There seems to be a lot of questions in regards to "is my passenger fan working?" Especially during the summertime months when it gets hotter outside and we run the air conditioning more often. Another question is "are both fans supposed to run when the air conditioning is on?"

This thread is particularly for cars with dual fans (1987-1992 cars). There is a small change in the 1987 cars compared to the 1988-92 cars.

100% stock systems work as such.

1988-1992 cars: The driver side fan will turn on at 223°F coolant temperature. It will also turn on and stay on when you have any of the air conditioning settings set (MAX, NORM, BI-LEVEL, DEFROST). When you have any of the A/C settings turned on, the fan will be on regardless of coolant temperature. If it's cold outside and you just did a cold start and have the defrost turned on, that will turn on the A/C compressor, which then turns your driver side fan on. Even though the coolant temp might only be 50°F (ambient outside temp)

The passenger fan will turn on at the stock "fan switch" temp (found in passenger side head between spark plugs #6 and #8). The stock fan switch is 235°F. It will also turn on when the air conditioning high-side pressure is above 233 psi. It will shut off around 190 psi.

This means that if you turn on your air conditioning, your driver side fan will be on. Your passenger side fan "may" be on. It "may not" be on. This depends on what the high-side pressure is.

In July/August with air conditioning on, chances are your high-side pressure will be 250 psi or more at all times. If you're in stop and go traffic/rush hour traffic, it most certainly will be 250+ psi. Pop your hood after getting home with the car still running and the A/C on and the coolant temp sky high and take a look. Are both fans on? Your fans are working properly.

But in spring/fall weather, particularly in mild climates, if you pop the hood and take a look with the A/C on, you may well find that the passenger fan is off. Or if will turn on and off every 30 seconds or so. This means that your high-side A/C pressure is varying back and forth between 190-230 psi. It rises to 230 psi. The passenger fan turns on. It lowers to 190 psi. The passenger fan turns off. This will repeat itself.

In wintertime, if you pop the hood and take a look with the defrost turned on/engine running. The passenger fan will probably never turn on. Because it's just too cold outside for the high-side pressure to build up.

Unfortunately, unless you have a set of A/C manifold gauges, you won't be able to know what the high-side pressure is. You can buy these at any automotive store, Ebay etc. You need the kind with two pressure displays (blue and red). The single displays that have blue only will only display the low side.

1987 cars with dual electric fans have a slight difference in that turning on the A/C won't turn on the passenger fan. It is only controlled by the coolant temperature in relation to the "fan switch." So if your car has 220°F coolant and the A/C is on, your passenger fan will be off......I suspect this was a one year only deal because GM figured out that the air conditioning worked better on dual fan cars if the A/C could control when the passenger fan turned on. I live in Texas and even during the peak of summertime hell, the car very rarely ever hits 235° coolant temps. But I need every bit of help I can get in terms of air conditioning performance. (My car is a 1989. I'm lucky in this regard.)

How can I change when the fans come on?

Driver side: The only way is to put a different computer chip in that changes the stock 223°F fan on setting to something lower. Hypertech, Jet, PCMForLess etc. Keep in mind, this is only for when your A/C is turned off. Because turning on the A/C will turn the driver side fan on regardless of coolant temp.

Passenger fan: Put a different fan switch in the passenger side cylinder head. Common options are 200/185°F (on/off) as well as 176/166°F (on/off)......Personally, I run the 200/185 switch on my car. It works well for summertime and wintertime. I get good heater output inside the car in the winter (fans will be OFF 99% of the time) while it keeps the coolant temp at 200° or less during the warmer parts of the year.

I find the 176/166 fan switch to be too low. Your fan will be on forever and never shut off (unless you really can get the temps to 165°F or colder). Too cold for winter and just going to wear out parts faster and use more voltage when you don't need to. If your car is at 170° coolant temps, do you really want to have both fans on? I don't.

