MGB & GT Forum

I’m obvi missing something. Stock 1967 Dynamo. How can I test it? When I connect a volt meter to the two terminals- 14 volts. According to some stiff I’ve read it should be 1-3 at 3000 rpm… seems low.

Best

Which two terminals? Simplest way to test the charging system (generator & regulator) is to connect your voltmeter to the battery posts. With everything off a fully charged auto battery should show 12.6V. 12.3V is 1/2 to 3/4 charged. Next start the engine and see what the voltmeter reads with the engine running at about 2,000 rpm. To charge the battery it should read 13.8v - 14.4V. Much less may not charge the battery. Increase the revs - the voltage should stay about the same. If it increases to 15+ volts the battery will be overcharged.

If it passes these tests the charging system is working as designed. Proper maintenance involves keeping the battery cables clean & tight (including the ground cable connection to the body) and adjusting the fan belt tension. Too loose will let the pulleys slip, too tight will wear the bushings prematurely. Just right is about an inch of deflection in the longest run of the belt.

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

14 volts is what generator-alternator in good working order is supposed to be.

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Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com

What are your reasons for not changing to an alternator ?

It’s a recent rebuild and has worked well. The regulator box burned out and I decided to test hence the query. A dynamo has fewer things to go wrong- no diode and can be rebuilt.

Well, it is true that it does not have rectifiers. BUT - it does pass the full output current through the brushes (whereas an alternator's output doesn't pass through the brushes). The regulator on a generator is usually mechanical, and involves several sets of contacts. Regulation is not as precise because the regulator typically only has a few levels of output whereas an alternator's regulator is continuously variable. Because the generator's limitations, its output is considerably less than most alternators. The 16ACR alternator is easily rebuilt.

If you want to run a generator for reasons of originality then by all means do so. It is adequate for an early stock MGB. But do expect your output to be low at low RPM. In fact, at idle or near idle, you probably have a power deficit if headlamps, heater fan and wipers are on. That is, the generator isn't keeping up with demand and the shortfall is supplied by the battery. An alternator is much better at producing power at low RPM (although a 16ACR may still have a slight deficit under worst case idle conditions).

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Alternators can be rebuilt, and quite a few you find at the parts suppliers are, in fact, rebuilt.

Checking voltage at the generator is likely to give you an erroneous reading since you may be checking output before the regulator. Field output if I recall correctly, is controlled only by rpm. You could see voltage as high as 18 volts or so at 3,000 rpm. Not sure what your "1-3" reference is unless it is a typo.

Checking voltage at the battery will check the entire charging system, generator, regulator, and battery. You will also find battery voltage in the engine bay so you don't need to expose the battery just for a quick check.


GMc

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Wait for me at the gate Tuffy, because without you they're never gonna let me in.

SDCH WTCH-X Twin Oaks Tuff Nutt
2008 - 2022

Hi,

Here's the Lucas booklet on testing the dynamo.

Herb

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