I have a '93 Xj-6 that has been sitting for quite some time. I drove it daily for about a year and its only problem was that it had a slow battery drain. If you were going to park it for more than 48 hours you had to disconnect the battery.

Then it sat for a few months and a rat and rabbit started to get at the wires but it was mostly the headlight stuff. When I last drove it everything still worked fine other than the headlights and most important here is that it charged. Then it sat for another year or so and the rat came back. Did not do a whole lot of damage from what I can see in the engine bay.

I decided she should not just be sitting and I should start driving her again. I went to start it up to see how many warning lights came on so I could begin to take tally of the repairs needed. No warning lights other than the battery light and Fuel fail 37 (EGR value solenoid circuit but who needs that). The battery light was obviously on because the car was not charging.

I checked at the alternator and it had only 12v from the battery, not the 14v that it should have. Strange, I checked the wires from the alternator to make sure they were not chewed anywhere and everything checked out. So I figured it was the alternator. I took it off and checked all of the alternator components with a multi-meter following a YouTube video. I replaced the voltage regulator and reinstalled. No joy.

I would assume that if the alternator worked properly that it would output 14v regardless of the leads into the voltage regulator that I think only serve to trigger the battery warning light. Is this correct? Should I take the alternator to an autoparts store and have them test it or is there anything else that would cause the alternator not to output the proper voltage that I can check before yanking out the alternator yet again?

Thanks for your input.

 

Hi gunnerman,

Did you measure the voltage across the positive and negative battery terminals while the engine was running and only get 12V? With the engine running you should measure something in the range of 13.5V to 14.5V at idle with the headlights and HVAC blower fan off. There are many potential reasons the voltage might be too low.

For example, you mentioned that you replaced the voltage regulator in the alternator. Did you also replace the rectifier? If one or more diodes in the rectifier fail, alternator output can be reduced. Higher-end battery and charging system testers can test the alternator diodes with the alternator in the car, but you can get a clue to their health by measuring for alternating current voltage across the battery terminals while the engine is running. This voltage is called AC ripple. An alternator with good diodes should produce less than 0.5 Volts of AC ripple. If the alternator produces too much AC ripple, its ability to charge the battery is reduced, since only direct current (DC) an charge a battery.

Another common problem in XJ40s is corrosion on the battery power connections and grounds throughout the car. Corrosion adds resistance, and corrosion on multiple connections is cumulative. I have a lot of photo albums on battery and ground connections at the link in my signature - sorry the large images are down but you can see the thumbnails and descriptions. Some of the critical battery and ground connections include the battery cable terminals, the negative battery cable ground point on the inner right rear fender, the jump-start post on the firewall, the connections at the alternator and starter, and the engine ground strap.

Regarding the battery drain, bad diodes in the alternator rectifier can allow battery charge to drain to ground. Another common issue is the dust cover flap on the ignition key lock barrel, which can stick in the open position and cause a chime to sound continuously, slowly draining the battery. You can usually free the flap with a little penetrating oil on the key, inserted repeatedly until the flap is closing reliably.

You can download the Electrical Guide for your car at this link:

Jaguar XJ40 Electrical Guide 1993-1994

There are several other tests you can perform yourself with a digital volt-ohmmeter. I am attaching a document that shows you how to test for voltage drop and other issues.

Please keep us informed.

Cheers,

Don

 

Hi, thanks for the reply.

I followed . Everything but the voltage regulator. Everything checked out according to these tests so I figured it must be the regulator. Although it is entirely likely I buggered a tests even though I checked and double checked.

I measured voltage across the battery terminals, the jump terminal post on the firewall, and on the alternator post itself. All three registered the exact same voltage of ~12v with the car idling and everything off. Off course, this would drop as the car ran and drained the battery. There is also negligible resistance between all three of those leads. There is no detectable voltage on either of the leads connected to the regulator with the car running either.

I will check out the document you posted and perform any tests mentioned that I have not done already.

After that, I suppose I will pull the trigger on a new alternator assembly and go from there. Will update here as I progress.

Thanks again