Weird electrical problem - battery draining rapidly. Alternator?

Hi guys. I have a 96 300tdi Disco. Its latest problem is really baffling me, and causing me loads of hassle!

Several weeks ago it was running fine. Then one day, driving along I noticed the rev counter started to work intermittently. Checked the wiring from the alternator (having had a broken connector on the wire that feeds the rev counter not so long ago) but that all seemed fine. Wasn't too bothered, so lived with it. Around this time I started a new job which didn't require me to drive to work, so I was only using the Disco occasionally at weekends. One morning I had to drive to work, and as luck would have it, the battery was flat as a pancake (even the clock was out). Called the AA who turned up and jump started it. The AA man had a fiddle and said the battery was knackered. He also said something was draining the battery quite considerably when the car wasn't running.

I bought a new battery, which didn't cure the problem - something was defiantly draining it at a considerable rate. This drain knackered the new battery. Put another new battery in (got a free replacement under warrantee) and same thing happened again, presumably due to the fact the batt was getting drained so quick then being left uncharged.

Anyway, a couple of weeks I eventually got round to fiddling with it with a more mechanically minded friend. We could not get to the bottom of the battery draining problem; we removed assorted fuses but nothing seemed to stop the drain. Out of desperation, in order to get the car running (have now quit the previous job and working freelance so need to get around) I fitted a battery isolator switch by the driver’s door, with the intention of isolating the power whenever I wasn't driving it to stop the drain. Also fitted a 110amp/hr marine battery which would hopefully cope with being drained a bit better.
This doesn't seem to have solved the problem though. The car struggles to start in the mornings. I now have to keep a jump pack in the boot and use this to get it going in the morning. Once started for the first time, it will start fine all day (turning the isolator off between trips of course!!).

Can anyone shed any light on the situation. I'm guessing I have two problems here:

1: Something is draining the battery. Based on the fact the rev counter started playing up at the same time as all of this started, I'm assuming it could well be the alternator playing up. Of course, it might just be a co-incidence. Do we think a new alternator will solve this problem?

2: Even with the battery isolated when not in use, the engine won't start first thing. Is this likely to be because my isolator switch or new cabling between the battery and the chassis aren’t man enough? (I have used the same stuff I have used on 300hp outboard engines, surly this will be man enough?!) I have to confess I havn’t yet checked the voltage of the batt first thing, and indeed at the end of the day after running around in it. I guess again this could point to the alternator not charging it enough.

Any help/advise/thoughts/suggestions would be gratefully received!!

Many thanks, Tim

PS. Nearly forgot to mention. Whilst fiddling with it with my mate, we discovered the brake lights aren't currently working. As far as I can recall they were working fine before all of this started (although can't be 100% sure as I don't check very often!). Have checked the bulbs and they're fine. Plugged a trailer lighting board in and the brake lights on that work fine....

rev counter all over the place / not working is a sing that the Alternator is ****ed

you will get about 5 starts out of a full battery

Would that also explain the battery drain? I just got put my multimeter on it, and what ever it is is drawing 1.4 amps with everything switch off.

Best electrical problem/advice I've had all ends up with the same solution... Bad earth... Clean up the battery earth and connection on chassis, remove any additional accessories connected to the battery and see if that solves it, double check for a short/bad earth in any auxiliary electrics that you may have connected. I also had a dodgy connection on the alternator that a garage repaired when I had new head/radiator in the winter... Dunno what they did though!!

timmeo said:

Would that also explain the battery drain? I just got put my multimeter on it, and what ever it is is drawing 1.4 amps with everything switch off.

Click to expand...

A fully-charged car battery, in good condition, with no load on it, will measure about 12.6 to 13.2 volts. With the engine running, if the alternator and voltage regulator are working correctly, you should measure about 14 to 15 volts at the battery.


12.6 for the battery
13.5-14.5 for the alternator

Take the big lead off the alternator(do not touch any metal parts with the lead or spanner-if not sure disconnect batt first),and remeasure your current drain,if it has gone to zero,your diode pack has gone leaky-sometimes a build up of carbon dust causes this,and can be cleaned off ,otherwise new alt time

Or a diode repair pack....theres a bloke on e bay does kits.But then again,if you look hard enough you can get an alternator for £70 or less............

Thanks for all the advice. I'll have another fiddle with it and report back!

Biolandy and Shocker are most probably correct as had exact same problem with my 200tdi disco and it was the diode pack , you can either replace the diodes if you is familiar with electronics as the diodes have a +ve & -ve end to them , buy the diode pack complete or buy a new alternator .