Remember though that even with a stock computer chip and a stock fan switch, that turning on the A/C will in most cases turn on both fans. (unless your high-side pressure is below 233 psi). If you live in Phoenix and have a stock setup, both fans will most certainly be on in July/August with the A/C on. (1987 cars will be different in this regard as mentioned above)

Various situations which affect the passenger fan

Like many here, I wondered if my passenger fan worked. It would never turn on. I lived in a small town in Canada. Summer weather was 80-85°F. Rush hour traffic was non-existent. I would let the car idle in the driveway for 20 minutes and the driver side would eventually come on. But the passenger side never would. Turning on the A/C, the passenger fan would stay off. There are 2 issues at work here:

1) Low Canadian ambient temps meant the driver side fan alone would keep the engine from hitting 235° coolant temps. When I put the 200/185 fan switch in the car, the passenger fan would start kicking on sporadically. (Those rare times when it would actually get to 200° coolant temps.)

2) I was low on R-12. My car had a leaky shraeder valve. Turns out I only had 1.8 lbs of R-12 in the car. (Stock capacity is 2.25 lbs of R-12). Once I fixed the shraeder valve and put the proper amount of refrigerant in the car, the passenger fan started working again when I'd turn on the A/C. But it wouldn't run constantly unless it was a really hot day. Hot enough to keep the high side pressure at 233 psi or higher........When you're low on refrigerant, the car will cycle the A/C too often for it to build up the high-side pressure high enough to turn on the fan.

Another situation is wintertime. Say it's -10° outside. You're parked outside. You turn on the Defrost once you start the car. But no radiator fans are spinning. That's because even though the Defrost setting should turn on the A/C compressor, in this situation it won't. That's because in this situation the temperature is below the "low pressure cycle switch" setting. You know how your compressor will cycle off on a warm day when the cold air gets to a certain temp? (typically about 40°F blowing out the ducts at you) In ultra cold weather, you'll be BELOW that temp anyways. This is why the defroster only turns the A/C compressor on when ambient weather is above 40° outside.

I've mentioned "high-side pressure" a few times. What is this?

Air conditioning measures both low-side pressure and high-side pressure. The low side is the cold side. (evaporator inside the firewall that's cold. Same as the evaporator inside your house air conditioner or the freezer compartment of your fridge.) Low side pressure typically runs 30-100 psi.

Our cars will cycle off the compressor at 25 psi, then cycle them back on at 45 psi...... 25 psi is roughly 40°F. So the person getting in their car at -10° in a Minnesota winter probably already has their low-side pressure at maybe 15 psi. This is why they can't get their compressor to turn on regardless of which setting they use. Their radiator fans will be off.

The high-side pressure is the hot side. This is the condenser in front of your radiator. The "big fan box" outside your home if you have a heat pump or central air conditioner. These shed the heat to the ambient. If it's July in Arizona or swampy Florida, your condenser will be maxed out and then some, trying to shed heat to an already desert hot outside or muggy sweaty hot outside. This means your high-side pressure will be super high. Ditto driving in rush hour traffic at slow/stopped speed in the summertime.

The above is why your passenger fan "may be" off in your car even with the air conditioning on. If it's mild weather and light traffic, the condenser will easily shed all of it's heat to the ambient. And running the defroster in the wintertime when it's only 50° outside will immediately send all of the heat to the ambient with ease. The high-side pressure will be very low in these instances. Maybe 150 psi. Not enough to turn on the passenger fan (233 psi). It will be much less than the 250-350 psi that extreme summertime weather and heavy traffic can bring.

that should answer a bunch of questions. good deal man. im actually glad i went aftermarket with a flexalite controller and ls1 fans.

Man, perfect explanation. I just converted my a/c today and ran it on max and sure enough just the driver side fan kicked on and stayed on but the temp never got up to 235 even though it idled for about 20-30 minutes. The passenger side fan never kicked on. I though maybe they were backwards. I guess I'll find out soon enough if that passenger side fan works. It was only about 75 today with low humidity. I did buy a hypertech fan switch to kick the fan on at 200 and turn it off at 185 but it will be overridden when I turn on the a/c. Thanks for taking the time to write this up.

