SDGuy

My A/C blows cold when the outdoor temp is over 75 degrees F. But my compressor won't engage if the outdoor temp is lower - even if the temp **** is fully CCW. Sometimes I want A/C for windshield defog or if it is a humid 70 degree day.

Is this normal, and if not as I suspect, where do you place your bet on the problem? I assume this is not a problem with the A/C relay or compressor itself. Isn't there both an interior and exterior thermostat for the A/C system? Where are they located and how hard are they to replace? Thanks for any advice. Howard

83na944

I don't know about late model AC systems, but the AC on early systems could be engaged at any temperature. I'd be suprised if the later AC limits this, either.

My guess is that your car is low on refrigerant charge and the low pressure switch is cutting the compressor out. You should be able to check this when it's below 70 by jumping the switch for a short period. Better yet, take it to an AC shop and have them check it out.

SDGuy

You are probably right, although I had a recharge 4 months ago. Today it was quite warm and the compressor clutch still is not engaging.

The 86 944 seems to have a single low/hi switch at the drier. For the life of me, I can't pull the two-wire plug off the switch. Is there a trick to this? The plug seems to be a molded type with the 2 wires coming out the top. How do I get it off? Do I just short the two wires at the plug? At least this will tell me if my compressor is bad so I know what I'm in for at the shop. Thanks!

Charlotte944

Late model climate control systems have an external temp sensor. Look under the blower box cover on the passenger side for a Bosch style connector that appears to be "dead ended."

When ambient temperature is high, the heater will not put out any "hot" air. However, if you set the temp control to max heat and hit the defrost button, the vents will blow REALLY hot air. This also engages the A/C compressor. With the A/C running the evaporator will pull moisture from the cabin air, helping to clear the windshield.

Charlotte944

Actually there are TWO switches. The one with the green connector is a high pressure switch (Bosch calls is a "refirgerant temperature switch."). This switch closes when the A/C compressor discharge pressure is too high. When this switch closes it triggers the radiator cooling fan relay to switch the fans to high speed. If you follow the wires you will find they connecto via spade lugs to two other wires. These wires go the the fan relay and are in parallel with the high temp switch portion of the radiator fan switch.

The second switch is black and "silver" (metallic) and has two male spade terminals that connect to two wires with female spade lugs. This is the compressor low pressure shut down. When this switch is open the A/C compressor will not engage.

SDGuy

Cliff,
Thanks for that great info. I had actually sent you an e-mail earlier in the day asking about this. What threw me off is this description (high,lo,binary) of the switch: http://www.paragon-products.com/prod....821.36.51.htm

So shorting the two spade teminals on the black/metalllic switch should force my compressor on - so that I can check the low side pressure? Is this switch also at the drier?

Charlotte944

The description of the switch is not correct. The switch is forward of the reciever/dryer. Pop up the headlights to get to the switch. Pull the wires off of the switch and short the wires together with a jumper. Then start the enigne and turn on the A/C. The compressor should engage. A/C technicians use this "trick" when re-charging the system after it has been evacuated.

Since you are having trouble with the system, I would not run it in this condition for any legnth of time unless you are going to add refrigerant.

SDGuy

Well, the compressor engages when I jumper the two leads, so the freon must be low. It doesn't blow cold. Since I had it recharged just 4 months ago, guess it is going to have to go to the shop to find the leak. Are 944 leaks most often compressor seals, condenser, or the hoses? Perhaps it is time to switch to R134a.

Here are 3 pictures. Cliff, double check my info on the hi/low sides.

smokey

Cliff, is it normal for the AC only to blow cold when the outside temperature is high, as per SDGuy's first question? My AC (1989 951) seems to work when it's hot outside (above 80 degrees) but does not do much when it's 70 or so. Does that mean that the temperature control is working properly? There are times in 70 degree weather where the AC would be handy due to humidity, but I don't know whether I have a problem with the temperature sensors, the mixing flaps or the electrical connections right now. How is the system supposed to work?

Charlotte944

You have the fittings properly identified. I can also tell that the system has not been converted to R-134.

Leaks can happen anywhere in the system, but front seal leaks appear to be the most common. After that it's service valves, hoses, and O-Rings. There is also a possibility that the condensor may be the problem. Because the condensor is directly in the air stream it is possible to pick up sand and grit from the road.

Dodge Durangos are particularly suceptable to leaking condensors because a portion of the front bumper acts like an air scoop and can actually sand blast the condensor. I had to replace the condensor in my step son's Durango because of this.

Charlotte944

I don't have the correct technical data available, but based on my personal experience, the operation of the climate control system is dependant on both ambient and passenger compartment temperature.

The ambient temp sensor is in the blower box on the passenger side near the air inlet. The cabin temp sensor is buried in the dash. In fact, the cabin sensor is "fed" air via the round grill next to the clock. A small fan takes a suction through the grill, blows the "sample" over the temp sensor and then sends the "exhaust" into the golve box.

As I understand the system (and I am by no means an expert on the subject), cabin and ambient temperature sensors and the climate control system temperature control are used to control the position of the mixing flaps, which in turn controls the temperature of the air coming out of the vents.

When you want heat, the climate control system opens the heater valve and adjusts the mixing flaps so you get "hot" air. How "hot" this air gets depends on what the actual cabin and ambient temps are. If it is hot outside, the heater will blow warm, and vice versa. When you hit the defrost button, the climate control system turns on the A/C, opens the heater valve, sets the fan to high speed, and adjusts the mixing flaps for max heat.

A/C operation is similar, but the heater valve is not opened. In either heat or A/C operation, cabin temp is controlled by the temperature control **** on the climate control module.

smokey

Thanks, Cliff.

SDGuy

Cliff, if you don't mind, I need a little more advice. I took my car to the AC shop and the system is not holding pressure. They checked for freon leaks with an electronic tester. No leaks at the compressor, condenser or lines, but the thing went crazy when they put the thing under the passenger dash. They said it is either the evaporator or expansion valve that is leaking. There is no way to determine which one without tearing the dash apart from what they tell me (although they don't work on too many p-cars). The expansion valve is $50 but a new evaporator is $800 - so you can guess which one I hope it is. Any advice on troubleshooting which it is without tearing the interior out? Thanks again. Howard

Charlotte944

To get to the expansion vavle you need to remove the cover over the blower motor, and then remove the blower.

Too get the cover off you need to remove the firewall to hood seal strip (it just pulls off) and remove the wiper arms. The cover may be glued or taped along the lower portion of the windshield, so be careful.

SDGuy

Thanks again. I ordered an expansion valve and drier from this place:
https://www.autohausaz.com/catalog/l...Token=30128745
Anyone have an opinion on which is more likely to have a significant freon leak - the evaporator or the expansion valve? Howard