TOPIC: Merc 1250 Tach Wiring

I have a factory mercury tach that came with the boat. I'm trying to reinstall, but can't get it to work properly.

The Mercontrol harness has white, brown & black. The white is already attached to the light. The tach has a jumper - one end I'm sure was attached to the mounting stud/case the other I thought was attached to the neg stud but after seeing what pics I could find, I'm afraid to attach it there.

This is how I wired it:
Black - Pos
Brown - Neg
White - Light
Jumper - Case to Pos

Any thoughts?

Thanks,
Mark

You need to be careful, because you never know how someone already may have rewired your boat, not necessarily using correct colors. Please take the following for whatever help it may or may not be. I do not claim to be an expert, nor do I claim to always be correct. I do know that this works on my particular setup.

I have a 1975 Glasspar with a 1975 Mercury Model 1500 inline 6. I bought it new, and I rewired it myself several years ago during a major refurbishing, so I know that no one else has messed with it. In my setup, here is how things were and are.

White has +12V WHEN the ignition is on, and goes to one side of the tach light.
Brown receives the tachometer signal from the switch box assembly.
Black goes to negative side of battery. The tach terminal receiving the black wire is also jumpered to the next terminal over (the other side of the tach light), and also to the tach post.

I suggest that you obtain the proper Mercury Manual for your motor, because probably you will be tweaking and playing with that motor forever. The manual is well worth the small expense, and is readily available.

Thanks Bill. I have a factory harness going to the Mercontrol and a factory tach harness coming out of the Mercontrol going to a factory Mercery tach.

If I understand you right, the jumper goes from the pos post to the neg post?

Thanks,

Mark

P.S. - I'm definitely getting a factory manual

I don't know what you mean by pos post to neg post, so I can not agree or disagree. Maybe the following will help.

On the back of my tach, there are four terminals. As you look at the back, as if you were behind the tach, then, running from the viewer's left to his right (or clockwise), let us arbitrarily call those terminals A, B, C, and D.

White goes to C.
Brown goes to D.
Black goes to A, which also is jumpered to B and to the tach post.

On my boat, using the tachometer terminals as junction points are also some additional wires related to the speedometer, the fuel gauge, the water pressure gauge, the vacuum gauge, and other things, but those items above are specifically related to the tachometer.

Thanks Bill.

Here is exactly what I have:
Harness: (1)Blk, (1)Brn, (1)Wht, (1)Blk Jumper
Tach: Pos, Neg, Wht

Here's a pic I took before I removed it, BUT, I started to remove the wires and put them back where I thought they came from before I took the picture. You see the jumper going from POS to NEG. This can't be right and I haven't tried it for fear of damage. My guess is the jumper goes from POS to a mounting post.

I found the following on another forum. I hooked it up exactly as shown in the pics, but it still didn't work. The light worked, but the needle just went to the negative stop when I started the motor.

Thanks for taking the time to help me with this
Mark

Mark, apparently your tachometer is sufficiently different than mine that my information does not transfer well to help you. Perhaps one of the other folks on this forum may be able to help. Good luck to you. You'll eventually get it solved if you never give up.

You need another tach. That one is a 53667A1 which should work with that engine but it is missing the calibrating resistor and I have no idea what the value of it might be to get a replacement. I have a couple of those tachs here with the same issue, but reality is it is easier to find a tach that reads off the rectifier AC terminal than it is to get yours to work properly. Back in the area from 1966 to 1972 or so Merc made a bewildering number of electronic ignition styles, some good and some not so good and nothing interchanged between them. It was a real mess until they standardized the designs with the ADI system in 1977. That tach you have is very specific to certian engines and not really worth messing with.

Ok. Thanks guys. This is the tach that came installed in the boat. There wasn't anything else attached to it other than the harness. Could it be anything else?

Thanks again
Mark

Yes there is one thing, try reversing the leads on the tach and see what happens, it might just work. I have done this in the past and it did the trick.

Ok. Thanks GF. I'll try that tomorrow. I thought that I had tried it, but maybe not.

My before pic shows the jumper going from the POS to the NEG. There's also some insulation missing that could be age or maybe to removed to go against ground post.

In any case I found two pics of Merc tachs that appear to have a jumper between the POS and NEG. I was thinking about trying this but I'm afraid of doing damage - mainly the switch box. Any thoughts?

Thanks,
Mark

Probably the caliberation resistor. Is it a stiff wire? If so it is probably resistor wire.

GF - Those are just pics I found, so I'm not sure.

I think I've tried every wiring combination and still can't get it to work. I thought maybe the rectifier or stator were bad so I did a simple check with a DVM and it's definitely charging. It's starting to appear that my tach might be bad.

I'm also getting voltage from the brown wire out the Mercontrol, but I have no idea what it's supposed to be. The only thing I haven't tested is the tach harness. I'll check it tomorrow. Does anybody know how to test it?

Thanks,
Mark