Excellent thanks. Now to test drive some in the FL swamp and see what happens.

thanks for the info. When I do the dual fan conversion this will help so I don't beat my head against the wall when the pass fan doesn't turn on.

QUESTION...do you have a schematic? I would like to see how the pass fan is wired

Wow, that explains so much, I always wondered why the passenger fan was never on when I checked! Thanks!

Took a look at my RS and noticed there are sensors on each head. So correct me if I'm wrong, the sensor on the driver head is for the temp gage, the sensor on the intake is for the PCM, and the sensor on the pass head is for the coolant fan? So then how do I hook up dual fans? does the sensor on the intake turn the fan on?

No. TBi, hvac controls or fan switch only turn the fan on.
A stock/lightly modded TBI will benefit more from a lower temperature fan switch more than it will dual fans.

as stated above only the switch on the passenger head turns on the single fan. if you want to put in dual fans you would have to wire in another relay. there are actually several ways to do it. i went with an aftermarket controller on my rs.

Click on my signature link at the bottom of post #1.

so if I wire in another fan and relay I can just tap on to that wire and it should work? I am going to put in the lower temp fan switch. I'm going to hook up the 2nd fan more for better A/C cooling so I will instead tap onto the A/C clutch so the 2nd fan comes on when I turn on the A/C

Not to be a dink, but do you have any schematics or troubleshooting charts to back any of this up? I've got old service manuals from before we closed our pontiac dealership, and I've personally had about 50 of these cars come through my doors over the years when I used to own a shop, and absolutely nothing support the behavior of your first post.

I'll be happy to scan and submit the troubleshooting charts and diagrams from the actual GM supplied text. I cannot however be responsible for cars whose electrical systems have been altered over the past 20-25 years.

-- Joe

Check the link in his sig...

Though, I've always seen diagrams show the fan pressure switch dictating weather the second fan runs or not w/ a/c on.

I don't know what manual that is. I've got one in my hand and the exact page says:



-- Joe

Since I'm speculating, I'd say '87 is the odd year out due to it being the first year for the dual fans (iirc) and the secondary fan/relay/wiring was a tacked on, last minute idea.
The following years ('88+), the A/C request was reversed to being open, then closing to ground @ 232psi.

The fan request input wire on an '87 stays grounded. The fan pressure switch is normally closed (reverse of 88+) and opens when the line psi reaches 232. The way this is setup, you can not use the fan pressure switch as a trigger to ground the secondary fan with A/C.

Also, if you have an A/C car, remove the A/C, the blue wire on the fan pressure switch has to be grounded or, the primary (or, main for single fan). An open ground signals the ECM the A/C has been turned on.

Wow man, excellent explanation. Thanks. It makes perfect sense to me since I live in Miami. I know now that mine have always worked perfectly fine. Thanks again.

thanks guys as always very helpful. I'm going to attempt hooking up the 2nd fan so it comes on when the A/C is turned on. Reid the schematic on your post works out perfectly!!! I noticed yesterday with A/C on around town driving the temp was at 210* and that little single fan was working overtime.

By all means, feel free to add in any GM texts/charts etc. My pictures are from 1991. But I assume that the 88-92 years were all the same as far as turn on temps, pressures etc. It coincides with how my '89 works.

As far as I'm concerned, the way GM set up the dual fans, it's a great system. They're on when they need to be and off when they don't need to be. It's a pet peeve of mine when people hack up the dual fan wiring to be always on. It's just not needed for cold starts, winter driving etc. I don't see the point of having a manual switch under the dash that can turn on the fans at will. Because just adding a simple 200/185 fan switch works perfectly for any climate. Now changing a single fan to a dual fan setup, I'm cool with that.

