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Right Tachometer for 1975 Merc 50hp 4 cyl

SaltyScot

Regular Contributor
I need a tach for this motor serial-4141159. It has the 3 wire plug-in on the front of the Mercontrol. Do i just need an old Mercury tach that fits this plug, or does it have to be from a 4 cylinder? I see one from a 6 cylinder that has the right plug, but doesn't appear to be switchable for the number of pulses. Any other alternatives besides the older tachs? Obviously, I'm not well versed in tachometers.:confused:

Stephen
 
Nope, modern one will work,set the adjusting switch to 6P. I do have a old one on my '69 Merc800, but they need to be wired a wee bit differently.
 
And if the tach doesn't work (or work correctly) select the other yellow wire.

Not sure why this happens, but I've seen it several times.

Jeff
 
It's actually going on my hydroplane. I'm going to add a gauge pod to attach to the deck with tach, water temp, and water pressure.
 
I'm pretty sure it's 6 poles, I have a tach on my 1975 850 that was made in the 80's so I don't see a problem.1980's-Tach.jpg
 
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Just realized this is type 2 Alternator Driver Ignition/ADI or Distributor Lightening Energizer. But as far as I can see it's still a 12 pole stator so should be 6P.
Maybe advance the spark a bit .
 
Well, I actually read the instructions now. :rolleyes:

d. Older model outboards (prior to 1977)may have the tach signal wire originating atthe ignition system though they are alternatorequipped. All alternator tachometers may beused on these systems by disconnecting thetach signal wire at the engine and connectingthat wire to the unrectified alternator signal atthe rectifier. Be certain the number ofalternator poles match the tachometer polesetting of the tach.

I still don't understand this completely. I have it connected to one of the two yellow wires. Should I connect it somewhere else? If each wire gives me 6 pulses(and I'm reading about half what it should), should I connect it to both yellow wires?
 
Don't connect it to both wires or you'll have a fire! The instructions are saying if you want to use the existing wiring in the control box move the tach signal wire from the switchbox to the yellow rectifier lead, I did this on my motor because I had the plug that goes into the controls.
Maybe you should change the setting and then run the motor to see if it falls into the rpm range. If you can't get the right reading ask a tech on this site for help.
 
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Don't connect it to both wires or you'll have a fire! The instructions are saying if you want to use the existing wiring in the control box move the tach signal wire from the switchbox to the yellow rectifier lead, I did this on my motor because I had the plug that goes into the controls.
Maybe you should change the setting and then run the motor to see if it falls into the rpm range. If you can't get the right reading ask a tech on this site for help.

It was 3. :) I appreciate all the input!
 
I finally got it out and tested it properly and it's showing 7000 rpm(35mph) at WOT. 1000 rpm at idle. That's with a 11x15 prop on a 11' hydroplane. It sounds real good at that speed and hasn't overheated on me. But that is way more than the 5300 redline. I've been running it for about 4 hours total at that speed about 40 minutes at a time. I had ordered a 11x17 prop but I don't think that is going to make enough of a difference number-wise. Might have to go back to a 35hp OMC on this boat. How can I confirm the results of this tachometer?
 
I simultaneously hooked up one of those cheap digital tachs that has a wire that wraps around the plug wire. After much adjustment, I got it to match the output of my main tach. So I'm pretty sure I'm running way over the redline. And even if I drop 500 rpms with the 11x17, I'm still significantly high. I'll post over on the prop section for possible prop selection.
 
So everything is fine until it isn't. I now have a 1968 Mercury 50hp with the Faria Beede 33005 set in the 12P (#5) position. And it was working fine for several runs, and then the tach just shut off in the middle of running. I tried switching to the other brown wire on the rectifier and same. I bought a new tachometer exactly the same and it's still not registering any rpm. I did put my multimeter on the signal wire while starting it and it seems to be giving some signal because the number changes while cranking. Any thoughts on where I should go from here?

Stephen
 
Do you have a battery connected to the main leads? Without a battery the rectifier will blow and this could cause no reading on the tach.
 
I have the starting battery to start it and I'm not disconnecting it while running. I'm not sure if that's what you're getting at.
 
Yes that's the idea, are you using a standard battery not a deep cycle? Deep cycle batteries are not good for basic marine operation.
With all the systems hooked up put a VOM set to AC + 20v on the 2 yellow terminals run the motor and see if you get a reading that varies with rpm - this shows the stator is functioning; put the meter (set to DC) on the battery terminals and see if the voltage goes above 13v while running.
Do this in the water in F not revving in neutral in the driveway.
 
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Thanks. I will try that. I run a regular marine battery. I took it out yesterday for 20 minutes and the battery tested 13.2V before and 12.7V after. Not sure if that's definitive or not.
 
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