Erroneous Temp Sensor Reading

[Horigan]

I'm having an issue with my OMC/Volvo engine water temperature sensor reading about 35 deg F too high. I've confirmed nominal temperature at the sensor with an IR gun (150 - 160F once warmed up). I've also confirmed the IR gun is reading accurately via a different temp probe. I've replaced the sensor twice and the dash gauge still shows about 35 degrees higher than the actual temperature.

An additional issue this is creating is that the ECM is seeing the erroneously high temperatures and it is triggering the SLOW (limp mode) system when the thermostat initially opens and I get a brief 25 deg overshoot as the thermostat is opening. The combined brief temperature spike, plus the sensor high reading, is triggering the SLOW mode, but this quickly gets resolved as the thermostat fully opens and the initial temperature spike is passed. Note that I have a half closed cooling system.

I've replace the thermostat twice and confirmed proper operation before installation (confirm initial opening at 160F and full open at 180F on the stove). So far I've checked continuity from the sensor to the engine harness and the wire reads good with no shorts to ground or high resistance. My next step is to check resistance from the engine harness to the gauge, then check the temp sensor wiring to the ECM. I have the OMC shop manual, so I'll need to review the wiring diagrams to the ECM. The specific sensor wire is pretty buried.

Have any of you run into this issue and can point me towards a likely area to investigate first? If my initial wiring checks don't find anything, I'll likely just buy another sensor and try that again. I've tried to find a resistance verses temperature chart for these sensors, but haven't been successful so far.

Thanks in advance for any inputs.

 

[alldodge]

The gauge has nothing to do with what the ECM is seeing. The gauge gets its reading from the sensor which provides a display

The ECM gets a ground from the Tan/Blue wire from Oil pressure and Water temp switches

 

[Horigan]

Ahhhh. Thanks alldodge. That makes more sense than one sensor supporting two functions.

When looking for the location of the temp switch (just starboard of the distributor) I see there is another temp sensor on the intake manifold just to the port side of the distributor. Does anybody know what this sensor signal goes to? I don't see it in the OMC manual schematics.

The OMC manual implies the sensor I've been replacing, between the thermostat housing and water pump, drives the gauge. The image in the manual looks like a single wire, stud and nut connection. The sensor next to the distributor is a multi-wire connector. I plan to disconnect the single wire sensor to confirm this is the sensor driving the gauge.

 

[alldodge]

This is what I have. The original to to large to upload

 

[Bt Doctur]

if all else fails simply add a resistor inline with the gauge wire. I`ll guess about 35 ohms. I do it all the time when the gauge does not read the same as the IR gun.You can determine what value using a 25k or 50 K pot from any electronics store. install it inline and rotate the knob to the correct reading , then remove it and measure the ohms required

 

[Horigan]

This is what I have. The original to to large to upload

Thanks alldodge. My diagram is similar.

Thanks Bt Doctur. That's a great idea!

 

[dingbat]

Temp sensors are pretty linear.

Measure the resistance with water at boiling and close to freezing then do the math to extrapolate the values in between.

Bear in mind mind that the IR gun is measuring the temp of the mass, not the actual water temp that the sensor is reading. Depending on the thermal dynamics of the system, the numbers may not be very close.

Certainly was the case for a customer I worked with last week. Ignoring alarms of a temp switch based upon readings from an IR gun cost him $70k

 

[Horigan]

Temp sensors are pretty linear.

Measure the resistance with water at boiling and close to freezing then do the math to extrapolate the values in between.

Bear in mind mind that the IR gun is measuring the temp of the mass, not the actual water temp that the sensor is reading. Depending on the thermal dynamics of the system, the numbers may not be very close.

Certainly was the case for a customer I worked with last week. Ignoring alarms of a temp switch based upon readings from an IR gun cost him $70k

Thanks dingbat for the input and the caution. On my half closed system my sensor is mounted to a four way tee plumbed to the bypass port of the Ford thermostat housing, so a stable IR reading of the tee should be representative of the water temp. I have a pot resister on order.

I'm finding the last couple of thermostats I've installed have more overshoot during opening than the earlier ones that were running low temp. I just need to make sure the overshoot is passed before getting on plane. I always do this during initial warm up, but the overshoot and brief SLOW activation has happened after the engine cools a bit while out on the water.