[Mk1] Why does Amp meter bounce up in the 'C' range?

Battery is good
Brushes in Generator are fine.
Belt is tensioned ok.
Regulator seems clean and points look good
At idle the gauge reads just on the D side (pretty normal)
However, when driving with the exceleration down the Amp meter bounce all over in the C range.

What’s going on here?

The car drives well but I just don’t understand what’s happening with the Amps

Underneath the regulator are several resistors that act to swamp out voltage and current surges as the contacts open and close. The MK1 uses an RB310 regulator, which has one or two, between the field and ground, and one between the field and the armature of the generator. If any are open they will produce the symptoms you are seeing.

Thanks for responding.
Pardon my ignorance but I don’t know what you mean by ‘open’. Are you referring to the three points?
In the attached picture the first two points are closed but the third (the one with the different copper points) is visibly open.
Is that what you are referring to?


This might be a slightly better picture.

Open means not conducting like a burnt out fuse in reference to the resistors Mike was describing.

You are seeing three switches, by underneath I understand you should look inside the metal hump they sit on, so unbolt the regular and look at the bottom, and measure the resistors you find there.

Here is a picture of the underside of two RB310 regulators from my junk box, which are slightly different from each other.

The cylindrical bars that you see are 100 ohm resistors. The regulator on the left has one resistor between the “D” and the “F” terminals, while the one on the right additionally has a resistor between the “F” terminal and ground. None of the resistors in the picture are functional, and I suggest that yours may also have suffered the same fate.

So, I’ll ask the obvious question, if we know the specs of these resistors can they be bought from an electronics supplier and fitted to these regulators?

That’s what I did many years ago.

Upon looking at my MK2 I see that I had replaced the “D to F” resistor with a pair of 75 Ohm 5 Watt resistors in parallel and the “F to Ground” resistor with a 1N5400 diode, biased for a negative ground car. The diode snubs the inductive kickback from the field winding and by so doing keeps the voltage regulator points from arcing and eroding. I probably used two resistors because I had them, but if you’re buying them a 33 Ohm 5 Watt resistor should work fine.
Mouser.com sells electronic parts.

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If I can digress for a moment and we assume that the regulator is working correctly; would it be normal to see the Amp meter reading further over to the ‘C’ charging side if the battery is at 11 volts. In other words does the regulator allow the voltage to continue until the battery and generator are at equilibrium?
Could this be a reasonable explanation for the Higher reading on the Amp meter?
Should I expect the needle to move back towards center once the correct battery charge has been achieved?
Note: this behaviour is only evident when drive and not when at idle.

If your battery is discharged to a point where the terminal voltage is 11volts, the generator and regulator will work to charge to the regulator’s settings. A generator works differently to an alternator and builds voltage with speed (the regulator manages charge). An alternator can maintain charging voltage at idle - something that a generator does not, in our Jags. I have two original generators in a '63 and a '62 and they both behave as yours: after a long break or multiple starts in short time the ammeter will run to maximum charge whist the cars are running at some speed and fall to zero at idle.

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Mike that’s absolutely brilliant.

The “why is my ammeter bouncing” question is a perennial one, and here, finally, is the fix!

I’ve really appreciated all of the responses here, so thanks everyone. Combined with this excellent little pamphlet, ( https://www.mg-cars.org.uk/imgytr/pdf/lucascourse5.pdf ) I think I have a better understanding now.

One last question:
If the resistors (under the regulator) are blown what is the impact of driving the car with them in this condition?

Here’s another Lucas publication from 1963 that might prove beneficial for troubleshooting a dynamo charging system:
Lucas_Generator_and_Control_Box_Tests.pdf (823.8 KB)
All the best,

The voltage regulator points will arc and burn out much more quickly.

Thanks Mike, this has been a very helpful thread.

I have similar symptoms with my 1964 MK2 3.4 MOD. For about the last 500 miles the amp gauge would read high charge once the car had been driving for 15+ minutes…the gauge would read normal after warm up just charge, then start flicking on the charge side, almost to the limit of the gauge. Then last week after travelling about 30 miles, about 3 miles from home, the gauge remained near the limit. The large resister registers within limits, however the resister between D & F registers less than 1 ohm resistance…would this be the reason for the amp gauge reading too high?


If you did not disconnect the resistor from the terminals then you were not only measuring the resistor, you were measuring everything else that was connected to the resistor. In this case, that includes the Voltage Regulator relay points, which are normally closed, which should give you a resistance reading very close to zero. Also, if the regulator was still connected to the generator then those windings were in the circuit too. If you were to disconnect the “F” terminal from the VR and put a piece of paper between the VR points then you would get a more accurate reading. Of course, don’t do any of this with the battery connected.


The Control Box is on the bench. Placed some thin cardboard between the contacts. Recorded the following values:

Contacts Resister 45ohms.
Page 25 of the Lucas Generator / Control Box Tests shows resister values as 37-43 for 12v generators with 4.5 ohm field windings and 55-65 for 12v generators with 6 ohm field windings. I understand the regulator to be the latter (as per the parts book), which means the value is below the range @ 45 ohms.

Other measurements leaving the cardboard in place:
D to F 45 ohms
F to G 68 ohms
D to G 25 ohms.

Perhaps the problem is elsewhere.


You’re right, those values sound good.