www.jec.org.uk

After 4 months of my 4 litre being cooped up since lockdown, I decided that with the hottest day of the year expected on Friday, I would get her out of storage and head off to a quiet beach for a swim.

The battery had been on a conditioner throughout the the last 4 months, so with a quick tyre pressure check (all perfect!) and fluids checked, I started her up, put the roof down and headed off with daughter to Hythe. The voltmeter was reading relatively low at just over 13v but it has always slightly under-read against the actual charging voltage so I didn't think anything of it.

After a lovely couple of hours on the beach, we headed home, the voltage reading was slightly lower, but again the car ran fine.

On Saturday, I took the car up to a Mercedes showroom about 20 miles away to look at a car for a friend and now the voltmeter was reading about 11v and also the temp gauge and oil pressure gauges were reading low. I was now getting a little concerned.

As any facelift convertible owner will know, it's nigh-on impossible to see any of the warning lights with the roof down, as I suspect I would otherwise have noticed that the red charge light was on!

When I went to leave the Mercedes dealer, the car tried to turn over but not sufficient to start the car. I borrowed the dealer's boost pack and jumped the car and decided to try and drive home. Voltmeter was now sitting at the minimum 9v reading. Suffice to say that the car dropped into limp-home mode, managed a stuttering 15 miles and then gave up trying to manage the incline on the motorway. So, with the support of the AA, I arrived home on the back of a recovery truck! Changes in legislation meant that I had to travel in my own car on the low-loader with ignition off and seatbelt on! I also found that I was fruitlessly pressing my own brake pedal going into some of the bends! Once the car was back at home and offloaded, I checked the battery voltage directly and it read 11.7v which seemed quite high not to be able to run the ignition and fire the injectors?

After a charge-up of the battery and restart of the car, today it seems the alternator is not producing any charge, so tomorrow I'll take it off and pop it into a guy I know who rebuilds alternators and starter motors. I know the alternator has never been changed in the life of the car so I can hardly complain.

Does anyone know a simple way of testing the rectifier pack so I can understand if that's the problem?

Moral of the story - don't leave an XJS for 4 months without driving it. I really should know better!

Paul

Ah Paul - what a pain! If you're going to get the alternator rebuilt then I'd assume that they will fully check out and replace anything that is below par so hopefully you will be up and running again.
My own cars haven't done very many miles but i have given them a few runs out just to keep their juices flowing.
That's done it now! I'll go out tomorrow morning and Sod's Law will strike with a vengeance!

Don’t know we’re you are in Kent, but can recommend https://www.bexleystarters.co.uk/ for the refurb.

Tks for your thoughts David. And tks for the recommendation cafcpete.

I'm going to do a few more checks this morning on the diode pack. I'm hoping the problem might lie there. After that, I'll try and pull the alternator out this morning. I've lent my workshop manual to someone but removal seems to only involve the pivot bolt (from below) and the adjuster link arm (from above) so I' hope it's not too problematic.

If the bearing seems ok and it is the diode pack, then I might try and source and fit it myself.

More to come later....

Paul

You did check the drive belt, didn't you? Could be loose or broken.

Tks Paul,

Yes, Drive belt nicely tensioned (sadly!)

Paul

This morning I decided to do some tests on the alternator to see if I could better pinpoint the problem.

I started by doing a simple diode check. Although I had no reason to suspect a parasitic drain, it's a simple check to perform. The test showed about 530mv with the negative probe on the B+ terminal with zero / overload when reversed, so that seemed fine.

With the engine running, I was only reading battery voltage of 12.8v when measured directly off the back of the alternator, so I decided it might well be a rectifier issue. With no apparent output current, I wasn't sure how I could check the regulator was functioning correctly, so I decided to take the alternator out.

Although removal from a 4 litre AJ16 is much easier than on a V12, Jaguar still conspired to make some things difficult for owners!

With the Air Filter and trunking removed, top access is fairly easy. However, removal of any of the nuts and bolts is a nuisance. And all of them will be tight!
I started by disconnecting the battery and removing the wiring connections.

I then loosened the lower lock nut on the adjustment arm. And that in itself is a pain to get at cleanly with a spanner! But with that eventually slackened off, I then slackened the adjustment arm mount bolt and then removed the trunnion bolt that attaches the adjustment arm to the laternator I then raised the car and started on the alternator lower pivot bracket bolt. And of course Jaguar designed it so you can't easily get to the back end nut without an offset spanner. And then the alternator belt and the fan try and stop you getting a socket on the front end of the bolt head. Don't believe the apparent clear access in the pic below! They are a pain to get at!!
Eventually they were loosened and removed.
A little bit of leverage with a pry bar and I moved the alternator out of its pivot bracket. It was then fairly easy to remove it from above.
Once out, I checked the front bearing which seemed very smooth. I then removed the back cover and checked the brush which seemed fine.
I have a very good auto-electrical specialist near to me so decided to drop the unit in there. I've asked them to confirm that there's no bearing play when run up to speed, check the rectifier pack and regulator and let me know how much to sort the problem. Weirdly, on the counter in front of me were 2 absolutely identical Denso alternators, apparently out of kit racing cars, that had just been dropped off by a customer for servicing. Coincidence or what?!

As I will change the belt whilst the alternator is out, it's a good time to also take off my aircon compressor and belt and put on the new compressor that I bought last year and still haven't fitted! And of course, I can probably find some other jobs to do whilst I'm in there!



To be continued...

