I've got a 78 gmc that I just picked up with a 350 that must be from another year. The temp gauge isn't working and I'm having trouble finding a sending unit. I can ground the wire and the gauge will go crazy and finally peg out way past hot. I bought a sending unit for a 78 and it's threads are too big. We matched one up from a mid 80s 6.2 diesel that fits but I still can't get the gauge to work. Am I checking my gauge correctly?
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Yeah, you need a 3/8" NPT sender.
Grounding the sensing leg lead - and getting a full high indication - was a good initial test. You proved that the indicator is reacting as designed. But you need to source the correct sensor if you want the dash gauge to indicate anything close to actual coolant temp. .
I think in 1979 GM redesigned the coolant temperature senders (and the cylinder head ports) used with the SBC's:
1. The ports and senders were reduced in diameter from 1/2" NPT to 3/8"
NPT.
2. The resistance range (from cold to hot) was changed to 1365 ohms to 65
ohms. Prior to that is was 350 ohms to 46 ohms.
Those resistance values are for gas engines only. I have no idea what they would be for a coolant temp sender used with a diesel. Don't try to use that mid-80's diesel sender, it will almost surely not be compatible.
Is this what your harness connector to the coolant sender looks like - except it's green?
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If so you should get a sender with this style terminal:
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It's called a button head style. It needs to have a 3/8" NPT body to thread into your mid-80's engine. But that combination (3/8" NPT with a button head) might be difficult to find. You might have to change the harness connector to a later model style.
