My XJS is my daily driver and the alternator gave out on me last weekend, I haven't had time to even look at my options until now. Charge has been intermittent, but I cleaned all the contacts and grounds with no improvement. Decided not to bother with the voltage regulator, because if I'm spending the time swapping it out I may as well replace the whole unit as the price is similar. Not inclined to put in another 75 amp Lucas unit, so my options are Later model Bosch, Land Rover, and GMCS30, I've found pretty detailed instructions on the GMCS30 unit, but not so much the other two, that seem less time consuming, which is a big deal for me at this point as a full time 18 credit student.

Land Rover
I'm Really inclined to go this route because Kirby's book says it bolts right up, you just have to replace the pulley. My only question is, will the Factory pulley fit on the LR unit? Has anyone tried this method?

XJ40 Bosch
With this option I would have to drill the Mounting bracket holes (10mm?) to accept the new alt, get the new Pivot bolt (BH110281J), and swap the pulley out. According to Kirby's book the shaft is larger than the Lucas unit, so I'd have to find a 3" diameter 10mm wide pulley, from what I understand, all bosh shafts are the same size (17mm?) any idea where I could find one? Are there any other steps to this?

Thanks in advance.

 

A late model 115 amp alternator is a HUGE improvement on the old 75 amp one. It goes in very easily. Remove the old one, unbolt the alloy bracket from the block, drill out the holes in it to 10mm, buy a 10mm long bolt, or make a bolt from some 10mm rod.


All the new alternator needs is the pulley from the old one put on it in place of the modern ribbed sort (your new alt may not even come with a pulley on it). Just use washer/shims to ensure the pulley on the new alt lines up with the crank pulley.
I did this, and it is not at all hard
Greg

 

That's fantastic to hear. So, I can use the existing Pulley? I keep reading that I can't and I've been searching everywhere (I've been at it for 7 hours now instead of doing homework) It would make my week If I could reuse it.

 

Definitely use the old pulley. No problem whatsoever. There was a theory that an more powerful alternator would make the drivebelt slip, but in an XJS V12 application it does not. As I and loads of others have proved from experience.
Greg

 

I can't tell you how much I appreciate it. I've spent so much time looking for a 3 inch v-belt pulley with a 17 mm bore that I'm pretty convinced they don't exist. Little did I know I already had one on the car. I'll be ordering the alternator right now then. Again, thank you so much.

 

An alternator produces drag in accordance with the electrical load put on it. Since no additional loads are created by swapping alternators, there's no reason to think that the drive belt would slip.

Having said that, since the stock alternator produces enough amps to supply the car's systems, there's nothing to gain by installing a more powerful one. If I was stuck for time, I'd just rebuild the existing unit.

 

That is what I was told, and in theory it is true. But in real life, the 115 amp alternator makes a huge difference to the car's electrics. For whatever reason, the 75 amp alt is insufficient. I now have 14 volts all the time, which I never had before, even with a whole series of new Jaguar alts, which used to regularly fail. Since I changed to the later XJS 115 amp, absolutely transformed. I wish I had done it years and years ago.
Greg

 

Generic Caveat:


Swapping the pulley may need more than the usual hand tools. If the pulley and fan are fastened to the shaft by a large nut, holding the armature/steady is an issue.


One solution: An air or electric impact wrench. A friendly tire shop will probably spin it off and back on the new one. Might take a 'six pack' of a refreshing beverage. Or adding an impact wrench to the tool array. Also great for over torqued lug nuts!!! Not good for putting them back.


Carl

 

My experience/observation with the older 62-75 amp Lucas alternators used in so many 80s vintage Jags was they got the job done...but could be overwhelmed when there was a stack-up of accessory use. Blowers on high, headlights on, rear defogger on, engine at idle....watch the voltmeter drop. With increased engine speed, all's well.

I'm not so sure that [lack of] total rated output was the specific problem...or was it something in the design of the old Lucas units that made them not-very-effective at idle speeds? I suspect the latter.

I've never felt there was a reliability or durability concern with them, though.

Cheers
DD

 

That's more volts, not amps. Other than a different reading on the voltage gauge, how else did it affect the car?

