Topic: gauges

[Sopcich04]

how do you test gauges on your cars? i had a 195 stat and the gauge read 220 so i bought a 180 stat and now im running 160.....? my alt and battery is running perfectly but im running low voltage on the gauge. also when i turn my dash dimmer my oil pressure and temp change. haha im so lost plz help!

[ta78w72]

I wrote a paper on how to bench test the temp and oil pressure gauges.  You need a variable resistor.  I think I used two.  A 100 Ohm and a 500 Ohm.  The variable resistor takes the place of the sender.  The bench testing process will validate whether or not your gauge is resgistering accurately or not.

If you want to test the gauges in the car, for the temp gauge, remove the wire to the sender.  The gauge should drop to zero (with the key on).  Then ground the sender wire to the block.  With the key on, the gauge should read way over to the right.  Same process for the oil pressure gauge, expect ground and unground positions on the gauge are reversed.

If you don't get those readings, then you'll need to pull the cluster and check the back circuit for breaks.  Usually problems are circuit related rather than a bad gauge.  The oil and temp gauges share the same 12 volt circuit.  The oil pressure gauge is first in the circuit, then the temp gauge.  So, it's possible to have a break in the circuit between the two gauges, whereby the oil pressure gauge works but the temp gauge doesn't (this happened to me).  If that's the case, you can run a jumper between the two gauges because there are nuts at the place where 12 volt enters both gauges.

You may have a grounding issue if you have problems that appear over various gauges.  The brake lights share 12 volt with the oil and temp gauge, and the cluster lights share ground with the gauges.

[Joker (§ir£Ğragon)]

To check the accuacy of the temp gauge, get a good thermometer that will read above 220 degrees. Remove your temp sender and wrap a wire around the base. Connect the other end of the wire to a good ground and connect the temp sender wire as normal. Boil some water and then drop the sender into it. Then you can compare the temp of the water using the thermometer with what your gauge reads. Leave it in there while the water cools and you can see if the comparison varies.

[ta78w72]

Wouldn't it be easier to put a thermometer into the radiator and compare the reading to the gauge?  But if you get a different reading from the gauge to the thermometer, you still don't know if the sender is bad or the gauge is bad.  The only information you get is that the readings are different.  That's why you substitute the sender for a variable resistor.  Then adjust the resistor to a known ohm value (those values are in the service manual) and verify that the guage is indicating the desired temperature at the proper ohms. 

However, in this case, since there are other things being affected, I believe the problem is electrical.  So, the cluster should be pulled and checked.  In fact, I think he has a ground issue or a circuit which is crossing another circuit (like the ground circuit).  At least that's where I would start, with a visual of the circuit on the back of the cluster.  Pin 5 powers the oil and temp gauges and pins 2 and 9 provide ground to the cluster.  I'm quoting for a 1978, but this should be similiar for other years.

[Eagle 1]

Russ, the water in the radiator would should be cooler than whats in the engine.
I agree to check the circuit board.  As you recall mine and I think yours too had a faulty circuit board.  Seems to be a common problem.