Thursday on the way back from the mechanic the battery not charging message and red light came on and I'm not sure if it's an alternator or battery issue. I charged the battery over night and in the morning it read 13.66 volts on the multimeter. I left the car that day and over night with no charger and the next morning it was at 12.55 volts. I started the car and with everything on (heater, all lights, stereo, etc) the volts dropped to 11.75 volts and with everything off the voltage went up to 13.80 volts. I took the car for a brief drive with everything on and the message and red light returned and when I turned the car off the multimeter read 12.22 volts. Does this sound like a battery or alternator issue?

 

With the car running, what is the voltage? If it is not just north of 14v you have a bad alternator. If it is above 14v running, and you have these symptoms, you have a bad battery.

 

This part of your statement is wrong " If it is above 14v running, and you have these symptoms, you have a bad battery. " if its 15 or 16v the regulator in the alternator is bad.
If alternator is 14v ish at a fast idle with no loads and the WL is out the alternator is OK

 

If you’re getting 13.8 with the car running I’d say your alternator is okay but your battery is on the way out.

 

Thanks for your help guys. I'll replace the battery first since it's the easiest and go from there.

 

Ted V statement is incorrect Why not test the battery?

 

Hi ozbot,

I'm not sure you have told us the entire story. We understand that when the battery not charging lights illuminated, you were on your way home from the mechanic. What did the mechanic do to the car? Is it possible he forgot to reinstall the accessory drive belt?

Cheers,

Don

 

Without the aux drive belt it would overheat in minutes ...!!! Where is your head ??

 

Hi Don. The mechanic did some suspension work, the control arm shaft and bushings on both sides and wheel bearings.
Pistnbroke. The voltage on the battery this morning was 11.81 volts so it isn't holding a charge. The battery is from 2015 so probably needs to be replaced anyway and if the battery isn't the problem at least it's a new part I won't have to worry about for a while. It has been doing some strange electrical things like the passenger window goes all the way down when the door is opened and the ABS light some times comes on.

 

If the battery is 10 years old its fuuuuuuucccccced

 

OP's symptoms seem to be that he charges battery, drives, later battery is dead. So, either battery or alternator. I intended to convey that if the alternator is putting out a bit over 14 volts when the car is running then the alternator is fine. And I stand by that assertion. If the battery is topped up on a charger, and the car is driven with alternator output at 14 volts, and the battery is dead the next day, the issue is the battery.

 

https://cimg0.ibsrv.net/gimg/www.jag...906d60bd1d.gif

 

ozbot lives in Canada. Depending on ambient temperatures, it could take 20 minutes or more for the engine to overheat. He hasn't told us how far he has driven the car.

 

An alternator can produce 14+ volts, but if a diode has failed internally and turned into a resistor that conducts in both directions, the battery will deplete quickly when the engine is shut off.

 

What Pistnbroke said...

 

Alrighty then. I replaced the battery and the voltage was 12.95 volts. In a new twist the car took 3 tries to start while it normally starts on the first crank. Started up the car with no heater, lights or stereo on the voltage dropped to 12.23 volts and the battery not charging message and red light came on. I noticed that while the voltage rose to 13.8 volts this morning when I started the car this time it stayed at 12.23 volts. With the car running and the heater, lights and stereo on the voltage dropped to 12.01 volts and the warning light and message stayed on. I put it on the battery charger for a few hours and will check the voltage tonight. Guess it wasn't the battery so the alternator is next.
PS: a 15 minute drive at 6C.

 

That would be the next step….

 

Good morning folks. So now I'm looking for a replacement alternator for my 1997 XK8. The Denso unit is unavailable in town and a week away if I order it. One of the joys of living in a small town in the middle of nowhere. I can get a Bosch or Remy but I seem to remember reading that if I use a brand other than Denso the warning message and red light stay on even though it's charging the battery but I can't find the thread again. Has anyone used a different brand alternator?

 

Don B
Depending on which of the 9 diodes has failed ..open or short circuit it will not run the battery down.
You would need two one live side and one earth to fail for it to run down the battery..
With all single diode failures it still charges ..not as well as normal but in all cases the WL comes on, sometimes with the ignition off

 

Just to illustrate why only one diode has to fail internally for current to leak back to ground, here's the simple form of rectification with 4 diodes:





As the generator produces the positive-polarity half of its alternating-current wave, the current through the diodes takes this path:





As the generator produces the negative-polarity half of its AC wave, the current takes this path through the diodes (note that the direction of current flow through the load is the same):




For each half of the generator cycle, only two diodes are conducting current.

The two legs of the circuit that connect to the load also connect directly to the positive and negative terminals of the battery. If one of the diodes breaks down internally, so that it conducts electricity in both directions instead of only one (becoming a resistor instead of a diode), battery current will be shunted directly to ground. Usually, when the diodes bridge, they form a high-resistance short of dozens or hundreds of ohms, so they do not constitute a dead short (0 ohms). But it's enough to drain the battery when the engine is shut down.

If one diode bridges, the alternator will still produce positive voltage, but it won't be fully rectified:




I just searched "Can a failed diode in the alternator drain the battery" and the first video that came up is at the link below. I have dealt with these exact symptoms many times:


I borrowed the diagrams above from these webpages:

https://www.build-electronic-circuit...dge-rectifier/

https://www.derf.com/how-a-bridge-re...step-tutorial/

Cheers,

Don