I did some research and found at least 3 threads on this but at best one answer was a bit unclear.

Quick summary, I sent my alt out to be rebuilt rather than getting new or an exchange. Due to life this job has gone on for nearly a month. So I forgot the exact placement of the wires.

BTW the rebuilder has a stellar rep doing al kinds of classics and only starters and alternators. Beautiful workmanship. If you happen to be in the Long Beach, Orange county area I will send his info. Total rebuild $125.

What I did: the 2 wire harness with one larger ring and one smaller went on the larger terminal stud with the nut and I put the small ring on a stud that uses the 8mm nut. There are 2, one is strapped with a bond to the alt housing body and the other I was told was for the instrument panel, specifically the battery light icon. I used the non grounded one as per a thread I found. The plastic plug of course was inserted into its spot. The car has been unhooked from the battery for 2 weeks at least.

The problem: when I started it (started right away and ran good, belts look and sound good) the alt gauge shows well into the negative zone. Did I wire this correctly? Some people describe the terminal stud I used as being lower on the back than the one that is grounded.

The two ring terminal are for noise suppressor. They are connected like this


Everything related to voltage control, exciting and charging lamp is in the round plastic connector. What i would do is check if i connected the thick positive wire onto the positive stud and check if i have ignition and battery voltages in the round connector and the connector itself is fully engages into the alternator

Thank you. I did find another source and you and it both have it the way I have it wired. I don't see how I can check the wire voltage in the plug in connection while plugged in. I did remove and shoved that connection in and out a few times to try and make sure it is all the way in.
I checked the voltage at the battery while the car was running and I'm getting 11.7v so it's on the charger for awhile to get my battery back to 12+ volts at rest. I know I should be seeing 14+ volts at idle.

I have to think that somehow I'm not getting that round plug all the way, but with the alt in place it's hard to reach. It's hard to get at the plug even when the alt is not in the bracket and turned where you can see the back. The harness is not very long to allow any slack. My red wire is correct as is my black wire with the white stripe.

After the battery is recharged I will try the plug one more time. Then it might be time to trace those wires in the harness and check upstream. It's a ton of work to change these out w/o a lift.

Edit: I'm beginning to understand the problem per this thread — https://www.jaguarforums.com/forum/x...-256001/page2/
I don't yet know how to fix this but I do have 12.8v at the large post on the firewall. So off I go to troubleshoot all the points.

I realize this is an oft posted topic and I've found several just scrolling, not even searching. Therefore it seems to be a bit of a problem. I haven't found anything yet tracing the wires. I have yet to clip onto to alt post and read the output and try and read the voltage at the battery to determine any difference. I will do that next and report. I'm beginning to think that I send out to be rebuilt a good alternator all the while the problem was elsewhere. To review, at first the amp gauge showed low until I drove a block or so and reached 2000 RPM. That got worse over time and the gauge was always very slow to react like the needle was suspended in glycerin as some are in industry..

Only posting and bumping this thread in case there is some unspoken wisdom.

Result: alt is putting out .06v. That's right point zero six. I might not be hooked up to the output. I have one wire alternators on other cars so the large ring terminal would be the output. Why such a large terminal for noise suppression? Is there output on the double nut terminal?

Another edit: According to this diagram connecting to the double nut post terminal should be producing voltage.


On going: now charging at the post terminal 14.4 v but the amp gauge is still showing like 10v. Need to put my battery tester on the battery and monitor that. Restarted and no gain at the battery. So check the DMM clipped to the alt and it's back to 0.05v now. I do suspect the 2 cables on the top of the starter motor but there is no way in this world to get at those.
Anyone following this or am I just talking to my hat?

What is the voltage on the post terminal when car is not running? Should be a battery voltage on it. And take out that round connector and check for the voltage in its terminals that have white wires. One of them must always have battery power, Another one powered with ignition on.

No voltage ignition on not running. I have so far checked continuity from battery to firewall terminal and alt to same. Seems like both were good but I'll check those again since I have done so many checks I'm losing my way. I really can't reach in there to take the round plug off the back of the alt. I was hoping to find the other ends of those circuits.

Then most likely your alternator output is not connected. There should be a thick cable coming from alternator post to the starter where it meets the battery positive cable. Find it and check its condition. Pay more attention to its terminals as they are known to fail. As your alternator provides current while running and the battery light is off you might not need to check the round connector.

