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Old 17-08-2018, 13:11   #1
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Unhappy Need Some Advice...Can't figure this out

Hoping somebody has an idea I haven't tried already. This seems simple but it has gotten the best of me so far.

I have a Catalina 28 with a Universal M25XP motor. When the motor is running, the water temp gauge stays stead on 160 degrees. Last time we went out for a sail, before we could get out of our canal, the temp started rising slowly above 160. I checked to ensure I opened the raw water thru hull and it was good. I pulled out my IR thermometer and checked the motor and it was at 140. Then the gauge needle went all the way to the right (no alarm sounded) so I shut down and dropped anchor. I checked the thru hull to make sure it wasn't clogged and it wasn't. Since the temp of the motor never went above 140, I figured it was either the gauge or the sending unit. We motored back into the slip for maintenance.
I checked the gauge by removing the sending unit wire at the motor and grounding it, the gauge needle went all the way right every time I grounded the wire, so figure the gauge was good. Instead of checking out the sending unit, I just went ahead and ordered one. Put the new one in, started it up, and the temp (on the gauge) started rising slowly then pegged out to the right. I checked the motor temp and it again never went above 140. So I figured it wasn't the sender so I cleaned up the old one and put it back in.
I pulled the instrument panel and checked all the wires for looseness/corrosion and all were good. I checked the gauge again by grounding the sending wire and the gauge pegs right. I checked the voltage coming to the gauge and it was a steady 13.0v
For the life of me I can't figure this simple issue out. Would appreciate any assistance provided.
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Old 17-08-2018, 14:20   #2
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Re: Need Some Advice...Can't figure this out

Where are you measuring 140?

Itís possible that the thermostat isnít opening and the sender is getting hot while the rest of the engine is not
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Old 17-08-2018, 15:32   #3
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Re: Need Some Advice...Can't figure this out

In case you didn't already know this, you can put the thermostat into a s/s pan along with a candy thermometer, and water to cover the thermostat and heat it. You can see when it opens and check the temp. Sometimes old thermostats no longer open at the temperature they're supposed to.

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Old 18-08-2018, 04:27   #4
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Re: Need Some Advice...Can't figure this out

If removing a thermostat to test you might as well replace it. The test would confirm the need.
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Old 18-08-2018, 04:45   #5
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Re: Need Some Advice...Can't figure this out

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Originally Posted by JPA Cate View Post
In case you didn't already know this, you can put the thermostat into a s/s pan along with a candy thermometer, and water to cover the thermostat and heat it. You can see when it opens and check the temp. Sometimes old thermostats no longer open at the temperature they're supposed to.

Ann
You can do the same with the temp sender to check it, leave it connected to the gauge, and compare readings.
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Old 18-08-2018, 04:45   #6
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Re: Need Some Advice...Can't figure this out

Greetings and welcome aboard the CF, Phil.


If a Sender Fails “Closed” (shorted - Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

Testing Temperature Gauges:

1. Disconnect Tan Sender Wire from Gauge (Terminal “S”) and turn Instrument Power ON.
The Temperature Gauge should indicate ZERO Scale (< Low Temperature).

2. Short the Gauge Sender Terminal “S” to Ground Terminal “G”.
The Gauge should deflect to FULL Scale (> High Temperature).

3. Measure Resistance from Tan Sender Wire (disconnected from Gauge) to Ground.
American Temp. Senders will read: 450 Ohms (Engine Cold @ 100oF)
or 29.6 Ohms (Engine Hot @ 250oF )
European Temp. Senders will read: 281 Ohms (Cold @ 40oC) and 22 Ohms (Hot @ 120oC)

4. Disconnect Tan Wire from Sender on Engine Block.

5. Test continuity of Tan Wire.
Zero Ohms from Block End to Gauge End - If not, repair or replace Wire.


6. Measure Resistance from Sender Terminal (on Block) to Ground on Block.
Should read as per (3) above - If not, replace Sender.

