Is there a way to test Mercury outboard overheat alarm with a pan of hot water?

**tlwjkw** · 04-12-2020, 10:37 PM

it makes ground to tha head.........................

**Bumblebeeman** · 04-13-2020, 12:10 AM

Does that mean it has to be in the head to set off the alarm when it sees the temperature that triggers the alarm?

**tlwjkw** · 04-13-2020, 12:23 AM

yes.. when it closes it touches tha metal head (which is grounded) completing tha ground part of tha circuit....

you might be able ta do it with a wire with alligator clips ta use as a jumper between one side of sensor and to tha motor ground heating up tha sensor.....

**tlwjkw** · 04-13-2020, 12:28 AM

if ya that determined ta make it sound, pull tha gear case and start tha thing... it will go off.

**Glastron1987** · 04-13-2020, 06:06 AM

I would think 212 should be enough to trigger it, but I am not quite sure. The bottom of the sensor does need to be grounded to the engine though either by it being installed or with other means, otherwise the circuit can not complete when it triggers.

**seansbaja1982** · 04-13-2020, 07:21 AM

It has to b grounded to cyl. head or any ground on the engine. Clip a alligator clip to it with a ground wire attached to block anywhere and carefully warm the contact face side that contacts the head with a heat gun. Keep the heat on contact side and slide the spring and seal up the wire away from u.
Won't take much to set it off...

**Glastron1987** · 04-13-2020, 09:43 PM

Just tried to pour water from a boiling kettle on an extra sensor I happen to have. It did not trigger. I think a heat gun would do a better job (maybe I'll try that tomorrow). Also, I think the trigger temperature is somewhere around 220 degrees Fahrenheit which is hotter than boiling water.

**H2Okie** · 04-13-2020, 11:21 PM

OT Sensor has a contact inside most like operated by a Bi-metallic strip , disk, or could be liquid filled with a diaphram but regardless of construction heat will have to slowly sink into sensor to get it to operate it's not going to be an instantaneous event . This is a thermal device an this usually means there will be a time delay for the heat to take effect . Whether forcing operation with hot water or heat gun you will have to allow a little time for the heat to soak in and actuate the device . There will most likely be a time delay on cooldown as well before the contact opens .

**tlwjkw** · 04-14-2020, 12:32 AM

his is a bi-metallic "spring".. tha temp will vary.. some will at 'bout 195 and up.. their QC plus age on 'em leave a little ta be desired and cause 'em ta vary somewhat...

**Bumblebeeman** · 04-14-2020, 02:24 PM

I want to thank everyone for their help on this. Just like H2Okie said it takes some time for the heat to soak in and it clearly takes a little more than 212 deg F of boiling water. I connected the spring to the cylinder head with a double-ended alligator clip. I put the silver tip about the size of a dime at the end of the spring in a shallow pan of hot water on a hot plate and brought the water to a boil. No alarm after 5 minutes of water boiling. I let the water entirely boil away thinking with the sensor in contact with the bottom of the pan and the hot plate on high the sensor could get a little hotter that the 212 deg limit of boiling water but still no alarm after about 10 seconds. I turned the hot plate off to avoid damaging the sensor. About 4 seconds after turning the hot plate off the alarm sounded. It took about 25 seconds of me blowing on the sensor for the alarm horn to stop sounding. If they are supposed to go off at 195 deg maybe the fact it is a 1996 engine the alarm doesn't work as well. Is this part replaceable? I Googled the part # on it, AIRPAX 9518, and nothing came back on that particular part. There are AIRPAX sensors, but I have no way of knowing what is the right part #. Has it been superseded?

**tlwjkw** · 04-14-2020, 02:36 PM

since you are being so picky with this thing tha absolute only way ya gonna get what you're lookin' for is with it installed, with gearcase removed anda "heat gun" pointed at it when this all starts ta measure temp when it does go off.. way to many variables with tha way you did it... its installed tight in tha head, its in an enclosed space etc.. it will probably go from 195 ta over 200 ina nano second once tha over heat starts its process.. jmo

**Bumblebeeman** · 04-14-2020, 03:00 PM

Another person mentioned removing the gear case. What does the gear case have to do with the cylinder head temperature measurement?

**H2Okie** · 04-14-2020, 03:29 PM

Sounds like it is operating a little above 212 deg from your testing . The hottest temp you will be able to achieve boiling water is 212 . If you want to know the exact operating temp you could use cooking oil instead of water and a cooking thermometer and try to slowly ramp the temp up above 200 deg. until the sensor contact operates .
The main thing is it operates (more of a go / no go test ) .

**tlwjkw** · 04-14-2020, 04:57 PM

Originally Posted by Bumblebeeman

Another person mentioned removing the gear case. What does the gear case have to do with the cylinder head temperature measurement?

that was me.. no water to get temp up.. won't burn tha pump up and get it to heat up quick.... one word of caution if you do it.. make sure you or someone you trust has an ear close to tha buzzer with a hand on tha key and/or kill lanyard.....

**Glastron1987** · 04-14-2020, 06:54 PM

Sounds to me that if you got the sensor to trigger so close to the boiling water temperature, the sensor is working properly.

Also, I did try to trigger my extra one with a heat gun. It took maybe 15 seconds the first time but I didn't really time it. After taking the heat gun off, it took a few seconds to untrigger. It triggered more quickly on subsequent tries because it retained the heat to a great extent. I tried to take a temperature reading with my infrared thermometer, but the shiny metal caused it to read falsely. In any case, it was too hot to really handle without a glove.