Mike;
Have had the same problem with my '49 GMC. Has a 261 Chevy engine in it and electric temp gauge. My sender is in the side of the thermostat housing and has never read out on the gauge correctly. The sensor is an NTC type or negative temperature coefficient. As the temperature rises, the resistance falls and vice versa. I got a model TS-6 sensor which was made, I believe by Standard Parts but it would read 135 when the real temp was 180. Tried installing a variety of resistors inline with the sensor wire but they either pegged the gauge or it wouldn't read at all. I have another sensor (original) which came off the 261 head I had but the engine is currently out of the truck for a rebuild so I don't know if it will read correctly or not. Can't help anymore than this until the engine goes back in and I get it started. I'll repost if I come up with anything.
