Thread: Brain buster!! Fans stay on high

A few days ago I installed my SLP headers. They were fighting me the whole way! Getting irritated as hell to get the front mani to seat flush and the crossover to slide into the front mani I started using a pry bar to give a quick shock more than a few times. Where the bar it self kept hitting a few times was the fan shroud in middle between both. Which leads me to believe I may have caused a problem??? I also had to remove the right motor heat shield (a little smack of the trim tool) (knocked the fasteners off) Maybe its one of those things that failed before and now I notice it? But I doubt it.

On a cold start my fans come on high and stay on and from what I can hear at idle. Ign on engine off they are not on though. Key off they are not on. So its not a short per say. I can command them on/off with HPT. The temp sensor is working as I can unplug it and it will go to -38*. Every now and then I use to hear the fans stay on with the key off (too hot/summer days)

Below is my fan tune. What could be the problem? I wiggled the harness and nothing has changed. I tapped the pcm and fan motors with a hammer and no change. I even put a different pcm file in with a timing tweak for starters with no change (incase it was a bad file)

Do these fan tables look ok?

I'm told a possible p0128 code will cause this perhaps? This code has been set to no error reported in my tune file I see. Being stone cold and the fans are commanded full does not make sense. Perhaps a relay? The hvac control is off so ac is out of the picture. The ac compressor does work on/off.

This isn't a brain buster. Some of us know you Pat...and last I knew you worked at dealerships. What does the schematic say? Are the fans being requested to be on based on the pcm?

I haven't work for GM since 2009.

Swapped Relay's with a known good car and that car did not act up meaning the relays are good. Even put them in my car and still acted up. Disconnected the AC compressor and no change. Even swapped back to stock PCM and no change.

Disconnected Pressure switch ofr AC and did not change



Cooling Fan Control 3.8L (L36)
The engine cooling fan system consists of two electrical cooling fans and three fan relays. The relays are arranged in a series/parallel configuration that allows the Powertrain Control Module (PCM) to operate both fans together at low or high speeds. The cooling fans and fan relays receive battery positive voltage from the underhood accessory wiring junction block. The ground path is provided at G117.
During low speed operation, the PCM supplies the ground path for the low speed fan relay through the low speed cooling fan relay control circuit. This energizes the cooling fan 1 relay coil, closes the relay contacts, and supplies battery positive voltage from the cool fan 1 fuse through the cooling fan motor supply voltage circuit to the right cooling fan. The ground path for the right cooling fan is through the cooling fan 2 relay and the left cooling fan. The result is a series circuit with both fans running at low speed.
During high speed operation the PCM supplies the ground path for the cooling fan 1 relay through the low speed cooling fan relay control circuit. After a 3 second delay, the PCM supplies a ground path for the cooling fan 2 relay and the cooling fan 3 relay through the high speed cooling fan relay control circuit. This energizes the cooling fan 2 relay coil, closes the relay contacts, and provides a ground path for the right cooling fan. At the same time the cooling fan 3 relay coil is energized closing the relay contacts and provides battery positive voltage from the cool fan 2 fuse on the cooling fan motor supply voltage circuit to the left cooling fan. During high speed fan operation, both engine cooling fans have there own ground path. The result is a parallel circuit with both fans running at high speed.
The PCM commands Low Speed Fans on under the following conditions:

•Engine coolant temperature exceeds approximately 106°C (223°F).
•When A/C is requested and the ambient temperature is greater than 50°C (122°F).
•A/C refrigerant pressure exceeds 1310 kPa (190 psi).
•After the vehicle is shut off if the engine coolant temperature at key-off is greater than 140°C (284°F)/ and system voltage is more than 12 volts . The fans will stay on for approximately 3 minutes. The PCM commands High Speed Fans on under the following conditions:
•Engine coolant temperature reaches 110°C (230°F).
•A/C refrigerant pressure exceeds 1655 kPa (240 psi).
•When certain DTCs set.

Bit of an update. Besides the fact my low coolant light does not work (I will have to put a bulb in it to see if the light is on) If I unplug the AC pressure sensor and wait 45 seconds the fans then shut off. I have no DTC's but did have a p0530 ac pressure sensor circuit code (but may have been from the other day when I unplugged it)

2009.. wow. This started when you did the headers you said....I'm wondering if you check the pcm output at the pcm for ground on the circuit, then disconnect the pcm connector and test again. Thinking you may have ended up grounding the wire somewhere.

I appreciate the input. I finally got around to working on it today. If the AC pressure is higher than 240 the fans kick on high according to description/operation. Well according to the scan tool it said 267! So I unplugged it and went to -14. Okay it sees that, so I evacuated my AC and still read 267! ! I bit the bullet and paid $55 to stick a new sensor in it and I fixed it!!! No more nascar fans at 5am...hahaha.

Upon removing the cluster I found that the low coolant light is powered by two inop leds. Anyone have certain ones they use?