confusion on voltage regulators

[lkadams]

In reading on swapping from a 912 motor to 911 motor, I've read "don't forget to swap the 912 voltage regulator for the 911 voltage regulator". Further reading has pointed at the voltage regulator needs to match the generator (i.e. Motorola or Bosch or what have you).

Now, I don't have the votlage regulator from the 911 that I got my engine from, but I have some others. If I recall correctly, my generator is a Bosch. How do I determine what the correct voltage regulator is to match my generator? Or does it really matter and if I grabbed say the Bosch 12V 420 W 30A regulator I have sitting on a shelf, will that work?

Colour me confused.

[lkadams]

John (304065) had posted this on Pelican but similarly here:

1968 911s had a Bosch 35A alternator and Bosch voltage regulator. 1970 911Ts used a Motorola 55A alternator. You are always supposed to use the same make of regulator and alternator according to the factory. So, get yourself a Motorola regulator, or something aftermarket that is adjustable like a Transpo, so you can set the output voltage yourself. You are clearly handy enough to use a voltmeter so this should be simple The problem with mixing brands is that the "set-point" for DF voltage is different based on the input voltage. Probably not a HUGE deal but a good idea if this is intended to be a long term fix

So I guess it depends on which generator/alternator (sorry I keep calling it a generator) I have???

[lkadams]

I should also say that my 911 motor is from a 1969 car, so would a Bosch alt have been the original type?

[304065]

Keith,

Your alternator (say it with me now, alternator! ) is probably a Bosch, probably a 55A. If you look at the data plate on the side, it's an oval tag that is riveted on, that will tell you. Another way to tell is to look at the connections for the alternator field and D+/61 terminals on the back-- if they are in a "T" formation it's a 55A-- if they are studs, it's a 35A. So if you are not doing concours, get yourself a modern solid-state Bosch regulator. These have a 14V set point and will work fine. Something like 901 603 206 02 or the equivalent Bosch number should be fine.

[lkadams]

Thanks John - realizing I have a box with a couple of 914 voltage regulators in there, which is the newer type, so I'll have to take a look. It's been about a year since I had my fan/alternator (see I said it)/shroud apart, so I'm hoping I took a picture!

[chiroracer]

Will a modern bosch solid state unit work also for a marchal sev alt. ? How would it know,maybe the electrons have the little cat faces on them.

[304065]

Will a modern bosch solid state unit work also for a marchal sev alt. ? How would it know,maybe the electrons have the little cat faces on them.

It doesn't know-- they are both regulators that work in the B circuit-- by altering the DF (dynamo field) voltage to achieve a particular "set-point" voltage on the D+/61 output. I don't know what the specs are for both, but I know they are different, at least back when the factory specified that "alternator and regulator must be from the same manufacturer" and felt stongly enough about it that they put it on a gold foil sticker on the engine lid latch panel. (the one with the big lightning bolt on it!)