TNT

Changed cyl heads on my 350. Had to move the original temp sender to the intake and extend the temp wire about 16 inches. Now guage reads just above 100 when warm. Checked engine with IR gun and it's at 180-190 degrees. So did extending the temp wire mess up the guage reading?
Guage read fine and accurate before!!

65GGvert

Extending the wire if a good connection is made will not cause a different reading. The higher the resistance to ground, the lower the temp will read, so you may have a bad connection in the new section you installed, a bad connection between the threads on the sensor to the intake, or the sensor resistance may have changed internally (not really likely). I have heard, but never experience that if an air bubble gets trapped so that fluid isn't touching the tip of the sensor, that the temp will read low too. Make sure you have a good connection at the threads, you can rule that out by running a jumper from a good ground to the brass body of the sensor and see if the gauge changes readings from what it was without the jumper. The most likely of the three is resistance where you made the change, if it read correctly before the change. Did you solder the extension, or crimp, or what? Are you using he original connector at the top of the sensor?

TNT

Origional connector and I crimped the new wire, great info i'ii do more checking this week. Thanks
Thad

7T1vette

The electrical 'ground' connection for the temp gauge circuit is between the threads on the sending unit and the threads in the cylinder head. If you used sealant or teflon tape on those threads, that's likely your problem.

If you need to seal any minor leakage, use a pipe fitting paste sealant. That will not inhibit electrical connection between meshing threads on that sending unit.

TNT

It was installed dry. Maybe the intake threads are a little rusty.
Thad

7T1vette

Before you go 'messing' with it, just get a piece of wire to act as a "jumper" and, while the car is at operating temp (engine could be OFF, but ignition ON), touch one end of the wire to the sender body and the other end to a good ground point. If the threaded joint is the problem, the temp gauge should then show the correct engine temperature. If it still reads the same, that threaded connection is not the problem.

If you changed the sending unit, it could be a different calibration. Various model years used different sender units [with differing calibrations]. If this could be your problem, Willcox has support videos on how to recalibrate your temp gauge. This is done without having to dig into the gauge cluster area.

Wish you luck!

TNT

It's the original sender. I tried the jumper ground a minute ago. No change. I'm stumped!!
Thad

cdl49

could there be an air pocket where the sending unit is mounted in the intake so that no water is touching it?
Dean

7T1vette

It is possible that you didn't get the total amount of coolant back in the system. The best way to fill the system is to remove the radiator cap and fill to required level. Then, with cap still removed, fire engine and allow to reach temp where the upper radiator hose begins to get hot. This means that the thermostat has opened.

When the stat opens, the fluid level in the radiator will usually drop some more, as air trapped behind the stat is released. If it drops, refill to the required level again, then install radiator cap.

No...with the cap off, it will NOT blow out hot water.

Try that. If you have to add much water/coolant, that may have been your problem. It is highly doubtful that anything in the gauge cluster area is at fault. The sending unit (just removing it could have damaged a very old sending unit) and wiring back to the firewall are MUCH more likely to be at fault.

TNT

After all testing it appears that the original sender is not functioning correctly. I'll do more testing later but thanks to all.
Thad

65GGvert

Just a rough reading, with engine at 180, the top of the sender should read around 120 ohms with an ohmeter to ground.

TNT

Thanks. Severe thunderstorms now in south Louisiana. I'll check it out in a day or so.
Thad