Low voltage at Temp Gauge

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zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Low voltage at Temp Gauge

Post by zman18 »

I'm trying to diagnose why I'm seeing high temp reading on my gauge but engine temps when checked with a laser temp gauge are normal. So far all I can find is that the voltage at the gauge is 10.2 instead of 12. Could this be the reason for the incorrect temp reading? Also any thoughts as to what could cause the low voltage?
Thanks
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: Low voltage at Temp Gauge

Post by wolfe10 »

John,

You are going to have to trace back to determine where voltage becomes unusable.

Could be bad battery
Could be bad alternator
Could be bad wiring connection (most likely)

Be sure to check the ground side as well.

Only your digital voltmeter knows for sure!
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
SeaBreeze
Posts: 91
Joined: Wed Aug 05, 2015 8:15 pm

Re: Low voltage at Temp Gauge

Post by SeaBreeze »

John
Catalina Direct has a troubleshooting guide to engine instruments. From their home page search “temperature gauge”. Then look for the link to “Engine instrument technical information”. You’re on the right track. Temperature sender changes resistance wih temperature changes. Inversely. So voltage at gauge changes with resistance change.
Rick
Sea Breeze
Rick Parish
Sea Breeze
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: Low voltage at Temp Gauge

Post by zman18 »

This has been an "interesting" problem. This all started when the temp gauge started showing high temps.
I did the following:
Checked both the raw water and coolant side of the engine for any blockages/issues - everything looked good
Replaced the impeller
Replaced temp sender unit and thermostat
Checked the new sender unit for ground - showed a good ground
Did the checks on the gauge to see if it was working - checks came out ok
Disconnected the wire at the sender and back of gauge and ran a temporary wire - gauge showed same overheating issues
Used a laser temp gauge to see what temps were actually on the engine when the temp gauge showed overheating - all temps showing in normal ranges

At this point I think I will replace the temp gauge even though it tested ok. It's about the only think left that I can think to do.
John Zoda
Compass Rose
Hull# 484
wolfe10
Posts: 587
Joined: Mon Dec 11, 2006 2:58 pm
Location: Ft Myers Florida

Re: Low voltage at Temp Gauge

Post by wolfe10 »

John,

Have you identified the source of your low voltage reading? I would sure do that first.

Said another way, I would want voltage reading at the gauge to be within a couple of tenths of a volt of reading at the battery.
Brett Wolfe
C350 #180
"Vindaloo"
Ft Myers FL
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: Low voltage at Temp Gauge

Post by zman18 »

Actually I did find the voltage issue. It was my error I took the reading from the sender wire to ground when I took the reading from the correct location I got my 12 volts. So still at this point the gauge seems to be the only thing left that is causing this problem.
John Zoda
Compass Rose
Hull# 484
chesworth
Posts: 11
Joined: Sun Feb 06, 2011 3:11 pm

Re: Low voltage at Temp Gauge

Post by chesworth »

John,
Did you finally solve problem? I have same issue. Temp gauge goes to 240 when turning on ignition and stays there. Did gauge test as per universal manual. All OK. Ran new wire from sender to gauge, no difference. Where did you check voltage?
Cheers
Alan
Taliesin
341
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: Low voltage at Temp Gauge

Post by zman18 »

Alan
Finally determined it was the gauge that was bad even though I did test it and it indicated it was working correctly. Replaced the gauge for around $50 and have not had an issue since it was replaced. Hope this helps.
John Zoda
Compass Rose
Hull# 484
User avatar
scott.monroe
Posts: 33
Joined: Fri Mar 15, 2019 5:13 am

Re: Low voltage at Temp Gauge

Post by scott.monroe »

Hi Alan,

Your symptoms sound like it could be from either a bad sender, a shorted sender wire or bad gauge. To test this you will need to test resistance not voltage and need a fair ohm meter. To test everything remove the sender wire from the gauge (S terminal). While the sender wire is removed turn on your ignition (sending power to the gauge). If you get 240 again, you have a bad gauge and replacing it should fix the problem. If the gauge reads 0 temp (not pegged to 240) then you can test the gauge by taking a wire and connecting the S on the gauge to the ground wire (NOT HOT!), ignition needs to be on. You should get the gauge to peg to max. To check the sender/ sender wire measure the resistance of the sender and sender wire with an ohm meter of the disconnected sender to a ground (negative wire on gauge or engine block) (but make sure the sender wire is not connected to the gauge, as it will read resistance through the gauge and give false readings). A cold engine should have a resistance > 200 ohms, normal temp range engine should be in the 100 ohms or less range. If you get near zero resistance values you have a short to ground either through the wire or sender. To rule out the sender wire, pull the wire off the sender and measure the resistance between the connector on the sender and the engine block. If you have less than 1 ohm you have a bad sender.

