1982 coupe, The charging gauge was always just above the red for years. I thought this was not good as it should be right in the middle of (normal), but the car kept running and starting so I didn't worry. Until finally the indicator started dropping and the car WOULDN'T start. Cars got 116k miles so I said, "Time for a (Working alternator)". I bought a good used Alternator on Ebay with 6 month warrenty from a trusted seller, put it in the car(never again), and same thing. When the charger is on the car the gauge reads right in the middle. Take off the charger (with a full charge) drops down to just barely above the red mark and upon driving slowly discharges as the engine uses up the battery power. I tested the two light brown wires in the back of the alt. and with the ignition on they show 12 volts + yet no charging connected to the alt. I've read my haynes manual and they tell you a complicated way of testing the alt. on the car but unless you're 3 inches tall there's no way you're going to get any leads down in the back of that thing. So, before I tell this guy that his alternator is bad and have to take it out again (kill me now), Is there anybody out there in xjs town that can tell me a simple way to test the alt., like say a volt meter on those brass spade connectors on the rear? Is it okay to run the engine with the wires disconnected? I have to finish fixing this car and sell it so I can drive my 91 classic edition convertible. I just keep taking two steps forward and one step back with this (in case it's listening ) GORGEOUS, car. Thanks for any help with my problem. Cheerio, Bud

 

Simple "test" is put a voltmeter across the battery, note the reading, start the engine, increase the revs so the alternator "excites" and read the meter again.

Engine off, you are looking for somewhere around 12.5v give or take.

Alternator "excited" and charging, should be 13.8v or better. This should NOT drop off much as it is loaded with lights, a/c etc.

The charge at IDLE will ALWAYS drop off, coz the alternator is not spinning fast enough to produce a charge.

I would NOT run it with the wires disconnected, as some alternators can "self excite" and damage diodes, found that out in the '70's when I worked for Lucas hahaha.

Also remove that small wire at the back of the alternator, plug the meter RED wire into it, and turn ON the ignition, you should have 12v in that wire, if NOT, the idiot light bulb is maybe burnt out, or some other bug is up its butt. You MUST have 12v in that wire to "excite" the alternator.

 

I checked that wire. It has 12 volts. Forgot to mention that the red light on the dash (top left ) goes out when the car starts. I thought that if there was something wrong it would stay on like any other idiot light? One last question. If the battery is showing under 12 volts but is still strong enough to start the car is this some sort of issue. Can't believe this guy sent me a bad alternator, But I've got to be sure it's suspect before I remove it for nothing. Bud! Thanks for your input

 

If your battery is below 12.6 volts with the car not running and no load on the battery it is not fully charged. It may start the car but the battery is not properly charged. If the alternator is working you should have around 13.8 volts at the battery when running and RPMS are 1500 or more. This should maintain even if you have a lot of loads on. (defogger, blower, A/C, headlamps, wipers, etc). If your alternator can't put out this voltage with no loads or drops off when the loads are turned on you have either a bad alternator or a large voltage drop in the vehicle's wiring.

First do the battery voltage tests. if they are good then the problem is with your gauge. If they are not good you need to measure voltage drop not resistance in the charging circuit.

Voltage drop is a measurement of very small resistance on large current circuits. A resistance check on circuits like this is not the way to test them. You need to voltage drop both the power and ground circuits of your alternator. You may have a bad ground cable somewhere or a power cable that is bad.

Take your voltmeter and with the car running and some loads on and connect one lead to the charging stud on the back of the alternator and the other lead to you positive lead on the battery. Then do the same from the case of the alternator to the negative post on the battery. In a perfect world you would read zero volts. However you would read .5volts or less on a good circuit. If it reads higher than .5 volts on either test you have found part of your problem and need to do more tests to find the problem.

I know this sounds hard but compared to taking alternators in and out it is a piece of cake. Plus it is nice to know what things are doing instead of just guessing.

Report your findings to us.

 

Thanks! Will do and will tell you my findings Bud. Oh by the way after finding that battery with low voltage,I put one in that read 12.2 volts and the voltmeter on the dash was hovering just above the red. I am going to buy a new battery tomorrow to eliminate it in the equation.I'll tell you my test results. Again Bud

 

I did the alternator to the battery tests on my 82 xjs (the one with the charging problem) and there was no voltage which is good, then it occurred to me, I have another xjs to compare to, so I did the battery voltage test and it read 13.8 volts. Then I attached an alligator clip to the output of the alternator and to the voltmeter and with the engine running it read 14.2 volts soooooo My 82 alternator most obviously is not charging as the battery readings are (with new battery) 12.2 and the alternator reads the same. I was reimbursed for the alternator but now I've got to do the job again. I learned my lesson...some parts should be bought rebuilt or new!! thanks for all the input. Bud

 

While you are about it, replace the alternator with a 115 amp one. The alternator on the mid 80s cars was not up to the job. My 85 ate them regularly until I put in a 115 amp one from a later V12 (thank Grant). The pulley needs to be swapped to a V belt, otherwise just the same. No squealing, should have done it years ago.

Greg

 

But I stil didn't get the answer to the question...why does the dash light still go out if the alternator is NOT charging. Isn't it supposed to stay on to indicate a fault? Bud

 

So you.. you're from France Greg ...are you ? WELL Bonjuor, and I'm glad to make your acquaintance! Merci por l'info!

 

Indeed I am. Glad too. The light does go out even though the alternator is not doing enough - it did on mine. The probblem is not 100% absolute dead circuit. It is that the alternator is not doing nearly enough. That was what happened to me, anyway, and until I listened to Grant and put in a 115 amp job, all the other experts told me everything was OK and i was just having bad luck with the 75 amp ones.

Basically, the 75 amp alternator has such a small amount of spare capacity, it soon gives up, is how I think about it.

Greg