Tac Dilemma

[capt bobby]

I have just purchased a 43' trawler and had to leave the boat in MD. for a while to have some work done so I could bring her south.
All the gauges have to be replaced as most of them don't work. I'm guessing that most of them will be no problem but everyone seems to be vague about what unit I can use to replace the Motorola units that are on the boat.
I have sourced an sierra Amega 68358P 3000 rpm diesel tac but am getting conflicting answers as to weather it will work. does anyone have a handle on this problem.

[jleonard]

What engine(s)?

[Halcyon]

Some diesels have tach sensors that pick up from a magnet on the flywheel, some get a signal off the alternator and I'm sure there are others. Knowing the source of the signal will help you fit the right tach.

[capt bobby]

The engines are 120 ford liehmans and im guessing they have a gear drive sending unit. Everything I read says the boat came with SW tacs but they have been replaced with Motorola. When I talked to Someone at Lauderdale speedometer they said that they had an sw gauge (pricy at 200) they said I could also use a VDO they had in stock (145.00) My question is if those three gauges work (sw /Motorola / VDO) then why wouldn't a sierra?

[denchen]

Having just had a look on ebay and Amazon it seems the Amega 68358P takes its feed from the alternator. If you do not have a tacko wire on your alternator, come back and I will talk you through what I did to get a pulse supply for my tacko.

[capt bobby]

Thanks for the info. I believe the pulse comes from a gear drive now. If there is a way to run it off the alternator that would be great. Thanks for any help you can give me.

[jleonard]

You are better off using a pulse from a gear drive if you have one. The Alt signal (from the R terminal) can change as time goes on and you'll have to re-calibrate.

[capt bobby]

Thanks to all.
Went with a 4000rpm sierra tac. Replaced all four of them and tied them to the r post on the alt. At 2000 rpm on the motor they read 4000. this was no problem on our 1200 mile trip as we ran the boat at 3800 on the tac
( 1900) on the motor. they were easy to sync up so no problem....now that I have the boat at home I will see if I can make them read correctly.

[jleonard]

Buy an IR tachometer and do the calibration yourself. Its easy to do. The tachs should have an adjustment screw via an access hole in the back. Do the calibration at around 1800 or so, that is where you need the most accuracy.

[mike66]

There may be a switch on the back of the tach for different alternator pole counts. Change the setting until it reads properly. Then fine tune with the adjustment screw if needed.