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Reading OEM Instrument Temperature Gauge


72-CLASSIC_RIDE

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Sometime ago I posed a question on correctly reading OEM engine temp gauge but no definitive answer was given. :no  I have noticed my temp gauge goes to a certain hash mark and regardless of outside temp (hot or cold), load on engine (prolonged high RPM's, pulling hills, this gauge seems to stay constant regardless of conditions once it achieves this hash mark reading!  So I start asking myself, what are those temps where hash marks are shown on temp gauge? :confused:   From 100 to 210 degrees reading, if I calculate the 4 divisions (hash marks) getting from 100 to 210 degrees, that would be 110 degrees divided by 4 which would incrementally be 27.5 degrees added to each of the 4 hash marks from 100 to 210 degrees.  Are you with me? :/   Using logic I would assume those calculations would accurately identify engine temp at those 4 intervals.  

Using the 100 reading as a starting point the 1st hash mark would be 127.5 degrees, 2nd being 155 degrees, 3rd hash mark being 182.5 leading up to the final addition of 27.5 making reading 210 degrees.

But from 210 to 250 it dispels using any logic to calculate temperature as the 27.5 number no longer applies because between 210 and 250, there is only a 40 degree temperature gap and still 4 hash marks to calculate (leaving the number 10 divisible vs. 27.5)!

So, can I assume the OEM temperature sender is calibrated to compensate for a shorter temperature variation at higher engine temp?  Bloody unlikely!! :->

 

So, can anyone explain what each hash mark reading was calculated to be as a number vs. the hash mark? :grin:

 

Here is a pic of my engine temp gauge with a arrow where my engine temp is running at 195 +/- 5 degrees. Again, logic would seem to apply here also.  The arrow shows at 2nd hash mark and appears reading lower than actual 195 +/- 5 degrees (based on my previous assumptions) of 155 degrees.  I'm getting very suspicious my temp gauge is not giving me accurate readings.  (Used a infrared to measure engine temp across and around thermostat housing area) so am pretty sure this disparity is real.  Looks like I cant rely on temp gauge at all!  Will have to try another temp sender (one designed to work with our cars) vs. new technology senders and see if that does not compare accurately with a infrared.

So I believe I found a problem with the sender but still would like to know what a number value is for each hash mark (if known)! 

 

Just today replaced the Thermostat Housing (leaking around gasket), thermostat housing gasket and 195 degree thermostat.  Has been a thorn in my side for some time as have not been able to prevent water leaks.  But over time I think it has finally narrowed down at the thermostat housing and if correct, I have finally stopped that pesky leaking.  This problem is what made me question the temperature gauge and seek some guidance on what value those hash marks have. 

 

Thanx for clearing this up for me (if possible)..........................

 

Doug

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As an old timer once said about factory gauges....Don't read to much into them. He's right. Even when I was working at the Chevy dealer we had bulletins on the accuracy of gauges. That is why most of them now don't have an actual temperature, just a range. 

 

With using the infrared temp you know where it should be on your gauge. Use that as your baseline. 

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Thanks for the clarification Scott.   Until otherwise discovered, I will make that 2nd hash mark my engine operating temp mark and wait for it to go beyond that if ever.  What concerns me is the needle does not vary once it gets to that mark.  With the old radiator, that needle moved from operating temp beyond 210 degrees reading on gauge.  Is my replacement radiator so efficient under all previous road/climate conditions that she holds that 195 degree sweet spot?  I will likely need to carry a infrared gun with me and intermittently read engine temp at different operating intervals to understand those characteristics.  I guess my suspicion leads to a caution that it is possible to over temp engine without any visual warning other than boiling over which in many cases can be to late.

 

Is there a gauge/sender that provides better accuracy?  I would consider a look alike gauge that takes the place of current OEM factory gauge but incrementally displays actual temp numbers on gauge dial if one is available.

 

Thanks again Scott.  Your explanation changed my whole mind set on how to deal with this issue.  :yay: 

Doug

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I would think that replacing the sending unit in the block would be a fairly inexpensive test, plus it would give you more information on the accuracy of the gauge. Just my thought process...........

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If your temperature stays within 10 degrees of thermostat your system is in great shape, I used an temp probe to measure mine at operating temp & put in a resistor in line at sender to have it read correctly resistor at radio shack cheap. It does vary at time of heavy traffic etc, but not a fast change.

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I had considered that Murphy.  I did replace temp sender at install of 402 CI motor less than 6 years ago.  It was at this early stage I discovered my old 350 CI OEM radiator was not capable of cooling that BB engine.  Specifically at lower engine RPM, engine temp would rise and did reflect it on gauge reading.  Not necessarily accurate but did indicate there was a problem.  Radiator needed recored at minimum so bought a aluminum radiator that to this day goes to that specific point on temp gauge and does not deviate.   What is clear is this temp sender (prior to replacing radiator) did adjust to those higher temperatures.  I will carry a infrared temp reader with me and check engine temps at intervals and extremes to get a better understanding of sender/gauge accuracy.  Maybe my aluminum radiator is that efficient to control temp at that 195 degree range.  Just want to be sure............

