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2003 Mercury 90HP 4 stroke voltage regulator issue

A

aegwell

New member
I have a low voltage reading at my battery (+12.3V) with the engine running. I measured right at the battery. I also switched the meter to AC setting and I read +25Vac.

Overnight, I charged my battery and the next day the battery measured +15.8V while running the motor. We went for a cruise and the battery reading got lower as the day wore on, eventually settling just above +12V.

Is the problem with my voltage regulator/rectifier? It seems to me that the rectifier is "leaking" AC voltage so that would be my first guess. However the replacement reg/rec is expensive and I don't want to just guess.

The current part number is 804278T11.
 
Is the problem with my voltage regulator/rectifier?
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Yes it is. In order for AC voltage to get by the R/R one or both of the rectifiers have shorted. Replace it and your battery will charge. Where did you find that part number? What is the engine SN? Carbed of EFI?
 
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The regulator part number I typed in original posting, is the one that came on my motor when new.
The motor is a carburetor type, not EFI (I wish it was).
My serial number:
OT644703.​
 
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I traced the part on this site...NLA. The serial number begins w/a zero, not an O. 0T644703. Does it have wires infused in it or do wires get connected to studs on it? A picture would help. Contact Mercury and ask them which rectifier will replace it.
 
Yes, that was a typo...it starts with a zero. The wires are potted in the unit. There is a good picture online if you Google Mercury Volt. regulator part number 854515.

I know which part number the motor now takes..804278A12. But I am looking to avoid paying for a brand new part. Yamaha makes the identical part for much cheaper, just a different part number.

If the problem was something other than the reg/rec I wanted to find that out first before buying one.
 
When you install the replacement make sure to smear the back w/a coating of heat transfer grease...can't think of the name of it. Any auto parts or electronic shop will have it.
 
when you start the engine the voltage was too high for a regulated output...it should read 14.8....it is normal to see ac on the output because the voltage is not filtered down to a flat line...in fact you are charging off the peaks of the ripple..i agree that you have a bad regulator...because of the cost i would meter the output directly on the terminal block coming from the regulator and compare that with the battery post readings..just to eliminate a bad connection or something...if your tach is crapping out as the voltage drops then you need not do this...you either got a bad regular or a stator failing...i agree that the cost of that regulator is high..
 
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