Have you tried looking at the main insulated positive terminal on the bulkhead, left hand side (RHD)). They are prone to corrosion (brass) with the consequences you describe. I know from experience happened to me. Needs a good careful clean. I bought one a couple of years ago from DM. Hope you get it sorted.
Nick

Tks Nick,

Luckily all 3 of my bulkhead terminals are beautifully clean and intact. I'm not sure how mine have survived so well when so many others are badly corroded? But, I'm not complaining!

Cheers

Paul

Part 2………

I had taken the troublesome alternator into South Eastern Auto Electrical Services in Maidstone.
Steve from SEAS rang me the next day and advised me that the bearings and brush were good, as I expected, but that at least one of the diodes in the voltage rectifier had failed and that the voltage regulator also needed replacing. I agreed that they should complete the job, as it seemed a false economy for me to collect the unit, buy the parts and do the job myself. Two days later, I picked up my shiny alternator.
The discolouration of part of the rectifier pack seems to give a clue as to at least one faulty diode.
I thought the alternator would go back in without trouble but how wrong was I!

The pivot bracket has a bush inserted at the forward end. I thought I could just gently tap the alternator into the pivot bracket. “ Gently” soon became “Firmly” which soon became “Determinedly”!
In retrospect, and even with the limited space available, I should have tried to drift the bush out a bit before trying to install the alternator, but rather foolishly I persevered with my stubborn approach. And because I had another car parked on the ramp above, every time I jacked the car up to work on manipulating the alternator from below, I then couldn’t open the bonnet to work from above!

So not only was I repeatedly jacking and lowering the car, even when I eventually thought I’d got the unit aligned inside the pivot bracket, I then found that it has to be PERFECTLY aligned in order for the bolt to slide through! So followed 2 hours of trying to insert screwdrivers, tapered drifts and every other bit of long thin metal that I could find in my garage to get the alignment perfect.

FINALLY, the bolt could be inserted from the front and with some gentle ratcheting so as not to damage the threads, I managed to wind it through the rear of the bracket. It had taken a total of 5 hours for something that should have taken 5 minutes!

After a beer to cheer myself up, I then fitted a new belt, set the tension adjustment rod, tightened the pivot bolt, refitted the electrical leads, started the car and watched as my voltmeter now read 14+ volts! Another beer followed in short order!
So, Lessons learnt:

It’s definitely worth considering a rebuild or partial rebuild of your alternator rather than the expensive cost of a replacement Denso unit or the dubious quality of the Chinese copy units currently on the market. After 25 years, my bearing had no play and the brush was fine. The expected failure points on these units always seem to be the rectifiers and the regulators. Even using an external company I only paid £115 for the work which including cleaning the casing and painting the end cover.

Unless your alternator just falls out of the pivot bracket (it won’t!), try and drift back the packing bush at the front end, before refitting the unit. You’ll be pleased you did!


Now I'll get on with replacing the aircon compressor.......

Paul

Well done, Paul.

The function of the diode pack function is worth perusal. Rectifier diodes are electronic devices that allow electrical current to flow in one direction only. Diodes are used to convert alternating current electrical energy into direct current electrical energy. In a DC vehicle electrical system, diodes can be found in several places, including the alternator. Alternator diodes perform three critical functions for the automotive charging system.

An alternator converts rotational mechanical energy into electrical energy by creating a variable electromagnetic field, converting AC to DC power. The field is induced across alternator windings, and creates an alternating current signal. Electrical devices in a vehicle, aircraft, marine or other similar application, require direct current electrical energy. The energy passes through an alternator rectifier, which is made up of six or more diodes. The rectifier converts the alternating current energy produced by the alternator into direct current energy suitable for DC battery system use.

If a part fails suddenly, there is typically a short burst of energy fed back into the electrical system. A fuse typically protects most vehicle electrical components from feedback. Typically, an alternator is not typically protected by a fused circuit. The individual diodes will only allow electrical current to flow in one direction, the energy feedback travelling only as far as the diodes, but not into the alternator itself. The diode pack acts as feedback protection and will be isolated from any energy returns which can damage the alternator unit.

Diodes allow electrical current to flow in one direction only, the polarity of the direct current electrical energy at one end of the diode chain is always positive, whereas at the other end of the chain, the polarity is negative. For a battery to charge while an alternator is in operation, the energy the battery receives must be of matched polarity, namely +Ve to +Ve and -Ve to -Ve. Diodes in a rectifier serve as a one-way check valve ensuring that polarity stays consistent while charging the battery and correct polarity charging is always attained. Its worth noting that an alternator must always be connected to the vehicle charge cables when the engine is running. Failure to ensure that will almost immediately result in diode pack failure. That said, diode burn out can also occur after many years of service or consistent high current charge conditions which can cause diodes to overheat and fail.

Those sliding mounting bushes are often a cuss especially if they become moved when a re-conditioner tightens mounting bolts on his test machine. Rarely does a tester move them back again to suit the vehicle mounting bracket! In my former life as a marine engineer, I would often find sliding bushes moved when I came to fit a refurbished alternator. I`d use various sockets, threaded studding/bolts, washers to shift the bush to a more suitable position. It saved the possibility of breaking an alternator casting if using a heavy hammer or more brutal techniques.

Now you have an as new alternator fitted you can forget about it for another twenty five years. If it does fail again, it will no doubt be someone elses problem!

Mike

Tks Mike,

Great summary of how the alternator and rectifier function. I'll include that in my magazine article!

Cheers

Paul