Electric Fan Conversion Question

[Rocket Man]

Yes, the extra relay I added in the harness is needed to convert the AC compressor clutch trigger wire to work with the fan harness. And yes, its a pretty tight fit under the fuse block stuffing the harness and relay underneath the fuse block, but it just BARELY fits. On the 00-02 trucks, there is no AC pressure switch that sends a signal to the ECM. But there IS a pressure switch on 03+ trucks. Since that pressure switch is there, it sends a signal to the ECM to let the computer know when the AC pressure is high enough to trigger the fans to turn on. Since the 00-02 trucks do not have this, you have to tie into the AC compressor clutch wire instead of the ECM. The clutch wire is positive, not negative. That's why the extra relay is needed, to change the positive signal from the clutch wire to negative. Then that relay sends a negative signal to the 05-06 OEM relay block which ends up turning the fans on whenever the AC compressor is running. This works okay for the MOST part. The ONLY drawback is it will still trigger a fan to turn on in cold weather when you have the defroster on, because the compressor turns on when the defroster is on. I hope all this makes sense. But I have had my fans wired like this since 2010 and they have worked fine ever since, other than a fan running un-needed in colder weather when the defroster is on.

That’s how I did my 02. It’s been fine for years.

 

[Rocket Man]

I got the rebuilt fan junction block from @ScottyBoy I 150% recommend working with him. Saved me grief of eBay (bad parts), scouring junkyard (they didn't have it), and rebuilding the block. He cleaned it up and it looks AWESOME, I mean like new!

Yesterday I got to installing it. Here's a few notes and questions. My goal is to have the EXACT or as close as humanly possible to the same routing & cable management as factory.

Pictures are facing the fender, left hand is front of vehicle.

View attachment 452276

QUESTION 1 The power lug for fans, I assume it mounted to the junction block left hand side?

I had to grind an ear off the right hand side of photo for it to fit.

View attachment 452278
QUESTION 2 The wire loom goes under the junction block parallel to the fender?

I tried routing it multiple different ways but that seemed to be the most logical. The red power line / lug was a touch too long though...?

Also it's a VERY tight fit with the relay which I located under that green connector.

QUESTION 3 I assume the relay functions as a OR essentially to turn on the fans when the AC compressor goes on? That it's not on the actual 2005+ trucks, they are computer controlled for AC fans?

View attachment 452279

QUESTION 4 These two connectors were originally held by christmas tree clips in the void the fan junction block now occupies. They are now loose under the junction block cover. How does the factory manage those in the newer trucks?

This is as far as I've got so far. The harness is now tucked away till I can figure out how it's routed to the fans themselves, get the pin in the PCM, and the AC clutch spliced in.

You won’t be able to do it exactly the same as the 05-up because your wiring harness isn’t the same. Do the best you can and don’t stress over it is my suggestion unless you want to find an 05 harness for the entire front end of your truck and take forever to switch it over and hope everything still works afterwards.

 

[Tonyrodz]

Yes, the extra relay I added in the harness is needed to convert the AC compressor clutch trigger wire to work with the fan harness. And yes, its a pretty tight fit under the fuse block stuffing the harness and relay underneath the fuse block, but it just BARELY fits. On the 00-02 trucks, there is no AC pressure switch that sends a signal to the ECM. But there IS a pressure switch on 03+ trucks. Since that pressure switch is there, it sends a signal to the ECM to let the computer know when the AC pressure is high enough to trigger the fans to turn on. Since the 00-02 trucks do not have this, you have to tie into the AC compressor clutch wire instead of the ECM. The clutch wire is positive, not negative. That's why the extra relay is needed, to change the positive signal from the clutch wire to negative. Then that relay sends a negative signal to the 05-06 OEM relay block which ends up turning the fans on whenever the AC compressor is running. This works okay for the MOST part. The ONLY drawback is it will still trigger a fan to turn on in cold weather when you have the defroster on, because the compressor turns on when the defroster is on. I hope all this makes sense. But I have had my fans wired like this since 2010 and they have worked fine ever since, other than a fan running un-needed in colder weather when the defroster is on.

What would happen if you disconnect the ac compressor during the cold months? Then it wouldn't trigger the fan to come on once the defroster was turned on. When I was younger I used to unplug the compressor during the cold months with no drawbacks.

 

[ScottyBoy]

What would happen if you disconnect the ac compressor during the cold months? Then it wouldn't trigger the fan to come on once the defroster was turned on. When I was younger I used to unplug the compressor during the cold months with no drawbacks.

In most climates, it would be just fine. But in very humid climates, like here in Louisiana, the AC compressor comes on to help the defroster to decrease humidity, and it that actually helps a LOT. But in drier climates, you will most likely not see any issue.
Another option would be to wire in an toggle switch on the AC trigger wire on that fan relay. Turn it off in cold weather, but then you will have to remember to turn it back on in warmer weather. I just left mine alone, and the only real drawback I've noticed is that sometimes in colder weather (belowe freezing) my truck takes a LOT longer to warm up.

