Cooling issues

KurtAyres

Cadet
Joined
Nov 8, 2003
Messages
7
My 1994 4 cyl. 40 h.p. Mercury is having cooling issues.

I had a short overheat warning a couple of trips ago. On the following trip, we ran her at full cruise power for three hours + and had no warning. The next one we had the horn almost continuously after running at full cruise power for 15 mins. and it continued even at low power. I changed the water pump, cup, and plate. The ground test went okay, but then we got the horn again while on the water. I disconnected the temp sensor on the powerhead, and the horn still sounded. Is that to be expected in a properly operating warning system, or should the horn go off when the sensor is disconnected? This model has no thermostat, so that's not the issue. Anyone have any suggestions?
 

dlplost

Petty Officer 2nd Class
Joined
Jun 6, 2008
Messages
117
Re: Cooling issues

First, Use your hand or one of those noncontact IR thermometers to check the motor temp to make sure it's a false alarm. It should never be hot enough to burn your hand, you should be able to hold your hand on it. It'll just feel hot at the most.

The Horn is powered thru the key switch. Any time the key is "on" the horn is powered. The temp sensor grounds the horn and turns it on when the temp exceeds the "set" temperature. If the horn sounds with the sensor unhooked, look for a shorted horn wire in the harness.
 

KurtAyres

Cadet
Joined
Nov 8, 2003
Messages
7
Re: Cooling issues

Thank you for the tech info and suggestions. I went out and ran her this morning "on the hard" at various power settings, and even restricted the water flow by lowering the pressure on the supply hose at times, and I could put my hand on the power head without discomfort throughout. One cylinder seems to be a bit warmer than the others, but only at low power, which seems odd.
 

KurtAyres

Cadet
Joined
Nov 8, 2003
Messages
7
Re: Cooling issues

One thing I forgot in my last post--I'm thinking about installing a water temperature gauge. I'm old enough to remember the age before idiot lights (or horns, in this case) when we used gauges to see problems coming instead of being stuck once the problem has arrived. I don't know if Mercury has a kit or something for an engine this small/old, but I thought maybe I could get an aftermarket setup and tee it into the telltale hose.
 
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