1976 Merc 85 HP Tachometer

[Darus]

So I've completly rewired a 76 Thunderbolt 85 HP. I've tried installing a Tach and am getting no reading. I have researched and researched and have had no luck. I know it is a brown wire, but not sure where I need to put it. Any help is appreciated.

 

[Chris1956]

A modern tach is driven by the stator pulses (AC voltage) Connect the brown wire to one of the yellow wires on the rectifier.

 

[Watermann]

Does this wiring diagram match your motor, should be close since yours is a mid 70's model. It always helps to include your motor serial when posting problems.

 

[Darus]

The serial is 4424962. It had to be completly rewired because the wires were chewed on and someone had spliced any wire color they could find after cutting it off at the plug. I'll try to get a photo to send if I can find a way to reduce the size.
But at this point the brown is connected to a yellow at the rectifier and still not working.

 

[Watermann]

Is the diagram similar to your motor? The diagram shows the brown wire going to the SB. Also verify what the pole setting is on the back of your tach.

 

[Darus]

The setting on the instrument is good. One thing I'm seeing is some diagrams are calling for red wire going to the starter solenoid. Could that be a problem?

 

[Darus]

I have attempted to connect to the SB but it does not work there either.

 

[Darus]

No Title

Figured out a way to get a photo.

 

[Watermann]

Ok, start at the tach so the pole setting is correct, when you turn the key switch to on does the tach needle jump and return to 0?

Are you using the brown wire from the plug in pig tail that comes from the front of the Mercontrol?

The reason you can post pics now is that by the rules you have to have 3 posts under your belt before you can post a pic.

 

[Darus]

I am using the connector on the control. Brown all the way back to the rectifier.

 

[Darus]

No movement on the gauge when turning the key though.

 

[Watermann]

Sounds like you're not getting switched power to the tach then, do some checking with a multi meter.

 

[Chris1956]

A 1976 Merc 850 was the year of switchover to ADI ignition. Older Merc tachometers used the brown terminal on the battery-driven distributor CDI ignition switchbox to drive the tach. That terminal does not exist on the ADI switchboxes, at least I don't think it does.

You should be using a universal tachometer, with +12VDC, ground and signal connections. When battery power is applied, the tach should zero. The tach should have a 12 pole or 6 pulse setting. Most have only a 6P setting. That is what you want to use. Check continuity from the rectifier, along the brown wire to the tach. If the rectifier is shorted to ground, the motor will not charge, and the tach will not work.

 

[Darus]

The switchbox is a Merc box (332-5772). I am getting 12v DC to the instrument from the key. I set it to 6P which it wasn't. I had it set to 4 cyl. Which I think was 2P. Moved my brown wire from the rectifier to the brown terminal on the switchbox. Still no luck.

 

[Watermann]

Time to run a direct wire to the tach and cut out all the connections just in case something inside the controls or another connection is faulty.

 

[Darus]

It seems to be getting a bit deeper. If you can see in the photo above there's a red wire coming off the left side of the switch box. I moved the terminal cover and it is labeled orange. That should be for the kill switch. So I may have voltage hitting the control box. It's all getting frustrating but I'm loving the challenge at the same time. I truly appreciate all the help though.

 

[Chris1956]

I do not think the brown terminal on the switchbox will run a 3 wire tachometer. You should be using the rectifier AC post for tach signal.

 

[Darus]

Ok guys, got some progress. The red wire coming off the switch block terminal that is actually labeled for orange, that should be the "kill" was jumpered to a ground point. It was also connected to the pos+ pole of the rectifier. Now when I turn the switch I see no jump in the tach, but if I let it turn over I get movement on the tach.

 

[Watermann]

So it was wired to be a direct short then in the SB, if so that's not good for any component that was connected.

Do you have a wiring diagram your working off of to do the wiring? I think the one I posted has a distributor and othrs said your version in 76 doesn't have the distributor.

 

[Darus]

I'm pretty sure I'm working with a good one now. I ordered a Seloc for this motor. I'll post the diagram that seems to fit with all the components I have. Friday or Saturday I'll get the boat out of the garage and fire her up and see what happens. I know another issue I'd been having was it ran fine at idle all the time but as I gave more throttle the engine would bog down and die in the water but was fine out of water.