OK, I got the sensor today, replaced it and STILL THE GAUGE does NOT work... so the original sensor was not defective....I checked the wiring...got a reading of 13 volts at both the sensor connector and at the gauge connector....does that mean the GAUGE has gone south?

 

it's a pretty simple circuit. 12v+ to the sender unit and the sender units provided ground via the threads when it's screwed into the block or manifold.
since swapping out the sender unit itself made no difference you should be able to rule out a grounding issue from the sender unit.
that basically leaves only a few options left.
1. check that you are getting power on the lead wire to the sender unit. Check the sender wire connector for corrosion where it attaches to the sender unit and along it's path back to the dash cluster and sender gauage itself.
C2's had a bulkhead connection that was very prone to corrosion issues but I don't believe the early C3's had this, the wires go straight thru the firewall thru a gomment i believe but also check for breaks in the wire.

2. Check the ground on the dash sender guage itself.

If the lead wire checks out OK and the gauge ground checks out ok it's probably the gauge itself

 

Get a 12v test light, take the connector off of the sending unit, put the 12v light in place of the sender. Clip it to ground and stab the point on the light inot the connector-- key on, the light will glow- very dim-- and the gauge should read around half scale- quick and dirty test, but it works everytime. Works on oil and fuel gauges too.

 

Thank guys but will this work (not very up on the electrical stuff).....I have a Actron General purpose digital multi-tester...it has a red and black test lead wires. When I place the red and black leads into the connector wire to the gauge, I get a 12-13 volt reading (with the engine running) Does that mean I am getting good power to the gauge? I did the same with the connector to the sensor...got the same 12-13 volt reading....does that mean I am getting a good ground and the wiring is "good to go" and the gauge is history?

 

There's a sure way to find out if the gauge is bad... disconnect the wire at the sensor, turn on ignition and quickly touch the connector to ground. The gauge should go nuts (I've seen them make a complete 360). Be sure you only quickly tap the connector to ground. If you hold it to ground it may toast the gauge.

 

So, if it goes nuts, it's a bad gauge or a good gauge..I want to make sure before I buy one...

Thanks!

 

the sender makes ground..the more temp the more ground is made. Intentionally grounding the lead just by-passes the sender. If your guage pegs doing this then it is a sign that your guage is most probably OK and the replacement sender is not the correct one. The problem with using a test light or volt meter is that both are only showing voltage, not amperage or resistance.

 

NO, IF at the sender unit....I presume on the driver's cyl head....you get 12 volts at that point, you got too much tape/goop on the unit and i'ts not grounding to the cyl head, OR you got a defective sender unit....to test the meter, just ground and unground the lead at the sender....meter swings widely....you fine....not the meter....

I been playing with making a Ford truck sender work with my stock '72 vette gauge here....I have 113 aluminum cyl heads, so a smaller sending unit hole that was not remachined for larger units....the old 1/2 NPT our cars had....I forgot to ask for that....oh well....too late now....

BTW, having fair success with the changeout, but I need more time, and take the meter shunts apart....

 

you also need to test the sender unit. Put the meter on OHMs, low scale and connect the red lead to the contact on the top of the sender and the black to ground and see what you get. It should be in the 90ohm range down towards zero with the temp up.

 

So, to get a correct reading of the resistance of the sender unit, I have to get the engine to operating temp and then connect the leads like it says? I just want to make sure I am doing this right.....

BTW, I did not use any teflon tape or liquid or anything. just straight threads when I replaced the sensor.... I did notice (when I pulled the original sensor out) that the "plug" on the end of the sensor was larger than the replacement I got at Ecklers...any thoughts?

The picture on the ltop is the replacement sensor and the picture on the bottom is the one I pulled from the engine yesterday....

 

I haven't looked at a wiring schematic but if the resistance of the sending unit goes down as temperature goes up it sounds like the sending unit is wired in series with the gauge. 12 volts is feed into the gauge and is grounded by the sending unit. The sending unit does not send anything, its just a variable resistor. With the wire to the sending unit disconnected it should read to the far left (cold) and if grounded it should read to the far right. One thing to keep in the back of your mind is that the gauge is an amp gauge, based on how much current is flowing in the circut. Instead of reading of much current is flowing in the circut (which would useless), the face plate is made so it reads temperature.
Again, I have not seen the wiring schematic so this could be all wrong.

