Merc 850 16v alternator output

[merc850]

According to my onboard volt meter and handheld VOM my alternator is putting out 16v. I know that 13 to 14v is the factory rating for a 1975
850; does anyone know what causes this and can you identify this part in the attachment (it was screwed onto the distributor front plate.)

 

[j_martin]

Re: Merc 850 16v alternator output

According to my onboard volt meter and handheld VOM my alternator is putting out 16v. I know that 13 to 14v is the factory rating for a 1975
850; does anyone know what causes this and can you identify this part in the attachment (it was screwed onto the distributor front plate.)

That's a shunt regulator. Actually, it is probably dead, and also really isn't needed. The battery is what's keeping it from going to 90 volts. In good shape, it'll hold it down to about 14.5 volts. It crawls up a little as the battery ages, like several years, and goes up drastically if you let the battery run out of water.

To recap. The stator design determines the maximum current available. Dead shorted at WOT, it won't supply any more or hurt itself. In order to charge at low rpm's, the voltage will freewheel to about 90 at WOT. The rectifier converts all the current available to DC, and the battery, load, and regulator hold the voltage down to a usable level. Bad connections and undersized wire can also allow the voltage to rise.

hope it helps
John

 

[merc850]

Re: Merc 850 16v alternator output

Thanks for your reply, the battery is 6+ years old and I will replace it after checking all the electrical connections. I have been running old Mercs for 35 years but they were all non Thunderbolt powered and all I would check for is 13v and water levels; I don't know how over-voltage would affect TB ignition and I had to replace the trigger already(bought motor on ebay).
Murray