From ➥ Basic Engine Gauge Theory and Testing
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Old 18-08-2018, 14:09   #7
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Re: Need Some Advice...Can't figure this out

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Originally Posted by JPA Cate View Post
In case you didn't already know this, you can put the thermostat into a s/s pan along with a candy thermometer, and water to cover the thermostat and heat it. You can see when it opens and check the temp. Sometimes old thermostats no longer open at the temperature they're supposed to.

Ann
That's a great trick Ann. Thanks for the tip!

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Old 18-08-2018, 16:23   #8
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Re: Need Some Advice...Can't figure this out

I was measuring on the radiator cap and the thermostat housing. During my troubleshooting today, I double checked my IR gun temp readings with a multimeter temp probe and the IR gun was about 20 degrees low. So the temp reading, assuming my multimeter probe is correct was around 160 (where it should be).
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Old 18-08-2018, 16:31   #9
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Unhappy Re: Need Some Advice...Can't figure this out

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Originally Posted by Phillih View Post
Hoping somebody has an idea I haven't tried already. This seems simple but it has gotten the best of me so far.

I have a Catalina 28 with a Universal M25XP motor. When the motor is running, the water temp gauge stays stead on 160 degrees. Last time we went out for a sail, before we could get out of our canal, the temp started rising slowly above 160. I checked to ensure I opened the raw water thru hull and it was good. I pulled out my IR thermometer and checked the motor and it was at 140. Then the gauge needle went all the way to the right (no alarm sounded) so I shut down and dropped anchor. I checked the thru hull to make sure it wasn't clogged and it wasn't. Since the temp of the motor never went above 140, I figured it was either the gauge or the sending unit. We motored back into the slip for maintenance.
I checked the gauge by removing the sending unit wire at the motor and grounding it, the gauge needle went all the way right every time I grounded the wire, so figure the gauge was good. Instead of checking out the sending unit, I just went ahead and ordered one. Put the new one in, started it up, and the temp (on the gauge) started rising slowly then pegged out to the right. I checked the motor temp and it again never went above 140. So I figured it wasn't the sender so I cleaned up the old one and put it back in.
I pulled the instrument panel and checked all the wires for looseness/corrosion and all were good. I checked the gauge again by grounding the sending wire and the gauge pegs right. I checked the voltage coming to the gauge and it was a steady 13.0v
For the life of me I can't figure this simple issue out. Would appreciate any assistance provided.
So I bled the thermostat housing and put a new sender wire direct from sender to gauge. Started her up and all looked good until the temp got to 160'ish then it pegged out to the right again (just like before). I checked the motor temp with my IR gun and it showed around 140. As a side note, since 140 is also not normal, I double checked my IR gun temp readings with a multimeter temp probe and the IR gun was about 20 degrees low. So the temp reading, assuming my multimeter probe is correct was around 160 (where it should be). Maybe Harbor Freight wasn't the best choice for an IR temp gun. The motor temp stayed steady at 160'ish but the gauge jumped back and forth between 180 and pegged out to the right. I hate throwing parts at a problem but I guess a gauge change is next.....this one is tough.
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Old 18-08-2018, 18:29   #10
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Re: Need Some Advice...Can't figure this out

IR temp guns are finicky because of the Immesivety (spelling?) of the surface that you are measuring. It can throw off the reading by as much as 90%. Shiny silver is the worst.
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Old 18-08-2018, 18:45   #11
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Re: Need Some Advice...Can't figure this out

We had an indicated over heat after engine warm up. I replaced the thermostat which did not fix it. It took a while but I determined the aux contact on the ignition switch was faulty. Replacing the ignition switch fixed it. I supposed there was too high resistance through the switch.
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Old 18-08-2018, 19:07   #12
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Re: Need Some Advice...Can't figure this out

