1991 Trans Am L98 TPI

I just charged the AC system in my car with R12a (Canada) to 38psi (per directions) with compressor running. The secondary fan won't come on when the AC is turned on. The fan/relay/fan switch in head all work normal. The fan will come on if engine temp gets hot enough, but just won't turn on when the AC is running.

Where does the signal come from to turn the secondary fan on with the AC?

The secondary fan comes on whenever ac is requested, D12 being grounded bye the ac pressure switch or if the engine gets to 236f the cylinder head switch grounds.

The secondary fan will come on if I jump the connectors on the AC pressure switch or ground the fan switch in the cylinder head. I also tried a different AC pressure switch with the same results. I charged up my AC system after putting it in the car with a parts store refill kit and I didn't put a vacuum to it either. Also the compressor never cycles off, and its not that hot in Canada this week.

I only have a gauge for the low side and wonder if the high side isn't building enough pressure to close the AC pressure switch and request the fan to come on.

How hot is it outside?

Today it was about 25*C - or 77*F

You're probably on the verge of it. R12a has a lower high side pressure than R-12 and much lower high side pressure than R-134a. It's quite conceivable that your A/C isn't getting the high side pressure up high enough to actually turn the passenger fan on.

25°C daytime temps for me is about November. And during that time of year, I might have the passenger fan on with the A/C. I might not. Depends on load (stuck in traffic), ambient temp, humidity, all sorts of things can influence the pressure readings.

The instructions that came with the R12a said to charge low side to 38psi while compressor running. I did that initially, it took 1 can of oil and 1 can of refrigerant to level it out at that pressure. I think it needs more oil still as the tester shows very low oil level. So the system only has one 6oz can of R12a in it.

After I checked the oil level a couple times and changed the AC pressure switch I lost a bit of pressure. Checked the pressure with it running after doing those 2 things and it was about 28psi. According to the test strips it could use more oil. I'm using the oil that comes in a pressurized can. If I add that will I still be able to add refrigerant and stay within proper low side PSI?

Does more pressure in the low side correspond to a higher high side pressure? Is 38PSI an accurate pressure for R12a?

I think the schematic is incorrect and the ac switch is n/o on f-body's.
Use this chart vs ambient temperature to figure out the high side pressure.
http://www.ackits.com/aacf/ptchart.cfm

I figured that switch was supposed to be n/o and closes at 190psi. It wouldn't make sense for it to be n/c all the time. If it were, then the fan would run constantly.

If R12a has a lower high side pressure than R12 or R134a then according to the conversion chart I won't even see enough pressure to close the switch on the high side.

Will not building enough pressure make the compressor run constantly? Is it bad for the compressor to never cycle off?

What happens in the system if I put a charge of the 38psi to my AC low side on a cooler day (65-70*F)? On real hot days will the system then have too much pressure?

You checked the relay by the brake master cylinder?

6 oz of refrigerant isn't much. My car took 2.25 lbs of R-12 originally. That's 36 oz.....The HC blends like Envirosafe, Duracool, and R12a tend to take about 12 oz to be equivalent to 2.25 lbs of R-12.

Does more pressure in the low side correspond to higher high side pressure? .....Generally yes. The high side is simply a measure of how much heat is being sent to the condenser up front. If the condenser is plugged with dirt, if your fan(s) aren't working, if it's hot outside, stuck in traffic jam, the high side pressure is going to go up regardless of refrigerant amount.

The refrigerant itself is the heat transfering mechanism. So if you're low on refrigerant, you won't be moving much heat. As such, the high side pressure won't be very high. I noticed when I had 1.8 lbs of R-12 in the car (low by 1/2 a pound), that the passenger fan would never kick on. It just wasn't moving enough heat to the condenser for the fan to kick on. And cooling performance wasn't very good. Only getting about 45-50°F coming out the ducts. The car would cycle the compressor on/off quite rapidly (due to low charge)

How much oil do you have in the system? 6 oz is full of oil. No need for more.

The trouble with using a specific low side psi to measure needs more/too full is that the low side pressure is going to change depending on ambient temperature. During the wintertime, the low side might be 30 psi. It might be 20 psi. This is why the compressor doesn't turn on at all when you turn on the defroster when there is snow on the ground. But the compressor does turn on with defroster or A/C use in the summertime. If you were in Arizona in super hot weather, the low side pressure might sit at 50 psi and never drop below that.

28 psi is pretty much certain to be too low though. Because the compressor should cycle off around 25 psi. And if it's sitting at 28 psi in the driveway, it absolutely will run lower than that when you're moving (and getting a rush of ambient air cooling the condenser up front. For this reason, I like to see 35 psi MINIMUM at the low side when checking the pressure. 40-45 psi gives you enough leeway so that when you're driving, it will have enough time to keep the compressor on without cycling rapidly.

Air conditioning isn't about creating cold air. But rather removing hot air. The more hot air that is removed, the more cold air is left over. So when you first start the car cold and turn on the A/C, the high side will rise as more heat gets shed to the ambient and the condenser gets warmer. At the same time the low side will fall as the cabin air gets colder. But once the condenser gets maxed out, the high side will keep going up while the low side will start to rise up as well. At this point your cooling is maxed out. This is why a traffic jam on a hot summer day will see low side and high side pressures both running high. The air blowing into the cabin won't be as cool as driving at nighttime with no traffic.

LOW REFRIGERANT: High side will be low. Your capacity to move heat is low since the refrigerant is what moves it. Car should cycle the compressor on/off rapidly (if you have a cycling compressor)......If you have a variable displacement compressor, it won't cycle. It will reduce it's compressor (variable) output.

TOO MUCH REFRIGERANT: High side will be high. Capacity is good. But you're over pressurized in the system. The high side will be maxed out before it should be. At that point your cooling performance goes down the crapper.

So overall it's a delicate balancing act between the low side pressure and the high side pressure. I would really recommend buying a true A/C manifold gauge. One with low and high side gauges. Not those cheapy red/green level indicators. But one that shows actual psi. After buying some manifold gauges, you'll notice that the high side hose won't fit on your car. You need a 3/16th adaptor to be able to screw onto the R-12 high side fitting on your car. If you had R-134a in the car prior and have the press on fittings, then a simple set of R-12 (screw on) to R-134a (press on) adaptors will do the trick.

Word to everybody: DO...NOT....INSTALL REFRIGERANT....IN THE HIGH SIDE PORT! Only use the low side. Installing on the high side will result in the can exploding in your face. Serious stuff. This is why the cheap low side only gauges are sold in stores. There is relatively low risk involved with them. But they don't tell you what's going on in terms of the high side.

To me, it sounds like you probably already have a good amount of oil in the car. I'd put the other 6 oz can of refrigerant in and see how it goes with that.

WOW, that is an amazing amount of information. You are a great resource to have on this forum.

I put the 6oz can of oil in the system and a day later I used the test tube to measure oil level. It still said it was low, but for now I will leave it as is.

Everything you said about pressures and compressor performance makes sense.

I think a friend of mine has proper AC gauges, so tomorrow I will ask him if he does. If not I will find someone that has them and measure both sides.

I also thought that the low side was low at 28psi and in the cooler weather up here doesn't help the high side psi.

I don't need the secondary fan right now, but I just want to make sure the system can function as designed if it does reach the proper high side psi.