Backstory. '56 DeSoto temp gauge not reading properly. Tested the gauge and sending unit using the procedure detailed in the service manual and confirmed both are working as intended. I'm running a factory dual quad intake on my 330. The hole for the temp sender is tapped to 3/8" NPT. From the factory I believe this hole was originally 1/4" NPT and the sending unit threaded directly into the intake without the need for an adapter/reducer. I'm guessing some time in this intakes life someone opened up the hole so they could run a mechanical temperature sender since those require an adapter/reducer where the probe sits in. Cut to today I'm using a regular 3/8 NPT to 1/4NPT reducer to fit a factory style electrical sending unit. With the added height of the reducer the sending unit doesn't sit down in the head as I'm sure it was supposed to. ON TOP of that issue the modern replacement unit looks to have a shorter probe than the factory one. Could all of this be compounding to create my incorrect gauge reading issues? An infrared thermometer at operating temperature confirms the engine is running at optimal temp.
What symptoms is the new sending unit exhibiting? I've never really had a problem with adapters on sending units because the thermal cycle of the probe eventually ends up relatively close to the liquid temp due to the prolonged exposure to the heated coolant, and thus the gauge might read a little lower (10-15°), but still provides a somewhat accurate indication of the engine's status. If I recall correctly, the manual doesn't list a specific Ohm range for the sending unit?
It shouldn't matter whether it is inserted in the head or not, the hot water is hitting the probe either way. Did you wrap it in teflon tape?
They make an adapter that doesn't raise the sending unit. It looks like a threaded piece of pipe that has inside and outside threads.
I suspect you have poor contact on the ground side of the sender. When adding thread sealant to the sender and bushing, not tape which is for shower heads, I leave the first 2-3 threads exposed so that they have good contact with what they are being installed into. Adding any resistance into the circuit will affect how the gauge responds since the sender is nothing more than a variable resistor itself.
Since you are this deep, it should be fairly easy to find the specs for your sender and compare. Something like this. Temp should provide resistance within spec.
this sounds like a very light weight way to start trouble shooting. i have thread sealant on both the reducing bushing AND the sending unit itself. i have to drain the water off to replace the water pump anyway so I’ll take these guys out and clean them up while I’m at it. give that a whirl and see what happens. thanks gents!
For shits a giggles try a small hose clamp and place it around the hex portion of the sending unit. While doing that stuff a wire between the clamp and sending unit then tighten the clamp. Now place the other end of the wire to a known good ground. Battery would be best. Now see what happens. Not ment to be a permanent fix, just a cheap easy test is all and no coolant lost. All though, I did leave the wire in place on a company vehicle. Could not get a good ground no matter cleaning or minimal sealant used.
Well that is a nice engine, also make sure the block has good grounds as well, that fresh looking paint makes a good electrical insulator. A toothed washer on the ground cable will bite through the paint or you could remove just enough paint so that the cable is in good contact with the block. A dab of dielectric grease will keep that exposed spot from rusting and spreading under the finish.
That's a good looking Hemi and why I mentioned the teflon tape in my post #3 above. I know because I have made the same mistake. Make sure that hemi has a ground strap or two, strip the teflon tape and I think you will find your issue is resolved.
The Jalopy Journal
