Right, so I would have to measure voltage over the three phases, in order to find out if it is the alternator itself that is at fault, or the rectifier within. Too much hassle for a $100,- part which then still isn’t repaired.
I’ll pull it and exchange it for a refurbished one from my local supplier.
If I know myself well enough, rebuilding the darn thing will cost me a lot more.
New brushes, new bearings, new rectifier + the amount of time…too expensive.
IIRC, there’s supposedly a way to test for a bad phase in an alt by checking the voltage with the VOM in AC mode. If all three phases of the alt are working properly, the AC voltage measured is miniscule. If a phase has gone bad, though – usually meaning a diode or two has failed – the AC reading jump dramatically.
I recall a story about some guy trying to repair an alternator who found the diodes were bad (IIRC there are six of them) but the correct diodes were NLA or stupid expensive or something, so instead he just bought a set of new dirt-cheap diodes for a Delco or something, mounted it on the fenderwell, and ran some wires from the windings in the alt to the diode pack. Worked fine, and apparently the diodes run cooler that way so may last longer.
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And with engine off and ignition ‘on’, Patrick - whar happens to the light?
Simple diode test; disconnect a battery clamp and measure current between the battery post and the (disconnected) clamp - doors closed, ign and all users ‘off’. You should read about 30 mA. Reading 0,5A means a bulb (usually boot) is still lit - high amp readings means faulty alt diodes…
Frank
xj6 85 Sov Europe (UK/NZ)
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I was never quite fluent enough in electrickery to understand why, but as I understand it the generators used in early cars produced voltage in proportion to RPM – the faster they turned, the higher the voltage produced. Then a voltage regulator was used to limit the voltage sent to the battery, and the excess voltage was simply wasted. Very inefficient. When the world switched to alternators, besides getting rid of a commutator and utilizing a diode pack instead, there was also a change in the nature of power generation: with an alternator, the faster it turns the more current is produced rather than voltage.
And while alternators are described as having an internal regulator, really it’s not the same thing; the generator is producing too much voltage and the external regulator just wastes the surplus, while the internal regulator in an alternator actually varies the excitation of the alt so that it only produces the voltage required, no waste.
Maybe some of you EE’s can explain what all I messed up about that.
You’re actually basically right.
However, even an alternator is only allowed to put out that much voltage a 12V battery can handle.
For cars with normal batteries, that would be around 14.3V.
For more modern cars, with Gel or lithium batteries, the charging current is raised to around 14.7-14.9V.
Those batteries are quickly charged to about 80% capacity and then the charging current drops to “trickle mode”, especially in hybrids with energy reclaim systems.
Upping the AMPS when switching alternators doesn’t really make sense to me.
It just strains the old wires and relays a bit more imho.
Seems like there are a few diodes shot.
I tested the output voltage again yesterday under full load (lights on, AC on, radio etc etc) and the voltage would not go over 12.6V. Although it’s not yet low enough to drain the battery while running, I will pull the alternator tomorrow and have it rebuilt. I am a sucker for original parts… 
Strangely enough, my Germany marketed XJ Sovereign V12 actually has the airpump mounted a top of the alternator…cannister is there as well in the right hand side wheel well…
Plus, in my experience, having it rebuilt locally increases the chances of success quite a bit. Off-the-shelf rebuilt units became too much of a gamble years ago. I’ve had nearly 100% success with my local rebuilder.
Not surprising, especially if you have a later car. The German-market spec changed over the years. I also have a vague (perhaps incorrect) recollection that, if desired, a German market customer could order a higher level of emission equipment.
Cheers
DD
Well, if you go to a higher-amp alt without upgrading the wiring, that’d be unwise. However, some of us add electrical loads – most notably electric radiator fans instead of the belt-driven fans which can add 20 amps of load, plus 100W headlights in place of the 55W OEM headlights which adds a bunch more amps, so as long as the alt needs replacing anyway might as well go for more amps. On the XJ-S, the wiring from battery to the buss on the firewall is no problem, it’s massive. The wiring from the buss to the starter is likewise sufficient. The wiring from the starter to the alternator, though, is pathetic; on my '83 it was looking charred carrying the OEM 75 amps.
Easy enough to fix, though; just add a heavy cable from the alt directly to the buss on the firewall.
At one time, Germany had the strictest emission requirements in the world. This wasn’t because they were overly concerned about the environment. It was because they wanted their automakers to become the world leaders in emission controls.
