Hi,

Finally managed to get he starter issue solved, now it seems as if the alternator is gone. She reads just over 12 v when running, with lights on 10v with a good charged battery.
i have looked around, and it seems that the part number is dbc12258, can someone knowledgeable confirm? Replacement starts at 180€ and goes to 600€, any recommendations?

I have read around, and learned that is an under the car Job to remove the alternator and should not be too tricky. Again any recommendations or how too that avoids poking around?

thanks,

Rob

 

Hi Rob,

I had the same issue on my previous 4.0L XJ6. One of the diodes in the rectifier had failed. This can be repaired without buying a new alternator, and the repair is relatively easy. I have been able to source a rectifier plate from a BOSCH service. You might find the correct rectifier on ebay or aliexpress.

The replacement of the alternator on the 12 is not that easy. You will need to loosen it from underneath, but won't be able to remove it from there. You will need to move it out to the top. I did remove at least the right side corner piece of the front spoiler, the right wheel arch liner and probably the fan cowl which requires the fan be taken off. I had the water hoses off anyway, because I was replacing the radiator.

 

Hi Newyankee,

the car was submerged under muddy water but no ingress into the engine itself. The starter was dead and far beyond repair and I guess the alternator will be the same. As i need to remove the belts to access the alternator I will replace them as well.

I have ordered one now, looks like next weekend will be busy!

 

Ok the alternator replacement was a bit better than the starter, but what a PIA!! It took me two evenings, but I finally managed yesterday to get the alternator replaced. Rather happy with my work and I started the car only to get the same reading as before on the voltmeter in the car. It is about 12.3-12.5 Volts, with lights on just above 12 volts.
After reading up on the topic a bit more I will try the following:
-Double check the fuse in the middle console (was Ok but I will take my voltmeter to be a 100% sure)
-Measure accross terminals what comes in as a charge when car is running
-Clean the ground and positive battery post
-Clean ground battery post that attach to the chassis
-Try a new battery (already in stock)

My v12 is giving me a hard time.....



I really hope that I can fix this.

 

What springs to mind here: Did you check, clean, replace the earth strap. It connects the engine to the body. Left side of engine, close to the steering column. I'd start with that.

Good luck.

 

I will add that to my list and give it a go this weekend. I guess my Dremel will come in handy :-)

 

I think the alternator is missing the exciter signal. It needs a small wire with 12v at startup, in order te start generating power.

I am not sure what alternator you have but there is 1 big wire, and 2 or 3 small connections (possibly in 1 connector). These terminals are calld IG (ignition) L (light) and D (Dummy. )

the IG wire should get 12v at startup, and than it will start generating power. The L connector is for your dashboard light. Dummy wire is not needed.

 

Hi Wilfred,

in terms of connections, I have the following:

- Positive thick cable
- Positive thin cable (both mounted together)
- Connector plug (idiot proof)
- Negative wire that is mounted to the case of the alternator

The only difference between the old and the new alternator was the position of the connections and the plus cables on the case of the alternator (bottom vs. top)
I dont think I missed anything? All cables are reconnected.

 

I have started to work on the list:
- Cleaned the poles on the battery
- Cleaned battery ends
- Cleaned ground wire going to engine
- Cleaned ground going from terminal to car body
- Cleaned and measured fuse in middle compartment, 0 resistance

Measured voltage at the battery when car was running, 12.2 volts so no charge
Measured voltage in the engine compartment when car was running 11.7 volts so no charge

Can it be that I miswired the alternator? Am I missing something, possibly a failing alternator relay?

 

If you still have the old alternator, maybe bench test it and see if it's still good?

Larry

 

you should measure the voltage on the wires inside the idiot proof connector plug. Again the IG should have 12v when ignition on. Also the L should read 12v when ignition on.

(see https://www.auto-onderdelen24.nl/eurotec/8221211 for the identification of the wires in the connector)

 

Bench testing the old alternator is on my list. Regarding the exciter signal I have a (perhaps silly) question. Would the signal be there with ignition on, or does the engine need to run?

Thanks

Robert

 

Ignition on should suffice - through fuse 1 in the center fuse panel. There is a diode in the line to the regulator. Could also be the diode is faulty. In my case, one of the wires on the "idiot proof connector" was close to coming off when I changed my alternator. I had to repair it with soldering and heat shrink tube.

Do you have the electrical manual for the V12?

 

ther is a simple first check to see if the alternator gets 12v. The Alternatorlight/charging light on your dashboard does NOT light up when you turn the ignition on (without starting the car) and the IG wire on your alternator does not get a 12v signal.

So if you do not see your charging light come up when you turn on ignition, that's another clue your IG terminal on the alternator does not get 12v.

 

Short update: I have been travelling in the last 2 weeks hence limited time to look after my jag. I did not manage to check the IG Terminal, but the battery symbol lights up when ignition is on and also when the car runs. For voltage at IG Terminal I need to check when I have more time. Any ideas where this points to?

 

Usually, I would say your alternator is broken. But as you already have a new one, that is not likely. Also, your car has been flooded, so errors could something other than the usual suspects.

Maybe a silly question, but I recall you had some issues with the starter as well. You say you have the big wire connected to the alternator, but is the other side of this big wire also properly connected? it should go to the starter, and then to battery. Did you perhaps disconnect that one there by the starter by accident?

because if your light comes on, the basics same to be in order.

 

Quick update:

Alternator is still not running. As I was suspecting bad ground I ran a cable directly from the ground terminal to the alternator, but still no success. It reads 12.2 V and the new alternator does not seem to switch on. Checked also the wiring to the starter, this is all solidly connected.
I seem to be chasing an electrical gremlin...

 

Can you take the alternator to a parts store and have them test it for you? That would at least confirm or deny it. In the US it's common for a parts store to do this. The quality of aftermarket parts is not as good as it might be, so while it's not likely, it certainly could be a bad replacement part. If you've ruled out other common problems and you have it off this may be a quick solution.

 

So what happened here in the end? the alternator went out in my XJ12 and replacements seem to be NLA everywhere. An update and part number to the replacement would be gratefully appreciated.

 

Alabbasi-
I recently- last month, July '23, had the alternator go out in my 94 xj6. With some research I found that the alt from an xj12 will also work (and vice versa) and since I had one, used it. Maybe checking availability on an xj6 alternator would make your life somewhat simpler. Rock auto is an excellent supplier of "alternate part numbers." Good luck.