Tachometer source of power

[MyPleasure]

Hi Fellas,
Our 36 Carver has twin Volvo Penta 740B gas engines. My port tach has stopped working over the winter, I checked the connections and everything looks good. Would anyone know if the tachs are driven from the alternator or the distributer?
Thanks for any help

[waybomb]

Power comes from the batteries.
The tech signal comes from the ignition.
Can you get to the backside of the tach easily? If yours are like mine, there is a screw that is used to select between 4, 6, and 8cyl engines. Turn that screw s number of times to all the settings, and then end it in 8. That may fix your issue.

[MyPleasure]

Thanks waybomb,
I can get to the back of the upper station tachs easily. I swapped the lower port tach to the upper port tach because the upper tach had stopped working, and now that tach has stopped working as well. I’m hoping that it’s not a problem with the wiring to the upper port tach.
When I swapped the tachs, I didn’t notice a setting screw for the number of cylinders, but I’ll have another look.
Thanks again!

[bud37]

Definitely check what Fred says and a question.....do you have an engine synchronizer gauge.......they can be connected to the tachs as well.

[MyPleasure]

Yes, there is an engine synchronizer gauge that seems to work fine, but I’ll check that as well.
Thanks

[MyPleasure]

Well, I tried moving the 468 setscrew around and it didn’t make the tach work. I guess that I’ll make sure that it’s getting power from the ignition next.

[bud37]

QR_BBPOST Well, I tried moving the 468 setscrew around and it didn’t make the tach work. I guess that I’ll make sure that it’s getting power from the ignition next.

Follow your wiring under the dash and see if the sync gage has a module that the tach signal is connected to....I guess/assume if the sync works as it is supposed to then there is signal from the engines somehow.

Can you tell what the maker of your gages is ?

[MyPleasure]

It’s funny that both port tachs now don’t seem to be working, but the synchronizers work at both upper and lower controls? So I wonder where the synchronizers get their signals from? I think that for now I’ll take the starboard tach from the lower station and switch it with the port tach at the upper station. Both tachs down below will no longer function, but I never really use the lower station anyway.

[waybomb]

The synchronizer gets the signal from the same wire as the tach.
On mine it goes from the tach to the synch. The synch was not a factory item on mine.

[MyPleasure]

These seem to be a factory install for the synchronizers. Since it just stopped working over the winter, I’m hoping that maybe the ground wire has some corrosion at the contacts. After Swapping out the other working tach from the lower station to the upper station last year, the tach that wasn’t working did not work when I installed it in the lower station, so I’m not holding out any hope that the connections are the problem. I’ll have to check the internet and see if I can find a way to test the tachs?