Hi guys and gals,
My name is Cole and I own a 1994 xj6 VDP. I purchased the car 4 years ago with 70K miles, and now it has 90K exactly. Car is in great shape, was owned by a retired attorney who owned three other jags, and was always dealer maintained. Anyway, the car runs and drives flawlessly, but I am having headlight/tailight problems. I think I have it nailed down to be the LLU (Logic Lighting Unit). It isn't the headlight modules up front as I can switch them and the problem stays on the same side. My question is... does anyone have a known good used LLU for a reasonable price (for those of you who don't know what this is, it is the circuit board that the headlight switch attaches to)? I will also ask in the wanted section. Also, I am in Mid Missouri, anyone knowledgeable in the xj40's live in my area and want to get together.
Thanks Cole

 

Welcome ,
You should contact ken (ken@britishparts.com ) otherwise ebay

 

Or if that doesn't return any results try a local junkyard.

 

Welcome to the site cole......Caddie gave good advice about Ken and I'd definitley try him and maybe Jagtechohio might be able to offer advice and parts.

Good luck

Jim

 

Hey, thanks for that Jim.... I called him last night, he had already managed to locate the part at a good price. I just hope he's getting his problem sorted, but he seems confident of the cause.

 

Thanks for the welcome's guys. I really appreciate JagtechOhio trying to help me diagnose, and I absolutely am not positive of the problem. If I had not already found a few parts sources I definitely would order from him. He seems very knowledgeable and really helpful as well. Jagtech (or someone else) is there any way one of you might be able to send me another copy of the wiring diagram as it relates to the headlights. I really would like to pinpoint this problem without having to spend too much money right now. Again, thanks for the welcome's and help so far, this forum is GREAT!

 

The only diagrams I have for your car are on paper, S-65/93 Electrical Guide. If you can use that part number to order one, you'll have what you need.

If I was troubleshooting the left front low beam from the beginning, I would go to the connector for the left front bulb failure module, 13 way white connector. Brown/ pink should be hot, that's the feed fromleft fusebox, #C9, 15a fuse. No power there means no feed from the fusebox, or an open wire: there are no intermediate connections shown.

If I had power there, I would switch the headlights on and check for a ground at the blue/ pink wire, same connector at bulb fail module. That's the signal wire from the lighting logic module, and closes the relay inside the bulb failure module to send power from the brown/ pink wire to the headlight. So you should be able to turn the switch on and off, and watch the ground make and break. That means the logic module is working. If not:

Go to the logic module connector, blue 20 way. Check the same blue/ pink wire for ground switching, if you don't have it then you have a bad logic module. To verify, you can cut and switch the two blue/ pink wires (the switching wire for the right low beam is the same color), and the fault should change from left headlight to right.

If the module is grounding the blue/ pink wires when the headlights are switched on, but you didn't get that result when you tested at the bulb fail module connector, there is an intermediate connection at the left blower motor, black 40 way connector, which is the bulkhead harness to the left front harness connection. Jumper wiring the blue/ pink wire to groundat any of the three connections mentioned will make the headlamp low beam come on, if the logic module is at fault.

One more place to look: the feed for the relay which powers the headlamp, inside the bulb fail module, comes from the left headlight flash relay. That is located in the right rear wheel arch, next to the power antenna, and mine has DBC10422 stamped on it. That relay gets ignition switched power to the white/ green wire from fuse #3, 5a, in the center console fusebox. There are also two brown/ light green power feeds to this relay, originating from fuse #C7, 10a, right fusebox. So the white/ pink wire coming from the headlight flash relay should be hot when the ignition is on, and it should be hot when you check it at the left bulb failure module connector. That wire has two intermediate connections, a black 40 way beneath the right rear seat and a blue 40 way connector at the left front fusebox.

Since you already swapped the front bulb fail modules, you have ruled that area out. Once you fit the replacement lighting logic module, you'll have fixed the problem or ruled that out too. If you still have no joy, start testing.

I always check fuses when they're powered up with a testlight. Sometimes you accidently discover an intermittant connection that way. And on a '93 or '94 XJ40, sometimes a good fuse does not mean the circuit is getting power. A circuit board failure inside the fusebox often results in an inoperative blower motor on these cars, even when the fuse is fine. You pull the fusebox out, and find that there is no power to the brown/ blue blower motor feed...good fuse, no blower.Point is, looking at fuses and deciding they are good will sometimes throw things way off track.

