1973 Mercury 850 85HP correct Tachometer?

[scottrbishop]

The tach in that came with my boat isn't giving me the right readings and I want to make sure I replace it with the right thing.

http://cgi.ebay.com/ebaymotors/ws/e...STRK:MEWN:IT&viewitem=&item=180104592828&rd=1

will this Quicksilver tach work on my motor? It looks the same but my manual doesn't give me a model # for the replacement. Will a 65794A2/73370A2 work, or should I look for a different aftermarket tach? Thanks!

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

I should probably also mention that this is a Thunderbolt Ignition model. I've read several threads on here about people having a hard time getting the correct tachometer for a thunderbolt ignition. Is there something special I have to look for?

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

I just tried to look up the part number on mercury parts express.com. It is no longer available. For the price of the tach on ebay , you can get a Teleflex or Faria here at iboats.com. They come with complete instructions on how to install.

 

[linckeil]

Re: 1973 Mercury 850 85HP correct Tachometer?

I don't know about the part numbers, but my 1973 sea ray with original 1973 merc 850 has a tach in it that looks identical to that one.

 

[Chris1956]

Re: 1973 Mercury 850 85HP correct Tachometer?

Scott, That motor will work with all the aftermarket universal tachometers. In 1973, merc made unique tachs that ran off the switchbox pulses. They wired their motors to use these unique tachs. The one on e-Bay may be that type, but I do not know for sure. However, you may easily convert your motor to run off the aftermarket tach instead.

When the older mercs came from the factory the brown wire (tach signal) was connected to the switchbox on the motor. If you move the brown wire to either of the AC connections of the rectifier, the aftermarket tach will work. Set it to 6P.

 

[pman7]

Re: 1973 Mercury 850 85HP correct Tachometer?

Chris, are you saying he should just disconnect the brown wire from the switchbox and move it over to either AC side of the rectifier? So in other words that brown wire has no other function than carrying motor impulses from the switchbox to the tach?

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

That is correct.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Thanks for the responses guys, I decided to go for the aftermarket tach. I see the brown wire on the switchbox and I see both of the yellow/red wires on the rectifier, but the brown cable coming from the distributor isn't long enough to make it to the rectifier on the other side of the engine. What should I do?

 

[linckeil]

Re: 1973 Mercury 850 85HP correct Tachometer?

its not the brown wire from the distributor you want to move. its the brown wire on the other side of the switchbox you need to move.

take a look at this picture for help.

http://www.maxrules.com/oldmercs/Wiring/1966ona/30.pdf

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Ahh, so the brown wire that runs from the engine harness? Disconnect from the switchbox and run to the rect? I'll run outside and check out the motor and report back.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Got her all hooked up and I'm on my way to the lake to try it out in half an hour. Just to verify, the AC side of the rect is the yellow/red wire(s), not the solid red wire, correct?

 

[linckeil]

Re: 1973 Mercury 850 85HP correct Tachometer?

yes, the 2 yellow/red wires run to the stator. you want to double up the brown wire on either one of the contacts where the yellow/red wires hook up to the rectifier.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Took it out on the lake today and it was a no go.. I think my problem is that I don't have a wire running to the IGN post on the tach. My old quicksilver tach the black wire running to NEG, the brown running to POS and another wire briging POS to the mounting bracket, and a white wire running to the light. What wire should I be running to IGN? It says 12-16 volts, can this be any 12 volt wire (take a tap off my CD player?) or does it need to run to the control box so the tach knows when to turn off and not drain my battery? I'd really appreciate a shove in the right direction, you guys have been awesome so far.

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

IGN on the tach is 12V+ from the key switch. When you turn the key on. The white wire on the ignition switch.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

So would that be the white wire on the control box that is labeled "tach Connector" on this diagram?

http://www.maxrules.com/oldmercs/Wiring/1966ona/30.pdf

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

Yes. That is the one.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

I tracked down the white wire and apparently it ran to the backlight of my old quicksilver tach. I'm leaving for radio shack to buy another ring terminal so I can chop the wire and reattach to the tach. I'll post back with results.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

She's alive!! One problem though, it's not reading the correct RPMs. At WOT It's only reading about 3600 when I know I'm going about 5200. The Teleflex Manual says to set the thing to 4P and everyone else tells me 6P. How can I tell how many poles are on my alternator?

 

[ricksrster]

Re: 1973 Mercury 850 85HP correct Tachometer?

6P is the correct setting. You have a 12 pole alternator which is 6 pulse. 6P should correct the issue.

 

[scottrbishop]

Re: 1973 Mercury 850 85HP correct Tachometer?

Looks like the back of the tach was already set at 6P. The voltage regulator is also brand new installed at the dealer and the battery is getting a good charge. What else could it be? Could the previous owner have installed a different size alternator?