84 XJS Alternator problem

I had a bad alternator on my 84 XJS and bought a new identical one from my local O’Reilly auto parts store. It worked for about a month and then started draining the battery dead and pulled 4 amps from the battery when the car was turned off. Suspect bad diodes so am waiting for a replacement they had to order. My question is that when I pulled the wires off there appears to be only 3 wires but there are 4 connections on the alternator. There is 2 big lugs with red wires connected to the same terminal. There is a smaller lug that goes inside the case with a smaller wire. And there is one more smaller lug on the outside of the case that I don’t seem to have a wire for unless it got tucked away where I can’t see it. Is there supposed to be wire connected to that one? And I’m not actually sure if the smaller wire goes to the internal lug or the external lug.

My 1979 coupe with the original Lucas alternator had a brown/black wire which is not a heavy gauge and is for the red ignition lamp in the instrument panel. My guess that goes to the small recessed terminal.
If you put that in the wrong place no harm done, it means your red lamp might not come on with the ignition switch. It should go out once the alternator is generating voltage above battery voltage.

You know the 2 big heavy wires go to the big post. The small post should be a sense wire, but to be 100% sure check with the supplier. Due to voltage drop in the wiring the voltage at the battery is less than the voltage generated by the alternator. The regulator inside the alternator senses and controls the alternator output voltage, but you can have a sense wire that might go all the way back to the battery so that is now an input to the regulator and makes sure the alternator output is high enough to give rated voltage at the battery.

Jaguar did not use a sense wire in 1979 and might not in your 1984. That is workable but not ideal, sensing battery voltage is better.
If your new alternator needs a sense wire and you do not provide it the alternator should go absolutely flat out with no feedback control.
That will be bad for your battery. So maybe that alternator is not a 100% drop in for your 1984 car.
Also not good that diodes have failed so soon. You need to deal with a supplier who knows about alternators.

If you do have an alternator with a sense terminal you can connect it to the big brown wires at the firewall terminal post. Not so easy because the terminal post is BSW thread. If there is room drill a small hole in one of the big lugs and tap it or use a small screw & nut. You only have to provide a few milliamps. Disconnect battery first, brown in Jaguar land is always battery +ve, not fused and a source of fright.

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Thanks, Great info, I appreciate it. I got a warranty replacement at Oreily Auto where I bought the original. It says Lucas on the back so I’m sure it is a remanufactured original. I don’t seem to have a sense wire but the alternator does have that unused lug on it. Maybe I should be connecting that to battery voltage. I’m just not sure. I have ~14v at the battery when it’s charging. What is the reason for having two battery wires? Do both go directly back to the battery? I don’t know why they used two instead of one larger one.

The main one carries charging current, which means it has a voltage drop over its length – which means that the voltage at the alt is higher than the voltage at the battery. IOW, the alternator is faithfully holding a proper charging voltage, but the voltage at the battery is a bit lower. By using a sensing wire, which carries zero current, the alternator can see what the voltage is at the battery terminals – and can increase the alt output accordingly to fully charge the battery.

In the particular case of the XJ-S, running a sense wire all the way back to the battery in the trunk would be a bit of work. However, one could achieve much of the benefit by simply running the sense wire to the buss on the firewall. The cable from the battery to the buss is massive indeed, presumably there’s not much voltage loss there – but the wires from the alt to the terminal on the starter are arguably inadequate and probably are the source of some serious voltage loss.

Doesn’t the sense wire need to be connected to a switched supply?

I understand the concept of a sense wire. But my original question was why is there two wires connected to the same alternator charging lug. Both wires have battery voltage on them when disconnected from the alternator so I assume they both go to the battery. If they both go the battery then one could maybe be used for the sense wire but it’s not. They are both meant to plug on the same v+ lug since the connector is wider than the sense lug is. Also is the sense wire on this version of the alternator optional? When it’s not connected is going to charge correctly. It came with no instructions.

Nooooooo! It needs to be connected as close to the + battery terminal as possible! Just as with the main terminals on the alt, it stays connected to the battery at all times. If it drains the battery, that means the regulator has failed.

Oh, yeah, different question. And a good question. The only thing I can figure is that’s the way Jaguar decided to handle 75 amps. Rather than one big wire, they went with two smaller wires. Does yours involve a pair of big spade terminals? Perhaps it’s because they wanted to use spade terminals but there aren’t any spade terminals that can handle 75A.

