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Old 08-12-2007, 12:43 PM   #1
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electric fan conversion and engine temps question

Ok, I am thinking of trying to fabricate the entire dual fan setup on my parts LT1 9C1 caprice to my pickup.

I found this information:
The B/D-car coolant fans operate under PCM control at the following engine temperatures and A/C system pressures:

Fan Mode Temperature A/C Pressure
Primary (RH) Fan ON: 107 C / 225 F 189 psi
Primary (RH) Fan OFF: 103 C / 217 F 150 psi
Secondary (LH) Fan ON: 111 C / 232 F 240 psi
Secondary (LH) Fan OFF: 107 C / 225 F 210 psi

Additionally, the PCM will turn off the fans at higher vehicle speeds (above 48 MPH I believe) since running fans can actually impede airflow through the radiator at high speed. Each fan also has a minimum running time. Once activated, the primary fan will run for a minimum of 50 seconds, and the secondary fan for a minimum of 26 seconds. Finally, certain Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one or both fans.

The primary is 150watt and the secondary is 100watt. Does this seem like overkill? It's pretty much the same engine in both vehicles.

I'm concerned with how to control when the fans turn on. I'll naturally take the relays from the caprice but the PCM activates them. I've seen mention of those saab thermal switches that you place inline on the lower return line and they only make a connection when they're at a certain temperature. How exactly do they work and what ranges do they come in? How do they know when to 'turn off' since it's at a different temp than when they turn on. Could I just use two of these, one for the primary and one for the secondary?

Additionally I would think I would want all fan operation to cease above 48MPH (as stated above), I've thought of using the TCC lockup as a good clue, does anyone know what reference wire to pull for this on a standard 700R4?

Lastly, what about AC activation, are there AC pressure switches that work like the fan temp switches? Would one of these be on the caprice I could just use? Or are they most likely just a pressure gauge and I will have to read that value somehow and decide when to activate the fans? How important is running the fan when AC is on? I rarely use AC when I'm around town or below 30MPH as I just open the windows, so I should never really need to worry about the AC being on and below 48MPH and overheating, but I'm just curious if I did forget and used AC while idling (very bad for my prone to overheating truck), can this hurt the AC or does it just lower the efficiency (as it's radiator doesn't cool as well)?

Lastly, what about the temps. From the factory my truck has a 195F thermostat as does the Caprice, but the fans are designed to only come on at much higher temperatures, it seems like at idle/hot situation the stock 9C1 setup would hold the radiator temps at around 220F. Is this too hot? I know running at 195F is supposed to increase efficiency, but is there ever a point where you start to loose efficiency or it's too stressful on the engine? I'm thinking with these fans the truck won't ever overheat even if it does stay at higher temps most of the time, because the fans should be able to cool it off quickly if it ever needs to. Thoughts?

Thanks all.

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Old 08-19-2007, 05:52 AM   #2
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Theres not many options since you don't have the PCM.

Use the simple temp switches to drive one fan, the second fan activated via relay from the A/C compressor.

Buy a commercial fan controller such as the SPAL PWM.

I'm installing a SPAL myself and updating this thread as I go, its taking some time though.

I do plan on using the torque converter control output of the ECU to prevent the A/C from running the fan. I found on the schematic the wires out the ECU that control the TC but have not tested it yet. After I get my fan installed I'm going to look into rigging a relay to interrupt the A/C control to stop the fan under TC lockup.
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