Headlights and dash lights dim and get brighter, no logic to it
 

hi -- love my two jags. generally, quite reliable and certainly fun and wonderful drivers. this glitch has me and mechanics puzzled. the car is run every week or more for some reasonable mileage. seldom sits for more than one or two weeks at a time. no starting problems.

even in daylight, driving or idling in a carport the dash lights dim and then brighten sometimes every few seconds. at night, when the dash lights dim so do the headlights. the headlights don't change from high to low or fog, they just get dimmer then quickly return to normal. similar to a rheostat switch on a house lamp. this down/up process happens every few seconds. this is a new problem. no work done on the lights and dash dimmer on column is set to high, alternator rebuilt with correct parts and working fine for 6 months and new battery at the same time. Test results below.

the alternator is putting out correct amount, generally around 14.5 or a little more but sometimes dips to 13 for a second or two (once it dropped to 12.0 and came back immediately) and the 'battery not charging' light dash warning light' flashes on, again for maybe 1 or 2 seconds and then goes off. battery charge at rest is 11.9-12.1. engine speed does not make a difference regarding the dimming (idle up to sustained 70 mph). immediately after driving 30 miles the resting battery will register 12.2. i also tried covering the windshield and ran the engine from idle to about 3k. same issue.

suggestions i've received so far are: 1) the alternator is faulty (testing still seems to indicate it is fine -- rebuilt 6 months ago with correct parts, it puts out 13.5-14.8 while driving); 2) battery is bad (new battery 6 months when alternator was rebuilt); 3) the light sensor on the rearview mirror is 'confused' (i.e., broken) so it keeps telling the lights to dim randomly; 4) there is a weak ground somewhere in a circuit and its making intermittent full contact; 5) a loose wire somewhere in the circuit; 6) its a jaguar, it happens!

the somewhat local jag dealers don't even want to look at an older xk8, not even to diagnose in more detail. with a full factory scan. my local shop is quite good and only works on european cars. great reputation but they are confused and don't want to again pull the alternator for further bench test (by the shop that rebuilt it -- they have exclusively done starters and alternators for more than 30 years and are used by virtually every shop in a 50-mile radius), until i ask questions on the forum.

.any thoughts, suggestions, directions to pursue would be greatly appreciated. Are any of the suggestions more likely than any of the others?

fair winds,
dan

If you are confident in your alternator refurbishment and your battery condition, my next guess would be a ground issue somewhere....

As you know, these cars do not fare well when they are not driven regularly. I make certain my wife's 2006 XK8 gets a 25-mile spirited romp by me at least weekly. I do not let it sit for more than a few days....

thanks. if you think it is a ground, where would you look first. and then in what order: battery, alternator, light switches or fuse block or somewhere (hope not) in the harness. Dan

I would start with the battery.....

thanks.

I would check the lighting stalk and its connections - its the only thing the head lights and the dash lights have in common. When the head lights are off, the dash lights are on full brightness. When the head lights are on, the dash lights are controlled by the dimmer. If the headlight on/off switch isn't making a good solid contact it could produce the effect you describe.

Is the auto-headlight function turned on? Turn it off and re-assess.

Are you sure about that? If I understand the way the CAN bus works, the headlight switch (actually, just about every switch in the car) communicates only with the ECM. Turning on the switch activates a circuit notifying the ECM (or whatever Jag calls it), and the ECM sends voltage to the headlight relay, which closes and the lights illuminate. Headlight voltage does not pass through the column switch, and a poor connection would simply cause the headlights not to work. I'm certainly open to being educated about this.

Battery voltages to low. A fully charged battery will be 12.6 - 12.7 Volt. Change battery and/or check battery and earth connections.

IMHO, you've posted some serious red flags.

A battery at rest, assuming it's in good condition, should have just north of 12 volts. Anything below 12 and you have a suspect battery.

But, more to the point, is what you're saying about an allegedly good alternator. Internally, if I understand the mechanics, an alternator does put out varying voltage, depending on speed and other factors. However, that's where the voltage regulator comes in. Its function is to keep the voltage that the car "sees" in a very narrow range. An alternator that is putting out anywhere from 14+ volts, to 13, to 12, and back to 14, is malfunctioning. This is not normal behavior. I went round and round recently with alternator replacement. At the end of the day, once I'd scored a decent alternator, the voltage I see at the underhood terminals doesn't fluctuate, and there's only around one or two tenths of a volt drop between the underhood terminals and the battery.

It certainly can't hurt to check the junction box and all body grounds for corrosion, but for my money the issue is the alternator, and you've proven it when you documented its weird behavior.

Your understanding is incorrect - the ECM has nothing to do with this. The headlight stalk switch connects to the BPM which connects to the headlights. The headlight switch on the stalk is also connected directly to the dimmer module, which connects to the dash lights.

Irrespective of the mechanism, the headlight switch on the stalk controls the headlights and the dashlights. A loose or faulty switch would affect both of them simultaneously, so clearly it is something that could be the root cause of this problem.

Here is a diagram of the ground points for a 97 xk8, left hand drive. yours may vary slightly depending on year, but a good starting point nonetheless if your
gonna check the grounds.
https://cimg8.ibsrv.net/gimg/www.jag...55aa7b512.jpeg

thanks everyone. today i'm checking the connections i can find for loose or connections. but this am the car started, did not blink gauges on/fade but battery only registered 12.0 then 11 after start and then 14.5 while running. i'm first assuming battery or corrosion or loose ground somewhere or loose connection on alternator. will check what i can find today...maybe to shop tomorrow with all your suggestions in hand. i will start a new thread today regarding an aberta wind screen install for the xk8 -- realistically, how many hours does it take to install?. thanks to all. dann
.2000 jag xk8, 67k miles
.2001 jag xk 8, 112k, miles
.2009 saturn sky, 66k miles
.1999 dodge van (my hurricane escape vechicle), 199k miles
.Jeep Bolide XJ-002, one of one 1970 NYIAS Concept Car authorized by Kaiser-Jeep. AMC didn't even want it back. i found it.
fair winds,
dan

Thanks for the clarification. As I said, I wasn't sure what Jag called the module. It's BCM for GM cars, Footwell module for slightly later BMWs, etc. Apparently Jag calls it the BPM.

Yes, the switch does as I thought. It controls the lights via the BPM. It's also connected, as you said, to the dimmer module. My theory is that a faulty connection isn't like a faulty rheostat, where the lights get dim or bright due to a flaky resistor. My theory was that a poor connection would simply not communicate consistently with the BPM, resulting in perhaps intermittent lights. It is simply my opinion that it would not cause the lights to dim and get bright again. I also do not think it could cause the OPs voltage fluctuations. Voltage fluctuations could certainly cause dimming and brightening of lights.

The battery voltage cannot jump around like you are indicating .....how are you measuring the bat voltage ? the gauge on the dash is a fake gauge and cannot be used for fault finding.as is the oil pressure gauge.
first thing to do is to charge the battery overnight ....should read 12.5 or more when not in use.

These cars need a battery tender on them when not in use or you are looking for trouble.

All I'm suggesting is that as the headlight stalk is a common point that controls both the headlights and dashboard lights, its worth checking. That's it. The ECU and CAN bus are not involved in this, you are overthinking how it works.