I need a new alternator?

My new battery when charged has 12 volts. Then goes dead.
When checking voltage at alternator with car running at 1500 rpm - voltmeter red on batt terminal ( with battery wire off) n black on alternator bolt for ground- zero volts. When all was connected n running it read 12 volts with no change between battery terminals n alternator terminals. So despite alternator looking new, it’s done? Worth it to try to fix it? I read where best to get a 100 amp one?

Not sure what you mean by battery wire off. Running an alternator with a battery disconnected can blow the alternator.

Your problem could be the voltage regulator failing, the alternator relay not working, a failed alternator, or wire connection problems

The repair manual suggests a way to check the voltage regulator.
Also, do you hear the alternator relay click when you turn on the ignition switch (but before you try start the motor?

69 OTS

Why do you want a 100 amp alternator? Around 60 amps is more than sufficient unless you are running some kind of high amp hungry sound system. .

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If your E-type has the factory ammeter, you’ll have to replace it with a voltmeter if you install a 100 amp alternator. The ammeter will only take 60 amps of input if I recall the figure correctly. If you ended up tapping into that extra juice, the ammeter would fail.


It was for a second n there was no voltage out of alternator. When alternator n battery were hooked up n read in series w voltmeter it read the same voltage at battery terminal as alternator- 12 volts- thus I assume circuit intact but not charging. Nothing coming out of alternator. Then volts on dash gauge goes down to 11, 10 then hard to crank etc.

I followed how Moss said to check it. Alternator - How to test using a Voltmeter - YouTube

No click on the cheap Chinese relay that has four black wires connected. The one near it for the horn was bad.
All connections were checked cleaned n are good.

Do you have a wiring diagram?


Did you say You have a 71? It should have a VOLT METER.
If not the others are correct.
Next I’m surprised more of these cars don’t burn down…
Look at the Home Depot wiring and connections. None original or up to snuff with MULTIPLE SLICES in the wires for resistance.
I would FIRST , Redo the wiring to the harness.
Next pull the alt and test to see if its pumping out.
Then move on to the regulator, would be interesting to see a picture of the wires to the regulator.
Can we see it?

68 model 4.2 dual v belt neg ground, voltmeter on dash

Makes sense- just a repro radio so 60 should be plenty YOURE right

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Ok is yours a volt meter?

Get a GM 1 wire and the whole abortion that you have there will be gone…
1 wire to battery post and your done.
imho of course


Here it is

VR n alt relay

Oy vey…Ive seen worse but, can anyone defend this?
Chris you have to clean house…Its not the cars fault or Malcom Sayers fault, its the prior owner.
You have work to do…
Or just put a 1 wire and get rid of it all, Is this a driver?

If you removed the terminal from the battery with the engine running you committed one of the big NO-NOs that are highlighted in the Moss Video. Hopefully you did not damage the alternator or voltage regulator, but time will tell.

If the alternator relay didn’t click when you turned on the ignition (before starting the engine), then the alternator will not be getting any voltage on its field wires, so it definitely will not work. I would suggest that you go to @Michael_Frank 's website here: Jaguar E-Type Alternator Wiring Explained and read up on the Lucas alternator circuit. At the bottom of the 2nd page is a link to a troubleshooting guide that should allow you to determine what is awry. Good luck!

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Yes a driver on weekends. i followed what moss said- battery was always connected - just checked voltage at alternator n then at battery. They were the same.

I’ll read up on the below link. Thanks guys

If its a driver, IMHO, ALLL that wiring goes, the relay goes the regulator goes.
You get the gm 1 wire run a single wire NEW from the alt post to the Battery post , tape back everything else and its clean and safe.
If not get a new harness, its an accident waiting to happen.
ps my 1 wire is I think 72 amp tops. nothing crazy.

Which gm1- will it fit right in?

Many years ago I purchased a replacement alternator from CJ. They provided a wiring diagram to ensure it was attached to the car correctly. Internal regulator. Touching wood but not had any kind of problem with it. As said, this was many years ago. If you’re sure about getting a modern replacement you could do worse that contact CJ…imo.

Moss had a WONDERFUL gm 1 wire WITH the double pulley, wire and all included.
They sold out in a day.
Supposedly they are remaking them now.
Its maybe a couple hundred dollars with EVERYTHING INCLUDED, The cj unit was the MOSS unit.
Its a simple conversion.
PM if you have questions, Good luck either way.
Once its done, YOU CAN forget about it for another 100 years,

The advice you’ve received here is good, I just want to be a little more emphatic, because this is a simple system and seeing it munged makes me crazed. First of all, there are multiple parts to this system, any one of which can cause failure to charge. The parts you need to check are the alternator, the voltage regulator, and the alternator relay. Never, ever, ever remove a wire, any wire, from a running alternator. Don’t remove cables from the battery while running, either. Your wiring harness is rubbish, to put a word to it. If you don’t want to replace the harness, you will need to patiently go through everything, fixing broken insulation and using insulated connectors. If you can’t find insulated connectors, drop me a line and I’ll send you a bunch.

I’ve done a lot of research in to 11AC alternator system, and I’ve posted a trouble guide that you should read. The Moss video is chock full of errors and oversimplifications, and it’s not specific to the 11AC system. Their procedure won’t help you diagnose the problem. My guide was mentioned above (thks), here’s the link again:


100 amp alternators are unnecessary, even with A/C, and a one wire is just silly. Before you substitute something else for something that is an original part of an increasingly valuable car, at least make an effort to figure it out.

So, let me cut through all the crap and start you off right. Take your meter and set it to read DC volts, 20V range if you have it. Disconnect the F+ wire at the alternator, with the car not running this time. Put one meter probe on the F+ wire terminal and the other on any good ground, and have someone start the car. ( if you don’t see any voltage, turn off the car and try again with the F- wire, since they’re probably mislabled) The next step depends on whether you’re seeing 12v there. If you’re not getting 12V, the problem is in the relay or the relay wiring. If you are getting 12v, try the voltage regulator test described in the FSM. If the VR tests bad, replace it. Otherwise your alternator is bad.