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Old 03-05-2013, 12:14   #16
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Re: Bad temp guage or sending unit?

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Originally Posted by sww914 View Post
Yes, it needs a ground. They generally supply a variable ground to the gauge depending on the temperature so make sure you have a great ground or it will give you inaccurate information.
................ .
What do you mean by a "variable ground"? Isn't the sender screwed into the head or block? Isn't that the ground?
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Old 03-05-2013, 13:13   #17
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Re: Bad temp guage or sending unit?

In many units yes it is. I the variable resistance inside creates what the gauge reads right?
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Old 03-05-2013, 19:03   #18
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Re: Bad temp guage or sending unit?

Yes, that's what I mean. For an idiot light, the sender is an on/ off switch. For gauges it sends a ground with different amounts of resistance depending on the temperature (or pressure, if that's what we're measuring). So a cold sender might have 20 ohms of resistance and one at 200 degrees might have 120 ohms or 300 or 5000, whatever. The point is that it changes.
Somebody can google the resistance range of temperature senders now and try to make me look stupid by citing exact numbers.
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Old 04-05-2013, 08:43   #19
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Re: Bad temp guage or sending unit?

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Originally Posted by sww914 View Post
Yes, that's what I mean. For an idiot light, the sender is an on/ off switch. For gauges it sends a ground with different amounts of resistance depending on the temperature (or pressure, if that's what we're measuring). .
It doesn't "send a ground". A ground is a ground. It cannot vary.

The 12 volt circuit runs through the gauge, then through the variable resistance of the sender to ground. The gauge is actually measuring current in a series circuit but it displays that current as temperature, oil pressure, fuel level, or whatever it is designed to display.

Quote:
So a cold sender might have 20 ohms of resistance and one at 200 degrees might have 120 ohms or 300
It's the other way around. The cold sender has a high resistance, the hot sender has a low resistance. Disconnect the wire at the sender and the gauge will show the lowest reading on the scale. Touch that wire to ground and the gauge will sow the highest reading on the scale.
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Old 04-05-2013, 10:45   #20
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Re: Bad temp guage or sending unit?

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The point is that it changes.
Somebody can google the resistance range of temperature senders now and try to make me look stupid by citing exact numbers.
Like I said...
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Old 04-05-2013, 10:48   #21
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Re: Bad temp guage or sending unit?

So did you ask because you wanted to know or are you just a troll?
You look like a troll.
You smell like a troll.
You act like a troll.
I'm going to go sail in nice warm flat water and 15 kts of breeze now, wearing nothing but shorts. Have fun under your bridge, troll.
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Old 04-05-2013, 10:49   #22
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Re: Bad temp guage or sending unit?

Unsubscribing now, troll.
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Old 05-05-2013, 05:29   #23
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Re: Bad temp guage or sending unit?

When measuring Resistance from (Tan) Sender Wire (disconnected from Gauge) to Ground:
American Temp. Senders will read: 450 Ohms (Engine Cold @ 100F)
or 29.6 Ohms (Engine Hot @ 250F )
European Temp. Senders will read: 281 Ohms (Cold @ 40C) or 22 Ohms (Hot @ 120C)

See ➥ Basic Engine Gauge Theory and Testing
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Old 05-05-2013, 06:16   #24
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Re: Bad temp guage or sending unit?

Temp senders are usually very reliable. They have a thermistor "Pill" in the bottom and the resistance varies with the temp. There are no moving parts unlike most pressure senders.

Look for corrosion issues in the connection, wire hatness and back of the gauge.
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Old 05-05-2013, 06:38   #25
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Re: Bad temp guage or sending unit?

I'll add one more possibility. If the sender relies on being screwed into the bock or head for the ground (negative) connection and the installer decided to use Teflon tape as a sealant, this could interfere with the electrical (and thermal) connection to the engine. If a sealant is needed, pipe dope is a better choice.

