I installed the Jabsco pump today – the original mounting nuts and bolts were ugly lumps of rust (who would use non stainless steel hardware on a marine engine?) and so i replaced those. Trimmed the ends of the raw water hose as they were looking ugly. tightened everything up and it runs much smoother. so far so good, although i expect it will start dripping soon. I’ll be damned if I’m gonna throw a $100.00 impeller in every year just to stop a little drip.
One thing that came up though, was that when i started the engine, the tach didn’t work. Damn, it just never ends. i pulled the instrument face and checked for tach signal. something like 1/2 a volt. not sure what it is supposed to be other than a pulsating signal, but i doubt that would be adequate.
Sigh. so i haul everything out of the port lazarette remove the engine cover and start fiddling with voltage measurements. I know that the tach signal is taken off the stator so with the engine off i checked for continuity.
Because the stator output is run through a diode (rectifier) bank, you must use the diode function on your digital multimeter. Strangely, everything checked okay. I checked the connector and the wire to the tach for breaks – it’s fine. Now I’m stumped.
I started the engine up and the tach runs for about 1/2 a second and then shuts down. I measure the signal and it rises to like 9 volts and then drops down to zero. Now this is stupid. I’m imagining a bad solder joint in the alternator is letting go under vibration and contemplate ripping the alternator out when I have one of my rare flashes of brilliance.
I run below and shut off AC power to the battery charger. After using the batteries to start the engine several times, of course it’s reading like 14.5 volts. Once the charger is shut off, the 12V system voltage slowly drops to normal. I run back to the cockpit and start the engine. Bang – the tach works fine!
What seems to be happening is the “smart” alternator regulator i have sees that elevated voltage and shuts down power to the alternator field windings. With no power going to the alternator field, there is no output and no tach signal. Once i removed that charging voltage supplied by the AC charger, everything worked as it should.
A lot of mucking about for nothing, but i understand her a little more so it’s worth it in the long run. I have a battery isolator here somewhere and I’ve toyed with running the output of the alternator through it. That way the regulator won’t be able to detect the voltage from the AC 12V charger. One day when I’m bored, i guess.
