how to test a temperature sensor on 71 mustang to make sure it lights up?

[71mustang]

it has a temp light no temp gauge, I replaced the bulbs with LED ones a few weeks ago but even before that the lights wouldn't light up when key is turn to start and they don't light up now with new led ones. the only one i can see light up is the alternator but that only lights up for a second when i turn the key to off. everything was hook up right in the gauge cluster but i do have to get a new instrument cluster circuit board as the left blinker bulb area was corroded and doesn't work.

I just want to make sure it works as i have never seen the light in the gauge go on, how many ohm's should i see when cold and when hot? and would their be any volts from the wire that goes to the sensor when the key is on?

 

[c9zx]

The "HOT" light should come on at 245 degrees. It usually means "fix engine now". It is just a bi-metal heat sensitive switch. It is open (infinite resistance) when cold and closed (zero resistance)when HOT. When closed it provides a ground to the light. The light should light when the key is in the START position, proving the circuit is good. With the key in the run position ground the wire attached to the temp switch. The HOT light should come on. From the wire to ground you should see 12 VDC IF the bulb and circuit are good.

 

[Bill73Ragtop]

We sometimes called those "time for a new engine" indicators when the temp got that high or oil pressure dropped to the point the light was really bright!

 

[c9zx]

"Too late lights" serve no real function other than to point out you neglected your engine to the point of catastrophic failure. It seems like someone's idea of dark humor. Chuck

 

[midlife]

The "HOT" light should come on at 245 degrees. It usually means "fix engine now". It is just a bi-metal heat sensitive switch. It is open (infinite resistance) when cold and closed (zero resistance)when HOT. When closed it provides a ground to the light. The light should light when the key is in the START position, proving the circuit is good. With the key in the run position ground the wire attached to the temp switch. The HOT light should come on. From the wire to ground you should see 12 VDC IF the bulb and circuit are good.

+1. Spot on.

 

[Tnfastbk]

We sometimes called those "time for a new engine" indicators when the temp got that high or oil pressure dropped to the point the light was really bright!

LOL you folks up North just coat it a little different than we do in the South. Around here they are referred to as "Idiot Lights".

 

[Don65Stang]

it has a temp light no temp gauge, I replaced the bulbs with LED ones a few weeks ago but even before that the lights wouldn't light up when key is turn to start and they don't light up now with new led ones. the only one i can see light up is the alternator but that only lights up for a second when i turn the key to off. everything was hook up right in the gauge cluster but i do have to get a new instrument cluster circuit board as the left blinker bulb area was corroded and doesn't work.

I just want to make sure it works as i have never seen the light in the gauge go on, how many ohm's should i see when cold and when hot? and would their be any volts from the wire that goes to the sensor when the key is on?

Welcome 71mustang!

Post an intro thread so we all can give you a proper welcome.