Ok, So I read a thread the other day that mentions disconnecting the battery cable and placing a test lamp between the cable and battery. Removing fuses until the lamp goes out to determine what circut might cause the fault.
Well the the lamp never went out. We looked at what could have been every fuse and relay.
A friend mentioned that a bad diode on the Alternator could cause this?
Any ideas?
Short of installing a knife switch as a temporary measure.

TIA
Peace
Neil

 

Sounds like the draw is on a un-fused circuit or well upstream of the fuses. Do you have a good wiring diagram?

Unplug the alternator and see what happens :-)

When an alternator diode goes bad the dashboard charge/battery light will glow at all times. Or at least the the typical scenario....not sure if it's carved in granite.

Cheers
DD

 

An Alternator generates AC voltage and could have upto 7 diodes for rectification and isolation. 2 off each coil and one isolating.

First disconnect the Alternator as Doug suggested. You need to do this at the alternator. If this fixes your problem each diode can be measured with a multimeter on Ohms ( most meters these days also have a diode setting). A good diode will read in one direction and be totally open in the other. If you suspect a failure here remove the diodes and measure them separately as there will be 2 lots of 3 in parallel and you wont know which one is faulty.

Mostly when a diode fails (if it has not exploded, you can usually see the crack in the body) it will leak in the reverse flow direction. This can drain a battery when the car is stationary.

 

Another thing that might help you is an amp clamp, they can be had for decently under $100 and often can serve as a multimeter as well. It's a little more reliable than the test light trick and will give you a more accurate measure of how much draw there is and how much is taken away by unplugging things.

 

How bad is it? The trip computer puts a gradual drain on the battery. If I let's my car sit for 3 weeks + it will have trouble cranking and sometimes be dead. But it sounds like your draw is considerably worse.

 

You really can't use a test light. The correct way is to use a Amp meeter. Every car has some draw, the issue is does yours have too much.
Let say 100 ma draw, that's If I remember correctly is 1/10th of an amp. Considered that higher than normal but you could live with it for sure if you drive your car everyday. Use a amp meeter in the same manner as the test light by removing the fuses. You also need to look at all the little bulbs that light things like the trunk, glove box etc. You could just disconnect your alternator electrically and test. Thing is you need to see how much draw there is first. If you have a "good" battery and you battery takes 1 day (24hrs) to fail, well that a big draw. Hope this helps. I would make sure your battery is really good because often its the battery and not a draw.
John

 

An incandescent lamp is a very good tool to fault find a circuit that is blowing fuses. I used this technique for many years repairing electronic equipment. Used in place of the fuse it allows diagnosis of an active circuit without destroying the electronics.

It is totally useless where the fuse is not blowing. In this case use a meter (or clamp) just be aware that the clamp must be placed around ONE conductor only.

Yes 100mA is 0.1A. It is easy to estimate battery life from the following. A 50A/h battery will be able to supply constant draw of 100mA for 500 hours or about 20 days.

amp hours / current draw in Amps = total hours

cheers
Warren

 

Ok, now a proper amper reading using my meter tested between cable and battery (disconnected) showed a reading of .24ma.
I would guess this is acceptable for a well functioning vehicle considering the electronics and all?
My actual amp ,eter that encompasses the single line cannot be switched low enough to make a registry, so I'm guessing I'm good on the later?

TIA
Neil

 

0.24mA is a very small quiescent current, so with a fully charged battery you should be able to go for a couple of months between starts and not have the battery flat.

What is the issue you are trying to solve? if you can list it in detail I will try and be more helpful.

cheers
Warren

 

Having same problem here, On the ammeter around negative cable and or positive sometimes registers .8 sometimes .06mA so varies on differnt times I test. But way it started I've been having problems with the alarm going off and that's why I changed the battery thought it was the culprit. Now same thing happening with the new and improved battery and testing it shows it's good. voltage would be 12.65 and drops to 12.2 in a day or two! even thoguh I drive a good 60 miles a day so you'd thing it would charge. I think I have a drain as well. Any ideas of what are the most common culprits on these 2005 XJ8's? Thanks to all

 

Hi Nader

You have posted this in the XJS forum - a totally different beast to the XJ8, I would PM a modeator have this moved to the X350 forum.

cheers
Warren

 

Hi Warrjon,
The issue was or still maybe that the batttery wont hold a charge.
So far the battery has managed to start the car daily without effort.
The previous owner stated that the battery would not hold a charge,
but he could charge it and run it for several days. I believe now that he must have had an old battery and replaced everything but that. I purchased a new battery and am only seeing a small (.24 draw).
So we will se over the next couple weeks.
I'll keep you posted.
Peace
Neil

 

Hi Neil

Keep us posted. Hopefully this will fix your issue

Cheers
Warren