1988 XJS charging issue - help!

OK, still no resolution to my charging issue…On the 1988 XJS V12HE.
Put in the 3rd alternator (which was bench tested and put out 14.3 volts).
Found a frayed alternator to starter cable and replaced it. But still not getting more than 9.6 volts at battery.
Question: How many cables/wires should be connected to the starter? My alternator cable connects directly to starter, so all output goes into the solenoid connection. How does power get to the rest of the system? There is a small wire connected to the solenoid which activates it… What am I missing here?
Any and all help greatly appreciated! Don’t wanna miss any more driving time.
1988 XJS-C V12HE (Nimbus)
1985 XJS-C 3.6
1976 XJ12C (August 1)

There is a main, large, battery cable which runs from the battery in the trunk to the engine compartment, with connections are each end as I recall.

Your description indicates that there may very well be a high resistance connection in the conductor path from the battery to the alternator. High resistance means voltage drop, and that is what you are describing.

Hi Joe,
You see the heavy electrical cable in the photo? That comes from the battery and runs forward on the LHS of the transmission tunnel, emerging on the LHS of the firewall.

There is a cable running from L to R across the firewall, engine side, and those connections need to be checked, battery disconnected. The connections are not simple, they are like a double connection- you will understand once you dive in.
The RHS connection on the firewall is hooked up to the starter , at the solenoid IIRC.
This is where you should begin looking.
It is also a good idea to run an extra cable from the alternator directly to the RHS firewall connection, which will eliminate poor conductivity.

hope this is of some help…

After looking at everything, I see that I do not have a connection from the starter to the right post. So I added a cable from the starter to the right post. All went well until I connected the battery. Got a large spark and everything is now dead… assume some main fuse(s) got blown. Will investigate this tomorrow. All I can now think of is that I somehow grounded one of the 2 ends of the new cable. I am pretty certain it is not at the right post, but will double check. And will dis-assemble and re-assemble connections at starter; fix the fuses and let’s see what we get.
At my wits end…

Hello Joe - you state that you ran a cable to the starter - hopefully you mean that you ran a cable to the starter solenoid - if you did connect to the starter, instead of the starter solenoid, then this would be the problem for the large spark when connecting the battery - Tex.

You may have checked this but the battery earth connection to the chassis in the boot is notorious for having corrosion at that point and causing high resistance. Easy fix and makes the world of difference in charging and starting.

Yes, Tex, I replaced the original cable connecting the alternator to the starter solenoid (starter). As there was nothing connecting from there to the rest of the system, I installed a cable from the starter to the right hand post.
Upon attempting to reconnect battery, I got large spark at the negative terminal. Now the entire car is dead. The battery is connected but there is no power anywhere.
What and where would have something blown to kill the entire system?

Will check the battery ground connection, too.

Joe, Is this vehicle new to you ? Your avatar shows you have another earlier model V12 also. How was it starting prior to this problem ? or did it never start since your ownership ? {missing wires} When connecting the negative cable you got a big spark and now nothing works, that sounds like a short somewhere or another wire somewhere improperly connected. The wiring on the starter/battery/alternator is not that complex to trace. You may now have created an additional problem to your prior issue of low voltage at the battery yet correct at the alternator. SD Faircloth

Nimbus’s history: Acquired him late 2021, after having sat for over a year (with previous owner). Has always been sluggish, running slightly hot, and many small tweeks needed. Last year finally figured out the distributor was the major problem as there was no advance occurring. Rebuilt it (incorrectly, unfortunately) and a weight swung out, jammed the distributor, and sheared all the teeth off the internal jack shaft.
As no one here would would perform repair, I opted to replace the engine. Nimbus has always started fine and the new engine fires instantly (still sluggish upon acceleration). But would not charge properly, running off battery power mainly. 3 alternators, and a second new battery later… here I am.
When I got him home, found the alternator was not fully connected. The small activator wire was there but had come off. Fixed that but still no go. I found that the cable from alternator to starter was frayed (like it had been chewed). So I replaced it. And that’s when I saw no wire to the right hand post.
So, I repaired the cable I had just removed and used it to connect to the right hand post.
What bothers me right now is: How did it start without being connected to the post? How was it possible to read volts to battery (9.6 or so) without that connection?

