20 Pole Tachometer Compatibility

[Bigprairie1]

Ok, I know there are a few tach threads rolling around...hopefully one more won't be too tedious. As well, I know that my 90hp Force needs a 20 pole tach.
However my question: would this (20 pole) tach be compatible with the 4cyl/5cyl Force outboards? (120/125/150hp motors?)
Is this driven by the rectifier or?
OR...does each of these motors need its own special Tach? I was wondering if Bayliner/Maxum/US marine made specialized tachs to cover off each of these or if the outboards separately or used a generic 20 pole tach which purely had a cylinder setting (3/4/5) on the back?
I don't have a tach in my boat (bayliner) right now and before I spend the cash I want to have my ducks in a row. I may buy used if I can find one that matches my original gauges...a new one would be a bit easier to source and of course guarantee from an application standpoint.
Any feedback on this is welcome.
Thanks guys!
BP

 

[Frank Acampora]

Re: 20 Pole Tachometer Compatibility

The 20 pole tach counts the 1/2 or full sine wave pulses off the 20 pole alternator stator. the purple tach signal wire connects directly to one alternator lead AT the rectifier. The number of cylinders does not matter.

SO: the same tach (Teleflex, for example) will work with any 2, 3, and 4 cylinder Force with a 20 pole stator. There is no setting button on the back. If the 5 cylinder 150 has a 20 pole stator, (and I suspect it does) it will work with that engine also. The number of cylinders does not matter because the signal is taken from the alternator, not the distributor (there is none), rectifier, or the coil. However, since both the alternator lead and the purple signal wire are connected together at one rectifier terminal, a defective rectifier WILL cause improper or no tachometer function.

 

[Bigprairie1]

Re: 20 Pole Tachometer Compatibility

The 20 pole tach counts the 1/2 or full sine wave pulses off the 20 pole alternator stator. the purple tach signal wire connects directly to one alternator lead AT the rectifier. The number of cylinders does not matter.

SO: the same tach (Teleflex, for example) will work with any 2, 3, and 4 cylinder Force with a 20 pole stator. There is no setting button on the back. If the 5 cylinder 150 has a 20 pole stator, (and I suspect it does) it will work with that engine also. The number of cylinders does not matter because the signal is taken from the alternator, not the distributor (there is none), rectifier, or the coil. However, since both the alternator lead and the purple signal wire are connected together at one rectifier terminal, a defective rectifier WILL cause improper or no tachometer function.

Great info (again) Frank!! I may have found one (US Marine) and the sellers says its applicable for a Force outboard...rather than an i/0. Is there a quick way to check on the back? A clear '20pole' marking or something that clearly identifies it for the outboards?
Many Thanks Again!!
BP

 

[hgmatt]

Re: 20 Pole Tachometer Compatibility

Frank, quick question. So a if I know that my tach guage works, but sometimes when I run the motor it will shoot up to 6000-8000 RPM (motors is definately not going that fast), is it reasonable to think that the rectifier is fried? If I remember correctly, the rectifier changes the AC current to DC to run electronics/charge the battery, correct?

For example, I started my Force '89 85 HP up last week, started right up and I noticed that the Tach was showing roughly 1500 RPM while the motor sat on muffs in the fast idle position. After it warmed up, I dropped out of fast idle and into idle and the tach showed roughly 1000 RPM. I rebuilt one of my carbs over the winter break, so I needed to adjust the fuel mixture screws. During the on again/off again process, I began to notice that the Tach would slowly creep up to the 6000-8000 RPM range; however, when I tested the voltage across the battery, it was showing in the 14.4 Volt range. Busted Rectifier? Busted Tach?

 

[Frank Acampora]

Re: 20 Pole Tachometer Compatibility

My Teleflex has no clear markings, however, there is a number and I believe that I was told that part of the number defines the type of tach. At any rate, the number is CE53743 if that is of any help.

I don't remember exactly how my tach was improperly functioning. All I remember is that when I changed the rectifier it started to function correctly.