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1500 Operating Temperature


Freshman Member
I have a ‘78 Midget 1500 with a rebuilt engine (less than 1000mi) and my temp guage tends to indicate overheat after 40min or so of driving. I’ve flushed the system, replaced the thermostat & sender unit, checked for manifold & gasket leaks and have no indication there’s anything wrong there. When the temp needle gets to just the edge of the red zone on the guage the temp at the thermostat housing (measured by laser thermometer) ranges between about 165-175 degrees and the block between 175-180. I’m tempted to believe the guage is boggled. The car has never boiled over or pushed coolant into the overflow tank.

Does anyone know what the operating range is for temperature? I’m planning on driving the car locally and keeping an eye on things but would like to know how hot is “too hot”.




There is no single answer regarding the operating temperature. However, when the needle is "centered" on 90 degree sweep Smiths temperature gauge it is typically around 190 oF (roughly 90 oC).

If you painted the t-stat housing black or put a square of electrical tape on it as a target and got a laser measurement of 175-180oF, AND if your car has a 180 oF t-stat installed, trust your measurements and suspect the gauge.

There are several possible fixes:

  1. Buy a new gauge and matching sender.
  2. Send your existing gauge off for calibration. While it is away, you can install an inexpensive gauge from your local parts store.
  3. Give up on Smiths and just use an aftermarket gauge and matching sender (like a decent VDO).
  4. Recalibrate the Smiths gauge yourself on the bench. (Look for online sources (including the link below) that discuss calibration)
  5. It could be a bad or incorrect temperature sending unit. On other British cars the sender changed a few times during production. Do you have a matched sender and gauge?
  6. Buy the "Gauge Wizard" from Spiyda Design in the U.K. and use it to tweak the match between your gauge and your measurements. or...
  7. Just map out with the existing gauge what the temperature really is (what is boiling, what is "normal") and just drive your car.

DIY Calibration: (See the link below AND realize you will need to put your sending unit in a pot of water which you bring to boiling and then slowly allow to cool as you monitor and record the sending unit's resistance curve. Do NOT use the values in the link below as they are for an MGB).

Spiyda Design Gauge Wizard: https://www.spiyda.com/fuel-gauge-wizard-mk3.html
The gauge wizard will let you match your gauge and sending unit without altering the gauge... but you will need to create the temperature/resistance curve as I mentioned above in DIY Calibration.
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