And that is the $64 dollar question. Initially, with no cable from the battery + to the starter solenoid, how did the solenoid get power, and thus, how did was the engine able to be started ? Something is wired incorrectly on this new to you engine in the starting system. You’re going to need a good wiring diagram specific to your year model. I found one, by Googling “1988 v12 jaguar circuit wiring diagram”. Type that into your Google engine and look for {http://jagrepair.com>images>Electrical (pdf)} It’s a large file, but the pages you need are right at the top. Note the reference to applicable VIN numbers. SD

Yes, thank you. Found that wiring diagram last night and have been pouring over it since.
And now that I am connect to the post, connecting the battery acts like there is a major ground fault.
With trusty volt/ohm meter in hand I go searching…

And…for the time being…disconnect positive and negative cables from your battery !! SD

And a visual search of wiring on the starter…Tis where I would start, with your wiring diagram taped to the fender SD

You may want to start by disconnecting everything from the battery posts in the engine compartment so as the only remaining connection to that post is from the battery, then connect the battery cables to the battery.

What voltage do you read on the battery?

What voltage do you read on the posts in the engine compartment? This should essentially be the same as what you read at the battery.

For the next step, I would get a battery switch and mount it to the negative terminal of the battery. That will allow you to open the switch when needed to make a change, then close the switch to do a test.

After installing the battery switch and leaving it open, connect one … and only one ping, one ping only please (sorry :innocent: ) … connect one cable to the post in the engine compartment, then close the battery switch.

No arcing (at least nothing of consequence)? Measure voltage at battery and at post - same? Good.

Open battery switch, connect another cable to the post, close battery switch, measure voltage … repeat until you find the cable which causes the arcing problem.

Is it possible that the post in the engine compartment somehow became grounded out to the chassis and that is what created the short circuit and the large arcing? Maybe verify that fist by disconnecting the battery and checking continuity with ohm meter?

The last paragraph of the previous post is where you will find the problem would be my bet.

OK, found my problem. The starter is (was) incorrectly wired.
And my apologies to Tex.
The mechanic connected the post cable correctly along with solenoid connection at the solenoid. Apparently this was done before he saw the alternator cable which he then conveniently attached a ground post of the starter. This explains why Nimbus starts but not charge.
And, yes, Tex, when I added my new cable, I just put it on top of the incorrect alternator connection (on the starter), directly to ground!
Simple fix if there was room to use 2 hands… After this short break (and beer) I’ll get back at it.
I’ll post final results. Thanks to all for the support!


Rewired the starter solenoid connection. No more arching at battery connection…

Now all I get is a click from the starter…uurrrgghh! Hope I didn’t fry it.
Any ideas? Where should I look next?

Usually, just getting a click from the starter solenoid indicates not enough voltage to the starter (not enough voltage to produce the current the starter takes to turn over).

Put a voltmeter from starter to ground and have someone try to crank it for you - how far low does the voltage drop?

Next, put the voltmeter across the battery and have someone try to crank it for you - how far does the voltage drop at the battery? Battery may now be an issue. If suspect the battery, put a charger on it and make sure it is fully charged, if same thing happens, take the battery out of another vehicle and try that battery - if the temporary battery cranks the car, it the battery you removed.

Ahhh…the sweet smell of success. Amazing what one can do with a bit of help and a wiring diagram. Starter wiring. Just as I indicated. Suspect now with your clicking starter solenoid that the battery voltage is low. Check as Jerry P. instructs. SD Faircloth www.jaguarfuelinjectorservice.com