chasing down the tach wires

[kyricochet]

i have a 1988' mercury 150 blackmax xr4<br />my tach quit working <br />yes my control has the plug<br />there is a black (ground) purple (ignition)and gray wire the gray wire is connected to the back of the tach also with the word "send" above it i assume sending unit ?<br /> the purple wire is hot to the tach when key is on<br />but im not getting anything from the gray wire <br />ive traced them all to there origin and ive found that the gray wire is going to a small square black box type item on the motor<br />the "black box" has three posts 1 with red wire and is hot when key is on another with 2 yellow wires that are hot when engine running and another with my gray wire and another yellow that is never hot <br />is my tach bad or is it the little black box could it be that the tach is bad and it is not sendind anything back to the blackbox or am i just a dumbass

 

[superman]

Re: chasing down the tach wires

The black bos is your charging rectifer which consists of 4 diodes making up a bridge rectifier. It is possible for one diode to short in the bridge and continue charging at a reeduced rate but kill the tach. Do you have a voltohmeter? If you do I can tell you how to test the rectifier

 

[kyricochet]

Re: chasing down the tach wires

i have no volt meter just test light <br />so you are saying this is wrong not to have any kinda of juice on the one post on the rectifier<br />you have to look over me im a lineman and not hardly a boat mechanic

 

[kyricochet]

Re: chasing down the tach wires

also this could explain the battery needing recharge after a couple of trips on the lake<br />correct?

 

[superman]

Re: chasing down the tach wires

If you have no meter and your tach quit and your battery is not staying charged sounds like your rectifier. I'd change it. It doesn't cost much. It sounds like some of the internal diodes are bad.

 

[mellowyellow]

Re: chasing down the tach wires

replaced my Rectifier this year and it was $35.00<br />one side charges, the other side supplies signal<br />to the tach.<br />good luck,<br />M.Y.

 

[kyricochet]

Re: chasing down the tach wires

would it be possible to jump a wire from hot post to the dead post on rectifier that runs to tach would this tell me anything or would it get tach working until i get the rectifier

 

[superman]

Re: chasing down the tach wires

Don't jump anything but you can do this. The 2 yellow wires are from the stator. They supply a AC pulse to the rectifier and the tach. In side the rectfier is 4 dioded that make up a bridge rectifier. Depending on which diode in the rectifier is shorted changes the results. The tach is connected to a terminal with a yellow wire going to it. I suspect the diode that connects from the terminal that the yellow wire and the tach wire go to is shorted to the ground connection which connects directly to the engine via the back of the diode which is the negative. If that is in fact the one that is shorted you can move the tach wire to the other yellow wire and if that is all that is wrong and is that is the only one of the internal diodes that is bad your tach will stat working, but that won't solve your charging problem. If this is the case and the diode is bad, the charge rate will be reduced or inoperative. Good Luck

 

[kyricochet]

Re: chasing down the tach wires

thankyou all for the input i couldnt have figured it out without you all<br />i went ahead and spent the $35 bucks for a new rectifier and now everything works great<br />thanks again