I have recently been plagued with burnt out on my 1967 Plus 4 4-seater. On average they have lasted about 150 miles, one only 30 miles and the best lasted 367. In all, I have had FIVE burn out. All replaced free of charge. After no. 2, we checked all the electrics. After no. 3, we replaced the regulator "just in case", although the old regulator had been checked as OK. About no. 3 dynamo, the supplier mentioned that they seemed to have trouble with this batch "made in India". I pointed out that the casing was stamped "made in England". "Yes," he said, "that is the casing, the windings are made in India." After no. 4, the electrics were again checked as was the regulator. Finally, after the fifth dynamo  packed up, Lucas themselves became involved, they were fed up with giving me new dynamos and I was fed up with (as was RAC)  in getting PUV 316F back home with no electrics and fittings the things.

Findings

Nothing wrong with the wiring or the regulator and yes, dynamo no. 5 burnt out, as had the others. But  the Indian armatures, although of supposedly same specification, were in fact producing more power than the specified, especially when fitted to a "higher revving engine" (presumably than the Morris Minor/Oxfords of India) helped by the larger lower pulley of the TR4 engine. So on prolonged running over 3500 revs, the power built up. Volts went sky high, the regulator earthed out, the dynamo, and then went tested the regulator was OK, it has saved itself but killed dynamo.

Solution

(We hope, the new setup is holding out at the moment, time will tell).

Lucas advised fitting an up-rated three coil regulator (part no. RB340) in pace of the normal two coil one (part no. B109 I believe).
 

The Red Light (Alternator vs Generator)
by Gerry Willburn at the eMOG Pub

The red light on a generator system is a much better indicator than the red light on an alternator system.

In a generator system, the red light is between the battery output and the generator output.  It will come on if the generator output is below the battery output.  It will also come on if the generator output is much ABOVE the battery output.  This is the case if the generator is working but the cut-out relay in the regulator does not pick up.

In most alternator systems, the starting power to the fields is through the red light.  Once the alternator output comes up, an internal switch in the regulator switches to the alternator output for field power and the light goes out because it has no "earth."  This can be a problem because, in an alternator, it is the field windings that go through the brushes.  When the brushes get so bad that they do not make contact,  The alternator will not work BUT THE RED LIGHT WILL NOT COME ON.  This happened to the 308 once and the first indication I had that anything was wrong was when the battery would no longer power the electric fuel pump (in the California desert).

ALTERNATOR KIT FOR THE TR4

I spent ages trying to find a way to replace my generator with an alternator. Many had suggested the workhorse Delco 10SI or 12SI series, they're cheap and easy to come by, but all the notes I saw about them discussed engine mods, which I wasn't interested in. (The Delco is also a "7127 AC Delco" alternator, and was used on a plethora of GM vehicles, you can find it at Kragen, Napa, etc.. for around $50)

I bought a 10SI and it seemed like the alternator body was too wide for me to be able to tighten the belt, and unfortunately most alternators are the same 5.25" width. I discovered the CS-121/CS-131 which were SI series replacements from GM. (SI is for "systems integrated" due to the fact that these were some of the first alternators to have an internal regulator, and CS is for "charging systems"). The 121 and 131 were the width of the stator (alternator body) in mm, making the CS-121 is about .5" thinner than the SI series alternators, but I wasn't sure of how well it would mount, and the alternators are harder to find since the few vehicles that used a CS-121 can take a CS-131 as well, so the auto shops only stock the 131. I tracked one down at an industrial alternator shop for around $200.

In the end, it didn't matter, because just as I was about to buy I found a company that sells an alternator conversion kit. They use a 10SI which supposedly has some simple case modifications (though I didn't notice any when I looked at it), and it fits and works fine, cranking out over 90A. Because of the high current, they recommend wiring direct to the battery which means your ammeter will only read discharge, but your charging light still works since it is on the field circuit. And presuming the regulator works fine, you can just look at the ammeter as a reading of how much power the car is using. Since the conversion requires a negative ground, you need to do the very simple ground conversion (see 2003/12/03 in the maintenance history), and if you don't flip the ammeter nodes, then this will just read positive anyways.

The shop that sells the alternator kit also has performance and street parts for british vintage cars, they're worth checking out:

Power British
http://www.pond.com/~britcars/
Phone: 610-270-0505
(it's a one-man shop, so generally you have to leave a message)