Hope this helps,
Scott
Scott Monroe
C350 #409 / Southern Yankee
North Kingstown, RI
chesworth
Posts: 11
Joined: Sun Feb 06, 2011 3:11 pm

Re: Low voltage at Temp Gauge

Post by chesworth »

Scott,
Thanks for your suggestions. I have done the suggested tests eliminating the gauge and a fault in the wiring. I have not done the resistance test however, I have assumed it is a sender problem and have order a new one. I'm hoping it will arrive by courier tomorrow, get it installed and solve problem. Thanks again for helpful advise.
Cheers
Alan
Taliesin 341
zman18
Posts: 60
Joined: Mon Sep 30, 2013 11:37 am

Re: Low voltage at Temp Gauge

Post by zman18 »

Alan
When troubleshooting my issue I also replaced the sender and thermostat but still showed the overheating at the gauge. If changing the sender doesn't work take a good look at the gauge. Even though my gauge tested out as being OK replacing it solved the problem. I have to admit the gauge was the last thing I thought was wrong. If you havn't already use a laser temperature sensor to look at temp reading throughout the engine. Doing this and not finding any temps that were high helped lead me to determining it was a gauge issue.
John Zoda
Compass Rose
Hull# 484
User avatar
TBOT422
Posts: 424
Joined: Sun Jun 26, 2011 5:36 pm
Location: Clearwater, FL

Re: Low voltage at Temp Gauge

Post by TBOT422 »

The gauge could be faulty even if it passes the Catalina continuity tests. First, you need to understand how the gauge works.

Electric Temperature Gauges

Basically, an electric temperature gauge is a voltmeter. The scale on the gauge face is reading temperature but the instrument itself is reading voltage. The gauge itself is comprised of a bimetallic (two different metals fastened together) "hairpin" assembly. This assembly is attached to the needle.

The gauge requires an electric circuit and a sending unit in order to read temperature. The sending unit is a temperature-sensitive material that is part of a variable resistance, water-sealed unit that sits in the coolant stream in the engine. As the engine warms up the resistance in the sending unit is lowered gradually until the system reaches maximum heat. The sending unit is the "ground" portion of the circuit.

In the completed circuit the battery voltage passes from one side of the gauge, through the bimetallic spring and onward to the sending unit, which is grounded to the engine. When the engine is cold the resistance is high, so little current passes through the gauge. This small current doesn't heat up the bimetallic spring, so the gauge reads a low temperature. As the engine warms and the sending unit's resistance lowers more current passes through the gauge and the needle reads higher and higher because the bimetallic spring expands further.

Electric gauges can fail to read accurately because the sending units fatigue or rust over, or simply lose their connection to ground. The bimetallic spring can also fatigue over time, rendering the gauge inaccurate or inoperable.

A theory on how the gauge could be bad and still pass the tests:

As the engine warms and the sending unit's resistance lowers more current passes through the gauge and the needle reads higher and higher because the bimetallic spring in the gauge expands further.

My theory is this. As the engine warms more current passes through the gauge. If there is a problem with the gauge that instead of providing a uniform expansion from the heat of the increased current, it instead distorts abnormally at higher current, it could 'peg' sooner than it should. Since it still has continuity, it would pass all the Catalina tests, or reading 0 with no ground and full scale when shorted to ground.
Gary & Janet
The Best of Times (Hull #422)
jbd3
Posts: 60
Joined: Sat Nov 22, 2008 6:44 pm

Re: Low voltage at Temp Gauge

Post by jbd3 »

After a lightning strike my temp guage reads 20 degrees cold. New sender made no difference. I was surprised there is no calibration. waiting for new guage as we speak. hull 325
chesworth
Posts: 11
Joined: Sun Feb 06, 2011 3:11 pm

Re: Low voltage at Temp Gauge

Post by chesworth »

It was the gauge!!!!!
After replacing the temperature sender, cleaning all the terminals, testing wiring continuity, and following the Universal test protocol for checking gauge (OK), I resorted to replacing the gauge. Eureka everything works. Now we can leave on our cruise to the North Channel of the Great Lakes.
Thanks everybody for your advise, a couple led me to replacing gauge.
Great Sailing,
Alan Chesworth
Taliesin #341
RFBaker167
Posts: 10
Joined: Wed Aug 03, 2016 12:52 pm

Re: Low voltage at Temp Gauge

Post by RFBaker167 »

Same issue on our Hull 167. Even though gauge passed initial tests, it was indeed the gauge. I also replaced the terminal block in the pedestal that had considerable corrosion. Probably the worst electrical compost I’ve seen on any boat, and far below standard.
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