Good evening all, I got to take my truck out for a drive today and it was awesome!! But, during the drive I saw something unique and wanted to see what you all had to say. Prior to the drive I noticed that my temp gauge was not working, and so I switched it out with an aftermarket gauge and sending unit. The factory gauge on the other hand went up to almost hot to cool and then climbed back up. It is not connected to a temp sensor so where my curiosity is, is how would it be operating if its not able to read resistance from a sensor.

1965 Ford F100 L6 300

steps I took to trouble shoot original sensor and gauge.

Replaced sensor
Checked continuity
Checked voltage ( peaked at 8 volts)
Grounded wire and checked gauge for movement (no movement)

 

Anyone got some ideas?

 

I have 66 250, temp and fuel gauges, oil and amp idiot lights, all working good. Today, Fri, I'll test my temp gauge, wire off sender, and then, touching lead wire to ground and see if gauge rises and let you know. It sounds like your gauge is bad

 

I've recently had to do some mods to mine also, it kept grounding whenever the electric fans turned on.
They're pretty simple gauges, power goes to it (via the dash voltage regulator) then goes to the sender which grounds the unit more and more the hotter it gets. You've replaced the sender which is probably always the first thing to do, and no change to the factory gauge when you grounded the wire. That's a problem because it should rise over and peak on hot. So there's a possibilty that the wire between the gauge and sender is bad, or as mentioned the gauge is bad or the power wire to the gauge is bad. When you say you checked continuity, did you check all the wires? You can check power is getting to the gauge with a test light /multimeter on the power wire to the gauge, which is the black wire green trace. It's piggybacked from fuel gauge, so it that works it probably isn't the power wire. If that doesn't work, I'd run a long temporary wire from other side of the gauge outside the door over the guard to the sender. (This is just to test the gauge temporarily) that way you don't have to muck around unpicking the wiring loom unless it is actually at fault.
If that doesn't work the gauge is shot!

See how those go and report back to us.

 

Shane, If all is removed and nothing it touching anything, it sounds like a grounding concern.


John

 

Shane, I did test, wire removed from sender, touch wire to ground, the gauge went up to hot.
Next, I hooked up my DVOM, set at 20V DC, red lead to the removed wire at sender, black lead to ground, At first I got no reading. This is crazy as all is working properly, there should be a reading, really as varying voltage, as the gauge regulator pulses. When I picked up the DVOM in my hand I got a varying reading of less than a volt to as much as 8 volts.

Now my DVOM has a rubber slip on case to protect it from whatever. If I put down the DVOM I got no reading at all. I took it out of the case and it worked with a reading described above. I can't explain that!!

So you could possibly test the wire at the sender for voltage that varies and know that part of system is OK, with pretty good assumption the gauge is bad.

 

John, if proper or lack there of a ground is the possible solution shouldn't it have been giving me erratic readings prior to the temp gauge swap?

 

Shane the sending being screwed into the block would create a ground. IMHO.

When I say grounding issue, could also mean lack of. Something is causing the needle to move.



John