Alternator question Re: pulley size (UPDATE)

In another thread , I asked about shaft size ....Here , I need to know if the alternator pulley diameter is the same for all MGB's....
My newer alternator (rebuilt), is showing similar signs to the one I replaced...i.e., the light flickers until I reach about 1500 rpm...I'm wondering if I have the wrong pulley size...A slightly smaller pulley, would keep the alt light out, and the battery charging at a more normal idle speed.
I used the same pulley for both units.
Thanks
Edward

I can't believe no on knows the answer to this....???

Do you have a seperate voltage regulator? If so, is it an ood, electro-mechanical unit with the adjustable coils inside? There may be an electronic VR that you could install/adapt to the system. I had to do that with my 66 mustang to cure the oscillating needle at lower rpms.

It helps to add the year of your car to your signature or avatar. I looked at your profile and it seems you have a 74.

Maybe someone will jump in with some more info about the stock pulleys. I have replaced the alternator more times than I wish to count on the 76 and have used the same pulley.

I don't have the info handy, but the manual does tell the ratio of engine revs to alternator revs - and the specification for maximum alternator speed is also published. If you can't find it, PM me and I'll look it up for you. I can measure my pulley too if that would help.

BUT - before you go tearing off, the real question is, once your warning lamp goes out (at 1500 RPM) does it stay out at normal idle. If so, then I think everything is fine.

Also, I would check your tachometer's accuracy. They are often out by a couple of hundred RPM - which isn't much error at 6,000 RPM but is noticeable at 1,000 RPM.

Note that dirty brushes can also make it difficult for the alternator to establish field activation.

I think the '74 has an external regulator, and you should start looking there.

As far as I know:
The alternator pulley is the same from 1968 to 1976 changing to a different pulley from 77-80.
The 77 and 78 used the same alternator as the 68 to 76.
The 77-80 alternator is different from the earlier one.
From 62-67 MGBs used a generator.

Anyone have additional information on this?

Sorry for the confusion....
My GT , is a '73....and has an internal voltage regulator...
I did have a '74 CB car, but it is gone.
When I got the '73 , it had what appeared to be a new pulley on the alternator...Since I had nothing to compare it with, I don't know if this pulley is the correct one.
The alt. that the car came with would not allow the lite to go out, until it reached almost 3000 rpm, and it would not stay out...
My rebuilt alt. replacement, puts the light out at about 1500, but the light flickers continuously below that...
Obviously something is amiss, but I can't seem to figure it out...Grounds are good...
This is why I'm suspecting the wrong size pulley...It would only take a slightly smaller one to keep the light out at 1000 rpm, which is where I like my idle.
If someone can measure the correct pulley, that would help.
Thanks
Edward

Oh man, now you are going to make me go look.

The pulley for your car should be 70MM (2.75" )for 219 mm circumference. That is the 36 amp alternator

On my 79 the pulley is 57mm (2.25" )for a 179mm circumference. That is the 43 amp alternator.

If you were to change to my pulley you would be running your alternator WAY faster.
Consider that the alternator runs at present 3-4X as fast as the engine.
The later pulley would turn your alternator about 20% faster.

While I am at it let me correct myself. The 73 does not have a voltage regulator per say it has a rectifier pack and it is internal to the alternator.

Thanks for the information...that will be very helpful...If this isn't the problem, then I guess I wasted my money with the "rebuilt" alt. that I got from a member....I really don't know where else to look.

Don't feel bad, I have a "brand-new for me" rebuilt alternator sitting in my garage that I never installed because I giggled the wires around on the back of my existing alternaor and giggled the wires under my dash, and the dim red glow of the ignition lite went away. It can be a little grounding problem under the dash. Do a search of the forum for how the instruments are grounded under your dash, and make sure your ground is to a spot you have sanded free of any rust and corrosion.

Good Point Dennis.

There are some interesting things that happen with grounds and shorts that are hard to figure out when it comes to things like alternators when running. Since it is a bad idea to run without a battery or without the alternator hooked up or to work on the electrical system while the car is running there are things ou cannot comfortably tell someone how to check when all assembled but you might find by testing components out of the car or with a component out of the system.

Your problem, as dennis pointed out by allusion, could be an intermittent short in the indicator circuit...

Is there a "standard" location for the instrument ground?

Well, tested the alt. output with a friend's "load" meter, and the "new" alternator is charging fine...14V+, and the battery is fully charged...
I still have to find the cause of the flickering alt. light...
One interesting fact, the light stops flickering if I turn on my parking or headlights...
I'm sure that's a clue...The circuit may be looking for a ground, and is finding it in my headlight circuit....
Now I guess I'll have to get under the dash....I still don't know where the ground is for the dash on my '73 GT...Anyone out there know where to look?
Thanks
Edward

On my 77, there are 2 or 3 wires with lugs on them going to a screw on the back of the firewall. Open your drivers door, and sit on the floor or ground, and then slowly slide your head under the dash with a flashlite in hand. You can get real comfortable with your head resting on the brake or clutch pedal.