which is correct settings for 20 pole tach

[friend22]

i bough 20 pole tach it has 1 2 3 4 settings on the back i have chrysler charger 4 cylinder motor which number i need to set my tach ? any idea ? what this numbers mean?

 

[fucawi]

Re: which is correct settings for 20 pole tach

1 2 3 4 cylinders

 

[friend22]

Re: which is correct settings for 20 pole tach

i put on #4 and my rpm way to hight on idle im getting 3700 rpm , put on #1 on idle is 1800 rpm whats wrong anybody can help?

 

[BobList]

Re: which is correct settings for 20 pole tach

The EXACT same thing happens to mine.

 

[fucawi]

Re: which is correct settings for 20 pole tach

the problem is that you have counted the number of coils under the flywheel and got 20 ....now poles come in pairs so some would say its a 10 pole...as its showing twice the rpm all I can suggest is a 10c diode from Radio Shack in the tacho wire ..ask for 1n4005 or 6 or 7 ...this will halve the pluses to the tacho .. you might have to try it both ways around ..let me know please if it works .

 

[friend22]

Re: which is correct settings for 20 pole tach

Thanks fucawi i got diode from Radio shack will try next weekend , the only concern that i had put 10 pole tach before i put 20 pole and had same results this is why i thought i need 20 pole , the old owner had changed the rectifier but i dont know if it wright one any idea how to check it ?

 

[fucawi]

Re: which is correct settings for 20 pole tach

well you did not say which motor it is ..if its CD then not all the poles will be for the charging system..subtract twice the number of cylinders from the coil count and that is the charging poles. Any rectifier will do if its rated hight enough ..if battery volts goes up to 14.4 then its working ....cannot guarantee the diode will work as I dont know the internals of the rev conunter ..but certainly ists getting twice the pulses it needs .

 

[friend22]

Re: which is correct settings for 20 pole tach

Its chrysler charger 1983 model serial 908h3a 4 cylinders i dont know if it has a CD

 

[Frank Acampora]

Re: which is correct settings for 20 pole tach

You have a CD ignition engine with the Prestolite ignition. It should have quick-connect fittings for the electronics.

Now: It is a 20 pole engine and the tachometer needed is a 20 pole tach, not a 12 pole tach like pre-1979 Chrysler engines. Although the stator has 20 poles, 0nly 10 are used for charging the battery and the other 10 are used to charge the CD boxes. HOWEVER: It is still known as 20 pole and requires a 20 pole tach.

Usually, 20 pole tachs are dedicated and do not have numbers and a switch on the back. However, again, I have seen one--only one-- that used #3 for 12 pole and #4 for 20 pole.

You can take signal off the purple wire attached to the rectifier and on these ignitions, that is the only signal source.

Double check that your tach is indeed a 20 pole tach--they are becoming more and more difficult to find and I suspect that yours is really a 12 pole tach designed for old Chryslers and OMC engines.

You can buy a Tiny Tach and pick up a signal for it off the #1 plug wire, just like a timing light. OR---if you are flush with cash, you can buy a Mercury tach. I don't know the numbers, you would need to investigate, but several Mercury models used 20 pole tachs.

 

[fucawi]

Re: which is correct settings for 20 pole tach

Everything frank says is good ....to let you know where my head is going with this as it seems a common problem and we could do with a solution.
The tach signal is AC from the battery charging coils.. each pulse charges a small capacitor in the tacho which is then discharged into the meter movement ( diode pump its called) the more often it discharges the more the meter reads ... dependent on the design fitting a diode in the line may halve the pulses and halve the reading....but read on .....due to component tolerances each unit will have to be calibrated hence inside you should find a trim pot ..this may have sufficent range to give you a true reading. If someone can post some pics of the internals then I would be able to advise better...usually there are not many components inside inside ....I built a tacho for a 50 hp force ...painted half flywheel edge black and half white..optical coupler ..diode pump and a 270 deg movement from an old car ....calibrated using a similarly painted blade on a circular saw running on the mains ......anything is possible ...

 

[TwoFish]

Re: which is correct settings for 20 pole tach

I had a tacho reading high a couple of years ago. I found some corrosion on the main loom plug at the back of the motor (1998 75hp Force). I read somewhere that corrosion can let other signals into the tacho wire. It fixed the problem. The pins either side of the tacho pin were earth & power for the fuel enrichment solenoid so I wouldn?t think these would be a problem. May have been a coincidence but I thought I?d mention it.