1972 Merc 500 Cooling & Wiring Problems

New to this site and looking for any help. I am restoring a 1972 Merc 500 for my father-in-law, and ran into a few problems. Just completed the install of a new water impeller, and ran the motor. I am not seeing any circulation out of the telltale, and concerned that water is not flowing freely. Ants were crawling out of the telltale before replacing the impeller, and they may have the circualtion system blocked. I understand this motor does not have a thermostat, but I am thinking the ants may have blocked the water passageways. Is there a way to clear these passages?

Secondly, the control box wiring is absolutley fried and brittle, and I would like to bypass the control box with a dash mounted key switch with choke. Would appreacite any help on a wiring diagram for a new keyswitch using the wires in the harness.
Thanks, Randy

Re: 1972 Merc 500 Cooling & Wiring Problems

You have a PM

Re: 1972 Merc 500 Cooling & Wiring Problems

Thanks to all for the help and advice, this is an incredible website. I purchased a dash mount on/off/choke switch and am attempting to wire the switch correctly. I have a drawing of the electrical layout, but it's not all that easy for me to understand. Here's what I have so far:
Red wire to "battery" on switch
Black wire to ground
Gray wire to choke
Yellow wire to selenoid
I have two connections left on the switch labeled "ignition circuit" and "magneto circuit", and three wires left that are orange, brown and white. Any help on how to complete this wiring would be greatly appreciated.
As a refresher this is a 1972 Merc 500.

Thanks, Randy

Re: 1972 Merc 500 Cooling & Wiring Problems

don't know about wiring, but take weed eater line and work it into the pee hole, ooohh that hurts. if it had ants, guaranteed there in a blockage.

Re: 1972 Merc 500 Cooling & Wiring Problems

The orange wire I believe is actually a faded salmon color. If so, on your diagram it will migrate through into your motor's internal harness and to the switchbox. This is the kill wire. It grounds out and kills your motor when you turn off the switch.

The brown wire is the signal wire to your tach plug in. The brown wire goes from the tach plugin into the motor and is connected via a jumper to the salmon/orange wire just inside the motor. So it is basically running to the same switchbox lead as the orange/salmon wire. The old fashion tachs got their signal from this switch box connection. I believe this old high voltage signal will burn up a modern tach.

The white wire is designed to supply power to the choke button and tach when the key switch is turned on.

You will likely need to hook up the orange/salmon kill wire, if that indeed is what your's is. I'm not exactly sure how you would wire it into your new switch. You'd have to use the diagram to figure it out. Your "ignition circuit" "magneto circuit" has me a bit puzzled. I don't believe my book has any reference to that but it may be what I'm referring to later as two separate two electrical systems.

I think you can forget about the brown wire for now until you want to hook up a tach later on and you will wire it differently anyway.

Your motor essentially has two electrical systems that share a common ground. The stator under the flywheel produces a 12+ volt charge that supplies juice to your battery and therefore the starter and choke and other acessories. There is also a spinning electricity producing magnet in what appears to be your distributor. It is called the ignition driver instead of distributor. It produces the high voltage charge that is sent to the spark plugs.


You might want to wait until you get someone else to confirm what I said. Ricksrster is pretty good at explaining these motors.

Re: 1972 Merc 500 Cooling & Wiring Problems

go to www.homestead.com/dolphinmarineservice/elec6.html.There are examples of most switches used on outboards and color codes used on mercury motors. was referred by another poster and used this to rewire a classic 50 control box.

Re: 1972 Merc 500 Cooling & Wiring Problems

sorry the address came out wrong. www.homestead.com/dolphinmarineservice/ele6.html

Re: 1972 Merc 500 Cooling & Wiring Problems

on the cooling problem i have a50/500 1977 and i found out all the ports was blocked....so i took the side cover off and cleaned out all ports....do not ave to take cove where plugs are this is just jugs....but side plate if i was rebuilding it ....would come off and clean.... i did mine and it cools 100% better....