Erroneous Temp Gauge Reading

[425HP409]

I finished the install of the 396 in my truck and installed a new temp sending unit. I also installed a new LMC temp gauge that has temp readings from 100-250 instead of the original gauge that reads cold and hot. The temp gauge only reads barely off of the 100 line when the engine is at operating temperature. I checked the gauge by opening end shorting the sending unit and the gauge responded correctly. Anyone have this problem with the LMC gauge? Or do I have a bad sending unit? I tested three different ones in water from cold to hot and they all read close to the same.

[RustyPile]

I have never used LMC as a source of parts, so I'm not at all familiar with their products. More than likely, you have the wrong sending unit.. Regardless of year/make/model, the gauge and sending unit must be a "pair".. Determine what sending unit LMC recommends for their gauge..

[dmjlambert]

Confirm with LMC it is intended to be used as a stock gauge and doesn't have a special sender that they sell for the gauge. If it is supposed to read same as stock except it has numbers on the dial, then: I had that symptom on my 1969 truck with very low temp readings on the gauge and I had the wrong sender.
Does your temperature sender look like one of these:

https://www.amazon.com/gp/product/B000C9SM22/
https://www.lectriclimited.com/tempe...ng-unit-117510

Here is a thread where davepl shows us the actual resistance readings the sender should have at various temperatures. You need to use some antifreeze on the stove to get the temperatures of the liquid over 212F, because water on the stove won't get beyond 212F and is not "hot" to an engine. https://67-72chevytrucks.com/vboard/...d.php?t=714839

If your engine does not have the 1/2NPT hole in the head to accept the sender, they can be turned to and threaded to 3/8NPT or some metric size if you need that, by a machinist. This has been discussed a bit in other threads, have the temperature sender machined down to fit the hole (in the case of my 1969 350 engine with 1990's heads). I had forum member A1971Blazer turn and thread mine. Other people had it done by brian mac. Here are a couple of threads where it is discussed.
http://67-72chevytrucks.com/vboard/s...d.php?t=703388
http://67-72chevytrucks.com/vboard/s...d.php?t=627880

[Sheepdip]

Get you a Harbor Freight temp gun and check it to the gauge vs the sending unit boss area.

[pjmoreland]

I believe the temperature gauges LMC sells are intended to be installed without the original external resistor that the factory gauges had. Installing the factory resistor when it isn't needed will increase the electrical current to the gauge winding that pushes the needle toward the cold end of the gauge. If you installed a resistor with the LMC gauge, try removing the resistor.

[MikeB]

I had the same problem with an LMC gauge meant for a square body truck, and ended up with a shunt resistor that sort of helps. Still, the needle barely starts moving at 160-170, then it's straight up at 190-200, but then moves rapidly to the right. Haven't got up to higher than 190 yet, but I think 210-220 would be around 7/8 of full scale.

I used a bi-metal thermometer in a thread port to do the calibration. Also used a potentiometer as the shunt resistor to determine 100 ohms was a good value. It's been a while, but I think I used a 5 watt wire-wound resistor.

Edit: I read in a few places that using a temp gun on the manifold surface isn't accurate. But I found no more than 1-2 degrees difference between my two guns and the thermometer, when I pointed the guns at the base behind the t-stat housing, out of the fan's airflow.