Got no output from my alternator. It's on a 29' 1990 Fountain . I believe it's a Prestolite 35A but not positive. Don't see any markings or a data plate. It was rebuilt by a reputable rebuilder a couple of years ago.

It has larger gauge orange wire and a larger gauge black wire for pos and gnd, but also has a smaller gauge blue wire and a smaller gauge brown wire going into it. The cap on the rear has an unusual rounded edge. Most pics I've found are straight across. See Pic.

Wanting to confirm what it is, if it needs an excitation signal and if it has a regulator/diodes that can be obtained/replaced. Thx!

Motor serial number please?

Not sure where/if the engine serial # is on the eng. Made a brief effort to locate it on the eng, but it's pretty tight in there. Guess I'll need to find the paperwork for the boat. I'm pretty sure I saw it in there somewhere.

Took off the rear cap and discovered the rear cap is the voltage regulator. It had a sticker inside that says "Transpo IY785 12V 093".
Looked that up on the net and discovered it's for a Mando alt and not really rare at all, just old. Appears they were used on lots of Mercruisers.

Disassembled alternator to the point I could check the diodes. Looks like you would have to unsolder the diodes to test them individually but at least one is working per my diode check function as witnessed by the 0.4V drop one way and open the other way. Only saw 3 thru-hole type diodes. I was expecting to see 6 diodes but almost all my alternator experience is with higher current GM alts (automotive) from the 60's thru the 80's.

Brushes and slip rings looked good and rotor measured ~4 ohms so that appears to be fine.

Tried bench testing the voltage regulator using a dual channel laboratory p/s with variable current and voltage. Connected one channel of the p/s across where the rotor connects (pos=green, neg=orange) and the other channel to the remote sense line (red wire) and gnd. Also applied 12V to the blue "ign" wire (to arm regulator). Only seeing ~120mA for the rotor and it doesn't change as the remote sense wire voltage is adj'ed above and below setpoint. Think it's safe to say the voltage regulator is bad.

Found a few regulators on the net, but in the pics, they all appear to be knock-offs. Eventually found an "new/old stock" Transpo IY785 on Ebay and scooped it up. Should be here in 4 days. Fingers crossed I get it wrapped up by the 4th!!

Serial number will be on a plate just above the starter

Don't know what motor but here is the mando alt
https://www.mercruiserparts.com/bam/...y/31808/961/40

That's it! Thx. I was on that site prev but somehow missed that page. Looks like the regulator is still available. Page also explains where the other 3 diodes are.

Ebay voltage regulator arrived. Installed and went for a test run. All is good.

Cool beans