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Voltage regulator not regulating, need help

Moss Motors
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73GeeTee Avatar
73GeeTee Todd Heimer
Hi gents,

Need some advice on what to test next:
My Lucas 12V positive ground system charges at 20-30 A and ~14 V when engine running and 12 V applied to the field.
But if I hook the regulator up normally (that is, NOT manually applying voltage to the dynamo field), it doesn't charge at all.
I've tried cleaning the contacts on the regulator; tried two other regulators (one old, one NOS) all with the same results (no charge).

1) what else can I test to determine why it's not charging?
2) are these regulators (ie Lucas RB340) polarity dependent?

(the vehicle in question is actually an Austin FX4 taxi, but I'm posting here because it's 12V positive ground, lucas generator, lucas regulator; like my MGA. The FX4 forum is pretty quiet)

Todd Heimer
Exeter, RI

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ghnl Avatar
ghnl Silver Member Eric Russell
Mebane, NC, USA   USA
1961 MG MGA "Calvin"
Click here: then scroll down to page # ET-120 plus the next few pages. Assuming your car's charging system is the same or similar to the MGA, you should find what you seek there.

Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

Blueosprey90 Avatar
Blueosprey90 Jeff Sienkiewicz
New Milford, CT, USA   USA
Google “moss motors voltage regulator adjustment” and “John twist university motors polarizing the generator” for some videos.

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copernicus Avatar
copernicus Nick Kopernik
Snowless Winter, CT, USA   USA
Todd, regulator/control box is not polarity sensitive. Moss Motors has 4 videos on testing, adjusting and replacing a regulator. Attached link will take you to their Youtube page; scroll down to about the 20th row and you'll see the 4 different videos:

p.s. I went thru a couple of bad regulators before I switched over to a solid state unit and have never had a problem in over 10 years.

18530 Jack Strobel
Saint Louis, MO, USA   USA
From your description it sounds like the cutout side is not working. If you go to From the Frame Up's website they have the complete Lucas training manuals listed in their tech section and the section on Generator_Output_Control is quite good. I've attached this section but go to the site and download the rest.

Lucas05_Generator_Output_Control.pdf    1.77 MB

ozieagle Avatar
ozieagle Gold Member Herb Adler
Geelong Victoria, Australia   AUS
1958 Wolseley 1500 "Wooly"
1966 MG MGB "Bl**dy B"
This Lucas publication may help you find the problem.


Questions about prostate cancer? Click here to join the discussion

Lucas_Generator_and_Control_Box_Tests.pdf    1.06 MB

73GeeTee Avatar
73GeeTee Todd Heimer
Thanks guys for the help. By following your leads I was able to find this:
which has 26 pages on how to diagnose and adjust my three coil Lucas RB 340 regulator. Problem solved, thanks!

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