This Tech Tip is From the Full Book, HOW TO USE AND UPGRADE TO GM GEN III LS-SERIES POWERTRAIN CONTROL SYSTEMS. For a comprehensive guide on this entire subject you can visit this link:
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I recommend Using the 1998 F body Camaro or “Three Wire” Temp sensor. with an ’81+ GM Truck Temp gauge. The way this works is as follows.
For Truck Years: 1967 – 1973
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 76 Ω
Right Line (Hot) when sender resistance = 51 Ω
For Truck Years: 1974 – 1978
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 350 Ω
Middle Line when sender resistance = 68 Ω
Right Line (Hot) when sender resistance = 46 Ω
For Truck Years: 1979 – 1987 (91)
The Temp Gauge Needle Should Point to:
Left Line (Cold) when sender resistance = 1,365 Ω
Middle Line when sender resistance = 96 Ω
Right Line (Hot) when sender resistance = 55 Ω
The LS Engines have only one Temperature sensor that has two wires(A-Black-Low Reference. B-Yellow-Signal) The ECM Recieves its temperature signal from this sensor. It then gets outputted from the ECM to the gauge of the vehicle the LS engine came out of. This output signal is a “serial” signal and can only be used by certain aftermarket gauge setups.
There is a Temp sensor that was used on 1998 F-Body Camaros that uses the same configuration as above but adds a third wire that went straight to the gauge on the ’98 F-body cars. Those cars used a Temperature gauge with the same ohm range as the ’79-’87(’91) GM Trucks.
To keep the factory GM truck gauge (and most aftermarket gauges) you can replace the original LS two wire Temp sensor directly with the Three wire Sensor. Along with the sensor, a new sensor pigtail connector will be required. These are both readily available at parts stores and amazon.
Using this sensor will allow the ecm to see its required signal, as well as provide the additional output for the original ’79-’87(’91) gauge.
Amazon Links for three wire sensor and pigtail;