Tom Austin

When I had my L79 rebuilt a few years ago, i put a brand new TU5 temperature sending unit in along with a brand new sending wire. All was fine for the first few years, but recently the gauge hasn't been reading correctly as it barely makes it to the 1st mark on the gauge after warm up. Today I installed another brand new Duralast TU5 temp switch (from Autozone) - (with just one wrap of teflon tape) . . . with no changes to the gauge - the needle still only barely gets to the first mark. I tested the gauge by grounding the sending wire - and the needle on the gauge pegs all the way to the right as I believe it should. Where do I go from here?

Tom

Frankie the Fink

Well get a $26 Harbor Freight I/R temp gun and shoot the thermostat housing with the engine warmed up and compare to the dash gauge - maybe its right after all...

It IS cool out there and maybe you have 160* thermostat or some such...

65GGvert

Yeah, maybe it's correct and your thermostat is stuck open

MarkC

I agree with the temp gun to verify. I use mine for lots of different uses. It is great for cooking on the grill. I have 8 temp senders I have bought from all the local parts stores plus one I paid close to $30 that is suppose to be calibrated. The temps range from 150 to 200 degrees with the senders either reading way to high or too low. I have a 66 l79 with the 250 temp gauge. The engine is running 180 degrees. Two months ago I picked up another one at our new OReily's store. It is reading about 10 degrees too high. I can live with that. BTW, I found another problem I had with the temp gun. Run it across your fuse block. I was melting the fuse for the blower when used on high speed. It was getting to hot because of a little corrosion. Cleaned up the contacts and some dielectric grease, problem is solved. The gun is also great for finding the hot and cold spots on your grill.

Powershift

Tom:

It could be a bad thermostat, but another possibility is corrosion and high resistance in the sender wiring back to the temperature gage. High resistance in this circuit will result in a lower than actual temperature reading.

This sending unit wire goes through the connector block on the firewall....and you may have corrosion in these connectors. Most of us do, unless we take them apart and clean. It is also possible to have corrosion at the temp gage terminal screw.

You can use a IR thermometer to check the engine coolant temperature and compare to the temperature gage. Buy or borrow one if needed. This will tell you if it is the thermostat.

The sending unit in the intake manifold can be checked with a multimeter (like a Fluke meter) at both hot and cold conditions and compared to known calibration charts. These charts are in the archives.........or I can provide. Again, you need a IR Gun to get the sender temperature and then read the resistance with the multimeter (sender wire disconnected).

If these all check out, then you have either a problem with high resistance in the sender wiring or in the gage itself. You can clean the wire contacts to see if this is the cause.

Larry

rwerth7

Assuming the gauge is calibrated correctly and the sending unit is accurate then the most likely cause is too high resistance between the sending unit and the gauge. Try running a separate test wire from the sending unit to the gauge. Don't forget to clean the connectors at the back of the gauge.

Tom Austin

Thanks guys for the input. I spoke to one of our techs @ work today and we believe that the thermostat my be stuck open . . . I'll try that 1st.

Happy New Year!

Tom