 

Doug

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I will watch more closely but cruising Woodward bumper to bumper my 350 will go one hash mark over the 210 easy. When it hits the 2nd mark past 210 I try to put it into N and rev it a bit. Once traffic starts moving, going North past the city, it comes down to one mark below 210. I don't remember what thermostat I even have in it but I think it is a 180. I replaced my original 2 for a 3 core 20+ years ago. Still have that 2 core in the basement. I did change to a 5 blade Camaro Flex Fan back in 1972. Didn't like the noise from the 4 blade and thought it looked cool. (no pun intended)

So far it never has spit out any coolant out the overflow so I'm happy.

Bruce

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  • 1 year later...

If your temperature stays within 10 degrees of thermostat your system is in great shape, I used an temp probe to measure mine at operating temp & put in a resistor in line at sender to have it read correctly resistor at radio shack cheap. It does vary at time of heavy traffic etc, but not a fast change.

Hi lasts. I am having the same issue. I installed a new 3 core Brass and copper radiator, A new fan clutch, new radiator hoses,180 Thermostat, New sending unit and my temp gauge is still reading 210+ We checked with a infrared thermometer and it shows 180. What resistor do I need and does it go on the gauge? My engine is a 350 that has been rebuilt with auto trans. I also noticed this engine has 2 sending units. 1 on each head. We unplugged the passenger side and it didn't change the gauge. Later found it was for TCS for emmisions.

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Mine also bounces around the 210 mark when running in traffic. Rarely runs above that and I also am running a 180 stat. I kinda gave up on the accuracy thing. As long as I am not bouncing around The 250 mark I am happy.

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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

The White 402 4spd I drove last time to the Western meet doug when she was up here 40 degrees outside she ran 230 all the time sometimes even up to 235.

now the 454 in the 71ss always runs a 210 no matter what temperature is outside and has yet to overheat.

 

Darren

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I checked and I already have a blue shunt resistor on the gauge. I cleaned the contacts and put it back on and it reads slightly lower than 210. I checked the resistance on it and it was 92.

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​I have tried a lot of different things to get mine running cooler. What worked for me was a 20 deg. cooler thermostat. I had run a 180 deg. with my set-up. 355 sbc, lopey cam, 3 core radiator, 7 blade flex fan that fits the whole shroud (within 1/8" of clearance), transmission cooler running thru rad. ,then to a auxiliary  trans cooler. It would run 200-210 all day long (not good) .  I then changed to a 160 deg. thermo stat. Now it runs at 180 all day in traffic and on the open road. cruising all day and up those Pennsylvania hills too! .Once it did get too 200 but came right back down in the open air. I have had the set-up since 2010, no problems since. that's my 2 cents

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I run a 180 stat on a 10:1 454 with 3.73 gears, I also run electric fans only

 

it runs down the road at 170 (auto meter gauge mechanical) without the fans even turned on

 

now when I stop the temp rises and the fans kick on at 210, start moving and the temps start to drop and the fans turn off at 185, even in the summer heat it runs 180 while moving no fans

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  • 4 weeks later...

I ran my Monte in 95 degree heat and it ran at 190 degrees all day In traffic. I am running a 180 stat and a non clutch fan. I am still thinking of adding the clutch to help with efficiency.

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"I think the gauge is actually linear even though the numbers make no sense linearly."

 

I so agree! I thought it was just today's cars but looks like I am wrong.

 

My air pressure gauge sure isn't like that.

Bruce

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the numbers on the guage absolutely do not reflect a linear movement-as was shown the first half of the guage spans 110 deg   with the 2nd half only 40!!  yikes! another old school GM engineering mystery... if you had a real  desire to know(And given your fellow members never ending  desire for information,yours truly included, we would welcome the exercise) you could sink the sender in several vessels of known temperature and measure the sender resistance-a plot of resistance vs temperature might be very revealing-easier would be  to just get a fully warmed up coolant reading with a lab grade thermometer and note the guage position-this would be the "relative normal"  and would allow to see if you were running hotter or cooler than "normal"-I dont know if my cheap aftermarket guage is accurate just know were it should be and is 95%  of the time-relatively speaking--this is interesting and forgive me if I,m rambling-just thinking out loud...

    As far as running cooler,I recently replaced an old stock style water pump  with what looks like the same-I dont think the new one has a higher flow rate but not sure-also changed the 10 year old 195 stat with a 180 stat and drilled the small hole in it-maybe the stat was the trick,cause now the guage reads cooler under low load and does not rise anywhere near as much as it used to under heavy load...a step in the right direction to be sure!!    Brian

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