I'm experiencing an issue with the following symptoms:

- Horn doesn't work (did recently, not sure what changed besides maybe a bad connection developing)
- High beams do not work (also worked recently)
- Main beams work fine
- When switching on the main beam headlights, #6 fuse blows immediately (#6 is for "Fog Rear Guard", though I don't have fogs since it's a US car)

I've tested a couple grounds in the vicinity of the headlight relay (C38616) and they seem to be fine.

I did read a post where someone had an issue where their lights were blowing a fuse because the bulbs were touching the chrome housing, but they were specifically talking about the fog lights touching the chrome, so I don't think that's the case here.

Are there other grounds that could be the culprit? I have to assume there's a bad ground somewhere but I'm not sure where to look.

Thanks for any suggestions!

- Anderson

 

You might have a couple different issues.

What year is the car?

A fuse blowing instantly is usually a "dead short" or "short to ground". That is, "+" voltage going to ground....like a hot (voltage) wire worn thru and touching a metal part

Cheers
DD

 

Interesting, I didn’t know that. It’s an 83. Thanks Doug

 

Unfortunately none of my diagrams show the circuit for #6 fuse so I'm not sure how/where it ties in with the headlight circuit.

I'll do some mulling.

When you try to operate the high beams does the headlight relay click?

When you try to operate the horn does the horn relay click?

More later

Cheers
DD

 

I can confirm the horn relay clicks when I press the horn, though I haven’t listened for the same click with the high beams. I’ll try that today.

Here’s the headlamp/fog diagram I have in the BGB:

 

Replaced #6 fuse and tried to flash the high beams without turning the headlamp knob on (if I do that the fuse will just blow). Didn't hear the relay click, though maybe that's normal without the main beams turned on? I poked around a little and didn't see a stray wire touching anything it isn't supposed to.

I also tried to use the horn without turning the main beams on, and got nothing, but can still hear the horn relay clicking. The horn is my main concern since I don't drive the car at night, but I imagine whatever the fix is will solve both.

 

The rear fog lamp thing is a mystery. Yours being a USA car the headlight switch wouldn't have the rear fog lamp internals...or so I would think. But apparently I'm wrong. Anyhow, it appears that there's a fault in the switch; the #6 fuse should not be powered by merely switching the headlights on. By rights you'd have to rotate the switch to the rear fog lamp position to power-up fuse #6.

Then there's the mystery of the fuse blowing. Look for red/blue wires in the trunk that might be pinched or cut, thus touching ground and blowing the fuse

The wiring diagram you've shown is different than what I'd expect to see on a USA car.

The horn relay clicking tells us the switch is OK and that the control side of the horn relay is getting power from the fuse. You'd need to check the the relay is also getting constant 12v (brown wire). And try applying power directly to the purple/yellow wires which go directly to the horns. Of course, clean all connections at the relay and the horns, and make sure the horns are grounded.

The headlight relay should click whenever the stalk is pulled back. Check and clean all connections at the relay. Also, at the base of the stalk switch is a metal ground tab which triggers the headlight relay. Sometimes the tab gets dirty. You'll have to remove he lower column shroud to see it. Circled in yellow in the picture but I've seen different style of tab over the years



Cheers
DD

 

Thanks very much as always, Doug. A great help and things for me to do now!

 

Update: Can confirm constant 12V at terminal 86 of the horn relay, and continuity between 85 and 86. When pressing the horn, the relay clicks (I've done this with two relays now and both do the same thing), BUT no voltage appears at terminal 30 like it should. I jumped the connections and still nothing. Naturally, my test of voltage at the horns themselves gave 0V. Ground connections for both horns are solid.

Doug - this makes me wonder if something is awry with the radiator fan wiring. I have a "lump'd" XJS with a carbureted Chevy 350 and a mechanical clutch fan, so no electric radiator fan for me. I'm not sure what the wiring for the radiator fan looks like...wondering if perhaps that wiring has a bad ground somewhere that is affecting the horn system.

 

Another update: I can confirm good continuity for the radiator fan ground connections at the fan relay.

 

And another update, this one more telling.

Neither of the two orange wires which run to the bottom of fuse #1 (radiator cooling fan fuse in engine bay fuse box) are getting 12V.

One of those wires runs to the horn relay, while the other runs to the cooling fan relay. The cooling fan relay has the orange wire, two black ground wires, a green/black wire, and a green/brown wire.

Neither the green/black nor green/brown had 12V. I think that's the problem. Now I need help figuring out which of those is supposed to supply a constant 12V and where they are getting power from exactly. Here's the cooling fan wiring diagram:


I'm awful at wiring diagrams, to the extent that I can't look at this and understand exactly where the power source is.

 

Solid brown wires are constant 12 volts on Jaguars....and other Bririshcars.

There's a tangle of wires below the headlight fuse box. I would disconnect the battery and unbolt the fuse box so it can be lifted away a bit, allow access to the nether regions. You might discover a problem; broken wire, dirty connections. Clean all the fuse caps and clips while you're there.

Cheers
DD

 

Being a Chevy swap car, there’s not really anything out of sight and everything looks clean and tight.

I can confirm that if I jump the horn wiring at the relay, the horns work. Tried three different relays and no difference so the problem is definitely elsewhere.