Lucas Generator and Control Box tests

(John Burke) #1

Over the winter I’m going over all the major systems in my new XK140 MC OTS. I’ve replaced some cooling hoses that were starting to crack, had the radiator cleaned and pressure tested, rebuilt the C45PV-5 generator and bench tested the RB310 Control Box. I’m now going through the “Lucas Generator and Control Box Tests” as described in the link below to make sure my charging system is within spec. I’m up to test 5 on the current regulator but am stumped as to how to measure the amperage. The test indicates a C45PV-5 Generator should put out 22 amps +/- 1 amp at 4,500 rpm (I assume that is engine rpm as opposed to generator rpm, but I am not positive). My conundrum is that my multimeter only goes up to 10 amps and goes off scale. I can’t even find a multimeter that goes up to the 40 amps DC indicated in the test. Do people just connect the ammeter in the car? If so which wires are those in the engine compartment? Any help from someone who has completed these tests is greatly appreciated.

(Roger McWilliams) #2

Ammeters come in a wide range of maximum current measuring capability. Multimeter ammeters maxing out at 10 amps is common. Using the ammeter in the car makes good sense since it supposed to cover the normal operation range. One potential drawback would be knowing the current to within 1 amp of accuracy or precision is a bit unlikely using the ammeter in the car. I would use the car ammeter in my car, I have confidence in it based on each past drive, but I don’t worry too much about getting the current to within one amp.

(Andrew Waugh) #3

Get a clamp on Ammeter. You can get inexpensive ones (mine was about 50$, but they’ve gone down in price). Clamp types aren’t terribly accurate, but they’re close enough for what you’re trying to measure.

You can also get a clamp attachment which you use in conjunction with your multimeter.

The clamp types are very convenient in use, as you don’t have to disconnect anything to take a reading.

(John Burke) #4

Thanks guys. A clamp meter is on the way.

(Clive Jervis) #5


I don’t think that calibrating the current cut-out is essential to begin with. As long as the voltage parts are correct, then you should be good. You can observe the charge current on the car’s ammeter.

Unless you are planning running at night in the rain with a discharged battery, I doubt you will need anywhere near the max output.

If the meter is pegging I would suspect the voltage cut-out is set too high. If you believe the battery is good and the voltage settings are correct and the meter is showing a large charge, then you can adjust the current regulator.


(John Burke) #6

I suspect the ammeter in the car isn’t hooked up properly. It shows about +5 to +10 amps when the car is running but does not move at all when I turn the headlights on, yet they shine brightly.

(Clive Jervis) #7


When the engine is not running and you turn on the lights does the needle show discharge?


(Rob Reilly) #8

I once had occasion to run my XK120 all night in the rain, although with a good battery, got lost in Kentucky looking for my reserved campsite on the way to the national Jag show in Franklin TN, but the car did fine. Lucas can be a Prince in the Darkness.

(John Burke) #9

Hi Clive, no it does not.

(John Burke) #10

Well, I got the clamp ammeter which worked nicely and I was up to test 6 when all the smoke escaped from of my freshly rebuilt generator! Now it doesn’t charge at all when wired up properly, the IGN light stays brightly lit.
My car was having charging issues all along so I think I’ll throw in the towel and try a Dynalite alternator conversion. It’s basically a small alternator and regulator inside the casing of a Lucas C45 Dynamo so it retains the stock appearance. They put out 40 amps from about 1,500 rpm and are available in both negative and positive earth. I’ll be sticking with positive earth to keep my car as original as possible and simplify the installation.

(Clive Jervis) #11

HI John,

Sorry to hear about the escaped smoke :frowning:

If you ammeter did not show discharge when lights were turned on, then either it is not functioning, or a PO had by passed it (because it wasn’t working?).

Maybe your generator can be saved - have you measured the resistance on the field coils? I think they should be 6 ohm. If field coils are good, then try flicking the pulley wheel and see if you can measure any current on its output - it will be milli or micro amps, but will tell you it is not dead.

Maybe the brushes got burned up a little and all else is OK. You will have to pull it apart to inspect brushes and armature.

Moss Motors have this video showing how you can test a generator by running it as an electric motor:

Best wishes,

(John Burke) #12

Thanks Clive, I will test the generator as you describe.

In the meantime I found this :smile: