older merc 800 newer tachometer

[martinezalfo]

Hi,can anybody tell me how to install a newer tach on a 1972 mercury 800 thunderbolt I have a mercontrol box with a three prong connection in front,Thanks.

 

[Moody Blue]

Re: older merc 800 newer tachometer

I have the same motor and installed a "newer" tach a couple years ago. You need a tach harness that plugs into the connector at the front of the MerControl box. They are frequently sold on ebay for about $20. P/N 52129A1. The tach uses three connections, +12V, tach signal from motor and GND. All three connections are provided through the MerControl connector. If you wish, you can hook the backlight to the other gauges backlight circuit.

You will need to change the connection point of one wire on the motor. There is probably a GREY wire connected to a terminal on the switchbox on the starter motor side of the motor. This GREY wire must be moved and connected to the rectifier. Connect it to either post where the yellow stator wires are presently connected. If the grey wire is already connected at the rectifier, then you are good to go.

Follow the instruction on the particular tach for making the proper settings. I think it is 12P if my memory is correct.

 

[martinezalfo]

Re: older merc 800 newer tachometer

Thanks but I dont have a grey wire going into the switch box they are red,green,white and brown will it be one of these?

 

[Moody Blue]

Re: older merc 800 newer tachometer

My bad, newer motors use the grey color code for tach signal. The brown wire is the one you need to move.

Red is constant +12v, white is switched +12V from control box and green is output to coil.

 

[martinezalfo]

Re: older merc 800 newer tachometer

Thanks Moody,here is a pick of the tach I will be using what settings should I use?

 

[Chris1956]

Re: older merc 800 newer tachometer

Try the 6C setting. If that is not it, you will know it as the idle will be too high to low. Your '72 will have the brown tach signal wire. They were normally connected to the switchbox, but could have been moved over the years. If the brown wire is connected to the rectifier yellow wire, it is correct. If not, remove the non-distributor brown wire from the switchbox and connect it to either of the yellow wires on the rectifier. The original rectifier was the large ugly rectangular device in the front lower cowling.

 

[mr roach]

Re: older merc 800 newer tachometer

i just got done ( litrerally ) installing a newer faria tach similiar to the one in your pics on my boat it has a 1977 merc 1150 i6 with the thunderbolt ign. engine on it andi used the 6p setting and it works fine but i bow to experience i just wanted to let you know it worked for me but when i set it to the 6c it pegged the tach