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Faulyt? Temperature Alarm. Mercury 2 stroke 75HP

C

CLimnatitis

New member
Hi all,

My 2 stroke Mercury (75 HP) sounds an alarm (most probably high temperature) when cruising in medium RPM (speed 10-12 knots) and especially when there are medium waves. If i bring throttle to idle and the boat rest for 1-2 minutes the "alarm" goes off. This situation is not happening when cruising very slow (6-8 Knots) or fast.
I assume the alarm is faulty because it goes off when i go to idle and rest position!

Can someone have an idea what it can be?
 
It could be the overheat sensor, they are not high-tech, they do fail and it's not uncommon to have to replace one.

However, it has been my general experience that when they die they die pretty good - so you get an alarm as soon as you power up - I haven't experienced them going off/on - working sometimes and not others.

So I wouldn't rule it out but I wouldn't rule out 1) the water pump - when was the impeller last changed? 2) the thermostat - ditto, has it every been tested/changed? these wear out too and if it's a little "sticky" you might be actually getting an overheat at low rpms before the poppet opens up and allows more cooling water to flow - and the poppet may need service/replacing some components.

If the answer to impeller, thermostat and poppet inspection/replacement is "a few years back" or "don't know", then maybe check them out before concluding the temp sensor is to blame.
 
More likely the float in the oil tank causing the problem. The distillates in oil causes glue that attaches magnet to float to loosen and cause false alarms. Unplug the 2 blues wires from tank and test to see if alarms stop..
 
galamb said:
It could be the overheat sensor, they are not high-tech, they do fail and it's not uncommon to have to replace one.

However, it has been my general experience that when they die they die pretty good - so you get an alarm as soon as you power up - I haven't experienced them going off/on - working sometimes and not others.

So I wouldn't rule it out but I wouldn't rule out 1) the water pump - when was the impeller last changed? 2) the thermostat - ditto, has it every been tested/changed? these wear out too and if it's a little "sticky" you might be actually getting an overheat at low rpms before the poppet opens up and allows more cooling water to flow - and the poppet may need service/replacing some components.

If the answer to impeller, thermostat and poppet inspection/replacement is "a few years back" or "don't know", then maybe check them out before concluding the temp sensor is to blame.
Click to expand...

Thank you for the response.

Impeller and thermostat replaced one year ago. So i will concentrate on the overheat sensor.
 
Thank you for the response,

I already disconnected the float in the oil tank but the problem persist! So this is also excluded as well.

I will proceed with galamp's suggestion to check the overheat sensor and come back soon!
 
CLimnatitis said:
Hi all,

My 2 stroke Mercury (75 HP) sounds an alarm (most probably high temperature) when cruising in medium RPM (speed 10-12 knots) and especially when there are medium waves. If i bring throttle to idle and the boat rest for 1-2 minutes the "alarm" goes off. This situation is not happening when cruising very slow (6-8 Knots) or fast.
I assume the alarm is faulty because it goes off when i go to idle and rest position!

Can someone have an idea what it can be?
Click to expand...
Mine has the same problem can you let me know what the issue was
 
If your 75 is like my 2002 90, it has a thermostat and a flow bypass valve.....both under the scabbed on casting on the upper left of the rear water jacket cover of the powerhead.....the big bulge end is the popoff. The stat controls cooling up to about 2500 RPM where water pressure unseats the popoff, larger cross sectional area as the stat, and allows more flow essentially bypassing the stat. If you have that problem as stated, expect your bypass valve is stuck closed.....was on my 90 with the same problem.
 
Texasmark said:
If your 75 is like my 2002 90, it has a thermostat and a flow bypass valve.....both under the scabbed on casting on the upper left of the rear water jacket cover of the powerhead.....the big bulge end is the popoff. The stat controls cooling up to about 2500 RPM where water pressure unseats the popoff, larger cross sectional area as the stat, and allows more flow essentially bypassing the stat. If you have that problem as stated, expect your bypass valve is stuck closed.....was on my 90 with the same problem.
Click to expand...
Thanks mate I’ll try that as I have done every othe thing else
 
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