On my '83 those wires were a bit charred. That concerned me a bit so I added a wire, something like 10 gauge, directly from the terminal on the alt directly to the buss on the firewall, thereby bypassing the stop at the starter. Then I ditched the Lucas 75 amp alt for a CS130 with its 105 amp rating and located where the air pump had been, so I wired it directly to the buss on the firewall and did away with the little conduit on the side of the engine block.

You can always add a 2nd alternator like I did just to buy you time.

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Ok I did some experimenting. When I first start the car, I only see 11.5-12v at the battery. It stayed that way for a minute or so and my gauge shows below normal voltage. I then race the engine and the voltage then jumps up to 13.5v. When I idle again it stays at 13.5v and continues to charge. If I shut the engine off and restart it is back to 12v again. So I hooked a sense wire to the sense lug (top left in picture). and started the car. It was at 12v but as soon as I touched the other end of the wire to the buss lug on the firewall it jumped to 13.5v. I hooked it up permanently and now when I start the car I immediately see my gauge move to the normal range. I’m guessing with it disconnected the sense lug would float and would eventually get high (or Maybe low) enough to kick in the regulator and racing the engine would make that occur faster but using the sense wire seems to work much better.

Nice colors! Your missing your air filter though It looks like

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Missing many things still. Much more color to be added. It looks dull in that picture comparatively speaking.

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That would imply that you’re losing 1.5V in the wiring between alt and buss when the battery is depleted after starting and it’s charging hard. Once the battery gets its charge back up, the charging current drops off and the voltage losses diminish. And by connecting the sensing wire, you’re getting the benefit intended, the alt is providing full charge even when pulling heavy current.

That’s all well and good except 1.5V loss seems like a lot to me. I’d expect wires to be getting hot with that much loss. Again, I’d be tempted to add another charging wire directly from alt to buss.

I don’t think I’m losing much voltage because the voltage at the battery is pretty close to the same as the voltage at the alternator. But I have removed the sense wire because it drained the battery. It was pulling 4 amps from the battery when hooked up and the car was not running. The alternator seems to charge ok but It has the same problem after 1st starting the car. It does not seem to start charging until I race the engine up to about 3500 rpm. Then I see my dash voltage gauge raise up to the center (13v) whereas before it was on the red line. After that it continues to charge whether idling or not. I don’t know if that lug is really for a sense wire but if it is it would appear to be bad. But on a new alternator I wouldn’t think so. If the sense lug on the alternator doesn’t pull any current maybe a high resistance resistor inline with the wire would work and prevent it from draining the battery while providing the sense voltage or I could put an inline diode.

If it really is a sense terminal it would not draw 4A.
Something is wrong.
The sense wire should apply enough voltage from the battery to the regulator to get the alternator going and giving a decent output at low RPM.

There are minor differences in configurations within alternators. If the sense wire does not get the regulator up and going, they all have self exciting diodes which can apply voltage for the regulator, but from a standing start it can take a fair amount of RPM before they become effective and you get serious voltage and current out of the main diodes. From then on even at low RPM you get power. The trouble is that if not properly regulated the alternator output voltage rises until it is above battery voltage, then it pumps as much current as it can deliver into the battery although the battery voltage does not rise much. That is bad for the battery. If you see around 13.5V on the battery it is charging without being overcharged, so the regulator is working.

From the model number can you find a datasheet or operating manual which identifies the connections/terminals of the alternator ?
If not, call in at any auto electrician shop and ask for advice.

Just looked back at previous posts on this thread.
The brown/black wire has the resistance of the ignition lamp and probably a parallel resistor. One end is on the battery thru the ignition switch, and it supplies the initial voltage for the regulator to work even at low RPM.
The series resistance limits the current draw, and once up and running the self exciting diodes can supply voltage for the regulator.
If you have the alternator terminal for this connected straight to battery it might easily draw 4A when you switch off the engine. If you do not connect it to brown/black wire you need enough RPM for the self exciting diodes to get the regulator up and going.

A true sense terminal on an alternator should only draw a few mA when connected direct to battery.
Maybe you can forget about a sense wire if there is no provision for it, just means battery charging is slightly less than optimal.

That’s not the sense wire, that’s the charging indicator wire. Goes to a light on the dash.

Of course, It should have been the brown/black wire.

Didn’t I read that many xjs charging systems didnt show a higher charge for a few seconds or so after charging. …Just recently installed new alt and it does that same thing

I guess we’re back to trying to identify the connectors in the photo above.