Mercury 850 1973 Aftermarket Tach

[zxjoe]

So I bought a cheap tach on Amazon, and I connected the tach directly to one of the yellow wires on the rectifier. I got a reading and was super excited, but when I disconnected the harness's brown wire from the switch box and connected the brown wire to the yellow wire on the rectifier, I connected the tach to the other end of the brown wire, but it wouldn't show a reading.

So, while the brown wire was still connected to the rectifier, I connected the tach to the same yellow wire as the brown wire was connected to, and I didn't get a reading on the tach. I moved the tach over to the other yellow wire on the rectifier and I did get a reading on the tach.

This made me think that maybe the brown wire was shorted along the way. I used an ohmmeter to test the brown wire to ground and I didn't get a reading. I also tested the brown wire to the 12v on the battery and again, no reading.

Any ideas?

Joe

 

[Faztbullet]

Test your rectifier

 

[Chris1956]

Fazt is suggesting that one of the diodes in your rectifier is shorted, grounding 1 of the yellow wires. This will half your charging rate and could cause a discharge of the battery when not running.

You should test the rectifier. It is simply a bridge rectifier with the -DC post internally grounded.

 

[zxjoe]

Thank you guys for responding.
I tested the diodes in the rectifier using this method:

Two of the diodes connected to the ground failed the test. Not sure if this is what Chris1956 was getting at but does seem to be an issue.

I can't buy an OEM replacement for the rectifier so I'm getting a little DIY with my 3D printer.

 

[Chris1956]

I am shocked that you cannot find a replacement. Sierra used to make them. Your rectifier was the old large beige ugly one with the three colors. A newer black square style will work just fine.

Alternatively, find a 15A-25A bridge rectifier on the web for $5 and install it. Just remember to ground the -DC terminal.

 

[zxjoe]

Are you saying that there is a place to mount one of the square ones?

I bought one from Amazon, it should get here sometime next week. I planned to mount it on a 3d printed bracket, then ground the rectifier by attaching a wire to one of the rectifier mounting screws.

 

[Chris1956]

A lot of the bridge rectifiers have a hole in their center. Find a spot on the front cowling support, above the switchbox and bolt it on. There are usually some holes around there. You could also probably find one of the original bolts on the rectifier will hold it. That way the wiring will definitely fit.