I have an 88 Iroc with dual coolant fans.When I ground the aldl the primary coolant fan comes on with the key in the run position.Is this correct?My factory service manual says if this happens check section 6e for ECM diagnosis leading me to think that the fan coming on is a problem with the computer.Also if I turn on the A/C or defrost the secondary fan does not come on.The A/C is inoperative so I know the pressure switch won`t turn the fan on but should it still turn on?Thanks for any replies.

 

Are your fans not working? The primary fan uses the computer to turn it on and the secondary a temp switch. The temp needs to be a bit over 220 to turn them on.

There is also a number of fan threads going on now that you can also look at.

 

99.9% no at this time of year. It's simply too cold outside to build up enough high side pressure (heat going to the condenser) in order to turn the secondary (passenger side) fan on.

With your defroster on, you'll probably only hear the A/C compressor run for about 5 seconds. That's obviously on a functioning A/C system. If your A/C isn't working, then the only way your secondary fan will turn on is if the coolant temp is higher than 235°. But at this time of year, that's very unlikely to happen too.

As for grounding the ALDL port under the dash. It's been awhile. But I know that grounding it turns on one of the fans. I can't remember which one though.

The majority of the time, the passenger fan works perfectly in our cars. It's just that the conditions for it to run happen so rarely. If you're low on refrigerant or it's not a heat wave, the passenger fan will be off.

 

I learned something new with this. When I first read this, I thought "no, that's wrong - the fan is controlled by the A/C switch, and has nothing to do with the actual pressure in the A/C system." But alas, I looked at the wiring diags, and you're right.

My cars were all TBI or carbed, and the system is much simpler on those - it has nothing to do with A/C pressure at all, the fan is just connected elctrically with the A/C switch - switch on = fan on, switch off = fan off.

It's very interesting that the dual fan cars have such an elaborate setup for controlling the secondary fan with regards to the A/C system (ie fan controlled by achieving a certain pressure in the A/C system).

 

The V6's will also have a fan A/C pressure switch and the TPi will use the MAF as part of the system as well.
Never looked for the answer as to why the system used a pressure switch. Just guessed it was to put that much less of a draw on electrical current. Lot of people don't realize that the alternator under a load can be more of strain on the motor than the AIR pump.

 

The pre 1988 cars used a fan switch only, to control the secondary (passenger) fan. The 1988-92 cars used both the fan switch and/or air conditioning "high side" pressure to turn the secondary fan on.

The primary (driver) fan will turn on/off whenever the air conditioning is on/off. No pressure psi numbers are used for this. It also is controlled by the computer chip temperature setting as well. Now the primary fan won't cycle on/off with your A/C compressor when it's cycling, because there is something like a 15 second delay between when the primary fan will turn off (from A/C being off) to when the fan actually shuts off........So what I'm saying is that switching from A/C to VENT or OFF or HEATER will turn your primary fan off. But the air conditioning system cycling generally won't turn the fan off. (The off cycle for the compressor is probably 5-10 seconds depending on various factors. Not long enough to turn off the primary fan)

Why the change in 1988 to add A/C high side pressure to the mix? There are a few reasons. First of all, air conditioning performance will be better when that 2nd fan turns on. We typically think of air conditioning as "cooling the air." But what's really happening is that we're "removing the heat." Air that's had heat removed is cold. It's the same thing that your fridge at home does. A fridge won't create cold air. It simply moves all the heat from the fridge and sends it to the bottom or rear of the fridge. What's left over stays in your fridge and freezer.

By turning that 2nd fan on, your A/C will remove more heat from the system at the condenser. But and here is the big factor. The stock 2nd fan turn on temp of 235° generally won't be reached. Even in August when the A/C is running constantly. Thus GM added the A/C high side pressure to control the fan.

So once the high side pressure reaches about 230 psi, the secondary fan will turn on. When (or if) the high side pressure gets down to 190 psi, the secondary fan will turn off. The higher the psi, the hotter the condenser is. So at 190 psi, the condenser is cool enough that the 2nd fan isn't needed. Thus saving you some electrical draw and a slight improvement in fuel economy. Volt meter reads a little higher. Simply put, at 190 psi all the heat will have been removed from the car.

Compare that to driving in 100° weather. The high side pressure may be running 280-400 psi and it still can't keep up with the heat load being put on the car's interior. The condenser is running at maximum capacity and still can't keep up.

I've hooked up gauges to the system and watched it all cycling in my driveway. With it on A/C, primary fan will stay on the whole time. "Low side" psi pressure will hover around 35-40 psi. The high side will slowly rise up to 230. Then the secondary fan will turn on. Then the high side will slowly go back down to 190. Secondary fan cycles off. It will do this forever.

Now if it's August in TX/FL/AZ etc, the high side may not come back down to 190 psi. So the secondary fan will run continuously. It's simply too hard to shed heat from the condenser to really hot and/or really humid air. Same way a human body can't cool off (shed heat) when it's really hot and muggy outside.

So all things considered, an 88 and later model should have better air conditioning performance than an earlier model. Although if you live in a mild climate, the difference may be unnoticeable or even non existent. The 2nd fan being controlled by high side pressure is simple yet very effective. It's only run when needed this way.