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Old 14-10-2019, 18:39   #1
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Temperature Switch - Help please!

Six months ago I bought a 210F temperatuure switch and a buzzer to add to my M25 Universal diesel. I have a temp sender and gauge, but wanted to the additional switch to protect the engine and so I don't have to watch the analog gauge. I was able to find a "T" fitting that will expose both devices to the coolant in the thermostat housing.

Now I go to install the switch and I'm puzzled...

The switch has two blade connections (and of course it threads into the engine). At room temperature, on the bench by itself, there is no resistance between the two blades - zero ohms on my multimeter. For me that is a "closed switch", conductive. In a pan of boiling water I'm still getting zero ohms between the blades. So it seems to me this is a closed switch regardless of temperature.

I don't understand. I have read that these switches are "normally open" at room temperature - which I take to mean infinite ohms between the blades - and then close (zero ohms between blades) at boiling temp.

So do I not understand? or do I have a bad switch?

Thank you for any advice!
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Old 14-10-2019, 18:59   #2
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Re: Temperature Switch - Help please!

Couple of thoughts...

-If the switch is made to be electrically isolated from the engine block, that is, the electrical components are isolated from the switch housing, then the switch is defective.

- If the switch is designed to drive two alarms at different locations ( dual station) then the threaded housing may act as the ground, and the two blades will always show continuity. If so, you need to test continuity between the blades and the housing.

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Old 14-10-2019, 18:59   #3
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Re: Temperature Switch - Help please!

Did you check continuity between the the threads and the blades? Some switches connect the device to the engine block which is connected to the negative side of the battery.
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Old 14-10-2019, 19:03   #4
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Temperature Switch - Help please!

There are of course two different devices that can look similar, sending units that often only have one wire, but sometimes two, they of course usually vary resistance with temp.

Then there are switches, which usually have two connectors but some only have one, they work by supplying a ground through the switch body if they only have one wire.
Some are normally open and are often meant for a light or alarm etc., when the set temp is reached, they close and complete the circuit.

However some are normally closed too, they are often used as “Murphy” switches, they allow power thru for example an electric fuel pump, engine gets hot, switch opens pump shuts off, engine stops, and some engines require power to run so when the switch opens, engine stops.

So in other words you could have several different things, but as nothing different happens when placed in boiling water It may be that boiling water isn’t hot enough or you have a bad switch, or even a sending unit.

You can test for a sending unit by seeing if the resistance changes with temp.

I assume your engine normally runs about 165F? If so I think I’d pick a lower alarm temp myself. 212 would be for an engine that normally runs close to 195 or so, of course I’m conservative too.

Try using oil and a thermometer and take it to 220 or so, everything has tolerances and sometimes a switch made for 212 may operate plus or mins say 5 degrees or so.
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Old 15-10-2019, 10:49   #5
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Re: Temperature Switch - Help please!

Quote:
Originally Posted by Cap Bill Des View Post
Six months ago I bought a 210F temperatuure switch and a buzzer to add to my M25 Universal diesel. I have a temp sender and gauge, but wanted to the additional switch to protect the engine and so I don't have to watch the analog gauge. I was able to find a "T" fitting that will expose both devices to the coolant in the thermostat housing.

Now I go to install the switch and I'm puzzled...

The switch has two blade connections (and of course it threads into the engine). At room temperature, on the bench by itself, there is no resistance between the two blades - zero ohms on my multimeter. For me that is a "closed switch", conductive. In a pan of boiling water I'm still getting zero ohms between the blades. So it seems to me this is a closed switch regardless of temperature.

I don't understand. I have read that these switches are "normally open" at room temperature - which I take to mean infinite ohms between the blades - and then close (zero ohms between blades) at boiling temp.

So do I not understand? or do I have a bad switch?

Thank you for any advice!
Yes! A bad switch.
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