tach problems

[jasona]

I have a 1989 Merc 80hp serial #0A752169. My tach is ready very erratic. I just replaced my rectifier because I tested it and it failed. After news rectifier, tach is stull acting funny. New rectifier passes tests and is good. Tach will read 4000 rpm when idiling, then as throttle increases, the tach drops to zero. I think I may had a bad stator. Would a stator affect the tach reading?
Any advice would be apprecited. Jason

 

[Barnacle_Bill]

Re: tach problems

A bad stator can certainly cause tach problems as the tach gets its signal from the stator. The connection is just made at the rectifier because it also feeds it. In the back of your tach is a "screw" where you set the tach to match the pulses your stator puts out. Turn this setting back and forth several times then stop exactly on 6P. You might luck out and clear some trash from the contacts inside the tach if there is any.

 

[RicMic]

Re: tach problems

Check the ground wire too, nodbody ever thinks about it, after all its "just" the ground right? Its at least as important as that other wire.

 

[asm_]

Re: tach problems

Switch your RPM signal tap to the other stator output wire.

What you observed is a common problem when tapping the incorrect stator/rectifier wire.

B

 

[achris]

Re: tach problems

Switch your RPM signal tap to the other stator output wire.

What you observed is a common problem when tapping the incorrect stator/rectifier wire.

B

It's a coil of wire... there is no correct and incorrect side. Either yellow wire will do the job....

 

[asm_]

Re: tach problems

I should clarify. It's not the stator side of circuit that's causing the problem. It's rectifier side. I know you'll probably say, it's a full-wave rectifier with four diodes. It shouldn't matter which side of the diodes we tap. But believe me, for reasons beyond my understanding, for some rectifier, it only work on one of the terminal, not the other.

B

 

[Barnacle_Bill]

Re: tach problems

I should clarify. It's not the stator side of circuit that's causing the problem. It's rectifier side. I know you'll probably say, it's a full-wave rectifier with four diodes. It shouldn't matter which side of the diodes we tap. But believe me, for reasons beyond my understanding, for some rectifier, it only work on one of the terminal, not the other.

B

That happens when one of the diodes is defective on that side of the rectifier. For all the tach cares you can take the rectifier out and throw it away.

 

[jasona]

Re: tach problems

Thanks for all the replies/help. I'll start with checking my ground, and ajdusting the "screw" on the back of the tach. If this helps, here is how I have the rectifier wired:
one alt post on rectifier has yellow wire from stator and yellow wire from regulator.
second alt post on rectifier has yellow wire from stator and yellow wire from regulator.
positive post on regulator has gray wire going to tach.
this is the same way the original rectifier was wired.

 

[asm_]

Re: tach problems

That happens when one of the diodes is defective on that side of the rectifier. For all the tach cares you can take the rectifier out and throw it away.

I'm not sure that's true, because the rectifier bench tested for each diode to be working correctly. At this point, I can only guess not all diode are created equal. Even within one single rectifier array.

B

 

[asm_]

Re: tach problems

Thanks for all the replies/help. I'll start with checking my ground, and ajdusting the "screw" on the back of the tach. If this helps, here is how I have the rectifier wired:
one alt post on rectifier has yellow wire from stator and yellow wire from regulator.
second alt post on rectifier has yellow wire from stator and yellow wire from regulator.
positive post on regulator has gray wire going to tach.
this is the same way the original rectifier was wired.

A picture would have work a lot better. But, unless I'm reading this incorrectly, you have the tach wire connected to regulator output? It should be connected to one of the rectifier post with yellow wire.

B