Hints and Tips - Dynamo - Conversion to Regulator

From: Bill
Sent: 14 June 1999
To: Dave Stewart; british-cars-pre-war
Subject: Re:MMM Dynamo

In a message dated 6/13/99 11:22:54, Dave Sewart wrote, and herein seeks advice and council:

<<Gents, We have had some real difficulty getting the dynamo in my 34 PA Airline to produce adequate current. It is specked as a 12 volt system, I think. We have had it out twice, worked it over, and it bench tested ok. We have "restored" the voltage "regulator." When installed, it will not produce adequate current below about 3200 rpm. As you might guess, I am completely in the dark about this aspect of the car. Any ideas?>>

Dave Sewart

From: Adrian Roger Twelvetrees
Sent: 19 June 1999
To: Scotty; Dwyer
Subject: Re: MMM Dynamo-Reply Dear All,

One of the problems with 3rd brush dynamos is that they need higher revs before they reach a controlled state than the two brush and regulator setup. Hence anyone running a car on a lower axle ratio than originally fitted(say 4.1:1 instead of 5.25:1) may well find that his charge is not keeping up with the current taken by the headlights when driving legally in built up areas. Conversion to two brush and regulator also gives your battery a much better life, since there is no risk of overcharging in the summer. However, I do think that the sight of a plastic regulator on the bulkhead spoils the car, so why not hide it somewhere?

Best wishes,

Roger Twelvetrees

Date: Tue, 15 Jun 1999
From: Dave Dwyer
To: british-cars-pre-war
Subject: MMM Dynamo


Your message about the PA system suggests that you don't have too much information on it. Pardon me if I'm teaching my granny, but note these points:

1. Definitely 12V

2. MAXIMUM repeat MAXIMUM current 10A (there's no significant cooling)

3. If you have the original system, what looks like a regulator is only a cutout.

4. The charge rate is adjusted by moving the "third brush", which alters the field excitation. You do this by removing the inspection band and loosening a small hex head screw whose head points down inside the top plate of the dynamo (on the left side of the car I think). DO NOT IN ANY CIRCUMSTANCES set the dynamo to charge at more than 10A no matter how fast the engine is reving (I set ours to a max of 8A just to be safe).

5. The other thing you need to check is that the field (third brush) terminal (the smaller of the two studs on the right of the dynamo) is connected through the ignition switch to the charge terminal. If in doubt about this link them at the dynamo to test. In the "winter" or "2" position on the ignition switch they are linked. In the summer or 1 position the circuit is completed by a resistance coil inside the top of the dynamo. If neither of these links is present you won't get a charge.

Again, my apologies if you knew all this and have a more complex problem.

Dave Dwyer

From: Scotty
Sent: 17 June 1999
To: Dwyer
Subject: Re: MMM Dynamo-Reply

On Tuesday, 15 June Dave wrote Subject: MMM Dynamo

>3. If you have the original system, what looks like a regulator is
>only a cutout.
>>>>>>>>>>>end of anip<<<<<<<<<

Dave, and the MMM afflicted, a LUCAS, Technical Service Bulletin [a copy of which I have in my collection of things interesting] from the Lucas factory is titled * Replacing, Third Brush Generators, with Compensated Voltage Control Equipment. typical wiring modifications.* There is no date on the bulletin but it does say, the average age of vehicles fitted with third brush generators is ten years or more. Any body want to guess the year of this publication? What's the point ? IMHO I think it is OK to modernise a vehicle to this extent, especially as Joseph Lucas and company endorse this Mod as they have with a tech service bulletin. My J2 will run on a two brush generator and RF95 box, which is the recommended one in the publication. When I get back to restoring it.---YMMV.


'33 J2 #4330.

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