 

mandm1200: That's exactly how it works. Except if you ground the sensor wire it will read way past hot as the sensor never reaches 0-ohms. The gauge may even burn out if you ground the sensor wire too long.

 

OK, when I do that I get a dim glow and the needle on the gauge goes all the way to the right (HOT ZONE). Is that normal?

 

I never checked it that way. In fact I never had to check a sender. Basic electronics tells me that a light bulb, especially a 12v bulb, has a very low resistance. A 12 watt bulb would be 1 ohm of resistance. I'm making an educated guess and think that would not be enough to make the gauge do anything else but peg it to the right(hot).
Sixfooter said the sending unit varies from 90 ohms to 1 ohms. I don't know if that is factual or not. If it is, head out to radio shack and get a some resistors. 20 ohms resistors should get you in the ball park and resistors are dirt cheap. One resistor would be 20 ohms. Two in series (one after the other) would be 40 ohms, and three in lenght will be 60 ohms, etc. I would think if the gauge was working, it should not peg to the right. As you add resistance it should move left.
To use the resistor take the wire off the sending unit and hold the resistor(s) between the end of the wire and ground.

 

OK, so if you ground the wire or connect it thru a light bulb the gauge pegs to the ritht, that means that the gauge and its associated wireing is good. Only thing left is the sensor. What is the resistance of the sensor when cold? Heat the sensor in some hot water, 180* or so and measure again, post what ya get

 

Here's the procedure to diagnose temp gage problems:

Tests
A. Disconnect the green wire at the sending unit and turn on the ignition switch. The gauge hand should stay against the left side stop pin.

B.Ground the green wire disconnected from the sending unit. With the ignition switch still "ON," the gauge hand should swing across the dial to the right stop pin.

Corrective Measures
If the gauge hand does not stay to the left as in test "A", either the wire is grounded between the dash unit and the engine unit or the dash unit is defective.

Test further by disconnecting the dark green sending unit wire at the gauge. Turn on the ignition switch. If the gauge hand stays on the left hand stop pin, replace the disconnected wire. But, if the gauge hand still moves, replace the dash gauge.

If the gauge hand does not swing across the dial as in test "B", there is an open circuit in the wire between the sending unit and gauge, the gauge is defective, or current is not reaching the dash gauge.

Test further by grounding the sending unit terminal (dark green wire) of the dash gauge and turn "ON" the ignition switch. If the gauge hand now moves, replace the disconnected wire. If the gauge hand still does not move, connect a test lamp to a ground and to the power (pink wire) terminal of dash unit. If the lamp does not light, test the wire between the ignition switch and the dash unit by connecting a test lamp to the "Accessory" terminal at the ignition switch and to a ground. The test lamp should burn.

If the gauge hand operates correctly as in tests "A" and "B" but the gauge does not indicate temperature correctly, either the sending unit is defective or the dash gauge is out of calibration. Replace sending unit one of known accuracy. If gauge reading is still incorrect, replace dash unit.

If the gauge hand is at maximum at all times and test "A" and "B" indicate that the wiring and the dash unit are in condition, the sending unit is bad and needs replacement.

If the gauge hand will not move, the dash unit is bad, or incorrectly installed. Correct the installation or replace the gauge unit.

 

cold=784

hot (212* for 1 minute) = 294

 

Hmm. that would put it way out of the range I told you so the gauge would not read if I was right. I'll check mine out, post in a bit

OK, well this surprised me a bit, cold mine is about 10k and ~200 it was 3.6k

 

They do (pass test A & B) and the unit STILL doesn't give me an accurate reading. BTW, I got the sending unit from Ecklers....they claim this unit DOES NOT have to be calibrated....

Quick (silly) question....does the Temp Gauge have to be installed in the console to test. The reason I'm asking is because I have taken the gauge cluster out of the car and plugged in the dash connector to the temp gauge by itself..I have been doing all the testing with the gauge out of the dash. Does the gauge have to be installed in the dash before I can get an accurate reading....something about another "ground"???

 

68 ohms of resistance should read around 220*
136 ohms of resistance should read around 150*