That plug setup Dave M mentions is almost always the culprit in later mopars, and trucks. I had the issue in a '67 Dart; it drove me crazy.The pins are swaged into a PC card, (starts in 63?) the card shrinks , loosening all the pins ,which are only a mechanical fit used to make the electrical contact . Tends to be intermittent, not just gauges but panel lights etc
The only real cure is not too bad, but it takes some dexterity.
Get a small box cutter or "ignition file" and shine up both the pin , all around 360 at board , maybe 1/8" up --and copper trace near bottom of pin . Has to be shiny or next step goes no where. If solder does not take, due to oxide or dirt--- you now got youself a big problem, as you dont get try two, makes a mess of it. . Box cutter can make scratches like tight criss cross pattern of scratches to get at good copper
Get a small solder iron , maybe 30-40 watt, or I have done with weller 100 gun if careful. Solder each pin to the trace , fast and with tinned clean iron. rosin core solder like Kester, ,being sure to not push sideways on pin with iron which gets you a crooked pin , wont go into plug. Be very careful not to overheat card as it will lift copper off it
Bulletproof .
On temp senders,a cold temp sensor are like the fuel gauge at empty Resistance is about 70-80 -90 ohms,Like E on fuel,as both gauges are the same parts. maybe a bit more ohms, and obviously temp is less critical. If no ohms, (open) = bad will stay at C , if low or shorted will go to H or mid when cold
NAPA ones tend to be slightly wrong resistance , readingsa bit off . This info is thermal gauges only. 1960 up. Ealrier gauges with wound coils have ohms like 240 , big difference. A NAPA is better suited to FGH etc ; in a C or D will read up on temp scale when it's cold. Ours (F up) are close to stude, which also used Atwater-Kent thermal gauges ,
I know I could never get correct one for a 300C new, always wrong ohms, with a long story about I must be wrong its the gauge etc I finally took to pulling one off any mopar junk engine 57-59, plymouth dodge etc for 392. always worked , not sure about 56 back. Stude even in 1950 are about the same, 100 ohms , as 60 up mopar
Hope this helps,
J
PS, you have to take dash apart anyway, On Dart, to do this right , to get card out . That gave me the idea of get another card online or from a JY supplier , fix that one , shine where the lights twist in too , I put ba little silicone grease there . Avoids part of dash lighting going on and off , or one directiojnal not lightng..sometimes. I got one on Ebay for dart
Then you just swap it in.