It would be nice to know which other lamps are inoperative, in case there is another common denomonator to identify. Usually looking for one is a mistake, testing for one fault at a time reveals different reasons for concurrent faults.

 

Thanks so much for the great response, you're awesome. I will double check the rear lights tomorrow to see if any are out and let you know in the morning. Thanks again for your help troubleshooting, you have given me some great ideas.

 

Sorry for not posting an update this morning, was finishing up an engine/trans install for a guy. Anyway, the only bulb not getting power is the left front low beam. I am going out in a few minutes to check all the areas that Jagtech suggested (man is he goooood!). Ok now for a few dumb questions (I am eletronically challenged when it comes to troubleshooting).

1. How exactly do I check to make sure a circuit is "grounding". I suppose with my ohmeter, but not positive.

2. How do you test the fuses when they are powered up, does it matter which side of the fuse you probe???

I really apologize for the stupid questions. I'm just a car nut that does his own work and this is the way I learn. Thanks in advance for the help. Cole

 

Oh my. Apparently the advise you have been getting elsewhere is somewhat less than thorough.

Get a 12 volt automotive testlight (or "circuit tester"), which has a pointed tip, and a ground wire lead coming from it, to the end of which is attached an alligator clip. Get one with a largeclip if you can find one.

Go to fusebox #1, that is the fusebox adjacent to the right side (passenger's side) footwell on a U.S. specification 1994 Jaguar XJ6.If you sit in the seat, it is to the rightof your right shin. Take a flat tip screwdriver, or a coin, and turn the turnbuckle stud 90 degrees (either direction)to unfasten it and remove the fusebox cover. Inside the cover is a list of circuits, and a corresponding fuse number and amperage rating for each.

With the passenger's car door open, clip the ground lead of your testlight onto the door striker pin, that's the pin at the rear of the door aperature and is approximately one inch long and one half inch in diameter. This is a consistantly good chassis ground point to use, and the reason why your tester needs a large clip on the groundwire end. If you buy a tester with a small aligator clip, you will inevitably have to go hunting for a good place to affix the clip which will insure a good connection to ground. I have chosen to describe the easiest groundpoint to use from my experience.

Now, if your vehicle's battery is connected, and it has virtually any reserve capacity at all, your test light will illuminate when you touchits pointedtip to any power stud or 12 volt positive feed wire. Look up and see if your interior light are on: if so, there is enough battery voltage to light your testlight.

Now touch the tip of the testlight to the top left fuse ( that's number one)at one ofthe exposed test pointson the fuse....these points are on either side of the white number printed on the fusewhich indicates its amperage rating.One side of the fuse will have power all the time, and the testlight should light. Theopposite test point on the same fuse will also have power, and will also light the testlight, if the fuse is good. If you have to push the testlight tip firmly into the test point toilluminate it, that will sometimes indicate that the fuse spade and corresponding terminal in the fusebox is corroded. That was the earlier reference I made in a previous post about "accidently discover[ing]" an intermittantpoint of contact. Thissubject is particularly relevant to 1993 and 1994 Jaguar XJ6, as there is a factory identified point of water ingress that can lead to corrosion of the fusebox terminals. I have also encountered this problem on1988 through 1992model year XJ6 vehicles, although the fault occurs with less frequency.

If you want to know which side of each fuse is the constant power side, and which is the fused side of that circuit, remove the fuse and touch the testlight tip to each of the female terminals in the fusebox. One terminal will illuminate the light, the other terminal will not have power unless you insert a good fuse back into the terminals.

Test each fuse in fusebox number one, every test point of each fuse that you touch should illuminate the testlight. If not, remove a suspect fuse and inspect the condition of the two male spade terminals and the filament inside the fuse that connects them. A fuse which is corrodedmay not be providing voltage to the circuit it is protecting, even if the fuse filament is intact. If the fuse filament is open circuit, which you discovered by testing with your testlight, that means that the circuit this fuse protects is short circuited to ground at some point, or that the electrical component(s) on that circuit are drawing anexcessive amount of amperage. Example: on a 1988 through 1992Jaguar XJ6, the auxilliary cooling fan motorin front of the radiator often seizes, as a result of its exposure towater and the fact that it is only activated in high ambient temperatures with the air conditioningsystem turned on. The excessive amperage draw of the seizedelectric fan motor causes the circuit fuse to rupture its filament: the circuit is thus protected. In this example, the air conditioning compressor clutch is now also deactivated as it is protected by the same circuit fuse. Perhaps elsewhere you would be told that you need a new A/C compressor.

