jury?

 

Jay, there's got to be higher current bulbs out there. Important question- does the fuse blow with car at rest for the few minutes? If yes, then I like your direction. If not, try it. There's just too much possibility that vibration is causing a bad connections to short.

Idea 2) put an ammeter (measures amps or current flowing) in place of the fuse. It will confirm your theory. Yes technically I should recomendations the meter go in series with a fuse holder, but the meters are internally fused and Jay will be monitoring current draw.
If fuse does not blow at rest, pick 1 side, disconnect bulb, drive. Blown? If no, the swinging short is on other side.


Btw it's pretty easy to change the bulbs Jay.

John

 

You might want to study the wiring diagram (available on jagrepair.com). From memory, there are a number of circuits on that same fuse. The true problem might be with one of those other circuits.

I suppose you could test your theory by unplugging one of the fog lights and see if an ORIGINAL fuse survives...

Best of luck, keep us posted.

 

You have a short, meaning somewhere from the switch to the fog lights, there's a bare wire touching metal. That requires visual inspection for the bare section of wiring. Get the wiring diagram and once you know the wiring colors, trace the harness. I really doubt higher wattage bulbs are the cause. I'd start at the fog lights themselves because they're exposed behind the grille.

 

Jay, there's got to be higher current bulbs out there. Important question- does the fuse blow with car at rest for the few minutes?
Yes the fuse blows at rest. I turned them on, went out front to verify they both were on, and when I turned back they had gone out.

Btw it's pretty easy to change the bulbs Jay.
From what I had read, the fogs are kind of hidden so I hadn't bother looking at them. I just wanted to get my ducks in a row before I start tearing into things.

Idea 2) put an ammeter (measures amps or current flowing) in place of the fuse.
I have a tester that plugs in to the fuse panel and then the fuse plugs into it. It can directly read the current in that circuit. The only problem is I have 2 (one for each of the 2 sizes of fuses on the market)...the one I need is currently missing...go figure!

The current actually being drawn is right at or slightly more than 20 Amps. I replaced the 15A fuse with a 20A and it ran for a while. Fuses are typically rated for twice the nominal current draw. (That way the fuse doesn't pop every time there is a current inrush or spike.) So I have a steady state 20A draw.

The circuit should normally run at 90 Watts (15A * 12V = 180 watts/2 = normal load) but it is drawing at least 240 watts (20A * 12V = 240W = blown)

Two 45 watt bulbs should run just fine on a 15A circuit.

You have a short, meaning somewhere from the switch to the fog lights, there's a bare wire touching metal.
If the wiring was indeed shorted, the bulbs would be dim or at least flicker. (The short would consume some or all the available current.) They were nice and steady bright so not super likely, but still a possibility.

This all will be put on the back burner since my radiator hose popped off the thermostat housing dousing the engine bay with antifreeze and tracer dye...for the 2nd time!

At least this time I figured out why. The thermostat housing nipple has a chunk missing out of it and the hose doesn't have enough material to hold onto. Got a housing and thermostat on order. I think I'll use some Permatex on the nipple so I don't have to tighten the clamp so tight like the previous owner.

 

Jay,
Your logic is good, and some sound advice given above.

The orginal bulbs in our generation are 55W: a pair of them should draw around 10 amps therefore the original 15A fuse is appropriate.

There are 100W bulbs available on Ebay (and doubtless every flavour in between) so entirely possible that someone has fitted higher power ones.

edit: You can get to the bulbs by removing the 'hatch' in the wheel well liner.

HTH,
Mike

Don't be tempted to fit any of the fuses from this kit:

 

Jay,

If you ever find the short or if you find that someone has put the wrong fuse in you Fog Lights. In other words, when you fix them. I would suggest you replace the bulbs with Sylvania Silver Star zKe's...They will cost you about $40 Bucks a pair. They are Pure White Bright and look like LED's but they are Halogen mixed with Xenon....I've had them in my Fog Lights for 6 months now and they really light up! Looking at them from the front, they are a whiter bright than the HID's and really standout! Another good thing is they won't turn-on your "Lights-Out" warning on the speedometer......If you don't have HID low-beams, these H1's will fit and really light your way....They also have 9005's for your High Beams at around $35 Bucks all on Amazon.....They are making a whole new World out there at night.......There are things you can see with these Bulbs that you have never seen before at night!!! It's Amazing!

Billy Clyde in Houston

 

I'm reading this in bed so I'm going from memory. By batch in the wheel well are you saying the hatch to get to the back of the headlights or is there another hatch I didn't notice?
Seems like the headlight hatch is quite a ways from the fog's.

Got my thermostat housing today and the thermostat is due tomorrow. I got the 183° (Couldn't find any discussion about pros or cons about the different temps) I was kind of surprised to see that low temp as original. Most cars I've seen lately have 205° or so to help the emissions by allowing slightly higher combustion temps. Will install tomorrow and get Heat Wave (Grandson named it after a red "Rescue Bot") back on the road.

 

The hatch is in wheel well in front of Tire. Take wheel off = make your life easier. 2 ensure you reattach hatch properly. The can fall off.

John

 

Isn't the heater aux pump on the same circuit? It's not unusual for the brushes to wear out and create a short in the electric motor, perhaps that is your problem. Try disconnecting it.
Regards,

 

I guess there are two possibilities - either a short or fault in the circuit between the fog-lamp relay and the lamps, OR a fault in the circuit that energises the relay.
Assuming the fuse you are blowing is the 15A fuse #17 in the engine compartment fuse box, there are two separate feeds to each lamp. Disconnect both and then connect each one back separately and switch on lights. If fuse still blows both times, it's probably a fault in the relay circuit. If it blows on one side but not the other, you need to trace the wiring on the bad side and find the problem. Fuse #17 ONLY feeds 12v to the foglamps and comes after the relay so it has no connection with any other circuit.


Edit : Having looked at the wiring diagram again, I think it unlikely that it is the relay circuit. Fuse #3 (5A) in the same fuse box feeds that relay and lots of other stuff, and the ground side is switched by the BPM, so not indicative with what is happening.
You could always take out the relay and put a direct 12v supply to the lamps - the fuse is after the relay so the circuit would still be protected.