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IR temp guns are finicky because of the Immesivetty (spelling?) of the surface that you are measuring. It can throw off the reading by as much as 90%. Shiny silver is the worst.
emissivity Is the spelling. What you described is right.
Itís a bit funky because the IR guns are measuring infrared wavelength light emission from the hot target. Think about Red the yellow then white hot. The hotter the object the more intense the glow. The color changes too.
But if the object is reflective or transparent in the infrared, such as polished metal, much less is emitted at any temperature. If the object is flat black like a graphite pencil lead more is emitted.
So most surfaces have an emissivity around .9 to 95. This table of emissivities it useful. If the emissivity is low your Thermometer may read low.
It is surprising that some materials we think of as transparent such as most glass are opaque to infrared.

https://www.thermoworks.com/emissivity_table

All this is to say that ir thermometers are fantastic but you need to know if you are trying to measure stuff that is very shiny or very transparent ( in the ir) you will get a false reading.
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Old 18-08-2018, 20:09   #13
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Re: Need Some Advice...Can't figure this out

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Originally Posted by Phillih View Post
So I bled the thermostat housing and put a new sender wire direct from sender to gauge. Started her up and all looked good until the temp got to 160'ish then it pegged out to the right again (just like before). I checked the motor temp with my IR gun and it showed around 140. As a side note, since 140 is also not normal, I double checked my IR gun temp readings with a multimeter temp probe and the IR gun was about 20 degrees low. So the temp reading, assuming my multimeter probe is correct was around 160 (where it should be). Maybe Harbor Freight wasn't the best choice for an IR temp gun. The motor temp stayed steady at 160'ish but the gauge jumped back and forth between 180 and pegged out to the right. I hate throwing parts at a problem but I guess a gauge change is next.....this one is tough.

IR heat guns are great if you have a baseline of measurements from when the engine is functioning normally. Then you are mesduri h changes, delta from normal etc. they are not so great at giving you actual accurate temperature readings of internals, such as the actual coolant temp, although some can be accurate. Thatís part of the benefit of measuring baselines.

Does the engine do this even at idle? When the gauge pegs, how quickly does the needle move? Is it a gradual building or a quick sudden pegging?
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Old 18-08-2018, 23:21   #14
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Re: Need Some Advice...Can't figure this out

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You can do the same with the temp sender to check it, leave it connected to the gauge, and compare readings.
Do that first, otherwise you are wasting your time. You don't need any thermometer - water boils at 212F and your gauge should read that within +/- 10 degrees, and steady, no jumping off scale. Just remove the temp sender from the engine, leave all the wires connected and put a big clip lead or hose clamp around the temp sensor with a wire to engine ground. Turn on "ignition" circuit, put sensor in pan of boiling hot water, and read the gauge.
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Old 21-08-2018, 12:24   #15
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Re: Need Some Advice...Can't figure this out

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Originally Posted by Phillih View Post
So I bled the thermostat housing and put a new sender wire direct from sender to gauge. Started her up and all looked good until the temp got to 160'ish then it pegged out to the right again (just like before). I checked the motor temp with my IR gun and it showed around 140. As a side note, since 140 is also not normal, I double checked my IR gun temp readings with a multimeter temp probe and the IR gun was about 20 degrees low. So the temp reading, assuming my multimeter probe is correct was around 160 (where it should be). Maybe Harbor Freight wasn't the best choice for an IR temp gun. The motor temp stayed steady at 160'ish but the gauge jumped back and forth between 180 and pegged out to the right. I hate throwing parts at a problem but I guess a gauge change is next.....this one is tough.
So to close up this issue, here was the ultimate solution summarized. After the gauge checked good, I put in a new sender unit (same issue occurred), bled the thermostat (same issue occurred), and put in a new sending wire (same issue occurred). Temp of motor was holding steady so I was going to start over in my troubleshooting. Checked gauge again, this time checked bad, swapped in a new gauge, and started her up. Runs perfect with temp holding at 160. Thanks to everyone who provided advice. I learn something new everyday.
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