theadmiral94

I found out today that my understanding (as well as many other forum members) of what turns on the SECONDARY coolant fan has been wrong.

Now, when I turn on the AC (auto button, C68 controls), BOTH coolant fans come on immediately.

Previously, ONLY the primary fan would come on when the AC was turned on (although I thought I remembered way back when both would come on, but doubted myself over the past couple of years).

I had believed based on the FSM that the secondary coolant fan would only come on based on coolant temperature over 238 degrees F.

However, while trouble-shooting a recent AC system compressor's short-cycling, I determined the system was just low on freon (cycling switch on low-pressure line was working perfectly) and added 12 oz of R134a via the low-pressure side, which stopped the short-cycling and returned the low-pressure side to a steady 30 psi at 1000 rpm in 85 degree ambient temperatures (which is correct per the 'performance chart for CCOT systems' in the FSM, page 1B-12).

BONUS -- now when I turn on the AC, BOTH coolant fans come on immediately, regardless of the digital dash coolant temperatures (both came right on at digital coolant temps of 176 and 201 degrees F -- well below the 238 degrees F when the secondary fan used to come on).

Interestly, the FSM section for the VIN (J) cooling fan control (6E3-C12-4), does mention turning on the coolant fan(s) when the AC high side reaches 240 (albeit relative to a 'A/C cooling fan switch' which the LT1s do not have).

Coincidently based on the 'performance chart' in the FSM, 240 psi is the high end of the high side pressure with ambient temperatures at 90. This makes me think that there are less differences between VIN 'P' and VIN 'J' than the FSM suggests.

So, moral to the story.

If BOTH your coolant fans are NOT operating when the AC is turned on, your AC system may need to be charged.

BTW, Now, with A/C on, and the car idling for 1/2 hour in the garage (with no extra fans blowing on the car), and BOTH coolant fans running, the max coolant temp was 201 degrees F -- AWESOME.

Red Dwarf

The fans on my 95 have 2 speeds, "lo" & "Hi". The fans operate in concert & donot turn on one at a time as some believe.

I have heard mechanics mistakenly refer to the "lo" speed as the primary fan & have also seen it referred to in that manner in the 95 FSM. Can be a little confusing.

theadmiral94

Red Dwarf,

You make a good point about the 95s and I believe the 96s too. GM added a 3rd fan relay which allows for the two-speed fans (whereas 94-92 had only two relays).

So, my comments are particular to 94-92 for sure, and may apply to the earlier C4s if they have a secondary (aka auxilary) fan.

Red Dwarf

I wasn't sure if that applied to the 94. Thanks for the update.

pcolt94

The 95-95 operates this way.

This is definitely a confusing post for a 94. Have never heard of fans operating as if they were a 95-96. If you look at the fan and relay circuitry configuration between the 94 and 95, they are completely different. On a 94 the PCM drives each separate relay for each separate fan. On a 95 it's configured so fans run series or parallel depending on relay switchology driven by the PCM.

It almost sounds like the PCM is bad or you don’t have a 94. But in consideration of what you said about it changing modes of operation in regard to the freon charge makes no sense to me of what to think.

If you have a FSM you can ID the relays, number of relays, and location of them.

I will also say that the 94 does operate as the FSM says and how it has been discussed in the Forum. But if you have ever driven around with a Tech 1 attached to the ALDL connector you will see things that are not advertised. At higher speeds all fans shut down. At lower speeds (temp 200) I have seen the secondary fan come on after getting gas.

I believe there is more software code and operational parameters in the PCM than anyone knows about or is in any printed mater. If you have a true 94, it just does not sound right that the secondary fan always runs. I will say it will keep it cooler but might be at the expense of the fan one day. Always has higher electrical load on alternator because the secondary fan draws more current that the primary fan (on a 94).

