0.1V DC multimeter battery reading when motor is running

[proslambanomenos]

I have a 1965 Mercury 1000 (100 HP) connected of course to a 12 V battery. When I connect the multimeter to the battery without the motor running, I get, as expected, 12.34V DC. When I connect the multimeter to the battery when the motor is running, I get 0.01V DC.

I'm not the world's greatest electrician, but my understanding of alternators is that the multimeter should register at least the same voltage if not more when the motor is running than when it's shut off. Is this correct?

Would one of you folks mind explaining my readings to me, and tell me if my alternator (does the old 1965 even have one?) is behaving?

Thanks,
Austin

 

[ThumbPkr]

Re: 0.1V DC multimeter battery reading when motor is running

You are correct,the reading should be higher if it is charging properly.
If it is a digital meter and it is not autoranging the higher charging voltage may be forcing it off scale which could account for the discrepency in indicated voltage.The output from the alternator is rectified AC which are really just pulses and are unfiltered and one of the functions of the battery is to provided the filtering which acts as a large capacitor to "smooth" the pulses.
If you have dirty battery cables the voltage reading might vary depending on where you probe the circuit.
I would try another meter or experiment with different metering points to try and resolve this.Ron G

 

[CharlieB]

Re: 0.1V DC multimeter battery reading when motor is running

Also switch your meter to the AC scale and see if you get a stable reading which could indicate a shorted diode in the rectifier allowing AC Voltage to reach the battery.

Check AC Voltage on the stator/alternator side of the rectifier, then remove the rectifier and test ohms thru the the rectifier BOTH directions. From each yellow to red, then again reversing your leads, red to each yellow. You should get readings one direction only, anything different then replace the rectifier.

 

[Yepblaze]

Re: 0.1V DC multimeter battery reading when motor is running

It's not uncommon for these older motors to wreak havoc on meter readings while running. Even some of the better digital meters go haywire when trying to get readings from these motors while running.

Try a different, better, or analog meter you may have better luck.

Or try taking your readings from an accessory power tap up by the helm. Sometimes once you get away from all the electronic noise from the running motor things seem to mellow out.

 

[proslambanomenos]

Re: 0.1V DC multimeter battery reading when motor is running

thanks folks, i appreciate all the info. i will try your suggestions this weekend.