A bit off topic, but if you want to see how new Corvettes do it, read post #2 here:
http://forums.corvetteforum.com/c6-z...n-come-on.html
This is pure genius as far as I'm concerned.

okay quick question..
ive got my a/c all out completely but i still have the HVAC plugged in..if i turn my "A/C" on, will my drivers side fan still kick on?

Here is the trouble shooting charts in a '92 manual. Can you look at this section in your manual? (8A-31) It indicates that with engine idle, and AC turned on to NORM, the fans should NOT operate until the pressure gets to 232* (1600kpa).




The chart you posted (6E-C12-1) Appears to indicate that the ECM will turn on the engine cooling fan when the AC is selected to on. While there is an A/C status bit in the code, I don't ever remember any code mask I've looked at turning a fan request on. I'm going to look at the $6E and $8D hack to verify. This is very odd because I've never seen an application work the way you (and that chart) are suggesting, yet some folks seem to report it working that way, others not..

-- Joe

I could not agree with you more.

I think the primary fan will come on when the ECM sees the A/C requested on, meaning it sees the compressor coil being energized. If your A/C is out try jumping the connector that went to the compressor clutch and see if it comes on when you flip the A/C lever to max.

Now that's a great post.

Found bad pax fan, replaced both while I was under there.

Stock 89 5.7 seems I get the opposite fan op from what Reid explains above.

1 . Cold start - driver fan comes on for a moment then off. (no biggie)
2. Cold engine with AC on - driver fan cycles, pax fan off (huh?)
3. Warm engine to 160 gauge indicated - driver fan off, pax fan on constant (huh?)
4. Warm engine to 160 gauge indicated AC on - driver fan cycles, pax fan on constant (huh?)

Sounds like what Reid explains but in reverse? Am I missing something?

How does it compare to the charts I posted in post # 20 ?

-- Joe

Joe - can I use the 1992 manual to diagnose my 89? I am awaiting my 89 service manual from ebay.

My fan setups are 100% untouched with the exception of the 200/185 fan switch. I also have a 180° thermostat.

I have had a few different chips in the car.

• A Hypertech Thermomaster (turned primary fan on/off at 176/166.....the same as their optional fan switch. Though I bought their other fan switch. The 200/185)
• PCMForLess chip. Had it programmed to turn the primary fan on/off at 190/185. This is my preferred on/off as it gives it a slight head start before the passenger fan kicks on. Can get away with just the driver fan on during mild weather.
• Stock GM chip. On at 223°.

I took out the PCMForLess chip and put the stock GM chip in because my car seems to run lean. And the GM chips are known for being a bit on the rich side. But here is the interesting part. Say I'm at 215° coolant temp with the A/C off. In theory that should mean that my passenger fan would be on but my driver side would be off.......But in actually, at 200° both fans will turn on. So it seems to me that the secondary fan turn on will override the primary fan being off.

This is yet another reason why I like the 200/185 fan switch. Because if I'm cruising at 215° coolant temps (with the A/C on) and then turn the A/C off, it will keep both fans on. Under a 100% stock chip and fan switch situation, both fans would turn off while the coolant was at 215°.

Having said that, I suspect that you have the connectors for the driver and passenger fan reversed. So in this case your passenger fan circuit is turning on your driver fan and vice versa. This would mean that if your car is like mine and the passenger fan ON overrides the driver side, that your car would have the driver side fan ON override the passenger fan.

3. Warm engine to 160 gauge indicated - driver fan off, pax fan on constant (huh?)
Reverse of mine (when I had the PCMForLess or Hypertech turning the 1st fan on early)

4. Warm engine to 160 gauge indicated AC on - driver fan cycles, pax fan on constant (huh?)
Reverse of mine.

Just for the heck of it, I went outside and hooked up the gauges and watched everything at work.

It's 84° outside. I'm parked in the shade. And I did a cold start. Both fans turned on briefly. Maybe 10 seconds. Then stopped. It's always done this.