Not quite. The battery light sometimes goes off but the amp gauge never moves even switching on high beams. The alt did produce voltage the time before the last time started. I guess this is what they call an intermittent problem although it is more often not working than working. Yes, I assume there is a loose cable somewhere. I can see the terminal stud and nut on the starter solenoid with a mirror. Visually it looks fine. Washer is flat. I've love to get a socket on that and maybe I can doing it blind from underneath. If I can clip an alligator on that I can check continuity both ways. IF.

Thank you for your continued help.

There is a small fuse for it takes power to make power , # 10 / 5 amp right engine bay fuse box , this position will eat fuses like popcorn if there is a fault so be prepared to double check after an engine run as the fault can trick you thinking move along to something else



The 2 battery cables that come together at the top starter solenoid terminal post can be seen between the intake manifold pipes , tighten from under the car , 13 mm on a 3 inch extension on a ratchet

Someone just recently left their's cable unconnected

@Parker 7 , I have visited the #10 5A fuse many times, even so far as checking them with a meter. I will try to tighten the nut on the solenoid but if that doesn't do the job I think that's it for this car. A '96 in less than excellent condition does not warrant the expense of shop work at the hourly prices quoted these days. I bought it in 2014 with just shy of 140K and now it's past 190K and given me a good run. XJ Jags FS around here just sit. All in all I may have taken a good alt out to be rebuilt when there was an underlying problem. And now it seems the be intermittent only occasionally working. The worst kind of problem.

Just in case someone in SoCal reads this, you can buy this car for parts value. Whatever that is.

i had loose nut on my starter. it also looked visually fine. was still a bad connection tho. i found this when playing around i checked voltage at the firewall and at the back of the alt.
14.5 at back of alt, 12.8 at firewall.
was clear that the alt was working but power was not making it to the battery.
so i used a long screwdriver and put it down the gap in the inlet manifold to bridge the connection and bamn, voltage at firewall increased.
un did it, cleaned it with CLR, and then did it back up.
ALL good now.

Older cars are a passion. you have to be prepared to diagnose and work on them. i am having huge issues with my cat not running right. but i couldn't scrap it. get rid of it and buy what? something newer and less cool? or something else old and cool and just get a different bunch of problems.

Thanks @Spud Matt. That gives me confidence to attempt to get to that terminal. I agree with you on cars and have been doing 99% of my repairs myself all my life, At one time I was a race car mechanic. That's no big deal because race cars are actually simple. But I'm 80 now and it's getting time to call it quits. This car has some cosmetic challenges. It's not worth anything on the general market, The deal is that I just passed CA smog and I pulled the spark plugs when I didn't really need to just to got barely over the limit. All six of the 11 YO plugs read perfectly, Runs like a top. It would make a fun track car.

jut remember to disconnect the battery when getting to tat terminal!!!!
it will arc and spark pretty fierce if earthed out!?
pretty easy to get to from underneath aslong as you can comfortably get under the car.

totally understand what you are saying bout age and working on cars.
i feel it now sometimes and wonder if i could still be doing it in 20-40 years time. I hope so but who knows.....

Yeah, I've been back and forth to the boot to disconnect the battery so many times I should have bought a battery switch. I had no idea this was going to turn into a complex issue.

One more review of events: The first thing that happened was I threw a belt (it was old). I didn't see the light as it's low in the instrument panel. I overheated it some, but luckily no apparent damage. New belt and that was when it would charge only after a block or so and some RPM. My understanding is that alts create voltage at idle but create more current as they speed up to a point, Depending, that is somewhere around 2-3000 RPM. Makes sense that with a higher current a connection could arc enough to establish a connection. That and heat expansion. But, as anyone would know, corrosion does not heal itself. So, all things point to the solenoid terminal by theory. This is about the last thing that I have not done that can be done. If the issue is in the harness somewhere I'm not going to chase it.

So all the money is on that terminal. I took yesterday off to rest my body. Might take today off as well. I see now that I can use the 13mm socket on a 6" extension and feel my way onto the nut. I'll loosen it, spray some vinegar on it followed by distilled water. This is the old tried and true German car electrical tune up except add alcohol to be used after the water to dry things out. I've used this successfully many times. It's nice to sand the ring terminals but if I remove them and clean them all while working blind, I may never get it back together. If I had a helper guiding me from the top, sure, I'd go for it.

I hope this thread along with others helps folks down the line that search for this issue. Someone ought to combine all the suggestions and put this in the sticky.

