Coolant temp sensor hook up

[Duncan K]

I have a 2001 LQ4/4L80 going into my 66 GMC. I am using the stock harness following the instructions from LT1swap.com. After depinning the harness my coolant temp sensor wires don't go anywhere. I am going to be using an electric fan, and had the ecu reflashed to run the fan. I am assuming the ECU has to get its info from the coolant temp sensor to trigger the fan?

My question is where should these wires go in the harness? I don't remember taking them out, and I cant find anything on the pin out?

[MDPotter]

I guess I'm not sure which wires you're referring to. I believe two wires go to the factory sensor which is on the front driver side head and that sensor provides the temperature reading to the PCM. If you're running a dual fan setup, you should have two different wires coming out of the PCM that are ground triggers for the fans. For a 2001 pickup, I think that truck had a single mechanical fan so hopefully you told Brendan you're adding dual fans so that he programs in that function. According to Brendan's pinouts, Blue 41 and Red 33 are the pinouts for the fan triggers and Blue 74 is coolant temp sensor input.

[Duncan K]

Originally Posted by MDPotter

I guess I'm not sure which wires you're referring to. I believe two wires go to the factory sensor which is on the front driver side head and that sensor provides the temperature reading to the PCM. If you're running a dual fan setup, you should have two different wires coming out of the PCM that are ground triggers for the fans. For a 2001 pickup, I think that truck had a single mechanical fan so hopefully you told Brendan you're adding dual fans so that he programs in that function. According to Brendan's pinouts, Blue 41 and Red 33 are the pinouts for the fan triggers and Blue 74 is coolant temp sensor input.

Thank you! I read the pin out 3 times but some how missed blue 74 being the location in the harness.

[Duncan K]

Ok so I went and checked my harness. Blue 74 is a yellow wire going to the MAF sensor. Not sure why it is different that what the pin out is saying?

[MDPotter]

It sounds like you may have repinned that wire incorrectly. As long as it's a 99-06, Blue 74 should be yellow, but go to the coolant sensor. I would check to see what you have at Red 31 as the pinout indicates that is also a yellow wire, but goes to the MAF.

When I redid my harness, I checked every single wire to make sure it's in the correct position in the PCM connectors and then checked them all for continuity. Do you have a tester you set to continuity mode to check all? It helps with what you're doing.

[Duncan K]

Red 31 also has a yellow wire but it goes to the transmission. Seems my harness is a bit different than the pin out. The donor truck was completely stock, never modified. I sent an email to Brendan to see if he has any idea