tachometer rectifier?

jamesh76

Seaman
Joined
May 14, 2008
Messages
52
I have a 1988 50 hp force.
I noticed towards the end of last year the tack would jump and not keep a stable reading. Had the boat out tonight and when you Turn the key on the tack will go to zero. Once the motor starts it will go straight down to the 6:00 Position and not read. I have checked the wires in the back of the tach and they all seem ok. Did a little research and read something about a rectifier on the motor. Anyone have any thoughts or ideas? How can I rule out the rectifier? What checks would you suggest. Thanks. James
 

pnwboat

Rear Admiral
Joined
Oct 8, 2007
Messages
4,251
Re: tachometer rectifier?

Make sure the wires on the rectifier are securely fastened including the ground wire. It may not have a ground wire. In these instances, the case of the rectifier is the ground connection. Make sure the case is touching bare metal and no corrosion.

You can check the rectifier with a meter. Pretty easy. See the link below.

How To Test A Bridge Rectifier With A Digital Multimeter DMM - YouTube

Replacement rectifier is available at Radio Shack P/N 276-1185 for about $5.

25A, 50V Full-Wave Bridge Rectifiers : Rectifiers | RadioShack.com

Don't rule out a bad tach too.
 

Jiggz

Captain
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Oct 23, 2009
Messages
3,817
Re: tachometer rectifier?

If you think the rectifier is faulty, you can disconnect the two yellow wires and temporary tape them off so they so do not short together or short to any ground while keeping the tach wire connected to one of the yellow wires. You can replace the rectifier with a universal rectifier/regulator or the cheaper radioshack version mentioned by PNW.
 

jamesh76

Seaman
Joined
May 14, 2008
Messages
52
Re: tachometer rectifier?

Thanks guys, I will do a test as mentioned above and see how that goes. Might just be as easy to spend $5 on one from radio shack and just replace it?

I did check the wires on the rectifier last night. All appear tight.

As for not ruling out a bad tach. Is there a way they can be tested as well?
 
Last edited:

pnwboat

Rear Admiral
Joined
Oct 8, 2007
Messages
4,251
Re: tachometer rectifier?

To test the tach itself, you need a way to generate an AC signal pulse to mimick what the stator on your motor generates.

If you've ruled out the rectifier as being bad (by replacing it and wired it correctly), and you're reading AC voltage on the signal wire at the tach, and it's still not working, the tach is probably bad.

Another way to see if the charging system is working like it should is to measure the DC voltage on the red wire on the rectifier. You should see around 14 - 15 volts DC at a minimum of 2000 - 3000 RPM's. If you see 12.5 volts DC, then you are just reading battery voltage and the charging system is not working.

The stator generates about 16 volts AC at 2000 - 3000 RPMs on the two Green/Yellow wires attached to the rectifier. The tach signal wire should be attached to one of these two terminals.
 

Jiggz

Captain
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Oct 23, 2009
Messages
3,817
Re: tachometer rectifier?

"I did check the wires on the rectifier last night. All appear tight."

So you disconnected the two wires from the rectifier while leaving the tach wire connected to one of the two Grn/Yel wires? Next test is to measure AC voltage between the two grn/yel wires and you should get some voltage reading above 12VAC even at idle. Note you will need some clips to hold the voltmeter probes to the wires and make sure the two wires do not short together or to ground. If you have AC voltages, most likely the tach is faulty. Or you can also do the test PNW mentioned which is to take DC voltage reading from the rectifier output wires which tests both the stator and rectifier outputs.
 

jamesh76

Seaman
Joined
May 14, 2008
Messages
52
Re: tachometer rectifier?

"I did check the wires on the rectifier last night. All appear tight."



No, I did not disconnect. Just checked to make sure they were all tight. Will check voltage readings tonight.
 

jamesh76

Seaman
Joined
May 14, 2008
Messages
52
Re: tachometer rectifier?

Ok everything checks out turned out to be the tach. Picked up one locally really hard to find a 20 post tach.

Only thing is and I noticed it before. After you turn the key off the tack clicks for about 5 seconds then stops. I assume this is normal?
 

Jiggz

Captain
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Oct 23, 2009
Messages
3,817
Re: tachometer rectifier?

Oh yes, 20 pole tachs are rare nowadays. I'm not really sure about the 5 second clicking although I know for a fact the ignition key when turned off, also turns off the power to the tach and the needle will drop down below zero.
 
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