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.

 

Reviving this since my horn still doesn't work and it's time for my annual inspection soon.

Right now the goal is simple: the solid brown wire running to Fuse #1 (engine bay/headlamp fuse box) is not getting any power. I need to find out why. My guess is that the wire is frayed or disconnected somewhere and is likely touching a ground point (I add the latter because fuse #6 to the high beams blows immediately when I attempt to turn on the high beams; perhaps the two are somehow related).

Maybe it's a silly question, but where does that brown wire lead? Ultimately to the battery, but what path does it take? Best as I can tell the brown wire runs into the driver (left hand side) footwell as part of a loom, though it enters the cabin at such a high location I'm afraid I'd have to remove the gauge cluster and other items to put eyes on it. Though from the cabin, does anyone know the next step along its path to the battery? My hope is that the fault is in an easy-to-access location along that line.

Thanks

 

In tandem with my last post, here's my confusion. I'm getting 12V at Fuse 11 (horn fuse). Getting nothing at Fuse 1 in the engine bay (aux fan fuse). My understanding is the two work together. As stated in my last post, I do not know the path the solid brown wire leading to fuse 1 is fed from (obviously the battery but the exact path eludes me - see below).

The 12V to both the horn fuse and fan relay are solid brown wires as expected. The 12V feed to Fuse 1 is also solid brown. But the wires running to Fuse 11 are white/pink.

I took the below picture of the big wiring diagram in the manual.

146 = Horn Relay
133 = Aux Fan Relay
77 = Battery Post

This is confusing to me. I would've expected the 12V to the horn relay to be coming from the #11 fuse (or even #1 fuse), but this diagram simply point to the battery. Meanwhile, I located Fuse #11 on the same diagram (notated 89 for anyone wanting to see it for themselves), and it points multiple directions but with no direct path to the horn relay.


Very frustrating.

I did notice something else though. In the above diagram, I see the light green (LG) wire running to the horn relay passes through two items I'm unfamiliar with: 145 = "Heated jet fuse" and 144 = "Heated jet ambient sensor". Could the fault lay with either of these? And what are they?

My greatest thanks to anyone willing to help me figure this out!

 

“Heated Jet” is the windshield washer nozzles; I believe that came with the HE. Ambient sensor is on the same radiator panel that the horns mount to, activates around 40 degrees. The heated washer wiring is pretty small, and easily damaged, pull the wiper assembly for access.
Brown wires are constant hot and connect to the firewall buss, and generally are not fused.
I also believe that the lead after the ambient sensor feeds the left and right door mirror glass heating element (at least I recall it does on my ‘88).

 

Still chasing the lack of 12v to Fuse #1 (engine bay fuse box). Recorded a short video of where I’m at; any ideas of where I should look next? Thanks

 

The only wiring diagrams that I have are MY ‘88 and ‘89; both show that the fuse that feeds terminal 30 of the horn relay is #6 which is a 15A in the black underhood fuse block. The feed for fuse 6 (brown wire) is fed from the left firewall buss stud.
The coil side of the horn relay (terminal 86, light green) is fed through the ignition switch to fuse #11 in the main fuse block (under the left dash); this is a 15A that also feeds the triple diode under the hood for the condenser fan.
I am guessing that you have been tracing the wrong fuse circuits and will find those fuses blown/corroded/missing.

 

Thanks but I’m confused. My engine bay fuse box has 5 fuses but you said #6. #11 under the dash has 12v on both sides so no issue with that one.

Now if you meant to say #6 under the dash, that would be interesting because it has been blowing every time I turn the headlights on, though it’s labeled “rear fog guard”. I’ve searched the forums for that one and even looked in my manual and can’t find much on that.

 

On my ‘88, the black fuse block under the hood has 6 cavities; first one isn’t used (closest to the radiator). Then there is #2 (10A), #3 (7.5A), #4 (7.5A), and #5 (10A)- these are for the headlamps. Last cavity, #6 (closest to firewall) is a 15A that is always live, and feeds the horn relay.
The light green at the triple diode is only live with the ignition on. I didn’t check what fuse feeds this, but suspect that the wiring diagram is correct and is fed from the #11 fuse.