Temperature Gauge Works Most of the Time

Hi Everyone, 

This temperature gauge might take the cake as the saying goes.  My Mach 1 has the original  wiring and gauges.  I had them rebuilt professionally about 5-6 years ago.  Engine runs cool and spot checking with temperature gun it appears to be right on spot.  I replaced the instrument voltage regulator about a year ago.

This weekend I drove it up into the Rockies to a car show about 3.5 hours one way.  Car ran great and no problems to include going over the 12,000 foot mountain pass.  The temperature gauge needle was barely past the first slash mark.  After 3 hour of driving the needle slowly moved to the off position.  It would then move about and 1/8th inch toward the first slash and slowly go back to the off position.  Elevation was 5400 feet at my house to 7400 feet at car show.  The other 2 gauges worked just fine the entire trip.

I was there from Thursday until Monday.  The whole time the temperature gauge would barely get to the first slash for 5 minutes or so and then over back to the off position.  It would do this numerous times while driving from 10 - 30 minutes.

So on the way home the needle does the same movements and mostly just sat in the off position.  The weather was 85 - 95 degrees.  About 1/2 mile from my house the needle just passed the first slash mark where it is normally  when driving and stayed there. Again the other 2 gauges worked just fine and the car ran great.  The engine temperature in various locations when spot checking with the laser gun were normal.

So the question is what would cause this temperature needle movement?    Thanks Mark

A bad sending unit, loose connection, or sending unit not making a good ground.

A bad sending unit, loose connection, or sending unit not making a good ground.

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Thanks Don.  I will check my connections and ground.  If good then the sending unit is the problem.  I put a new one in about a year or so.

If you used Teflon tape when you installed the new sending unit, it may not be getting a good ground to the block.

Try this test. Take the wire off the sending unit and ground it. Then turn the key on and see if the gauge goes to full scale.

Try this test. Take the wire off the sending unit and ground it. Then turn the key on and see if the gauge goes to full scale.

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Don't leave the sending unit wire grounded for very long, as it will burn up the gauge.

Hey everyone,  sorry for the slow update.  Life keeps getting in the way of my Mustang time.  It appears to be the temperature sending unit.  I purchased a Motorcraft brand about a year ago.  I purchased the best NAPA has and so far so good.  I talked to the company that re built my gauges about 3 years ago and they said they have seen a very high number of sending unit failures.  Thanks Mark

No matter the brand name, all OEM style aftermarket sending units do not meet stock specs and read all over the place. The Auto Meter senders seem to be accurate and reliable. Chuck