Okay so, i'm having a really weird issue that I've never come across before. I have a mechanical water temp gauge with a typical thermocouple. It will only read temperature when the ignition key is off. Turn the key on, and it goes to zero. The only wiring to it is for the little light, which is working fine. Speedway plastic housing gauge. I have a headache. Anyone got any ideas??
The only possibility I can come up with is this...I have a SBF with an HEI distributor. The thermocouple for the gauge is right underneath the distributor. Like I can barely get it in there. I'm wondering if it is somehow picking up a static charge being so close to the distributor. I believe it's grounded good, or it wouldn't be working at all.
Plastic gauge housing.....try grounding the cap tube with a jumper wire and see if that makes any difference. Try it close to the gauge, if possible.
I would say you have ground issues. Ground the engine to the frame, ground the body to the frame, ground the dash to the body. That should fix your problem.
Unplug the HEI after getting up to temp and turn the key on, see if it reads correctly then. That will tell you if it's the dist causing the issue.
You said you have a mechanical gauge, which only has a sensing capillary, not a thermocouple, so there is no electrical mechanism (besides the light). The gas in the capillary gets hot, expands, and the bourdon tube inside the gauge transmits mechanical movement to the pointer. A mechanical gauge will read regardless of what the ignition switch is doing.
As stated mechanical temp gauge should read no matter if it’s in a bucket of hot water or your engine , or if you remove the battery from your car . Electric should have not one issue to change the gauge reading . Murphy has moved in your private space .
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