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How to test water temp sensor / gauge


mp3mike05

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Anyone know how to test the water temp sensor / gauge on a 1990 Malibu Skier (5.7l 350).

The sensor appears to be in the thermostat housing and just has one wire going to it. I have a mediocre digital multimeter. Since I bought the boat the water temp gauge has never moved off the bottom. Just trying to find a good way to narrow it down between the sender, wiring, and gauge before I go buying useless parts. Thanks.

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You can ground out the wire going to the sensor for starters, that will tell you if you gauge and wiring are ok. A boat that old like mine did the same thing and my t stat was stuck open,a cheap easy fix. Discount inboard marine or Bakes are great places for parts.

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You can ground out the wire going to the sensor for starters, that will tell you if you gauge and wiring are ok. A boat that old like mine did the same thing and my t stat was stuck open,a cheap easy fix. Discount inboard marine or Bakes are great places for parts.

Should grounding out the wire peg the gauge? Wouldn't surprise me if the thermostat was bad but figured I would start with testing the sender.

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Should grounding out the wire peg the gauge? Wouldn't surprise me if the thermostat was bad but figured I would start with testing the sender.

Yes, grounding wire will cause the gauge to peg. That tells you all you wiring, gauge ect are working as they should.

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Yes, grounding wire will cause the gauge to peg. That tells you all you wiring, gauge ect are working as they should.

If it pegs, unground the wire immediatly. Pegging the gauge like that with no electrical resistance in the circuit will damage the gauge. Edited by electricjohn
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  • 6 months later...

Yes, grounding wire will cause the gauge to peg. That tells you all you wiring, gauge ect are working as they should.

Ok so bringing back from the dead.

I grounded the wire coming off the sensor and the gauge pegged so I took it off immediately. I purchased a new water temp sensor over the winter, and when I took the boat out the gauge will still not move (maybe a TINY bit but never above 100).

I used an infrared temperature gun, and the t-stat housing measures in at 142 degrees F.

If I ground the "body" of the sender the gauge will hover on up to around 200, but not peg. The sender was slightly longer than my old one, but I don't see why that should matter. Is my gauge busted?

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Check the thermostat man.

I didn't physically look at it, but the housing measures 142 degrees, and it's a 140 degree thermostat (i think) so that sounds spot on to me.

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nother way to test the gauge would be in boiling water, it should read 212 deg F. Also, look at where the sensor is located and make sure it gets water temp just before the thermostat, you might have a dead spot or even an air pocket.

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  • 2 months later...

nother way to test the gauge would be in boiling water, it should read 212 deg F. Also, look at where the sensor is located and make sure it gets water temp just before the thermostat, you might have a dead spot or even an air pocket.

Is the sensor supposed to ground itself via the threads in the thermosatat housing? If I remove the sender to put in boiling water, the gauge doesnt budge at all. If I ground the housing and put it in water, the sensor appears to register a reasonable value.

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Yes, the sensor must be grounded to work. Run a ground wire out of the boiling water along with the sensor wire. I figured most of the boats have a 160F thermostat. Thermostats fail more often when exposed to an open cooling system as in a boat.

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