SunCr

Cycling, orifice HVAC systems are designed to operate at head pressures that average 200 psi. Static, non operating pressures, are dependent on the outside air temp and rise with air temp. Once the compressor engages, the pressure immediately rises and that also creates heat - about 100 degrees at the condensor for this pressure. Cooling is provided by fan(s) which for most applications are turned on and off by the ECM and the ECM needs to know what the head pressure is in order to maintain the ideal pressure. This provides the best vent temps and by turning the fans on and off (which it should be doing if it's much below 50 degrees out), minimizes cycling of the compressor clutch. Since 1990 - for those vehicles not using a mechanical fan - information is provided to the ECM by a pressure sensitive sensor or thermistor mounted on the High Pressure line. It's fed a 5 volt reference from the ECM and each volt returned is approximately 100 psi. The ECM, based on this signal, will then ground the fan relay(s). Exact operating parameters should be outlined in the FSM - try looking in the electrical schematics under Fans or Compressor if it isn't in Driveability and Emissions. The easiest way to troubleshoot a problem is with a scanner. That will show you the a/c pressure sensor signal in volts (better ones also translate this to pressure) and you can observe the fan(s) to make sure that they are operating as designed. A couple of notes: Below 50 degrees, it may take some engine heat to raise system pressure to the threshold for fan operation - best to test when it's 70 degrees or better. Extremely high pressures - much above 260 psi for R134, might be indicitve of a restricted system - even though both fans are cranking. Hook up a manifold gage set so that you can observe both the high and low side pressures.

theadmiral94

PCOLT94,

sorry if it is a bit confusing, I was tired when I typed it, but wanted to get the info out given the recent heat wave.

It is possible the PCM is not a 94 or was programmed with different logic (we're the 3rd or 4th owner), I have never really checked, but I don't think so.

It has only two relays, both mounted on driver side of radiator shroud, just as indicated in my 94 FSM.

Without the AC turned on, the two fans operate as normal and touted by most on the forum for a 94 (i.e. primary/driver side fan comes on at 228 -- and secondary/passenger-side fan would also come on at 238).

With the AC turned on, but previous to adding freon, only the primary/driver side fan came on (regardless of coolant temp), but once the coolant temp rose to 238+, the secondary/passenger-side fan would also come on.

Now, with the AC turned on, BUT after adding freon to prevent the very fast compressor cycling (and no cooling inside to speak of), both fans come on immediately (or so far at least with coolant temps at 176 plus -- haven't tested it yet starting up cold.)

The part of FSM for LT1s (VIN P) which I found not clear/incorrect is that the PCM appears to turn on the secondary/passenger-side fan based on AC pressure too (in addition to coolant temperature).

However, there is a mention in my 94 FSM of AC pressure having an affect on fan operations for the VIN J.

So, to your point, there are many things that are not documented in the FSM -- and I may have found yet another 'feature' we didn't know about (I really like the fans coming on after getting gas, very interesting). -- as if it wanted to burp

I agree that the secondary/passenger-side fan running whenever the AC is running might draw extra amperage (except over x mph, under certain temps, when passing gas stations ), but at this point I am not sure what was 'designed', it is possible that both fans were intended to run in hotter air temps, but many of us have slightly under charged AC systems which has been preventing this 'designed' feature.

SunCR gave me an idea too -- as I do not have AC manifold gauge set (yet) I should be able to use the diagnostic mode of the C68 system or the PCM to check the 'high' pressure is not too high.

BTW, my understanding of the 95-96 fan operation via the PCM is they didn't really change the PCM's wiring or the number or fan circuits within the PCM, but introduced a 3rd relay and changed the type and wiring configuration of the other two relays. Thus when the PCM activates the 'low' (or previously known 'primary' circuit), it activates both fans on 'low speed', and when the PCM activates the 'high' (or previously known 'secondary' circuit), it activates both fans on 'high speed'.

theadmiral94

Suncr,

thanks for the response -- very well written and a great explanation of how the AC system works --

Your explanation makes me wonder if I may have slightly overcharged the AC system, causing higher than desired pressure on the high side (low side was correct, based on my inexpensive AC charging hose/gauge from interdynamic from wallyworld, or at least I think?).

Though I do not have a manifold gauge set, I recall reading the ability to check the high-side pressure sensor's output in the PCM or C68 diagnostic mode -- so I will check and follow-up.

I'll have to double-check, but though I have been through my FSM many many times, I don't remember it providing specifics about the fan operation as it relates to AC pressure (for the LT1/VIN P), hence the title of this thread. But stay tuned, if I find it, I'll advise its location.

Mr. Peabody

To your original post ...... that is why many of us who use our vettes as daily drivers run with the AC on in stop and go summer traffic because with both fans on it keeps the coolent temp lower than without the AC running