The Stovebolt Site Forums The Shop Area The Electrical Bay 3/8" npt temp sending test & problem

I am testing a new 3/8"npt sending unit. The ohms appears ok. Problem is when I connected the guage assembly to a 12v power pack (made up of AA batteries) I used for light checking. The guage will react to the 12v on quick contact but will not react to the curcuit with the sending units installed. I have the units setting in the opening of a clean pop can with water. There is a food thermo in another hole. I warm the water with a propane torch to different temp and check each sending unit ohm reading. Both the original with the 1/2" npt and the new 3/8" units check very close at high temp. Taking the sending unit out of the line and jumped the guage reacts but with either unit in - nothing. Does the battery pack not have enough strength to operate the guage with the sending unit resistance?

Dont know. Sigh one more test - try it again and check for 12 V on batteries. For this test if fairly fresh they should do OK for a minute or two.

Uh you do have a complete circuit on the sender ie one wire to terminal and one to the case ie pipe threads area?

Dan Bentler

The cirtuit is complete. If I take the sending unit out and touch the wires together , the guage reacts. I have checked voltage at the top of the sender and at the base and both show voltage. Something here I am overlooking. Hope someone can see the hole in this system or logic.

sounds like you may not be completeing the circuit - power to the gauge, gauge to the sender, sender body to ground on the gauge case .... maybe you need to just install it and watch it work as designed seeing as you've tested all the bits of it OK

Bill

Bill - You think if I ground the guage cluster itself(also), this may solve the hole? I was only grounding the circuit by way of the battery ground and the cluster was not grounded to anything (just setting on the bench). I am hoping this sending unit works, it will allow those with older heads to use an electric guage.

Bill - went out and grounded the cluster also - no difference

When I calibrate a new sender I go with an ohmmeter test first. With sender in the heated water I write down the ohm values for a serier of temps. (cool to hot)using a known good temp. instrument for reference. Then I duplicate those ohm values with a variable resistor hooked to the gauge sender input with other side of resistor grounded to case. This allows me to add a touch of resistance in series with sender to get gauge to read exactly. A one turn pot 0-250 ohms will work in a pinch. I use a 10 turn pot salvaged from some past project just because I have it. Remember these gauges don't react to change very fast so give the gauge a half a minute or so to stablized.

Now this guy does not fool around and does it right and complete.

Dan Bentler