Note:

Going back to the original post
Quote:
....... my temp gauge on the 89 Yanmar HM35F is showing 230 degrees at 2500 RPM. The thing is, a laser gauge shows the hottest portion of the cooling system is 175.....
The problem is that the gauge is reading higher than he believes is correct. Any corrosion in the connectors or incomplete connection to the block would raise the resistance of the circuit and cause the gauge to read lower than it should.

This points to a problem with the gauge or sender. A remote possibility is a partial short to ground in the wire between the gauge and the sender.
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Old 11-05-2013, 16:37   #26
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Re: Bad temp guage or sending unit?

So I have almost the opposite problem:

The temp indicator indicates normal under all phases of running. Push the engine hard and it slowly goes into the red like 90% of 4-108s. Ease of the throttle and the temps goes back into the green, about 182 degrees at crusie.
The sender has been changed, the gauge has been changed, the thermostat has been changed, in other words: All is normal, everything behaves as it should.

Then a surveyor climbs onboard the boat and started "shooting" the engine at various spots with his laser temp gun.

He concluded the temp gauge readings on the boat was off 20 degrees and he advised the buyer it was safe to operate this engine far into the red because the gauge was showing 20 degrees low and his laser gun was to be trusted 100.%...

Don't think so, the engine could overheat and self-destruct in no time at all.

So, where do they shoot the laser beam? On the tank? On the heat exchanger inlet, on the sender or what?

Would you thrust one of them Home Depot guns enough to run your expensive engine in the RED for one hour?

He said any marine engine should be able to run wide open throttel for 1 hour and told the buyer it was safe because the REAL temprature was 20 degress less than indicated..Scratching my head here...
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Old 11-05-2013, 18:13   #27
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Re: Bad temp guage or sending unit?

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So I have almost the opposite problem:

The temp indicator indicates normal under all phases of running. Push the engine hard and it slowly goes into the red like 90% of 4-108s. Ease of the throttle and the temps goes back into the green, about 182 degrees at crusie.
The sender has been changed, the gauge has been changed, the thermostat has been changed, in other words: All is normal, everything behaves as it should.

Then a surveyor climbs onboard the boat and started "shooting" the engine at various spots with his laser temp gun.

He concluded the temp gauge readings on the boat was off 20 degrees and he advised the buyer it was safe to operate this engine far into the red because the gauge was showing 20 degrees low and his laser gun was to be trusted 100.%...

Don't think so, the engine could overheat and self-destruct in no time at all.

So, where do they shoot the laser beam? On the tank? On the heat exchanger inlet, on the sender or what?

Would you thrust one of them Home Depot guns enough to run your expensive engine in the RED for one hour?

He said any marine engine should be able to run wide open throttel for 1 hour and told the buyer it was safe because the REAL temprature was 20 degress less than indicated..Scratching my head here...
Your engine is overheating under load. There are many possibilities including bad impeller, partially obstructed water intake, partially opbstructed or scaled heat exchanger and others. Find the problem and fix it. Hire a good mechanic if necessary.

Don't listen to the surveyor.
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Old 12-05-2013, 03:58   #28
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Re: Bad temp guage or sending unit?

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Find the problem and fix it
Easy to say: It needs a larger heat exchanger and there is no room for one.

My point with the above post was to question the accuracy of one of them Laser Temp Guns, not to trouble shoot the engine as I already knows it slowly over-heats under full throttle: The danger here is the surveyor saying the indicated temps on the panel is wrong the to ignore them..
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Old 12-05-2013, 05:48   #29
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Re: Bad temp guage or sending unit?

CSY Man, the infrared thermometers are very accurate. One item to note about them though is the spread of their beam. I lent mine to a dockmate diagnosing an overheating issue and he brought it back saying it didn't work very well. He was shooting from three feet away and assuming the temperature reading was only from the red dot...

Simple solution here: keep the CSY.
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Old 12-05-2013, 06:07   #30
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Re: Bad temp guage or sending unit?

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Easy to say: It needs a larger heat exchanger and there is no room for one.
If this is something you cobbled together yourself, that's quite possible. If it's a production boat that didn't overheat from day one, something has happened over the years or all of a sudden that causes it to overheat. Find what it is, correct it, and the froblem is fixed
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