Taco problem with 115 merc

[of2fsh]

Hi, just a quick question as im having a problem with my 115 merc 2004 model.
Ive posted here and not on the similar thread so as not to hijak the other blokes thread


Today i lost the taco on the way home from fishing,the taco needle sits hard down when the motor is running.When the ignition is turned on with the engine off the needle rises up a bit.I checked the charge rate with a multi meter and it seems only a slight increase when the engine is running.from 12.4 volts to 12.9 with the engine idiling.So would i be correct in saying i should replace the rectifier.Im having to repair this my self as here in New Zealand at this time of the year its impossible to get the boat to a mechanic as they are very busy.

Cheers in advance, Wayne

 

[sschefer]

Re: Taco problem with 115 merc

Hi, just a quick question as im having a problem with my 115 merc 2004 model.
Ive posted here and not on the similar thread so as not to hijak the other blokes thread


Today i lost the taco on the way home from fishing,the taco needle sits hard down when the motor is running.When the ignition is turned on with the engine off the needle rises up a bit.I checked the charge rate with a multi meter and it seems only a slight increase when the engine is running.from 12.4 volts to 12.9 with the engine idiling.So would i be correct in saying i should replace the rectifier.Im having to repair this my self as here in New Zealand at this time of the year its impossible to get the boat to a mechanic as they are very busy.

Cheers in advance, Wayne

I won't rule out the rectifier but I wouldn't replace it because of a Tach problem. The Tach is on the simplest side of the rectifier circuit and there's little that can go wrong. However, they do fail so it can't be completely eliminated.

The voltage readings are correct but you're looking at DC voltage and you want to see pulsed DC. In order to see that you should use the A/C scale and be reading 5-6 volts A/C on idle from the S (signal or send) post of the tach. If you are, then the rectifier is probably O.K. and the tach would be suspect.

Check your multi-meter, it may have a tach mode built in. I have a high end Fluke and a cheapo Radio Shack and they can both be setup to act as a tach. Both of them setup differently so consult your manual.

It's important to remember that a Tach doesn't care about voltage levels as long as they are within and acceptable range (9-15volts). What a tach interprets for you is the number of voltage pulses it gets.