Help with temp sensor reading low

[JGross]

I just started up my 5.3l swap and the engine started fine, however the coolant temperature is reading low. I have a scan tool the plugs into the OBD port and the PCM is reading 122 deg when the actual temp is 187. I have a digital thermometer in the radiator tank as well as the original temp sensor that drive the gauge in my 65 C10 so I’m sure the problems is with the PCM reading.

The first thing I checked was the voltage going to the sensor (yellow wire) 4.96V

Then I jumped across the sensor connectors which should have given me a temp of 250 but it read 172

The wiring diagram shows the gray wire as a “low reference” I’m not sure what that is.

Thinking the low reference might be a ground I checked it to ground and got 1.2 ohms


Here are my Questions:

1. What is meant by low reference
2. Where does the gray wire go? It doesn’t seem to go to the PCM
3. Any Ideas what my problem is?

Thanks for the help

[JGross]

Originally Posted by JGross

I just started up my 5.3l swap and the engine started fine, however the coolant temperature is reading low. I have a scan tool the plugs into the OBD port and the PCM is reading 122 deg when the actual temp is 187. I have a digital thermometer in the radiator tank as well as the original temp sensor that drive the gauge in my 65 C10 so I’m sure the problems is with the PCM reading.

The first thing I checked was the voltage going to the sensor (yellow wire) 4.96V

Then I jumped across the sensor connectors which should have given me a temp of 250 but it read 172

The wiring diagram shows the gray wire as a “low reference” I’m not sure what that is.

Thinking the low reference might be a ground I checked it to ground and got 1.2 ohms


Here are my Questions:

1. What is meant by low reference
2. Where does the gray wire go? It doesn’t seem to go to the PCM
3. Any Ideas what my problem is?

Thanks for the help

Never mind I found out the scan tool was bad it was reading 40 to 50 degs low.

[71chevystep4x4]

Thats awsome!!!!