 

[Matthew Jeschke]

@ScottyBoy makes perfect sense about the AC Compressor clutch. If I have to tear apart the AC system again I'll add that pressure sensor maybe lol

My friend has a 2005 Silverado. I asked him if I could look under his hood and he didn't even give me an answer. He kind of looked at me weird

 

[ScottyBoy]

@ScottyBoy makes perfect sense about the AC Compressor clutch. If I have to tear apart the AC system again I'll add that pressure sensor maybe lol

My friend has a 2005 Silverado. I asked him if I could look under his hood and he didn't even give me an answer. He kind of looked at me weird

I remember reading in a thread over on Performance Trucks about adding a pressure sensor and then reconfigure the computer software to recognize it and everything. So it can allegedly be done, but in my opinion it's not worth the hassle. Like I said, I have been daily driving my 2001 like this since 2010, so that's 15 years now and it's been doing perfectly fine except for VERY cold days, which honestly doesn't even happen that often in Louisiana. And with you being in Arizona, I'm guessing you would not have very many sub-freezing days as well. The ONLY difference between the harness you have and a completely stock harness in a 2005 is the AC trigger. Yours is now triggered by the AC clutch wire, (so anytime the AC compressor is enagaged, one fan wil be on) and a 2005 is triggered by the ECM, and the ECM triggers it when it senses the AC pressures are above a certain threshold. Both harnesses are triggered exactly the same as far as the temperature. When the ECM senses that the coolant temperature is above a certain threshold, it triggers both fans to come on at low speed. But if one fan is already on because of the AC, then the other fan will turn on at high speed, therefore having both fans on at high speed. When the AC is off and the coolant gets up to temperature, both fans are wired in series, so they come on at low speed.
Also, if you haven't already upgrades your alternator, I highly suggest that you upgrade to a more powerful alternator. I upgraded from a 130a to a 145a, and I still sometimes see a sharp voltage drop for a second when the fans kick on. But that usually only happens at idle. At higher engine rpm's, I don't notice any voltage fluctuations. I also added a smaller alternator pulley to help the alternator spin faster, which helped marginally but not a lot. I bought it on Ebay for like $15 shipped, so at that price it was worth it l guess.

 

[Matthew Jeschke]

I got everything wired up... @ScottyBoy Did you intend the AC clutch signal wire be fed through the supply / power side of the relay to the compressor clutch? I hooked it to the control / trigger wire for clutch relay and it doesn't work... Otherwise I can short the relay and the fans go on.

I can also command the fans on from the PCM (although not individually, as I see only the low temp fan is wired up in harness). I just cannot command on the fans when the AC turns on.

...

Studying schematics for the 2005 Tahoe. I think I may have a less expensive way to do this...

The pressure sensor is a voltage divider. It's giving a fraction of 5V proportional to the pressure. Not sure what the impedance of the sensor is, but probably doesn't matter. I'll try a 100k ohm potentiometer

PCM Pinout

C1 pin 45 (Gray) is the 5V reference (Supply voltage)
C2 Pin 14 (Red/Black) is the signal being read (divided voltage)
C2 Pin 80 (Black) is the reference ground

I can connect a potentiometer in there and figure out which voltage commands on the fans. Only thing is, I'm not 100% sure but assume the PCM looks at the AC clutch being commanded ON AND not JUST the pressure. Otherwise, we're back to needing a relay.

If this works a couple cheap resistors can be used in lieu of the relay... additionally, I'm assuming you only used the low temp fan trigger from PCM as you had to splice the two fan relay triggers together to use the relay method w/ AC clutch.




Found sensor on Rock Auto. It's a three pin confirming schematic above. I think those wires are dedicated to just that sensor (the reference power is not shared with any other sensors?).

More Information for GM GENUINE 1551343

I hooked up a pot to spare PCM in my lab and it worked as I'd thought. Additionally, given HPT looks like the PCM does discern Pressure sensor from AC clutch command signal. Tomorrow I can try it in the vehicle with the potentiometer.

I'm guessing 50% divider say two 50kohm resistors (2.5v) will work fine but don't have the scalar / multiplier for voltage to PSI on the sensor.





This webpage says 0V is 0PSI and 5V is roughly 430PSI.

A/C Pressure Sensor Function

How do automotive A/C refrigerant pressure sensors work? How to test A/C refrigerant pressure sensors.

www.freeasestudyguides.com

Stock settings for pressure AC refrigerant pressure command on in PCM from 2005 chevy tahoe.


DTC0530 Less than 100mV or greater than 4.9v for 5 seconds trips code.

This indicates the divider may have a resolution of 2%... 1:500

It'd be nice to have one of the sensors available to measure impedance.

At 50kohms only 1uA is entering the PCM signal line. Probably the PCM prefers more current to measure the signal?