Connections for tach on 1981 Mercury 90 in-line six...mystified.

[Bannister 6905]

I recently became the proud owner of a classic bass boat that came with a 1981 Merc 90 "tower of power" outboard.

The engine was equipped with a tach, which does not work.

The connections on the rear of the tach are "S" (I assume signal - It was connected to the gray wire coming from the harness, which I believe is the signal wire.), "GND" (ground) and a bulb that is connected to switched power.

Most tach I am familiar with also have a "POS" (not that POS!) that connects to switched power. This one does not have such a connection.

Seeing as how my tach does not work, are there any aftermarket options that will work with my motor?

Thanks!

Bannister 6905

 

[GA_Boater]

instrument lighting power is usually separate from power for the tach. If the light comes with the switch, it sounds like an oddball, but you never know.

Can you post a pic of the back of tach and a front one may help. Any brand markings on the tach?

 

[Bannister 6905]

No Title

Thanks for the quick reply - here are the pics. The light pushes into the hole seen in the close up. As I said, there's no connection for 12V - at least, not as far as I can see.

 

[racerone]

I believe the tachometer would be driven by the alternator signal.---The grey wire in this case.---And of course a ground is needed.

 

[Bannister 6905]

That's how it was connected, thanks. What's the correct 'pulse' setting?

 

[GA_Boater]

The setting should be 6 for a Merc outboard.

All I can only think the switched 12 volts for the light is also used for tach power.

Medallion is still making instruments - Maybe they have a help line?

 

[Faztbullet]

Check the rectifier as tach may be good........

 

[Bannister 6905]

Thanks. I am in contact with Medallion and they are working on it.
The battery is being charges when the motor is running. Does that indicate that the rectifier is functioning, or do I need to measure the output voltage from the gray wire? That's AC, correct?

Bannister

 

[GA_Boater]

Yes, the gray wire is AC. You can check the output of the rectifier by measuring across the battery. With the motor running, the voltage should rise to approximately 14-14.5 VDC as RPM increases. If the motor does not have a regulator, not sure on your motor, it may go as high as 16 VDC or so.

 

[Bannister 6905]

OK, so the weather here turned much nicer than predicted and I did start the boat. The voltage at the gray wire measured 10 - 12V AC, depending on RPM.
The Medallion folks tell me that they don't make this style tach anymore and they can't repair it, so I guess I am SOL.
Regardless, thanks for all your help.

 

[Bannister 6905]

The voltage measurement across the battery with the engine running is 13V DC to 14V DC, so it is charging.

 

[kck8385]

Bannister,

If you have not done it yet check the continuity on the gray wire from where it connects to the tachometer to where it connects to the rectifier. It's possible that you are not getting the signal from the rectifier all the way to the tachometer. Just a thought from experience.

Kevin

 

[Chris1956]

Bannister, If memory serves, Merc used to have a tach that was driven off the brown terminal of the switchbox, on battery powered CDI ignitions. The tachometers had just two connections. My '73 merc 850 had the tach signal wire connected to the switchbox, versus the rectifier AC post, so I think it was meant to work with a 2 wire tach..

My '77 Merc 1500 had the tach signal wire connected to the yellow rectifier AC post, so somewhere in between those years the change was made.

If I am correct, your ADI switchbox will not be able to power that older style tach.

The good news is that your Merc will power any number of modern tachometers made by Teleflex, Faria and others.

 

[Watermann]

A couple things, when connected to power does the tach needle jump slightly when the key is switched on? If not check for 12v getting to the tach. Your tach shares the light power so there isn't a secondary power wire for that type tach and maybe the bulb even needs to be installed and not burned out. Is the cylinder selector switch on the back set to 1? If so that could be an issue as well.

 

[stevehendo34]

My rectifier was bad. Disconnect stator and check with ohm meter. I think you can remove output wire from rectifier and check for 12v there while running.

 

[merc850]

Don't remove the 12v wire from rectifier that's how you ruin them and possibly the stator, just take V reading at battery terminals. I would discard that tach and buy a new alternator driven one.

 

[Bannister 6905]

Issue resolved - the plug going from the binnacle control to the harness to the dash had worked itself loose. Reconnected, checked tach and it now works. Thanks to all for the input.