temp gauge/send unit help-electric gurus!

[Macdon221]

Good Day all,

First I have read everything that I can find on this I think...

My Temp gauge in my 68 charger always reads low. Between the 120 and 170 mark.

At operating temp with an infrared temp gun on the pump, housing, temp sending unit area etc...temp consistently reads 170-190 degrees.

Voltage limiter test checks out fine.

The gauge when grounded slowly goes to full. Careful to stop before it gets all the way.

I have read the resistance on the gauge display should be...

Hot 10 ohms
Med 23 ohms
Cold 74 ohms

I put resistors in line and grounded the sending unit wire.(10.6 ohms each). I get with 1 resistor (10.6 ohms) the needle pointed exactly at the 230 degree mark. With 2 resistors in line (21.2 ohms) I get the needle exactly on the 170 degree mark. Multiple checks, multiple grounds.

Does the above reading sound about right?

Now going across the sender when cold....I get over 400 ohms. I have not done it operating hot yet.

If I look at specs for a few temp sending units online with the correct part number...I see the specs are.....

0 ohms at 220 degrees
24.7 ohms at 100 degrees
152 ohms at 0 degrees

Does this make sense?

The readings I am getting with resistors inline seems to make perfect sense for cold to hot. If that is the case...shouldn't the resistance of the temp sending unit specs kind of match?

Is there a temp sending unit (make model manufacturer) that is the perfect match to the gauge if the gauge is functioning correctly?

Any help appreciated?

 

[70chall440]

I think your readings are good, but I also think the gauges are pretty inaccurate to be honest. I don't know that the answer is overall. I know this isn't helpful...

 

[493 Mike]

You may have to have the gauge recalibrated. I recently had my 65 Coronet temp gauge recalibrated as it wouldn't move far enough to make it into the operating range.
Mike

 

[RJRENTON]

IMO...
Have you tested by substitution, the temperature sending unit? It could be tested in hot water and its resistance checked as the sending unit was heated. Bear in mind that the sending unit is negative temperature coefficient...resistance goes down as the device is heated. I guess you would need to sample several different sensors to arrive at an average resistance and then use a series resistor in the gauge line to get close to the indicated readings. This resistor would need to be a 1% glass filled precision resistor. Person #3 suggested getting the gauge calibrated. How? Mopars instruments are thermal devices which use the 5 volt output from the IP voltage regulator combined with an internal heating element to cause the pointer to deflect in response to the resistance of the sensing unit. After many years of being heated/cooled, I'm sure the accuracy of the instrument is comprised.
Personally, these Rally instrument panel gauges are superficial at best. For my GTX, I use an under dash mechanical temperature gauge and oil pressure gauge (AutoMeter). On start up, the mechanical pressure gauge responses immediately while the Mopar gauge takes 30-40 seconds to move. Likewise, the coolant temperature gauge is really a pressure gauge calibrated to read temperature in response to the expanding gas pressure due to heating of the coolant and the immersion of the resevour. Again, the dash temp gauge is very slow to respond to the temperature variations, compared to the mechanical instrument. I also use the RTE solid state IP regulator to provide the 5 volts....its just Mopar instruments that are an antiquated design. Now if someone could come up with a cost-effective direct replacement for the Mopar gauge based on an electronic design (Perhaps when it has been unequivocally determined that Hell has frozen over).....
Just my opinion of course.
BOB RENTON