My old Corvette has a 37 amp alternator which supplies everything adequately. I would gain nothing by installing a bigger one.

 

V=IR, quite so. Therefore, if my O level physics has not led me astray (quite possible) if the resistance is the same and the volts have gone up, then the amps must have also.


I am not trying to get into a contest, all I am saying is that on my car the bigger alternator made a very great improvement. I am not saying your Corvette needs or would benefit from a change, nor that anyone else's V12 XJS would. Only that my personal experience has been that a succession of 75 amp Alts were not up to he job and that a 115 one fixed the problem. This is response to the OP who asked for info.
Greg

 

This makes sense. The output characteristics of the alternator meant that it was insufficient to keep up with maximum potential demand while at low engine speed. The fix is not a more powerful alternator, but one that maintains sufficient output even at idle.

Possibly Jag got the pulley diameters wrong?

 

I went through the whole "alternate" alternator game when mine went out and went through the whole Kirby gamut of land rover and related alternators.

In the end, the easies option was to take your existing alternator to an old school "alternator and starter shop" and have them rewind it for 115 amps. Otherwise getting the mounting bosses and such to line up was a problem I did not want to deal with.

Also be aware, that the higher in amps you go, it will not charge at idle.

And I'm suprised that Greg in France has not mentioned that you'd be well advised to run a separate ground cable from the alt to frame, I did just that.

The one problem that I haven't looked into on my rewound alt is that the red battery light on the dashboard is off at idle and gradually comes on as the rpms increase. My ciggy lighter VM indicates around 12+something volts at idle and then goes up to 14V+ as the rpms come up from idle. Looking at the wiring diagram, I see that the one end of the dash light goes to 12V battery, other goes to the alt exciter terminal, and as the alt starts charging I guess the voltage on that terminal is supposed to come up to 12V thereby extinguishing the light. Mine does the opposite after the rewire I suppose

 

I forgot to ask this earlier, am I going to have to use spacers on the pulley or anything? If so, do you by any chance remember how many?

 

You are quite correct here, what happens if the current draw of the car exceeds the 75amp ability of the alternator the system effectively goes into Constant Current whereby, as the current requirement goes up the voltage must drop until the voltage avalanches and the alternator dies, or more than likely the diode pack. Which is what I think the OP's issue is.

A 115 amp Alt is a good idea on an XJS with the amount of electrics this car has. Cheap *** Jaguar just cut corners.

 

This is not correct, All alternators need to be excited before they will charge, if it does not charge its as simple as putting in a larger ALT lamp as the XJS use this to excite the alternator. This is why the ALT lamp lights for a short time when you start the car.

Larger alternators need more current drawn from them to excite that is all.

My 115amp charges fine and so does the 185amp in my mate BMW.

 

I just used large diameter washers to get it right. You can always measure the position of the old one first from a known point (eg the front surface of the front lug on the alt where it is against the bracket) to the pulley and insert washers to get the new one the same. Warlock is 100% correct about an extra earth strap direct from the alt strap to the chassis frame. Well worth doing, but clean the frame to bare metal where you attach the earth, then grease it up when the strap is fitted. You can often find unused holes with captive nuts on the lower chassis rail to use.


Greg

 

Update: Got the alternator off and the pulleys removed. Unfortunately the Lucas pulley is smaller than the Bosch unit, I've heard different things about wether or not they are interchangeable, so maybe at some point during the production run the shaft diameter of the Lucas alt was changed to 17mm? I guess I'm gonna throw the old pulley on the drill press tomorrow and hopefully get it to fit. Will report back with results. Wish me luck, because at this point I could really use some.

 

This is why I went the alt rebuild route using the current alt body, else you have to deal with swapping out pulleys, machining spacers, worrying about whether you've alined it right etc. nothing insurmountable, but needless hassle for no additional benefit.

with regard to high amp output alts not charging at low speeds, That is what the alt shop guy told me, and I can see on my car that idle volts is pretty low.


Hmm, so the high amp alternators can charge at low rpms with additional excitation amps, huh? So where physically on the car is that resistor located, I trust it is in series with the battery warning light and need to be replaced with a lower resistance by say 25%.

 

I seem to recall that it was located behind the IP, but that could be early onset dementia in effect or something.