Need Tach clarification for 73 Merc 850

Jumper · Sep 9, 2002

Trying to get my tach working but can't figure out if it's set properly. There's only 4 "poles" on the back of the tach, but looking at the teleflex website, it suggests "6". Looking at the back of the tach, it's a Medallion Instruments Inc. Right now it's set on "1", but is this the correct setting?

rickdb1boat · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

I believe 6 is correct. Maybe others will have more input.

Jumper · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

That's just it, I don't have a "6" on the back of my tach

It only goes up to four. But here's a little more confusion now. I printed out some installation procedures off of the teleflex website. In them, it says for Mercury motors, to connect the gray wire to the rectifier or the alternator at the terminal switch. However, the gray wire coming out of my wiring harness goes to the choke solenoid (according to my manual). The brown wire from the wiring harness goes to the switch box assy. Is the switch box assy also the terminal switch? This same brown wire is also the one my manual says is for the tachometer? Confused yet? Cuz I sure as **** am. Reading the Teleflex instructions, it says I can run a wire straight from the rectifier (either yellow terminal) to the tachometer. Is this something I can do and if so, should I disconnect the brown wire from the switch box assy? Or did I forget to add a "jumper" wire in there some where? HELP!

rickdb1boat · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

I looked around at some other sites and if that motor is a 4 Cyl. then it should be set on 1. As for why it's not working, it could be the Tach. or something in the wiring. I am not that familiar with the older Mercs, so maybe someone here can HELP with this. And yes, I am confused!!

Jumper · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

Thanks for checking rick!

It is a 4cyl and it was set on 1 so at least that's the correct setting! Now if I can only figure out the wiring confusion

snapperbait · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

Run a new wire from the correct terminal on the tach directly to the yellow terminal on the rectifier.... This is probably your most simple and direct route...

Leave the brown wire to the switch box..

mikebob · Sep 9, 2002

Re: Need Tach clarification for 73 Merc 850

I have a 73 merc 850 and i just hooked my tach to the +/- on bat. and to the brown wire coming out of the plug on the side of motor. check your control, in the front of it it should have a plug with three wires. A brown, white, and black wire ( these three wires are for dash mounted tach). the white wire is +, the black wire is -, and the brown wire is distributer or tach. If its not there there is three wires coming out of the dist. hook the signal wire from tach to the brown terminal.

Jumper · Sep 10, 2002

Re: Need Tach clarification for 73 Merc 850

Thanks for the info guys!

I'm going to run a wire directly off the rectifier to see if this tach is working first (the boat and motor have sat for 10+ years) Mike, My control has the plug (female end) that you're talking about, but my tach is hard wired to the console/motor. I had to buy an aftermarket wiring harness for my motor and according to the directions it came with, the brown wire ran to the switch box and the gray ran to the choke solenoid. But like I was saying, its vice versa according to my manual. I wonder if I've got those two wires hooked wrong. If the tach is working, I may go ahead and switch those two wires around and see what happens. Thanks for the info!

One other question for ya mike since we have the same motors. What RPM does your motor seem to run at when it's WOT? Any other suggestions?

mikebob · Sep 10, 2002

Re: Need Tach clarification for 73 Merc 850

My grey wire goes to starter. But if that plug is their and wired even with differ colors you should be able to use volt meter find +- and the other one would have to be signal. I posted a quest. about that one myself because books say 4800-5000 at wot. In the water forward gear mine only runs 3200 wot, so i dont know if that spec is in neut, or forward, or my prop might have too much pitch for higher rpm since i have a prop for trolling. I will tell you this though at 3200 rpm it gets thier fast and still keeps pretty good speed at about 30mph

rickdb1boat · Sep 10, 2002

Re: Need Tach clarification for 73 Merc 850

For Mikebob, That spec. is always at WOT IN FORWARD GEAR. Never Rev. your Motor that high in neutral! Either your Tach. is off also, or your underproped. Just a little info I thought you could use. Good Luck,

Jumper · Sep 10, 2002

Re: Need Tach clarification for 73 Merc 850

Ok, update on my tach: it works. However, it doesn't zero out when I turn the motor off. Instead the needle stops at 200 (it shows about 1100 rpm's in neutral at idle speed). I also noticed the needle "free floats"--if I move the tach around, the needle moves. Is this tach no good anymore or is there a way to zero it out? I know on some of the newer model tach's, you can calibrate it using a master tach, but is it possible with an older model tach? It does have a couple of "dials" inside. Are those for calibrating?

mikebob · Sep 10, 2002

Re: Need Tach clarification for 73 Merc 850

thanks rick i appr. it. I am leaning towards prop myself been looking ing here on prop info. mine has a 13-19 prop everything in here says i need a 13-21 prop but thats only about 400rpm which would make 3600. I may just leave it alone it runs great and i dont ski just fish and drift anyway

Jumper · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

any suggestions on the possibility of calibrating a tach?

rickdb1boat · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

For Jumper: Most Tachs do not return to Zero, when the ignition is off. My Tach also sits at about 200, until I hit the key, then it sits on Zero! Does yours Zero when you turn the key to "On"?

rickdb1boat · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

Mikebob, You would want to go "Down" in Pitch on your Prop to gain RPM's. A 13"X21" will put you at "2800" Rpm's! You need to go the other way! I would drop to a 13"X15"Pitch, to get you a Rpm of around 4000, which is still low, assuming your Tach is correct and the Motor is running well. I would look into a different Tach or borrow another to verify readings before you change the Prop. If indeed you are running that low RPM'S, that is not good for your Motor! It will have a short life lugging around!

Rex · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

Find some one that's got "FLUKE VOLT METER" WITH A INDUCTION CLAMP PUT IT ON NUMBER 1 WIRE. PUT REMEBER IS A 2 STOKER NOT 4. THAT WILL TELL IF THE IS RIGHT OR NOT SIMPLE REX

Jumper · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

Rick, The tach doesn't even zero out when I turn the key and add battery power. But like I said, it registers RPM's just not sure if it's the correct RPM's or not. But I initially thought the same thing: once I turn the key and get battery power on it, it'll zero out. You know what....I just realized, I don't think I even have battery power to it

. I ran the wire to the rectifier and connected the ground wire. Do you think that could be the culprit??

Jumper · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

Question just popped into my pea sized brain, but I don't think I have three terminals on the back. I only remember having a "Send" and "Gnd" terminal. Where would I hook up the 12v source wire if that's the case?

rickdb1boat · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

The 3RD teminal is just for a Backlight for nightime running. You may or may not have it! Check again, but that will not affect the Tachs operation! Does it move at all when you turn the key to "ON"? Can you check your WOT throttle RPM on the Water and tell us what it is? If it's up around 6 Grand, then it is probably off! I did read that some Tachs have an adjustment to correct the readings. Maybe that's the key, but you would have to verify the "Real" reading to know how to set yours.

Jumper · Sep 11, 2002

Re: Need Tach clarification for 73 Merc 850

It's got back lighting but it's on it's own wiring. I haven't been able to put the water on the boat yet to find out what the WOT rpm's are either. I'm hoping to do that Friday after work. But I don't remember the needle moving at all when I turned the key. Just stuck on 200. I know idle rpm's are around 1100 and the manual says the rpm's in gear while idling are around 500-600. I may put the motor in gear tonight and see what it reads. Or I may just forget all this headache and buy a new one!!

Need Tach clarification for 73 Merc 850

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