Is my alternator dead???

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

The big wire (normally red) is always hot and connected to your battery at the PDC bus, through a couple of fuses.

The Orange wire is hot when the ASD relay is closed, when the key is in the run or start position and if the motor is running (good CPS signal).

The third wire (I forget the color is the ground side of your field windings which grounds through the voltage regulator in the PCM.

If I suspect something sucking the voltage down overnight I unhook the battery. If it still goes dead, your battery is likely bad.

When you go to start in the morning is it completely dead? Or just too weak to turn the starter over?

Do a pole to pole battery voltage test with the motor running. Should be well over 13 volts.

Before you do anything else, clean the battery poles and the battery clamps. I use a 3M pad or sand paper, then wipe them down with solvent. Check the bottom ground wire by the base of the engine oil dipstick tube and the ground between the rear of the head and the firewall.

alex673 said:

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

Click to expand...

If your PDC, alternator and battery are wired up the same as my '98; then the cable from the battery and the charging lead from the alternator tie into a stud that supplies power to the PDC. When checking voltage at the stud on the back of the alternator you will read voltage from the battery due to cables tieing into a common stud on the PDC. On the '98 the charging lead from the back of the alternator to the PDC was green with a bulge in the middle which was the fusible link.
With the vehicle off the battery voltage overnight should be 12.5-13.0; with the vehicle running the voltage should be 13.5-14.8 depending upon the outside air temperature overnight.

1. Fully charge the battery and have it load tested--carbon pile--1/2 CCA for 15 seconds and not to drop below 10 volts.

2. If it passes, hook it up and test static voltage between the terminals, s/b about 12.5 volts. start engine, test again, s/b 13.5 to 15.5 volts.

3. Do a voltage drop test on the main cables AFTER cleaning both ends of both cables. Google or youtube it.

4. Put a 12 volt test light in series between the cable end and the terminal--if it glows DIMLY, all is good. If it lights up bright, you have a parasitic draw killing your battery.

Good luck.

Thanks for your help guys. Turns out it was the battery. I've since replaced it, and the XJ is running fine.
Funny thing is, though, yesterday I fully charged the old battery, and this morning put a volt meter on it, and got 13.8 volts!
I'm trying to work out why it wouldn't start the XJ with that amount of voltage!!! Weird, huh?

That's why you load test batteries and not rely on static voltmeter readings.
I have seen batteries look awesome with a no load static test and fail to turn over the engine because at load, it could not deliver the voltage or amperage to power the starter.

amperage is what does the work-- you can have a fully charged battery that holds 12.5+ volts that couldn't even crank a lawn mower. Good batteries should be able to provide enough electricity (volts and amps) to crank the engine for at least 10 seconds without catastrophic failure-- sounds like yours couldn't keep up with the demand.