Temp sensor wire issue

[stevel]

Trying to trouble shoot issue with temp gauge not functioning. When i remove the wire ti the temp sensor and ground it to body I get a strong spark. When test light is connected to the wire I get voltage.
Is there supposed to be voltage to the sensor?

[kwmech]

The sensor acts like a resistor controlling the amount of ground through the gauge. So yes you will see a small spark, not like a red wire from the batt though. That's why when you ground the sensor wire the gauge pegs

[stevel]

Thank you. Not getting a movement in gauge when I ground the wire. Suspect bad gauge. Worked fine up to a month ago. It is the original gauge.

[dmjlambert]

You can use an alligator clip jumper wire to connect the temp gauge to the sensor.



davepl posted this chart of what the resistance should be of the sensor at various engine temperatures. Use an ohm meter to check it out.

thread with discussion

[stevel]

I have the gauge out. Can you tell me how to bench test? Not sure what terminals to use. Thanks again.

[RustyPile]

Originally Posted by stevel

I have the gauge out. Can you tell me how to bench test? Not sure what terminals to use. Thanks again.

Before you start jumping down rabbit holes, check the resistor on the back of the gauge. If the resistor is bad, the gauge reading won't be accurate. It's the little ceramic "bar" connected to the terminals on the back of the gauge. You have to remove it from the gauge, you won't get an accurate test with it connected. The resistance should be ~ 85 ohms. The replacement resistors are available but hard to find.. A common 3 watt flameproof axial lead resistor will work just fine.

[dmjlambert]

Here is a thread with info about the connections to the gauge.
http://67-72chevytrucks.com/vboard/s...d.php?t=648701

[stevel]

Resistor test perfect. Applied voltage across terms and needle pegs out. reverse polarity and needle returns to 0.

[pjmoreland]

What does your temperature sensor look like? Does it look like the one below? Just making sure you don't have a temperature switch for a warning light or some other style of sensor from a different year. Is your sensor installed without sealant so it makes good electrical contact with the engine?

[stevel]

I replaced the sensor when I noticed the the gauge stopped working. It does look correct. ,did not use thread sealer.

[pjmoreland]

I reread your posts above, and one thing stands out to me. When you grounded the sensor wire at the engine, the gauge didn't move, but the needle did move when you applied power and ground directly to the gauge. I'm thinking you have one of the following issues:

1) Inadequate ground on the bottom post of the gauge. Could be oxidized. There are also several screws on the back of the gauge cluster that are responsible for connecting the gauge sub-panel to the main metal cluster tin and the tin to the flexible printed circuit. Then there is a black wire on the main gauge cluster connector that should be clipped to the dash or choke cable, depending on the year and model. You could try simply adding an additional wire to that bottom post that is connected to a known good ground.

2) Faulty electrical connection to your temp sensor. This will require some debugging with a digital multimeter in the "Ohms" setting or a test light to verify electrical continuity between the temp sensor connector and the post on the back of the gauge that dmjlambert labeled in his post above.

3) No power at the gauge on the post on the driver side of the gauge. Is your CLUSTER FEED fuse good? I kind of doubt this is the issue since you got a spark when grounding your sending wire.

If you want to bench test your gauge, I recommend the following (refer to the attached sketch for post numbers):

1) Make sure the ceramic resistor is installed on the gauge, and the nuts are snug

2) Attach ground to post 4 (bottom)

3) Attach 12V to post 1 (driver side). The gauge needle should move to the C position. If you move the needle by hand away from C and then let go, it should then return back to C.

4) Get another resistor that is approximately 85 Ohms. This emulates the temp sensor at around 195 degrees. Attach one side of this resistor to post 4 (ground), and attach the other side of the resistor to post 2 (passenger side). The gauge should move to a bit past the first mark on the C side. I got the resistor I used out of an early '80s Chevy van. Those gauge clusters have a bunch of these resistors in them. You could also borrow the resistor from your fuel gauge.

There are two windings inside the gauge. One winding pulls the gauge toward C, and the other pulls the gauge toward H. They fight against each other. The ceramic resistor on the back of the gauge feeds the C winding. The temp sensor feeds the H winding. You can measure the resistance of these windings to get an idea of the health of the gauge. I don't know how much the resistances of these windings vary from one gauge to the next, but if your measurements are radically different than mine, I'd be concerned. It is easy to get erroneous readings on an old gauge due to oxidation of the posts and nuts.

Grounding post 2 and having the needle go to H isn't a conclusive test of the gauge functionality. I had a gauge once that passed this test, but the needle wouldn't move if I connected post 2 to an 85 Ohm resistor that was connected to ground. This was due to a damaged H winding. I believe it had an internal short.