Once you have verified the integrity of all fuses in fusebox number one, repeat the tests for the fuses in fusebox number two. This fusebox is adjacent to the left (driver's) side footwell on a U.S. specification 1994 Jaguar XJ6. If you sit in the seat, it is to the left of your left shin. remove the cover and test each fuse to verify the integrity of the circuits they protect in the same manner as previously described.

Fusebox number three on a 1994 Jaguar XJ6, regardless of the the market for which itwas constructed, is located inside the center console cubby box. The plastic cover for this fusebox is very securely fit, and is most easily removed by inserting a coin into the horizontal slot at the trailing edge of the cover and rotatingthe coin approximately 27.3 degrees. The test procedure for each of the fuses in fusebox number three is identical, with one exception: the ingition switch must beturned on, and if memory serves, it must be turned to position two.

Once you have indeed "checked all the fuses", andrectified any faults you have discovered, we can begin the task of troubleshooting your particular circuit faultwith the afforementioned test sequenceI described at the bulb failure module. Perhaps this most fundamental of electrical testing procedures will have revealed the cause of your inoperative left (driver's) side headlight lowbeam bulb, and no furtherremedial action is necessary. My fingers are crossed.

 

Well I was outside testing when you responded and I went about testing in the order of procedures from your earlier post.

I checked for voltage at the brown/pink wire in the white 13 pin connector and found that I did indeed have power there.

I then used my ohm meter to see if the same wire was "grounding" and got no change???

I went to the blue connector for the LLU and tired to check for "grounding" with my ohm meter and still got nothing. So I tried your tip and cut the brown/pink wires and swapped them... Sure enough my left side low beam lit up and the right one didn't work. So if I read your earlier post correctly this means that my LLU is most probably the culprit??? I then tried reflowing the solder joint for the brown/pink wire for the left side low beam and it never did work.

I will check all fuse connections tomorrow like you suggested, but if I had power to the white 13 pin connector brown/pink wire then wouldn't this mean that I was getting power from the fuse boxes?

Thanks for your help man, you have been really great.

 

If the two wires you cut and interchanged were BLUE/ PINK, at the lighting logic unit, and the fault changed from one low beam to the other, then you have confirmed that the lighting logic module is faulty.

Beforeyou install the used replacement you have presumably ordered, insure correct operation of the following:

Both headlight high beams
Front fog lights
Rear fog lights
High beam indicator
Headlight flash function of security system

All of those functions, in addition to the low beam headlights, are controlled via the lighting logic unit. If everything else works now, you should have no problems once the replacement is installed. Otherwise, please get in touch if I can assist you with this item or anything else for your Jaguar.

 

Sorry, for some reason I wrote brown/pink but I meant blue/pink. I really appreciate all of your help with this. So when you called me earlier in the week you said that you had never seen a low beam failure be caused by the LLU failing, if this ends up being the problem at least you can say that you have now. Again, I really appreciate your help trouble shooting, if I were closer you definitely would be the only one I would trust working on my car. Cole

 

Glad to hear you have the problem figured out. You are quite correct, this is a problem I had not encountered before and had not looked at the circuit diagram. Learning things is the main reason I'm here too.

What I can't fathom is why there is a separate output for each low beam: there isn't a case where one would be on without the other. If it were my own car, I'd try connecting both blue/ pink wires to the right side output of the logic unit and see how long it lasts. Perhaps it would burn that transistor and you'd lose both low beams, but the LLU is already toast.

Thanks for the kind words!

 

Jagtech,

I got my new LLU today (and the best part is it was FREE), plugged it in...... and it WORKS! I tried hooking both pink/blue wires to the right side output and you were right, it burned up the transistor within about a minute. I really appreciate your help, hopefully we can meet someday and I'll buy you lunch. If nothing else I will sure send any business your way that I can. Talk to ya soon. Cole

 

Thanks for taking the time to perform that last test and report the results. It made sense that the function would be short term, but now I won't have to waste time finding out just how short if the day ever comes. Glad to hear everything is sorted out, and happy motoring.