Remember the terminal post down the inlet manifold ( starter solenoid top post ) is not threaded steel so caution on how much torque

Yep , gotta disconnect battery positive post , never overtighten

There is the terminal post on the inside engine compartment right wheel well very near the engine rear firewall , this is the battery cable to the tie point on the starter solenoid ( starter cable , but serves a function of passing power back from alternator to the battery and rest of car )

Will have more for you Monday

O volts point a suspect to ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

never mind you have 10 volts

Been using that terminal for checking since it was first mentioned. Under a big boot, 2 large wires. I did disassemble that one and checked continuity back to the battery, Seems to have continuity to the stud terminal on the alt as well. I realize that may be going through the starter. That's why this is maddening.

Thank you all. I'm not done yet. Even if I do get rid of the car, I like to solve problems. In retirement I have spent 10 years restoring antiques for the trade (dealers). Not just furniture, all kinds of metal, wood and plastic items.

I'm sorry gents, but I can't get to that nut. I tried for 15 minutes and the risk of more injury, especially to my shoulder replacement, is too high. I managed to spray the vinegar on the terminal so if that is the problem it should connect at least enough to let us know that was the issue. If not, then it really is beyond me. I hate to wax philosophical, but that may be my last time on my back and side under a car. Too late in life to buy even a low rise lift.

I'm disappointed, of course, after working on what seems to be 100's of cars for better than 60 years. I remember when I knew only what I learned at the go kart track when I was 14. My dad was not interested so a neighbor drove me and the kart to the races on Sundays. I graduated to a Corvette, my first car, and been at it ever since.

It's been a good run. Thanks to you that made suggestions. I have to say being here for 10 years I've noticed a drop off of some of the members I used to see in my early days. I may be back, who knows, but so long and farewell.

On_a_Jag, out.

did you try and get to the nut from underneath or above?
underneath i found was easy enough to get a socket on there. can't recall if i used an extension or not.
if you want to test if that is the problem or not you could try what i did that led me to know it was my issue.
and that is,
connect your multimeter up to the firewall stud and an earth point.
then with a big long metal pole (screwdriver or similar) you should be able to reach down and bridge the connection with that.
if there is a no connection at all you will likely see sparks when you do it and then get voltage increase.
if there is a slightly dodgy connection you may not see sparks but you should see a voltage increase as you are completing the circuit

I haven't had any time to get rid of this car. In fact, I started some minor rust repair just for grins and the fact that I can after many different car restorations.

So since I'm back for the moment I will ask about the noise suppression capacitor. I see that the loom goes from the B+ post to the cap and then to ground as most caps do. Has anyone simply disconnected the cap to see if it has shorted? By that I mean running the car, not actually checking the mF of the cap. I can do that too once I isolate the cap. Seems to me that in the beginning with my alt only failing at start up and idle and then coming on line that might be consistent with a faulty cap robbing voltage from the alt and sending it to ground until the amps cam up to reform the cap or otherwise work at speed.

I'm going to try this but I thought I'd throw this in for posterity. This subject seems like an underdeveloped troubleshoot and diagnostic procedure for alternator malfunction.

A quick recap of the trouble shooting done:
Checked continuity of all cables from boot to alt including firewall post and starter solenoid. They check out individually but not as a whole string. I even jumped the terminal ***'y at the solenoid with a long screw driver and took a reading off the screwdriver shaft. IIRC. it showed 14 v running.
Checked and cleaned the #5 fuse and connections below
Determined the alt will charge using jumper wires at 14-15 volts
Checked belt tension on new belts and cleaned pulleys.

Only thing I have not investigated is what if anything is on the cabin side of that through-the-firewall terminal. That would be up behind the glove box, I presume. Anything in there?

Thank you.

Some X300s come without the large round cylinder noise suppressor for whatever reason but would expect some low level of interference on an AM radio station

For simplicity you can remove the small suppressor wire from the large B + alternator post and leave the other suppressor wire connected on the back of the alternator , the back wire has to find a path to the car frame ground through the back alternator connection that was hard to square with me at what back alternator point

Come to think of it , the noise suppressor has a 2 wire connector ( short pigtail ) on the way to the alternator for a quick disconnect

The through the firewall battery cable terminal post can be tightened as a post for both fwd and cabin interior cable with a trick only using the fwd engine compartment only

I tried getting on these firewall terminal post from the cabin side and is impossible even for a thin framed person like myself

In the alternator contribution flow of current the large battery cable on the engine compartment side right wheel well sheet metal inches away from the rear firewall is in running car the first tie point after the starter solenoid tie point , that single on the wheel well side cable ties into a bus bar / mega fuse under the rear seat , and then comes fwd to the engine firewall 2 terminal posts ( along with tying into the rear seat fuse boxes )