The 'E' Type Forum

Ok, so I’ve got the car running sweet, new dizzy cap cured an ages old intermittent misfire, engine is turbine smooth, idles at a solid 800 rpm, has loads of torque, absolutely fab.. Well, 1 hr and maybe 40 mikes into this morning’s journey I noticed the volt meter had dropped from a steady 13v to a flat 12volts, I also noticed that the indicators were becoming slower all the time, the ignition light did not illuminate, but obviously the alternator/ regulator seemed to have stopped working, after another 15 miles the car suffered fuel starvation, obviously the pump couldn’t be receiving any juice to keep it pumping, anyway, after recovery got the car home, I put the battery on charge for an hour, car then started once fuel pump had filled the carbs and the voltmeter is now showing a healthy 13-15 volts!
My guess is the regulator…. Or maybe the alternator??… any thoughts??

Are all the belts ok....tensioned and not slipping?

Yes, the belts are fine, no squeaks, no slip, the change from healthy charging to discharging was instant, almost like throwing a switch, I’m thinking that the Regulator could do this…

There is a test procedure in the S3 service manual for alternator and regulator....Steve

Cheers Steve…. I’ll get on that tomorrow, it’s surprising how sitting on the hard shoulder of a busy motorway for 4 hours can be!

Quick update, took a drive to Nell’s cafe this morning, now voltmeter is registering 16 volts, indicators are flashing very fast….
I’m sure this is going to turn out to be the regulator

Clean up all your main earth cables

Good call, just watched a you tube video of the same kind of problem… it was an earth fault

I have to agree with the earth cable check. For me it was the one down by the regulator itself. I think in the end I fitted a nut and screw/bolt, after cleaning up the metal behind it, then making sure there was a good earth, then painting over the head of the screw/bolt. This left the end of the (fairly long) screw/bolt protruding as a sort of earth stud.

Hope that helps and prevents you from going down the expensive route of buying a new regulator or alternator.

The big transistor doing the switching of the field circuit is a 2n3055. If you look at the pinout data for that then you'll realise two failure modes are possible:-
1/ the transistor insulating pad is compromised and the body of the transistor conducts to the casing (which is earthed). That'd mean the field circuit is always at least partially on and the voltage would thus be high, no matter what.
2/ one of the transistor legs carries (the emitter) the field circuit path to earth via the circuit board and then the wiring. If that is compromised then the field circuit never properly completes and the voltage will always be low.

A good earth on the regulator is essential.

kind regards
Marek

Good evening gents, your advice is invaluable, cleaned up the earths on the regulator, now Charging at a steady 14 volts!!
Thus far with the help from this forum I’ve got the engine firing on all 12 cylinders, the carbs balanced, the gearbox changing gears correctly and now the Regulator working as it should, the car is running as good as it was when I bought it 8 years ago, but now I’ve the confidence to tackle most jobs myself, many thanks
Regards
John

Hi ,dont want to upset the purests here ,
can i suggest fitting a modern alternator ,
this will be alot more reliable.
Rob Gill

macman wrote: ↑

Mon Jun 02, 2025 3:23 pm

a modern alternator ,
this will be alot more reliable.
Rob Gill

I'm not sure that it would be more reliable. My 1969 Daimler Sovereign is still using the alternator it left the factory with. It has had no repair work in the 25 years that I have owned it.

Modern alternators may be more efficient, but that does not necessarily imply better reliability. In fact, some people might argue the opposite.

John's problem turned out to be a poor earth connection, which can happen to any type of electrical equipment.

it was good result for John, I think.

Kind regards

Absolutely spot on Stu, don’t want to tempt fate, but the old girl is running so well (atm) that I’m using it as a daily driver ( luckily enough I don’t go far )… tomorrow morning I’m doing a breakfast run down to the coast, round trip of approximately 120 miles, all thanks to you boys

Fitting a modern alternator onto a v12 isn't straightforward. The Butec unit is fed by a small double pulley with thin belts, pointing backwards and any replacement will be a single pulley forward pointing unit. This will have to be put in a place which doesn't foul the other belts and can be packed in amongst the much more heavily populated engine bay. You'll need to weld up a new alternator bracket, then bend the cabling around to meet the new unit and source a belt.

kind regards
Marek

Breakfast run was a success thanks all

Back again!!!! Sorry to be a pest, but after 3-4 days of the regulator behaving itself it’s now giving a constant 18 volts, I intend rebuilding the regulator, however that will not be in the immediate future, my question is, if I put my lights and blower on the voltage drops to about 14-15 volts, would it be safe to run it like that for a week or so without damaging any electrical circuits???

Hi John, the 18 volts is too high for several items on your car, the battery, ignition and any in-car entertainment items could be damaged. Are you sure that the 18 volt reading is accurate, are you using the cars voltage gauge or a Digital Voltmeter test instrument?
I have made a test device to test and set up the Butec voltage regulator, its not portable as it uses my Snap-on Digital oscilloscope. If you can manage without your reg. for a couple of days you can send it to me and I'll test it for you (foc) Brushes are a likely problem in the alternator and worth checking at this point, regards another John.

Wow, what a fabulous offer, cheers John, I have checked it with a digital meter and it is 18volts, but after running for 30 minutes or so, it drops below 13 volts, but if i stop and restart… 18 volts again, if you could test my regulator that would be absolutely fantastic… I can do without the car for a while, I’m about to remove and paint the intakes…

Morning John, I would be able to test it for you as soon as it arrives, I would use the Royal Mail as most of the others have a habit of being late or loosing the item, Evri especially. I'm guessing that you have adjusted the potentiometer on the front of the reg. and it did not do anything? I Will return it as soon a possible, if you drop me an email on j.favell@hotmail.co.uk I will supply my full address unless you can find it on the forum cheers John.