Alarm horn??

[fitz73222]

Ryan,
The senders are resistance based, just like a fuel sender.  Variable resistances give different gauge readings.  Resistance too high (or too low)?  The circuit is closed and the horn sounds.  My "guess" is that, being resistance to ground, once the resistance gets to a certain point (over temp), it shorts to positive and sounds the alarm.  Much the way a kill switch works by shorting to the ign positive and grounding the coils...

That's exactly how it works Bob; and one additional note for Ryan is a temp gage on an outboard is pretty worthless because outboards run to cool to get the gage to even move past "C". I have them on my 115's; they both work and never budge past "C". Most marine inboards run at 160-165F where as our outboards run at 110-120F so the typical temp gage you buy has a higher threshold before the needle moves. I've searched high and low for lower range temp gages but to no avail. So I've had to condition myself for awareness that if the gage should ever move past "C" I may have a problem. My hot horns self test everytime I turn the key on, a little annoying but I know they're working when I lite the engines up. A water pressure gage is much more beneficial for reading water pump health and flow rate if your running through grass or pick up a plastic bag over the intake holes and the gage will nose dive immediatley but if a thermostat sticks shut; you'll never see it on a water pressure gage; you can fry an engine with her peeing like a race horse. So having a horn or temp gage AND a water pressure gage will cover both failure modes.

[Aquamaniac]

Thanks to all.....
I like the idea of multiple 'failure warnings'.....so might have to find a water pressure gauge/switch if possible to add to this old beast. Guess the temp gauge might in fact work.....just haven't seen it move!
Will try to make sure alarm horn works at the very least. Farley, if you do run up on a lower range temp gauge...please give me a shout. This motor seems to run quite cool, but am not impressed with the pisser flow. Lots of flow through discharge though. Think someone told me Mercs often did not have good stream until higher pressure.(????) But not much on muffs.

[seabob4]

Tom, you might want to check the condition of the wear plates in the water pump housing.  On motors this old, where just impeller changes have been done over the years, the wear plates "wear" (jeez, go figure) and reduce the pressure of the water flow out the water pump.  That can cause both a weak stream and an over heat condition...

[fitz73222]

One thing to remember and I wish all outboards were designed this way and that is the tell tale (pisser) is plumbed from the outlet side of both thermostat covers. Which means that the waterflow out of the pisser varies with the degree of thermostat openess. So at idle the thermostats are nearly closed and the outlet water diminshes fairly dramtically. At high speed the thermostats are more open and the pisser volume increases. So in this case, water pressure from the pump has little effect on the pisser flow rate. The extra water so to speak is routed through the above water exhaust relief ports at the rear of the midsection or out through the prop as you describe. This is unique to V6 Mercs of the era. So what this does tell you if you don't see any water coming out of the tell tale after warm up you have either a waterpump issue OR a thermostat issue which prevents you from baking the engine if you just have a visual to go by. Most other outboards including smaller Mercs with thermostat controlled cooling have the point of tell tale origin before the thermostats which means you have no way of knowing that a thermostat has stuck but your still showing water flow out the tell tale (got tired of writing pisser). That said, both of the cylinder head thermostat covers are plumbed into one tell tale outlet which means a thermostat can stick on one cylinder head and get hot while the other is open and leads you to believe everything is OK when it is not. What most race folks do is plumb both discharges separately through the back of the engine pan so they absolutley know that both thermostats are opening and closing properly and have two tell tales. That's why you see high performance 2.4 and 2.5's with two tell tales.

[Aquamaniac]

Indeed, took her out today and in the water all warmed up, the pisser flowed well. We have already checked pump and impeller ...good. Replaced both thermostats. Interestingly, PO had wrong ones in....and were not recently replaced as he claimed. Surprise,surprise.
Motor ran well today and did not notice the same throttle issue. Was a bit notchy (I assume the high/mid/low carb  'levels' but might just take some getting used to.

[seabob4]

Well it's a beauty day to be out Tom.  I, on the other hand, am also on a boat...painting a bilge, with a whole bunch of chit in it.  Not exactly the same... :(