Fan Cycling
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From: Gushikawa Japan
Fan Cycling
Does anyone know what the average fan cycling time should be? This is my first C4 (91), so I'm not on the up and up for what's normal and what's not. I have the fans jumped so that they both come on with the low temp switch, but it seems like they run an awful long time before shutting down. Mayba a 2 to 1 ratio for on to off (5 minutes running to 2.5 minutes off). I haven't checked the cycle temps or anything yet. There's so much good info on this forum that is's easier to just ask. :confused:
Thanks, Matt
Thanks, Matt
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From: Gushikawa Japan
Re: Fan Cycling (Motorworx)
Revised: The fans stay off for about 1:20, and run for about 7 minutes with the hood closed, 3 minutes with the hood open. The temp starts dropping right away when the fans come on, but has to fight for the last couple of degrees to turn the fans off.
Sound like someone installed a low temp switch with a slightly higher thermostat? For instance; thermostat closing at around 170, but the fans don't turn off until 160. 170 degree coolant being held in the block, not letting the switch turn off? :banghead:
Sound like someone installed a low temp switch with a slightly higher thermostat? For instance; thermostat closing at around 170, but the fans don't turn off until 160. 170 degree coolant being held in the block, not letting the switch turn off? :banghead:
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From: The Top of Utah
Re: Fan Cycling (Motorworx)
At first I was convinced it was an air circulation problem. Particularly in light of the hood opened/closed difference. It still may be. Pull your radiator and clean out all the junk that gets packed between the rad and condensor. A good thorough cooling system flush wouldn't hurt, either. Mostly, I think your fan switch is too cool. Mine turns the fan in at 205-210 and off at 195. It never runs, except while idling or in stop and go traffic. Good luck, and...
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Le Mans Master
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From: San Diego, Ca
Re: Fan Cycling (Motorworx)
If your checking it with a/c on, the fans are controlled by the a/c pressure switch. I don't know the parameters for the '91, but most a/c systems try to maintain a high side pressure in the range of 170 to 230 psi at idle which will keep the low side somewhere around 25 to 28 psi which provides the best vent temps and as a side benefit, keeps the operating temps within a range of the thermostat to about 212 degrees or so. For example, when the PCM receives a voltage signal indicating a high side pressure of 230, it grounds the fan relays turning the fans on until the pressure drops to 170 psi, at which point it turns the fans off. How fast the pressure falls is dependent upon the outstide air temp and the moisture content of the air. I think some of the later Vettes also use a rheostat on the secondary fan that boosts the fan speed depending on the pressure input, but the key is that the on time will vary depending on temp and humidity.
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From: The Top of Utah
Re: Fan Cycling (SunCr)
If your checking it with a/c on, the fans are controlled by the a/c pressure switch. Yadda, Yadda, Yadda.
I have the fans jumped so that they both come on with the low temp switch...
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From: Gushikawa Japan
Re: Fan Cycling (CFI-EFI)
OK. The fans are turning on at 174 and off at 163. I think that's too low. Does anyone know the location of the switch? It's not between 1 and 3. The one wire thermistor for the gauge is between 6 and 8, and the one for the ECU is in the front of the intake manifold. I can't see any other switch. :confused:
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From: The Top of Utah
Re: Fan Cycling (Motorworx)
With 174* on and 163* off, it's a wonder the fans ever shut off. The fan switch is a one wire affair, also. When it reaches it's design temp, the contacts close and it completes the relay primary citcuit by grounding it in the head. My parts book doesn't go to 1991. What IS between the #1 & #3 spark plugs? I'm not familiar with a dual fan set up that dosen't have the fan switch in one of these locations or the other.
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From: The Top of Utah
Re: Fan Cycling (Motorworx)
Like I said, I'm not familiar with a C4 with an aux fan that doesn't have the aux fan switch in one of those locations. The main fan, of course, is ECM controlled. The cars I know of, that have the plug, were origionally single fan cars. You may have to dig out your service manual or start tracing wires. If this is common in later years, maybe someone with a later car will pipe up. Again, good luck, and...
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Le Mans Master
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From: San Diego, Ca
Re: Fan Cycling (Motorworx)
Didn't mean to mislead you -diagnosis of the dual fan system begins with the a/c pressure switch - it provides primary control. In fact, the trouble charts begin with grounding the diagnostic connector to see if both run; then, removing the ground, turning on the a/c, and then with a manifold gauge set, monitoring system pressure, see if and when the fans comes on. Otherwise, here's the parameters for fan operation (a/c off) as commanded by the PCM. There is no fan switch per se, just input from the Engine Coolant Temp switch (mounted on the intake below the throttle body):
"The PCM will command fan 1 "ON" at 104°C (219°F) and "OFF" at 98°C (208°F) and, fan 2 "ON" at 109°C (228°F) and "OFF" at 103°C (217°F)."
Given your temps; I'm guessing either the a/c is on or the PCM has been reprogrammed.
Hope this helps a little.
"The PCM will command fan 1 "ON" at 104°C (219°F) and "OFF" at 98°C (208°F) and, fan 2 "ON" at 109°C (228°F) and "OFF" at 103°C (217°F)."
Given your temps; I'm guessing either the a/c is on or the PCM has been reprogrammed.
Hope this helps a little.
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From: The Top of Utah
Re: Fan Cycling (SunCr)
Are you saying that both fans on a '91 are ECM (or PCM, if you prefer) controlled? I'm about to LEARN something here. If so, what year did this start?
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Le Mans Master
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From: San Diego, Ca
Re: Fan Cycling (CFI-EFI)
Yes. It started it when both fans became suckers instead of a pusher and a sucker - '90 I believe, but at least by the last L98 or '91. There's no fan switch for the aux fan in the left head (which were also redesigned for '91), but the provision is still there since both heads are interchangeable and the right side still has the temp gauge sender. The a/c system was also redesigned, replacing the fan switch with a pressure switch that communicates pressures from 0 to 450 psi. The ECM/PCM uses that info for fan control; compressor operation and will spit out trouble codes if system operation falls out of design parameters - something the earlier C4's wouldn't do, or what they can tell the tech is extremely limited.
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From: Gushikawa Japan
Re: Fan Cycling (SunCr)
That would explain why the fans don't come on immediately with the a/c switch. There is a short delay until the system builds up pressure.
This has been very helpful. Thank you to everyone. I have ordered the 185/175 switch from Eklers, and will rewire the system appropriately. The ECM goes away this summer in favor of a stand-alone, anyway.
This has been very helpful. Thank you to everyone. I have ordered the 185/175 switch from Eklers, and will rewire the system appropriately. The ECM goes away this summer in favor of a stand-alone, anyway.