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weak compresser

MXHOWES on Mon June 16, 2003 4:18 PM User is offline

Year: 1992
Make: Chevy
Model: Astro
Engine Size: 4.3
Refrigerant Type: r-134a
Ambient Temp: 70
Pressure Low: 23
Pressure High: 140

These questions concern a Harrison HR6HE.

After fully charging a empty dual air system (4 cans) high side will reach 140, low stays about 23. Given enough time (maybe 3 min...sometimes never) low will drop to 21, compressor cuts out for maybe 15 sec and cuts back in at 43#. I get vent temps of 44 with an ambient of 70. There is frost right after the orifice and on the lines going in and out of the accumulator.
So, I am thinking that either the compressor is weak and can only pump out 140# or there must be a restriction on the low side at the compressor.... Right? Wrong? Or?
The compressor is very quite.
After the car has been sitting over night the compressor spins quite easy (using one finger can turn it over).
Driving on the freeway makes little difference to the above mentioned specs.

The car cools adequately for my use, but with the compressor engaged 90-100% of the time I’m sure it will affect my gas milage and the life of the compressor(what’s left).

Thanks, Jim

Dougflas on Mon June 16, 2003 10:06 PM User is offline

Try to add more refrigerant. It should not frost after metering device. I'm betting that you're undercharged. Add a few oz at a time. The high side is too low also. More "gas" should correct this.

Mitch on Mon June 16, 2003 10:48 PM User is offline

I have a different opinion. If you charged with 48 ounces of 134a, that is the the spec for a factory dual air 1996 Astro, so I think you are OK on charge.

The last thing you would want to do is raise the high side by adding refrigerant above the spec'd amount.

What was your engine speed and were you on inside air or outside air? Was the rear air on or off? High or low speed if it was on. If at idle, everything is great. Even above idle, rear air systems have a hard time pulling down to the switch setting. See my post titled "My theory on rear air" a couple of weeks ago,
originally posted on May 22, last comment on May 23rd. About page 9 of the subjects concerning General AC Questions

It sounds to me like your system is working fine. A quiet compressor, resonable high side pressure, and good cooling is exactly what everyone is looking for in the AC game.

Edited: Mon June 16, 2003 at 10:55 PM by Mitch

MXHOWES on Tue June 17, 2003 10:44 AM User is offline

Mitch, Good post on the rear air, several years ago when my system worked perfect I would agree with your conclusion. In looking at the way the rear air is piped (return line not going thru the accumulator) I just wonder if that has something to do with it.

But back to my Astro.... since the system calls for 3.75# of r-12, 4 cans of r-134a is exactly 80%. At 1500 rpm and only the front on low I can get it to cycle on 45sec. off 15 sec. As soon as I add higher fan speeds or rear air fan it stays on constantly. (maybe cycles after 4 or 5 min).
When I get to some place where the ambient is over 70 I will look to see if I get a higher high side reading.

Thanks, Jim

Mitch on Tue June 17, 2003 10:59 AM User is offline

The first symptom of a low charge on CCOT systems is excessive cycling and you are not having that problem. Rear air systems add a little confusion because the GM rear air usually uses a TXV at the back.

As you are thinking, a weak compressor will result in low cooling under high loads, but if you can pull 23 psi low side and you can freeze the accumulator, everything is fine at that ambient temperature. The compressor has the capacity to handle the load at that temp.

The frost on the accumulator tells me that your charge level is pretty close. A warm accumulator would indicate a low charge.

When the weather gets hotter, if you can't get the low side down to mid 20s, compressor won't cycle, and cooling is poor, I would agree with you that the compressor is weak.

Edited: Tue June 17, 2003 at 11:02 AM by Mitch

MXHOWES on Tue June 17, 2003 8:32 PM User is offline

Let’s see if I understand what you are saying. Just like a water pump, which may only be able to suck 20' of head can pump 150' of head. The job of the compressor is as much creating a low pressure on the suction side as creating a high pressure on the high side. But the high side pressure is not as important as the low side pressure (except that the higher the pressure, the hotter the gas and the easier it is to dissipate the heat). Maybe I am starting to understand A/C 101. (maybe not)

Thanks, Jim

P.S. There is a small 'bomb' between my compressor and the condenser (looks factory), is this an in line filter?

Mitch on Tue June 17, 2003 11:13 PM User is offline

The main job of the compressor is to create a pressure drop across the expansion device (fixed orifice or TXV) and provide a flow of refrigerant so that the liquid refrigerant can expand into the low side and absorb heat.

Low side is very important because that sets the temp of the evap core and the temp of the air at the vents. The high side is less important. It needs to be only high enough to push the liquid across the expansion device. There is a certain minimum, but generally, the lower, the better.

In MVAC, the high side rises to whatever temp difference it takes to reject the heat picked up at the evap. The high side can be all over the place, but you want the low side to around 26-27 psi average for the best cooling with 134a. Non-cycling systems are set to achieve that pressure. Pressure switch cycling systems cut out at 21-23 and cut in at 39-42.

Stationary systems are usually designed to have a condensing temp of 30-35F above ambient. At 70F that would equate to a condensing pressure of 125-135 psi. If you can get that on a vehicle, it is great. Usually pressures are somewhat higher, but it depends on a lot of factors. As I recall the 96 Astro had about 150-160 psi at ambients around 85F and I remember thinking it was very low (a good thing). Cooling was fine and vent temps were as low as 37F.

The can or bomb is almost certainly a muffler. Some are empty chambers, some are baffled chambers.

Remember the cardinal rule of hydraulics " Pumps don't put out pressure. They put out flow. Pressure is resistance to flow"

Edited: Tue June 17, 2003 at 11:24 PM by Mitch

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