New tach, old motor

[dOb]

I?m not much for wiring and rewiring, so I have a plan to install a tach, let me know what you think.

Here?s what I?ve got - 1974 50 hp Mercury 500 Thunderbolt with original Mercontrol. And a Mercury marine tachometer (2004)

Here?s what I want to do ? Last summer I had installed a new sender for the fuel gauge. It was set up so that the fuel gauge was on a switch. You would flip a switch and the light would come on, and the fuel gauge would come to life. That way there wasn?t a drain on the battery. It worked fine, but the tank was old and dirty. I have since removed the metal tank and installed a plastic one so the fuel gauge is not being used.
I was wondering if I could jump off of the 12v line and the ground line from the fuel gauge to the tach, and then run the brown wire from the Mercontrol box to the send on the tach.

I?ve read that the Mercontrol tach plugs are hard to come by, and that they had a danger of shorting out the ignition. This way the tach would be on a switch, and not wired to the ignition. What do you think?

Thanks for your help.

 

[Silvertip]

Re: New tach, old motor

Why would you put the tach, or any other gauge on a separate switch when the +12 volt line feeding those gauges should come from the "A" (accessory) terminal on the igntion switch. There should already be a wire for that hanging from the boat harness. When you turn the key on the gauges then come to life. Do it right. When the key is off there is no drain on the ignition system. Besides, the ignition system is a magneto system. The battery is used only to spin the starter.

 

[dOb]

Re: New tach, old motor

Thanks for the quick reply.
I haven't seen a separate wire hanging from the harness. When I rebuilt this boat, I assumed I would have to rewire the whole thing. I started reading up on rewiring and found several posts that basically said, old wiring isn't necessarily bad, and if it ain't broke don't fix it. When I hooked up a battery, the lights, bilge pump, and gas gauge all worked. Everything had its own switch.. That's just the way the PO had it set up.
I know it's not necessarily the "right" way to do it. But this would be the only other gauge I would need. I guess my question is, is there any danger to doing it this way, or any reason this wouldn't work.

Thanks again

 

[Chris1956]

Re: New tach, old motor

That MerControl should have a 3 prong rubber connector on the front of it. That is the hook up for the tach signal and power and ground for all gauges.

 

[dOb]

Re: New tach, old motor

That MerControl should have a 3 prong rubber connector on the front of it. That is the hook up for the tach signal and power and ground for all gauges.

Yes it does. The brown wire (tach signal) I mentioned earlier was from that connector. However, I had read that they stopped using that because if the tach failed, it could fry the ignition. If that's not a real danger, then I can hard wire them in, as I don't think I will be able to find that plug. Also, I may need to replace some of the wires inside of the control box. Some are rotted and cracked.

My idea was to use the tach signal from the control box, and the power and ground that already exist from the fuel gauge. These would be the only two gauges. The dash isn't big enough for any more.

 

[Chris1956]

Re: New tach, old motor

A modern tachometer (which yours is) takes the signal from the stator. The brown wire under the cowling should be connected to one of the yellow stator wires, at the rectifier. if not, move it....

After you leave that switch that powers your gauges "on" a few times and kill the battery, you will then use the switched power from the ign switch at the MerControl, I predict.

 

[dOb]

Re: New tach, old motor

I know you're right. That's the obvious danger. But while researching tach info, I ran across this kind of thing a couple of times -

The reason Mercury quit using the old thunderbolt tach was if they failed it could short the ign. system and kill the engine.

And wondered if that's why the PO bypassed it. Who knows. More than likely it'll bug me enough to get in there and put it back the right way.

So just to be clear, I can use the power and ground from the front of the merControl as common power and ground for all lights, pumps and switches, so that everything is only powered when the key is turned? Is that correct?

 

[dOb]

Re: New tach, old motor

A modern tachometer (which yours is) takes the signal from the stator. The brown wire under the cowling should be connected to one of the yellow stator wires, at the rectifier. if not, move it....

I was reading up on this when I decided to buy the tach. I knew there was a brown wire in the control box, but I hadn't seen one in the internal harness. Then I found this thread:
http://forums.iboats.com/showthread.php?t=307559

This explained why I couldn't see a brown wire. There's actually a link in that thread to another one. Both are very helpful and well written. I'll post it here in case someone else is searching this information. The brown and orange wire are connected and taped over in the harness.

I had to do some minor surgery to the internal harness to dig out the other orange (brown) wire and run a connection to the rectifier. That all went pretty well. But I think that if I ever need to get in there again, I'll need to get a new harness. I had to wrap some crumbling wire insulation.
Then I had to get into the control box to connect to the tach send. I discovered that the original 12v wire and the ground that lead to the plug were toast. The white/power wire actually looked like strands of green hair. Since I was already there, I went ahead and replaced those (I also figured out why my choke wasn't working and fixed that as well).
I temporarily hooked up the tach, set it on 6p, and...IT WORKED!

At this point, I just have the power, ground and tach send wires running "through" the plug at the front of the control box. I didn't want to rebuilt the plug. I will probably run the wires under the dash, and then use a different three prong plug to connect everything. I'll post back when I get that figured out.

Thanks again. This forum really is the best place for information!

 

[Chris1956]

Re: New tach, old motor

You may run low current devices off the ign accessory wire. it was not designed to carry large current loads like pumps and lights. it is also un fuzed.

 

[dOb]

Re: New tach, old motor

You may run low current devices off the ign accessory wire. it was not designed to carry large current loads like pumps and lights. it is also un fuzed.

Thanks, I?m glad you said that. I?ll cancel the mirror ball and stick with the gauges.

Just to wrap things up, I ended up rebuilding the plug after all. I found that a 1 inch PVC cap fits really snug over the rubber end that sticks out of the control box. The male end of the plug is actually more like three little plugs with a cover. But it does the job. I just painted the PVC and drilled a small hole in the top.

The tach fit perfectly in the cutout from the original 1974 dash. I guess they have a universal size for large gauges, or I just got lucky. It is set at 6p. When I turn the key, it jumps to zero, and seems to be accurate from idle to WOT. At this point, the fuel gage is still on a switch. If I ever hook it up again, I?ll put it on the same power as the tach.

(note: the oversized bolt holding the controller to the sidewall is just temporary. It has a wing nut for easy off and on. When everything is done, I?ll replace the original nuts and bolts.)

Thanks for the help with this. I do have some questions about motor performance, but that will be another post.

 

[phebus1]

Re: New tach, old motor

Very clean install, that looks great.