Hi all,
I'm at a crossroads and could use some guidance. This is for my 1988 XJ6 (US). My passenger's side low beam lamp is out, although all four lamps light up when the brights are engaged. I have swapped bulbs and modules to isolate the problem and it stays on the passenger's side (I have also verified that both of my BFM's are functioning). I also have an intermittent issue with my turn signals (this is secondary and not that important to me, but it may help explain some of the voltages I've recorded).

I unplugged both of the 9-pin harnesses from the BFMs (black and yellow) and took voltage readings while the light switch was in all three positions and am attaching an image of the voltages I've recorded. Anything below 1V I recorded as 0. I'm a little baffled by the results and wanted to share what I recorded to see whether the community could help point me in the right direction... I don't have wire colors indicated here but would be happy to write them down if anyone wants. What I can say is that the Blue/Pink wire that supplies voltage to the low beam reads 12V on both sides, so I'm at a loss as to what's actually happening here.
I have a copy of the Hayne's book that isn't very helpful, and the electrical diagrams I've found elsewhere aren't good enough for me to feel confident enough to start tracing wires back through the firewall without feeling like I'm stabbing in the dark, so any help is greatly appreciated!
Another piece of data I'll mention is that I had the BFM pulled apart and plugged in, and I could manually engage the relays and turn on the low beam.
-A

 

Hi Albert,

If you measure 12V to ground on both of the terminals to the low beam bulb, perhaps the ground side wire is shorted to 12V power. If a 12V source is present on both sides of the bulb, no current will flow through the bulb filament and you'll get no light. It could be that when you turn on the high beams a ground path is created for the low beam bulb so it can illuminate. This is just a wild guess based on what you've reported. The Electrical Guide doesn't show the arrangement of the wires in the electrical connector so I couldn't identify the wires from your diagram, but if you'll add the wire colors to your chart I'll be happy to have a closer look.

The official 1988 Jaguar XJ Electrical Guide can be downloaded here:

1988 Jaguar XJ40 Electrical Guide

Cheers,

Don

 

Thanks for the link and for your response, Don. After posting I realized that there's very little utility in posting a diagram without wire color codes, I'll update that shortly and will also test the lifted ground theory as I did not investigate that directly (which is a little embarrassing because I had noticed that the bright switch engages a separate path to ground, so it should have been top of the list...)
Again, thanks much for your help and I'll be back soon with updates.

 

Ok, After reviewing the document Don was kind enough to share, it appears that the black wiring harness contains the wires relevant to the headlamp, so I have included those in an updated image. I also checked the ground connection and can confirm that ground is continuous from the headlamp terminal to the battery negative.
My thinking at this point is that white/purple wire #2 is not supplying voltage to the headlamp relay which is causing my problem, does that seem like a reasonable hypothesis?

 

So you have continuity from the negative terminal of the headlamp bulb connector to the negative battery terminal. Do you measure any voltage between that negative terminal and the negative battery terminal? If so, the ground wire may be shorted to a power wire.


I think you're right. The white/purple wire carries ignition-switched power, so with the engine running you should have 12V on that wire. The power comes from Fuse 2 in the fuse box under the arm rest in the center console, so have a look at that fuse.

Cheers,

Don

 

The plot thickens:
I confirm that I'm not getting 12V on fuse 2 in the console box... Oddly enough, when I pull fuse 1 (the complement circuit for the left hand light module) it doesn't kill power to the headlamp, which I would have expected.
Going back to the ground issue for the headlamp, I have a little more information that supports your assumption that it's the main culprit, Don. With no power to the BFM (key in ignition, light switch at position 0) I measure 0.6 Ohm path to ground. When I turn on the lights (switch position 2) the resistance jumps to 7 Ohm, which seems high enough to indicate a significant fault.
On a separate but related note, I have had three instances now of short term power failure while driving. On two occasions the power went out for a moment while the car was moving but came back on within a second or two and the engine recovered from the stall. The third time there was a full stall even after the power came back on. On that third occasion I was pulling into the garage and barely moving so I presume there wasn't enough inertia to keep the engine turning. My initial suspicion is a ground fault, and with this new information about the headlamp ground, I'm thinking it might be time to take a step back and begin by testing and cleaning all of the mechanical ground points in the car... Not the most glamorous task, but a stalling car is no joke so I'd better start there, and perhaps I might be able to get in front of multiple issues at once... I'll check back once I've made some headway, wish me luck!
-A
P.S. thanks again very much for your guidance so far, Don!

 

Good luck Albert.

 

Just wanted to check back in and report that I got the headlamp working again (to great relief!)
I removed fuse box 3 (center console) to investigate. I had voltage on fuse 1 but not fuse 2, and since they are supplied from the same source I figured the fault must be localized to the fuse box somewhere. I couldn't figure out how to remove the circuit board from the box without breaking something, so I just thoroughly cleaned the leads, reinstalled, and tried the lights since the board looked to be in good shape, and the light fired right up.
Hope this helps someone out in the future. Thanks again to all for the help and encouragement!
-A