Turning on MAX A/C, both fans kicked on and then the passenger fan would cycle off at aprox 190 psi and back on at aprox 230 psi. The passenger fan cycling would occur maybe every 15 seconds or so. There isn't much load on the system right now. The interior is cool. The engine is cool.

The driver side fan stayed on the entire time......Finally switching the setting from A/C to VENT, both fans turned off. At this point I shut off the car.

Coolant temperature was 100°F or colder the entire time.

Reid -

This could not have hit the message boards at a better time! Took my car out last night to go to dinner about 3 miles down the street. Traffic in town was backed up for a local celebration along our harbor area and the car overheated (never has done this before). Ran home from the restaurant with no symptoms, so I parked it for the night.

Went out this morning and ran it up to temp. Thermostat openned OK but neither fan was on as the temp rose to about 230 on the dash gage. Obviously, my suspect problem was the primary fan or the relay/circuit leading to it.

Your troubleshooting tip to turn on the AC proved to me the fan was not functioning properly and, since I lost no coolant when it went to overtemp last night, I can now focus on the fan circuit for repair.

Thank you for a very clear and informative posting. And, by the way, can you post something else the day after my next part fails? I could not believe I just logged in to research my problem and your posting was the first on my "unread" list. You have great timing!

Based on the conflicting data I've seen, I'm not sure a 1992 manual can be used to diagnose a 1992...

-- Joe

Reid thanks for the comments. Tell me, in your cooling fan diagnosis link in post #1, that is from an 89 service manual right?

And I agree what you propose that my relays might be driving the wrong fans. But they could not be cross wired at the fans b/c the harnesses at the fans are diff lengths. So they could be crossed at the relays? Do you know if the relays are side by side in the charcoal canister area I could check? I know the pax relay is there since i heard it clicking when I grounded the head switch but I'm not sure about the other. Thx!

Yes perfectly the opposite of what mine are doing. Thus my post #31...

If only it were always that easy.


The stuff in my signature is from a 1991 F-body manual. Not sure if it was a service manual or something similar to that. I have a Haynes book for my 89 (The book is the 82-92 Pontiac Firebird Haynes), but it's very sparse in terms of what/how/when the fans work.

Unfortunately that's all the information I have on the 1991 fan diagnosis info.

I'm afraid I don't know where the exact location of the relays are. I've never had to swap mine out.

[QUOTE=Reid Fleming;527950
I'm afraid I don't know where the exact location of the relays are. I've never had to swap mine out.[/QUOTE]

In post #20, my chart shows the component (relay) locations.

-- Joe

Joe,

Post 20 chart says for VIN F relay is LH front of dash. This is vague to me b/c there should be two relays for what the manual says heavy duty "if so equipped" right?

Besides that my 89 "heavy duty" relay is at the charc canister. Not sure where the other is. I'll get my manual this week and post back what I find out.

On another note the PO must have a cooler therm and/or fan switch b/c at 160 gauge indicated the fans kick on. Unless the gauge if off :-/

Cheers.

I don't know if this is true but I was told that the stock single fan pulls 3,000cfm...? how much air do the dual fans pull when both are on?

I hate to be a tard...but the actual relay/switch. At the local auto parts store they only show a realy for the a/c compressor. The staff says the relays are all the same. And they do look the same and fit. I have 3 relays driverside upper firewall and one on the pass side by the blower motor. Unplug the one by the blower-motor and the a/c goes off line. Two of the relays on the driverside have the black with pink-stripe wires that go to the two fans. I replaced both of them with the relays that were listed as a/c comp switch at the parts store. This causes the passenger side fan to come on whenever the a/c is on. The relay for the driverside fan never turned on the fan. After 1/2 hour idle with a/c on and drive around the block..and it was very hot out...the temp started to really rise. Test light for the power going into the relay is hot. Removing the relay and jumping the two female prongs for power in and out of relay causes the fan to come on with ignition off. Is it possible that with all the different set-ups and relays...over the years there are some unlisted relays that are no longer stocked that might be internally wired slightly different and we are being sold a relay that is supposed to be for the fan associated with the a/c. I noticed that the relays have unused contacts that would be for diffent set-ups... example...my relays have a three large male prongs and two smaller gauge male prongs where the attach point in the car only uses two of the small ones and two of the large ones. The third big male blade is a blank female with no contact in it. I have been going through this cooling problem for three years now. I'm thinking there is a different relay than normlly listed...maybe? Seems pretty simple that removing the relay and jumping the two females together kicks on a fan even with the ignition off and the key in my pocket...but wont ever kick on on a dang hot florida day with a second brand new realy in-place. I'm getting a headache ...haha.

