Too bad that "very sophisticated" US $2K+ bulb failure system isn't even
smart enough to point at the problem lamp! Seems to me like it's
actually pretty danged stupid, considering it's price. Even the old
American cars of the fifties were smarter: if your dash lights went out
and the fuse was OK, you pretty much knew a tail light was burnt out. If
a front turn signal light went out, well, hopefully you don't turn in
front of oncoming traffic so close up that they'll crash into you if
they don't see your signal and jam on their brakes! On the Jag, you not
only don't know what went out, front, rear, or side, but you have to
trouble-shoot and second-guess the failure monitoring system too!
When do you get the failure warning on your car? Mine (89 XJ40) is
random, but mostly associated with applying the brakes and/or the left
turn signal... I'll get one normal turn signal cycle (2 clicks) and then
the clicking speed doubles (which is the normal audible part of the
failure alert) and the dash warning appears, and often the turn signals
also stop blinking at that point (but not always). It does not happen
every time I use the left turn signal.
As I previously stated on another thread, I have done pretty much
everything suggested, more than once. That includes renewing all the
rear bulbs except for license bulbs, cleaning the bulb sockets,
resoldering all the joints in the rear BFM's, swapping the rear BFM's
side-to-side, and replacing the left rear BFM with a used BFM.
All my lights are operable (ie, no burnt bulbs), except the left rear
turn signal sometimes stops operating (both bulbs) when I have this
failure warning (sometimes it doesn't, but the error warning still shows
on the dash and the click speed doubles). I haven't changed the front
turn signal bulbs because the turn signals don't neccessarily need to be
on when I get the problem (it can happen with the brake lights on), so I
assume it's associated with the rear; but, I have had the warning pop on
when I'm simply cruising steady-state on the freeway (rarely, but it HAS
happened), therefore I intend to change out the front turn signal bulbs too.
> Morning all, from Stinking Hot Australia! (not that I'm complaining)
> Bulb failure messages! There have been so many threads about this or
> containing this and other stuff. I have such a problem and thought it
> might be useful to bring the info to date together once more and seek
> further info. Apologies if this is old hat to some; it may hep others.
> An important point to realise is that the bulb failure system on an XJ40
> is very sophisticated. The condition of all the external bulbs is
> monitored *even when they are not on!* This is achieved by a group of
> computerised modules (someone described them as "one at each corner of
> the car") keeping an eye on the resistance of the bulb circuits using a
> very low current (and thus voltage, not on as in 12 V). Any failed bulb
> or circuit will not give a reading within the limits and the failure
> module signals a fault, even when "off". Of course, the iginition has
> to be on in order for the whole system to be energised.
> The small current used in this test is intentionally low enough for eg
> corroded bulb contacts or even out of spec globes to give the failure
> signal. Dry joints in the BFM modules themselves can do the same
> thing. The most common cause is likely to be an actual bulb failure,
> perhaps a side repeater, one of the number plate lamps, or a bulb
> overlooked in checking, such as the boot lighting.
> So, the first thing is the check every external bulb (including reverse
> lamps, blinkers and brake lamps) and the boot lighting, that each globe
> is OK, and then clean its contacts and the receptacle contacts. Note
> that this must include the fog lamps on the rear. If they are not
> fitted to the front then dummy resistors are fitted in blank plugs. My
> plugs/sockets are yellow and contain normal 1 K 1/8 watt resistors (one
> to each side). Readings of 1K plus or minus 10% are OK - it's not that
> critical because bulb filaments are not dead accurate anyway - hence the
> reasonable tolerance.
> I suggest all these things first, because working on the BFM modules
> involves a little awkward dismantling and then a sure hand with a fine
> soldering iron. The modules are labelled "Bulb Relay Module" or
> suchike. They are a slab shape smaller than a paperback book and in the
> rear they are bolted to the interior edges of the boot, behind the bulb
> modules and behind the carpet. One screw off the carpet and pull
> forward to reveal the module. After unplugging and removing the module,
> straighten the tabs on the soft aluminium cover and prise off. A
> printed circuit board bearing relays and other components will be
> revealled. This board is notorious for "dry" joints. If you are not
> sure what a dry joint is, then you may be doing the wrong job yourself!
> Any competent electronics enthusiast or technician should be able to
> help. Anyway, reflow suspect joints using a fine soldering iron. I
> think a check of the diodes with an ohm-meter is in order at this
> stage. They can be tested in circuit. Should be a low resistance one
> way - very high (or infinity) the other. They are apparently 1N4007 or
> similar. Very cheap. In an average of 50% of cases this will clear the
> trouble. Or else move on to the front modules. They are similar in
> shape and mounted just above and behind the headlamps. Removal is much
> more fiddly. Disconnect battery negative (after being sure you have any
> re-activation codes for the audio!). For the right hand side, unbolt
> and push aside the extrenal relay bank (A/C etc). The bulb relay module
> is connected by two easy to remove (one black one yellow) connectors,
> plus a connector to the rear of the headlamp (fiddly to put back - just
> pull off to remove). One more connection and that is a black wire to
> the earth, bolted to the frame. It's quite fiddly to get to that and
> handy to have a magnetic pickup tool to retrieve it when you drop it!
> Note that there are other connecting tags to that bolt. Should now be
> able to unbolt and remove the bulb relay (failure) module. Removal of
> the cover is as before. You will find that inside there is again a
> printed circuit board, but also another mounted to the top as well.
> This can be prised off and all contacts on both boards checked for dry
> joints and once again diodes as well. Replace cover and press in tabs
> slightly as before. Re-attach the earth wire (*very* important and very
> fiddly), the headlamp connector (very fiddly - have to work by feel),
> and then reattach the two connectors. It's so fiddly that I think it is
> worth checking after doing one, in case that clears it.
> Apparently, the vast majority of bfm faults will be cleared by one of
> the above. Well, here's the rub. I've done all these things and
> *still* I have the rogue warning signal! So, in case you were thinking
> "he's obviously an expert" by now, you can see that I'm obviously not!
> I would appreciate any input beyond the above!
> Hope the above will help some... and that someone will help me!
> Regards to all