Aye, there was a time that generators had only one control. The cut out. Open with ignition off and engine not running. Closed when ignition and engine running. Rev the engine and watch the amps go up on the ammeter. On long runs, it was common to turn on the head lights, even in day light. prevent over charge of the battery.
My T generators had a moveable third brush. Altered amp production by moving it? I always called for all out on amps…
On one trip to NM in my 37. Dark back country roads. Drove in low or second to get brighter lights to see the road edge!!! No guard rails. steep fall. Trailer loaded with Xmas trees, staked high!!! Tail wagging the dog…
Carl
Kirbert,
not that I’m aware of. For a long time the only emission restriction was that the exhaust fumes mustn’t obstruct visibility. Only in the course of the 1980s with the acid rain debate exhaust testing became compulsory in Germany, but still with fairly high thresholds (4 % CO). Cars registered before 1972 are still exempt from emission tests.
Catalytic converters became compulsory in Germany in 1984 (and, indeed, in opposition of European neighbors), but there was a five-year merging phase for manufacturers to adapt. During this period it was common practice to offer cars with or without cat in parallel. It was kind of nice to directly compare power losses and increases in fuel consumption. The Jaguar XJ12 was one of them (output falling from 295 to 264 DIN hp). Of course, later progress in engine management technology largely cured these issues. I’ve never heard of US or Cal spec cars with emission control being offered for sale in Germany (probably because of the light issue) nor that manufacturer offered the US or Cal spec emission control as an option.
Best
Jochen
75 XJ6L 4.2 auto (UK spec)
Upgrading the lighting would for me consist of a XENON or LED upgrade…a lot less power needed there.
But again, I’m a sucker for original…
Verzonden vanaf mijn Galaxy
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I was referring to the phase from the late 1970s, when California had mandatory catalytic converters, and the phase during the 1980 when Germany and Europe didn’t, but most vehicles were produced both for Europe and California in obviously different standards. At that time European H4 headlights were not allowed in the US, while US sealed beams were not allowed in parts of Europe - during this period XID or LED lights were not even featured in Star Wars.
Best
Jochen
75 XJ6L 4.2 auto (UK spec)
Well, that’s why my XJ12 has a green “Umweltplakette”. (German anti-pollution system with stickers in various colors to put on the windscreen)
Verzonden vanaf mijn Galaxy
-------- Oorspronkelijk bericht --------
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That would only affect the heavy connection between the alternator and the battery, Patrick - the current to the various users remain the same, irrespective of alt output…
And alt output is limited by battery voltage - with a fully charged battery and users ‘off’; little current is delivered. As more and more users are turned on the capacity of the alt comes into play - but if no extra users are fitted; increasing alt rating is sort of wasted. However, with a depleted battery the capacity will be used, but high charging current to the battry is detrimental…
On the other hand; as more users are added, or a depleted battery, drawing more current, the alternator output voltage drops. In a way; it responds like a battery - which also drops voltage in response to current demand…
Frank
xj6 85 Sov Europe (UK/NZ)
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Hi Frank,
that makes sense to me, but as I will not be adding any users to this car, there is no need for an “upgrade” to 115A. I want to keep the car as original as possible, That’s quite a thing with appraisers over here in the Netherlands. The more original the car is, the higher the appraised value.
I made one exception to this: when we repainted the car, we upgraded to metallic black pearl paint (Audi Orca Black, LC9X) instead of the standard uni-black. The appraiser didn’t even notice that this is not an original Jaguar colour…
And that makes sense to me, Patrick…
Frank
xj6 85 Sov Europe (UK/NZ)
**
Update:
the air pump as well as the alternator are out of the car at the moment.
The alternator is being rebuilt as we speak, by an auto electronics expert who just lovingly stated “Oh, a dinosaur”, when he saw the Lucas Alternator sitting on top of his workbench…
The same guy rebuilt my starter last year and did one hell of a job on it.
Hopefully, I’ll have a working original alternator back in the car by tomorrow afternoon… yay!!
I’ve been using the same rebuilder for years. One time I walked thru the door with a Ferrari starter in my hands. From 20 feet away he exclaimed “Ahhhhh…Magnetti Marelli”.
I guess when you do nothing but look at starters and alternators year-after-year you become pretty adept at identifying them 
Cheers
DD