I would think if you wanted to run dual fans it would be very simple to Run a Jumper wire with a vamp tap from the grn/wht wire on the main relay to the Gry wire on the heavy duty fan relay this way no matter which fan is triggered they both come on for whatever the reason beit temps or a/c.

I don't have a problem but why does my 92 with single fan have an a/c high-side fan switch if that's only for the 2nd fan in a dual fan setup? I used it a long time ago to wire a manual fan switch when I was too lazy to figure out why my fan wasn't turning on at all. With it disconnected the fan properly comes on when the a/c is turned on.

(what book did you get those pictures out of or do you have a digital copy of it thanks in advance )

Hello all,

I have been reading your posts on here about cooling fan issues. thank you very much for the great info. I bought for my 17 y/o son, a 92 Camaro rs, t-top, 305CI auto tran. she is green with gold stripes. It was sitting in a yard under some pine trees for a few years now. so I rescued her.

id been racking my brain as to why the cooling fan will come on when I put the a/c on. (single fan) but wont when idling and starting to over heat.

I replaced the sensor on the top of the engine. (intake man) I drained the coolant and checked the sensor aka temp gage sensor drivers side head wall. took it out, cleaned it up. reinstalled. fired her up, still no fan. but it will work when I put the a/c on??? I replaced the t-stat with a 195F. put in some new antifreeze 50/50%. again. fired her up. no fan???

I just read on here about the sensor on the passenger side wall.

"THANK YOU"

I will check this out tomorrow and get back to you guys.

BTW i'm Dennis Fleming I live in central mass. a little farming town. between Worcester and Springfield by the Quabbin reservoir. call Barre.

again, thank you so much for sharing the knowledge. this is great.

good night

hello AM I correct in saying that if the ac is out of r12 , the passenger side fan will not turn on at all? Reason I ask is that my drivers fan is always on and for the first time ever, my car ran to almost 250 degrees and the pass fan did not come on at all. Once I started to move, the temp dropped right down. your thoughts?

The passenger fan should still turn on when it gets to the fan switch temperature (235°). It shouldn't matter if you're out of R-12 or not. You could have the car on VENT or HEATER and the same thing would happen. It would turn on solely because of the fan switch.

The A/C high pressure sensor just has the ability to turn on the passenger fan regardless of coolant temp. I could drive 3 blocks from home with a cold start and the A/C on. And the high pressure sensor would turn on my passenger fan, even though the coolant temp might only be 100-120*°F

so if my pass fan is not working, where would be the best place to start looking? relay and which one? the fan itself? I don't remember if it was working prior to changing the vent control panel in the car which I am sure has nothing to do with it. I originally thought both were coming on at the same time when I first got car.

I'm fighting the SAME problem right now. I can jump both relays and get the fans to come on...

i have 2 relays on the pass side of the car. one right next to the radiator and one on the fender. the one on the fender operates the passenger fan. Once I replaced that relay, the fan came right onand now I have both working.

Hi - 1988 IROC 5.7 - in my situation my passenger side fan does not come on at temp or when the AC is set to max. So I guess its accurate to say possible bad fan or relay. Could you direct me where the relay is for the passenger side fan. Thanks