Tachometer hook up on mercury 850 85hp

[b0horman]

i have a 1974 mercury 85hp 850 inline 4 that i am trying to hook up a tachometer to. I bought an aftermarket tach. Fiara i believe. i looked at the wiring diagram and the sending wire for the tach should be a light brown wire / brown wire coming from the main harness out of the engine which runs up to my console. the same harness has all the starter / key wires. i hooked up the tach with power and ground and the wire for the sending signal but i get now readings.

how can i test the wire to see if i am getting the right reading to pick up RPMs. Should i look at voltage or ohms?

 

[turtles11756]

Re: Tachometer hook up on mercury 850 85hp

is the rectifier ok ?

 

[Chris1956]

Re: Tachometer hook up on mercury 850 85hp

Tach wire is brown. Medium brown and the only brown wire. Tach also needs white wire for power and black wire for ground. A lot of MerControls have a three connector female plug on the front end. This is place to connect tach.

 

[b0horman]

Re: Tachometer hook up on mercury 850 85hp

i don't have the throttle control that has the tachometer hook up. my boat is a center console. i just used power and ground from my console to hook up the tach.

how can i determine if the rectifier is good? where is it located? is it the little black thing near my switch box on the right side attached to the front plate. it has a brown wire that attaches to switch box along with another brown wire.

 

[Chris1956]

Re: Tachometer hook up on mercury 850 85hp

A motor of that vintage may have the older style rectifier, which was like two packs of cigarettes in size. The newer rectifiers were much smaller, black with three connections. Both will have two yellow wires, a red wire, and usually a brown wire.

 

[b0horman]

Re: Tachometer hook up on mercury 850 85hp

this sounds very similar to the switch box that is located in the front of the motor. has the 4 colors you listed on the left and brown, black and yellow for the distributor on the right. i am assuming that is good because my engine is running. just at a high idle.

 

[Chris1956]

Re: Tachometer hook up on mercury 850 85hp

Nah, Switchbox has about 7 connections, and no yellow wires. Rectifier has only three connectors and two of them are yellow come down from under the flywheel. Rectifier can be in front lower cowling.

 

[merc850]

Re: Tachometer hook up on mercury 850 85hp

One of these should be the rectifier on your motor, you hook a wire to one of the yellow leads and attach it to the SENDER terminal on the tach. Hook the other leads to + - etc.

 

[b0horman]

Re: Tachometer hook up on mercury 850 85hp

i have the smaller rectifier. i took the brown wire from my switch box and hooked it up to one of the yellow wire. I am still not getting a signal. Is there a way i can test the sending wire with a volt / ohm meter. want to check to see if iam even getting a signal with my wire. it runs underneath my deck so it could have a short.

 

[Chris1956]

Re: Tachometer hook up on mercury 850 85hp

Yes. Place the ohmmeter on the tach wire and ground under the dashboard. At the motor disconnect the tach wire from the rectifier and ground it. The ohmmeter should read zero with the wire grounded and infinite with the wire unconnected.