Tachomoter resister

[richardv]

I have a tachometer for a 1972 Johnson 85 HP 85ESL-72R that has a burnt resister right at the alternator lead. I can tell that the resister is a 1/4 watt but it is too charred to identify the OHMS rating. It is an OMC Tach with the numbers 121614 on the back side. I have not found a local dealer who has any information on this tach. If anyone knows the correct resister size please let me know and I will get one at Radio Shack. <br />Thanks

 

[javsam]

Re: Tachomoter resister

w=v(squared)/r. <br />r=v(squared)/w.<br />r=144/.25.<br />r=576 v=12 w=.25<br />576 ohm resistor should work.<br />javsam

 

[Seasport]

Re: Tachomoter resister

I don't think this is quite right javsam. What you have worked out is the resistance value which will dissipate 1/4W in heat with 12V across it. If the resistor is rated at 1/4W under normal operation usually it should dissipate less than this.<br /><br />The resistor will be used to match the tach impedance to the alternator output. Have you tried measuring the resistance with a meter (with it disconnected)? Might still be OK.<br /><br />Hard to tell what the value should be without further info, but if it is any help Teleflex specify a 2000ohm 1/2W resistor between send and ground on some Evinrude/Johnson motors. Might be a red herring. If in doubt start on the high side and reduce the value until the tach starts working.<br /><br />Maybe someone will have a similar tach which they can have a look at for you.

 

[javsam]

Re: Tachomoter resister

good point seasport. thanks<br />javsam

 

[Mod2]

Re: Tachomoter resister

To Electrical/Electronics.

 

[richardv]

Re: Tachomoter resister

javsam & seasport<br />Thanks for taking a look at this. It looks like I need to buy a new tach and not worry about fixing the old one.<br />Rich