11 AC Lucus Alternator

Hello sorry if this has already been posted i have looked back but not found yet
I have a 11 AC Lucas alternator and I am not getting any output
Charge light on dash illuminated and no increase in voltage at battery when engine running before I go out and replace it is there anything else on a series one 3500 that I should look at before replacing alternator
Thanks Roy


Well-Known Member
The 11AC charging system has 4 components. The alternator, a 4TR voltage regulator located in the passenger side glove box,

a 3AW warning light relay located in the driver side glove box (might look a bit like this)

and the early cars may have had a 6RA relay.

I would check that all the connections are clean and secure first then try the 4TR voltage regulator. Failing that, the alternator brushes might need replacing or worst case the alternator windings might have failed.

Thank you Dave I will have a look at the regulator
I have had a look at the brushes and they look fine is there any type of bench test I can do to check the alternator before re fitting
photos and locations are a great help


Well-Known Member
The 3AW units are quite delicate. You can get modern solid state ones now (the plastic cube next to the one in the picture is one). You can test of the battery is charging by testing voltage while engine is idling. It should read ~ 13+ volts if the alt is charging. The 3AW unit just controls the charging light on dash.
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With the alternator fitted and car running if I put a volt meter onto the largest connection on the back of the alternator I only get a voltage of 12 volts same as battery voltage
Update I had an hour before going to work alternator back on still no sign of increased voltage at rear of alternator no increase of voltage at aftermarket dash volt gauge and ignition light on
Thanks to Dave's photos I found my 4 TR unit gave it a gentle tap and all came to life stopped again another tap back to life so a new 4 TR unit required I did check the connections and all seamed in order
Thanks again Roy
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Hi Roy, Mine did exactly the same a couple of years ago (1971, 3500 auto) replaced the 4TR unit and all is well. I bought one from Holdens for about £55, unfortunately they are not the "Lucas" red ones but it works. Happy days.
got my 4TR from wins international today not lucas original as above but as long as it works £ 49.36 plus postage which was next day
I got an issue with my battery (6 years old) and most likely with the alternator. Output of the Lucas is 12.8V. What do you think is this sufficient? The books speak of 13.8 - 14.1 afaik.


Well-Known Member
12.8V is not nearly enough to charge a battery. It should be around 14V. I don't think there's any adjustment available to the charging system so the alternator brushes would be a good place to start.

Sorry to raise this necro thread but it came up in a search for me. My question is, if my battery is discharging overnight (and the system isn't charging when engine running - alt light flickering intermittently), which would be the most likely cuplrit? I ask because I assumed it was the alternator. I yanked it out and took it down to the sparky - but he gave it the all clear. I have a Series one v8 - I believe the 6RA is on the drivers side wing in the engine bay and the 4TR is in the passenger glovebox area, 3AW Drivers glove box. How do I check these components are working properly? I reinstalled the alternator and everything seems fine. But I don't trust the prince of darkness. I wonder if my battery will be flat in the morning?


Well-Known Member
You should test the alternator for charge at idle. 13.8-14.1v is considered normal.

I would take the battery to a car parts store to have them test it too. They can test it under load to see how it’s holding up.

I would also check for a parasitic draw on the system. You can do so by making sure everything is switched off and the doors are closed. Disconnect the boot light (you may have to check that also separately). Remove the positive lead from the battery and bridge the lead and terminal of the battery with an ammeter. You shouldn’t see anything larger than about 35mA. Anything about 100mA and there might be a problem.
Thanks mate. I suspected the interior light but that wasn't it. Will check the draw as you suggested- thanks for that I didn't know you could do that!
Charge into the battery was over 14v. Was 12.3 before sparky checked the alt, hence why I removed it. Nothing more frustrating than an intermittent fault lol


Well-Known Member
On my car (with an AC11 alt) it takes about thirty seconds for the light to go out if it’s sat for a week. But it’s still charging at close to 14v. In my case I’m fairly sure it’s the regulator (I switched to an electronic 3AW light relay years ago).
Well the battery drained to 8v overnight so I still have a leak somewhere...it would be nice just to drive the car, but apparently that's not my lot.


Active Member
Recharge the battery on a charger or that will be scrap if left at such a low voltage . Modern batteries will not stand a deep discharge without some degradation
I charged it to 93% and then tested the draw as described above. Draw was .05 on the 200ma setting on my multi-meter....what does that equate to? I'm starting to think it's actually the battery itself...perhaps...?

Anyhoo, it's back on charge now, not connected to +ve terminal in the car. Will test it before I try using it in the morning...I wonder what will happen.


Well-Known Member
If you reconnect the battery (when it's fully charged but after any fumes have dissipated) then connecting the last terminal (earth) slowly you should see that drain as a tiny spark. If so I'd remove the interior and boot light from the system and see if it's still there, and after that remove the fuses one by one until it disappears and you should ba able to narrow down the source of it.