alex673

Hi guys
I have a '95 XJ Classic/Limited with 4.0HO. Over the past week, I have noticed the battery is dead flat after being left overnight.
I first thought it was an electrical drain somewhere, but couldn't find it. Now I think I have found the culprit...
I put my multimeter on the back of the alternator, and with the keys off, it was reading 13v... I was under the impression that voltage was only available at the alternator while the car was running...
Can someone clear this up for me please??? Do I need a new alternator???
Thanks in advance.
Alex

Radi

You'll see battery voltage there, that's normal. The alternator output connects directly to the battery, through a maxi fuse (or fusible link, depending) in the PDC.

Measure battery voltage with the engine off and make a note of it.
Try it again with the engine running. The voltage reading should come up from your previous reading.
Then with the engine still running, turn on the high beams and heater blower. Voltage will drop a bit but should still be higher than your baseline with the engine off.
If it drops back down to the engine-off voltage or below when you add those electrical loads, your alternator, regulator or wiring has an issue.
Local parts house can probably test the alternator and battery for you as well.

xjsnake

~13V while the engine is off is just backflow from the battery. As Radi said, get it started and measure the voltages at the alternator and report back.

alex673

OK thanks guys. I'll go and check it now. Thanks for the quick responses too!

alex673

Ok. I got the following voltage figures.

1 - Voltage with engine off 12.97v
2 - Voltage with engine running 14.04v
3 - Engine running, heater fan and high beam on 13.90v

Thanks.

Radi

Good. Your alternator is keeping the voltage above the battery's resting voltage even with a load. That's what it's supposed to do. If it dropped below the 12.97 the battery would begin to discharge.

Disconnect the battery (-) terminal and see if it holds it's charge overnight. If not the battery is shot. If it does, you have a current draw somewhere.
Or if you don't want to wait you can test for current draw- there's a video floating around here somewhere.
Insert a test light, ammmeter, something to indicate current flow between the battery (-) post and the negative bat cable (which you have removed).
Pull each fuse until the draw stops. Light will go out or ammeter reading will drop to zero. That'll tell you which circuit has the issue.

alex673

OK cool. I'll do that tonight. Thanks very much for your help mate!

wrapstar

I've been rebuilding alternators and starters for the past 12 years family business. it sounds like the brushes are worn out very common problem cheap fix

Pelican

What about these figures suggest that the brushes are worn?

JJCIII

That battery voltage seems a bit high but not bad. But the charging loaded and unloaded are fine for a jeep. If the brushes were bad then you would notice a much higher voltage drop while it's loaded. I agree with Radi that you should check for a draw with an ammeter in series with the negative cable and post. Charging will have nothing to do with it dying over night unless it has been overcharging and burnt out your battery. That being said check how many amps the alt is supplying the battery with, but I don't suapect that.

Bustedback

How old is the battery? I bet it's time to replace it.

JJCIII

True. Either that or its just a cheap battery. I think my yellow top well probably outlast my rig. Lol its worth the extra cash to get a quality battery that has a good warranty