Water Temperature, Graphics vs.Numbers

I tired of not knowing what temperature “N” or “R” really represented on my temperature gauge. If the needle goes to “M” or “A” am AI in danger of frying the engine? Who knows? I didn’t, so I performed a little experiment this afternoon.

I drained 1 gallon of coolant from the radiator. That put the coolant level below the temperature sending unit. Next I removed the sending unit and set up a test rig as shown in the below picture. I used a camp stove to heat the water. The thermometer is an internal probe cooking thermometer of known accuracy. I ran wires to the stock wiring harness and grounding point. Disconnected the coil and turned the ignition to run

I heated the water as high as it would go, 211 degrees F, and noted the position of the gauge needle. I shut the stove off and noted the position of the needle at various temperatures as the water cooled. I took pictures about every 8-10 degrees. For example.

I also made a picture of what I was seeing.

It’s nice to know that when the temperature gauge needle goes to 'ell the coolant temp is only 210 degrees F.

I’d bet these readings are probably close for any car with a properly working temperature gauge and sending unit.


Nice work. I’ve wondered this myself.

Yes - a good thing to know for sure. I am not as confident that there is not a lot of variation among the gauges and senders in use.

I arrived at BritCar coffee the other day and someone ask me how I was. I told them “I have 2 gauges on the dash that say I’m normal - can that possibly be correct?”. They agreed it was doubtful.


What a good idea
Thanks for sharing it and the results
Are there any numbers or par number on your sender?

Thanks, it was fun to do. There were numbers on one of the hex flats but I didn’t write them down. Il look and see if I can read them.

After I de-lumped an S2 OTS I was a bit concerned about the apparent inaccuracy of the water gauge (reading low as I recall). It was just a hunch at first, but a new sender read closer to thermometer temp. It occurred to me that bent sellers might save low-reading parts for fitting to hot-running sales cars.


The gauge movement is absolutely identical to the earlier gauge with the numbers, so you can simply divide the range into quarters, and you’ll have:

Bottom of scale = 30C
1/4 = 50C
Mid-point - 70C
3/4 = 90C
Top of scale = 110C

“Normal” operating range for a healthy engine is anywhere from a bit under 70C (in cold weather), to something over 100C (in VERY hot weather and/or very heavy load).

Ray L.

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On one of the flats:

TT 4201 /

That probably means something to someone.

Like when I drove your car, way back in 2005?

Driving around town today, stop and go traffic, normal small town stuff, with the ambient temperature reaching 98 F I was running a maximum of between 185 F and 192F. No guessing or interpolation involved.


John. Very good to know. Thanks for doing this. When I first got mine it would creep up toward the red on 90 degree days. After getting radiator, water pump etc freshened up and especially Coolcat fans it now hangs out around M and A so I guess I am OK. As a Vega owner I am always concerned about engine heat especially since both Vega and E have different head/block materials. I think my good luck and longevity with my Vegas is helped most by dramatically upgrading the original inadequate radiators and fans so that I am always running between 170-190 in Houston heat and traffic.

I would love to get the E a little cooler even though it sees little action from June-Sept since it is a non-AC car. I have thought about adding a 3rd fan in front of the radiator. Any thoughts if this might be worthwhile ? I do know I might have a mental disorder about cool running engines.

68 E-type FHC

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You really do have a suspicious mind.

Then there is this option:

An in-line rheostat for the sender wire. What temperature would you like it to have?

Given the number of threads on overheating and questions about wonky gauges I’d guess you aren’t alone. In answer to your question, I don’t know. You’d be giving up some square inches of radiator blocked by the fan for the gain of increased airflow. The stock puller fans only suck through the top 60% or so of the radiator. I’ve often wondered if there was any gains to be made if you could get air sucked through the entire radiator.

I haven’t looked at Mike Frank’s CoolCat site in a while. I wonder if he experimented with supplemental pusher fans in front of the radiator.

Concerning possible hyperventilating, about “overheating”…

  1. it’s an overheat ONLY when the system boils over;
  2. any decent water-cooled engine, at the most stressfull time (traffic, hard into the loud pedal), that maintains right around ~200F, will be JUST fine.

Or something melts. :rofl:

If mine is running too cool I can always change the temperature at which the “otter switch” turns on the fans.

While on this subject take a close look at a temperature gauge or the drawing I made above. There is a short line below the “N”. It’s maybe an 1/8th of an inch long on the gauge. Why is that there? I had a thought that it might indicate the area in which the thermostat started to open.

I added a third fan as a backup with the CoolCat fans. It does nothing that I can tell other than maybe improving A/C performance. It is mounted on mesh in front and low as possible.



That, and other marks, are used in calibrating the gauges at the factory.

Ray L.

Thanks Ray. So like the dot below the 0 MPH peg on the speedometer.


John, this is good info for future reference. Thanks for taking the time to sit and wait for the water to cool off. I know it takes a while! :wink: