A/C Condenser Fan

[The_Full_Monte]

Hi, all! The Full Monte is my 1999 K2500 Chevy Suburban (7.4 Vortec, 4L80E, 4.10s) and has been my daily driver for nearly four years now.

Question regarding A/C condenser fan/fan relay: ever since I purchased this rig, I've noticed the condenser fan only kicks on when temperatures outside and in the engine bay are super hot. Is this normal?

To me, the condenser fan should kick on whenever the A/C is on, regardless of outdoor and engine bay temperatures.

Thoughts?

 

[The_Full_Monte]

This looks feasible, but for me, that usually means a lot of f---ing work. Thoughts from the peanut galary?

Dual 16-17

Electric Fan: Universal 12v high performance 16" inch electric radiator cooling fan. Upgraded 130 watt motor draws 12 amps, producing over 2852 CFM. Configured as an air puller by default or reversible to an air pusher. Includes mounting bracket feet and complete cooler fin tie strap kit...

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[packer0440]

As is set up from factory, the auxiliary cooling fan (official name, only factory installed on trucks with a 454), is grounded through a temperature switch threaded into the passenger side of the block, although location may very slightly depending on vortec/tbi. This switch closes when the coolant temp reaches 225 degrees, meaning it really only turns on during very intense conditions (since t-stats are usually 195), and is not connected to the air conditioning system at all. To make it come on during AC operation, you could splice into the wire going to the temp switch and make some sort of switch that would supply ground when the AC is turned on.

 

[yevgenievich]

On vortec 454 aux fan is ecu controlled. Aux fan relay is near the ecu.

 

[1998_K1500_Sub]

As is set up from factory, the auxiliary cooling fan (official name, only factory installed on trucks with a 454), is grounded through a temperature switch threaded into the passenger side of the block, although location may very slightly depending on vortec/tbi. This switch closes when the coolant temp reaches 225 degrees, meaning it really only turns on during very intense conditions (since t-stats are usually 195), and is not connected to the air conditioning system at all. To make it come on during AC operation, you could splice into the wire going to the temp switch and make some sort of switch that would supply ground when the AC is turned on.

The above may be the case for earlier models, but the later models (my 1998 anyway, and I believe all 1996+) use the ECU to control the fan, along with an AC high-side pressure switch, the vehicle speed and the engine temperature.

The first triggering mechanism is the "Compressor Pressure Switch" mounted on the hoses' mounting block, near the compressor (on the 5.7L anyway, presumably 7.4L similar) that closes at around 350psi and opens around 250psi. See the attached segment of the 1998 service manual, the highlighted text, and look for "Compressor Pressure Switch".

Other aux cooling fan triggers exist related to engine temperature and vehicle speed, see this post and others in this thread:

https://www.gmt400.com/threads/fan-clutch-staying-locked-on.57440/post-1232391

This is not to say that one couldn't enable their own method, e.g., by installing the same fan and wiring it via a relay to the AC clutch, or adding a temperature sensing switch to the condenser, or a pressure switch on the high-side AC line, or... use your imagination.

 

[GoToGuy]

Yep, aux fan installed to increase efficiency of ac.

 

[1998_K1500_Sub]

Question regarding A/C condenser fan/fan relay: ever since I purchased this rig, I've noticed the condenser fan only kicks on when temperatures outside and in the engine bay are super hot. Is this normal?

Yup, that's how it works from the factory on your 1999, because the OE "Compressor Pressure Switch" has such high "turn-on" pressure. You could replace that switch with one that operates at lower pressures to effect fan turn-on much sooner / at much lower pressures. This is something I've been planning to do myself on my 1998 but haven't. Read on...

To me, the condenser fan should kick on whenever the A/C is on, regardless of outdoor and engine bay temperatures.

The fan should turn on whenever there's a benefit, e.g., to the AC or engine cooling. When ambient temps are low and the AC's on, for example, the aux cooling fan's hardly necessary as the engine fan's airflow keeps the condenser cool.

Much like I've seen on other vehicles (not GMT400), the aux cooling fan could turn-on once the high-side pressure exceeded 200psi or so, to aid the AC condensing operation.

The aux cooling fan might also turn-on to cool the engine bay after engine shut-off in hot ambient conditions. What's this got to do with AC? A lot. Keeping the engine bay cool during shutdown lowers the pressures within the AC system, which are (of course) affected by the underhood temps. The benefit of keeping the underhood temps low after engine shut-off is that, on subsequent, hot engine start and immediate AC turn-on (a pretty typical occurrence) the AC high-side pressures won't go sky-high. Ask @PlayingWithTBI about the time his system blew a hose on the high-side after hot start-up and it dumped the contents of the AC system (oil and refrigerant) into his engine bay.

 

[packer0440]

That's right, my mistake. I forgot about the major AC system changes in 96-up.

 

[1998_K1500_Sub]

That's right, my mistake. I forgot about the major AC system changes in 96-up.

There might have been some compressor and fan control changes made in 1994, as that's when R134a became standard. I know the 1994 (and 1995?) systems (not sure about 1993 and prior) have a high-pressure switch mounted on the condenser output, as well as a high-pressure cut-out switch on the back of the compressor. I'm guessing these condenser-mounted switches weren't present in the earlier R12 systems, and I'm also guessing they are used to control the auto-recirc function and possibly the aux cooling fan on/off in those 1994-1995 models.

Any light you could shine on this would be appreciated. I really need to download the FSMs for those years and study them

 

[packer0440]

There might have been some compressor and fan control changes made in 1994, as that's when R134a became standard. I know the 1994 (and 1995?) systems (not sure about 1993 and prior) have a high-pressure switch mounted on the condenser output, as well as a high-pressure cut-out switch on the back of the compressor. I'm guessing these condenser-mounted switches weren't present in the earlier R12 systems, and I'm also guessing they are used to control the auto-recirc function and possibly the aux cooling fan on/off in those 1994-1995 models.

Any light you could shine on this would be appreciated. I really need to download the FSMs for those years and study them

In my 1990, there was a single wire cut-out on the back of the original compressor that I had to change to 2 wire when converting to R-134, which I think had a similar function to the later systems. On my 92, no such switch is present (and there is no wiring for it either). Wiring manuals for both years match up with this reality. No switches are present at all on the condenser output on either. I think that the recirculate function is controlled by this switch in the 1990. When turning on the AC, it will blow for about 20-30 seconds, then you can hear the air start to recirculate. Since the recirculate option was added to the control head in 92, this was likely the reason for dropping the switch in that year (although the compressor is the same and just has a plug in place of it). When trying to source lines for the 1990 truck (NOT easy for a 454 since they have a unique mounting orientation and high side valve location), the only two that I could find were for 92 and 94, and the pictures did look like the 94 had an additional port like you mentioned.

Both trucks have the auxiliary cooling fan as well, and the wiring looks identical. Power is wired directly from the power block with a fusible link to the relay, ignition power is sent to the other contact, and ground is provided through the switch in the head. Simple! The 1994 AC controller and ecm seem pretty similar to earlier models, despite the switch to R-134, so it seems unlikely that the operation of the fan would be completely different (especially if it involved making special wiring accommodations just for 454 trucks). Now, 95 is an oddball in all directions, so I'm not so sure on that one, and I can't find a 95 wiring manual to study this. When inclusion of the cooling fan expanded beyond big blocks, along with improved computer functionality, it makes sense to make more efficient use of the auxiliary fan.