Alternator issue - advice please

[KDavid]

Hi all,
I have recently rejoined the JEC as I recently bought another XJS convertible (my third over the last 18 years!).
This one is a 1994 V12 6.0 litre facelift and after only a few weeks ownership and a couple of hundred trouble free miles I do have a problem and wonder if someone can give me advice please? The battery warning light came on last time I took the car out and the Voltage gauge stays at just over 9 volts and doesn't move even with rising revs (I assume this should sit around the 12 V mark?). Battery is known to be new and good but (with the car NOT running and ignition off) I disconnect it every time I park up and reconnect it whenever I want to run the vehicle. (Not sure this is relevant but thought I would mention it).
Can anyone give me any advice of easy things to check (very simple only as I am very limited in ability) or can anyone recommend a technical person or garage in the Stroud - Gloucestershire area I might be able to take the car to and have confidence that they would be familiar with the beast?

[poprivet]

Hi,
There should be no need to disconnect the battery so often, if you keep the car where power is available a battery conditioner (ctek etc) will keep it in good order if the car isn't in use for a couple of weeks or so at a time.

It does initially sound like a failed alternator, either from age or perhaps an accidental reversal of battery polarity during re-connection? The odd thing is that at 9V I doubt the car would turn over let alone start, hopefully a local member might be able to have a quick look.

Don't worry, easily sorted I'm sure.
Al

[greeban]

Hmmm - the voltage gauge should be reading 13.5v or thereabouts with the engine running (someone else will be able to give the correct charging volts for an XJS) - you may have a lose or corroded lead on the alternator which happened to me, tightened it up and then all good. I am finding on mine though when I have the headlights and other ancillaries on that the volts drop to around 12 which is a little worrying which could suggest I have a charging problem?

[KDavid]

Hi
Thanks to you both for your input. My gauge sits very still at just over 9v and doesn't move whether the engines running or not? Battery is good as engine turns over well and starts well enough. I assumed if the alternator had failed it would not rise when engine runs as its not charging but from what you're saying it should reflect better the battery state at a level 12 - 13v until a draw is made on it such as lights etc?

[poprivet]

That now sounds more like a gauge problem, when you said the battery warning light is on do you mean the red ignition warning lamp? Some models had a load dump module on the alternator to stop it glowing, I don't know if a '94 has one. You need a multimeter to see what is really happening.

[KDavid]

Thank you poprivet.
Red ignition light does stay on and voltmeter does rise and stays a little over 9 volts indicated. It does drop a little when the engine runs so I'm not sure if gauge has failed or not.
Ill have a look underneath when I can get it safely jacked up and see if there's anything obvious otherwise I'll have to find a local garage who can assist.

[ptjs1]

KDavid,

I can't see how the car would start and run repeatedly if the battery wasn't being charged when the engine is running. If you're able to repeatedly start and run the car without offline charging of the battery in-between, I suspect it must just be an instrument problem that you've got rather than an aleternator problem.

I'd run a very simple check: Connect a multimeter to the battery in the boot. Check the battery voltage. Start the car, now check the voltage. Get someone to fast idle the car and again check the voltage.

Let us know how you get on.

Cheers

Paul

[KDavid]

Hi John,
Thanks for your advice and results are;
Battery disconnected 12.0v
Battery connected 11.9v
Engine running 11.4v
Engine running fast 11.4v

Looks like alternator I guess but still interesting if gauge in car should show nearer 12v to reflect battery level perhaps??
Thanks
Kim

[ptjs1]

Kim,

A couple of thoughts:

If you've measured direct off the battery terminals, it does indeed sound as if the battery isn't being charged. Just to remove any likleihood of teh wiring from the front to the back of the car, try performing the test of teh battery voltage when the engine is being run by connecting the voltmeter to the +ve and -ve terminals that are located on the front bulkhead. (You have both on a 94 car).

Don't rely on any reading off the gauge! There's a couple of reasons why why you just can't trust that gauge on a facelift car! Do all your readings off the battery and / or bulkhead wiring connections.

Good luck

Paul

[KDavid]

Sorry Paul
Can’t find terminals under bonnet can you orientate me please left right back front etc. thanks but even after checking online I can’t seem to find them.
Thanks again

[ptjs1]

Kim,

The terminal points are on the bulkhead on the left and the right about 2/3rd of the distance from the centre to the inner wing (I think!) They'll probably still have their rubber protection caps on them. Certainly on the passenger side, there are two, 1 +ve, 1 -ve. One is slightly higher than the other but I can't remember which is +ve and which is -ve. You'll need to check that with your meter.

If I get a chance, I'll have a look at my car tomorrow and take a photo to explain.

Cheers

Paul

[KDavid]

Hi Paul,
Thank you for your further advice but still no joy i'm afraid so finally given up and calling in automotive electrician via local garage for this Thursday morning. I know its odd thing to say but i'm hoping he just condemns the alternator and i can get on with £150.00 refurbished replacement and get back on the road. If not the alternator i'm a bit nervous of what it light be or the pain of diagnosing it without an XJS expert on hand. Wish me luck.

[RichardSEL]

Sounds like one or more of the diodes (there's a diode pack at the end of the alternator) have gone.
They are single diodes (or in a sealed pack) connected in a "bridge" that take the alternating voltage generated by your alternator spinning and converts it to DC for charging. Generally that goes via a regulator of some description (I've never worked on a XJS) that ensures that the battery is never over charged.

Diode_bridge_alt_1.svg.png (10.79KiB)Viewed 2505 times

By removing the large red cable from the alternator, and measuring on the alternator's terminal when the engine's running you've effectively removed the possibility of the (seperate) regulator being at fault. If there's a DC voltage at the alternator when the engine's running -- and that volts increases with engine i.e. alternator speed -- then your fault is not with your alternator but more probably the (seperate) regulator unit.

Depending on year, the regulator unit can be housed with the diode pack that's underneath the plastic cover at the end of the alternator. Steam cleaning an engine used to be the enemy of alternators -- removing the cover and using a switch cleaner like Servisol with a toothbrush to clean there can often restore

This is why you've been advised about battery terminal volts rising with engine speed -- the regulator cuts the generated charge at about 13 - 14V DC-ish

Any help?

[KDavid]

Hi Richard
Thank you for your comprehensive post but sadly most of it went over my non technical brain.
I’m not sure I can even get to connection wires which plug in at the rear of the alternator via a plug so will leave it to the expert coming tomorrow. At least I can’t add to the trouble by trying to access circuits and diagnostics way above my pay grade.
Thank you again

[ng4xjs]

Hi David

I'm with Paul on this. Exactly the same problem I was having a couple of years ago to the extent I ran out of electricity leaving Norfolk bound for Kent and had to call upon one of the better known Breakdown Services. Their Man told me the alternator was kaput (but I knew it wasn't). He charged my battery sufficient for the remainder of the trip home. When I reached I did examine the nearside (passenger) bulkhead terminal only some 9.5V measured, removed and replaced with a new item. NLA at JLR but DM did have one (DAC10368). Did the trick for me. It's a fairly long brass threaded rod with two nylon insulators for passage through the bulkhead. On mine the two nuts and washes engine side had badly corroded causing the electrical maladies.


Hope you get it sorted soon.

Nick