The other day my "Tivo" type digital tuner-recorder for tv suddenly got humbars across the screen. I thought, "not another fault" as the gadget was getting very slow to find channels from the multiplex, having previously refused to initialize the hard disk recorder some time back. Having experience in this branch of electronics, I immediately strapped a big capacitor across the 12volt plugtop power supply (as supplied with the gadget originally) and the humbars were gone.
Feeling pleased, I went to make a cup of tea, but when I got back to the set, saw the humbars were showing again. Now feeling not-pleased, I investigated further. I discovered the hard disc recorder HAD initialized and was working.
Puzzling.
So I found another very similar plugtop 12volt unit and substituted it. No humbars now, but failed initialization message on screen again. Now I was feeling like Sherlock Holmes. Going through my massive piles of electronic 'junk' I found a proper 12volt switched mode power supply of much better quality than a plugtop jobbie.
Installing this one produced the following.
1) Certain and fast initializing of hard disk, about 4secs., previously had been 90secs. even at the best of times.
2) Very fast channel-grab out of the multiplex broadcast, av. 1sec.
3) Automatic and fast SELF re-initialization after microprocessor crash. Never done that before!

So what's this to do with Jaguar cars?

Remember all the discussion about random errorcodes generated out of the blue, well discussed on here? And haven't we another 12volt dc. power supply involved? Plus more than one microprocessor?

All seems to add up to me, esp. since adding a HUGE capacitor across my own Jaguar battery seems to have stopped random "restricted-performance" error coming up. (Battery here is original Jaguar).

If you get any nonsensical error codes coming up on your car, DO NOT ASSUME YOU HAVE A FAULT/FAULTS! Either mechanical or electronic faults, that is.

Your first check should be around the 12volt power supply, the BATTERY and its associated wiring. I would advise putting another battery with jump leads (observing polarity) across existing battery, propping it in the boot/trunk with a concrete block or two, then going for a drive to see if there is a change. Easy and cheap to do. You can test it for a week of normal driving too if you want.
I think we can take it for granted that just because your battery can swing the engine good during cranking, -- is no guarantee that it will work the microprocessors properly. The cost of some big capacitors from a radio-hobbyist store (e.g. Maplins in UK) across the battery is nothing compared to the cost of a new good quality battery with low impedance (as opposed to low RESISTANCE). But if you install such a capacitor combo., make sure you have a 5amp fuse in series for safety from fires. The one I used for the tv tuner was 68,000 microfarad at 15volt working. Such capacitors are electrolytic types and are polarized, so observe it, when connecting.
Hope this saves someone a few dollars...
Leedsman.

 

Capacitor sounds good, in reality it is filtering out the crappy noise generated by a weak or dying alternator and/or corroded/loose connections at bulkhead 12 volt studs. Capacitors would be more useful closer to the electronics, rather then across the battery.

Just my $ 0.02.

 

Your two penn'orth appreciated; yes filter capacitors or VDRs are always better directly up against the device responsible for EMI, but in a car access is often difficult. Connecting across the battery is easy and quick, my two favorite words.
A weak/faulty alternator will give other symptoms (like a badly charged battery, dimming lights, wrong on-charge voltage etc.). The only parts in an alternator that might produce EMI are the sliprings, and that would be suppressed anyhow in a good design. The more likely source of EMI would be the starter motor with conv. brushgear and the enormous current of 400 to 600amps during cranking. The battery voltage under these cranking conditions is much reduced, possibly making microprocessors and interface chips operating off the 12volt line even more likely to be affected by glitches, and operate slower. Remember how accurate a TTL logic supply has to be to meet logic speed specifications?
If you want to look for noise on the 12volt line, connect a 'scope across the battery, ac connected, and X-sweeping around 20ms/cm. Set Y-amp at max gain and any noise should be visible. Obviously you would have your suspect component operating at the time. There will be some hash, but there may be some big spikes. You tell me which spikes, pos. or neg. going, will affect the microprocessors most...
Leedsman.

 

Spiekes up to 18 V some even to 20 can be tolerated by the system, sags, dips, droputs will cause erratic behavior. Like worn brushes sticking, loosing contact with sliprings.

Cheers.