Charging 2002 Blazer AC

[reefgeorge]

I am experienced with residential and boat HVAC systems but have never worked on an automotive cycling system so I need some help setting the charge level. I know that the fool proof approach is to identify and fix any leaks, evacuate, pull a vacuum and charge with 28 ounces of 134a but I want to identify the best way to "sneak" up on the correct charge.

My refrigerant charge level fell below the low pressure cutoff limit and my compressor would not cycle on. I put a set of gauges on the system and shorted the low pressure switch to get some measurements. Low side was 0 PSI, high side was 70 PSI. I put in 12oz of 134a. Ambient was 75 F. With the compressor forced on full time I got 15 PSI low side, 135 PSI high side. In normal compressor cycle mode the low side cycled from 52 PSI on to 23 PSI off, with high side PSI at 135 PSI maximum , 46 F center vent output. There were 4 cycles per minute (6 seconds on, 8 seconds off).

I realize that the spec is 28 - 32 PSI on the low side and 160+ on the high side and this system is probably still undercharged. Do you set the targets with the compressor cycling and look for the cutoff to fall within the 28 - 32 PSI range (with an eye on head pressure maximum) or do you set the charge with the compressor forced on?

Thanks,

George

[old_master]

Older systems using R12 with either an STV, POA, or expansion valve, there was a sight glass in the liquid line at the receiver drier. You could charge them until the bubbles cleared and you were done. On the CCOT, (clutch cycling orfice tube) systems there is no sight glass because there would be bubbles almost all the time making the glass useless. On CCOT systems, the ONLY way to know how much refrigerant is in the system is to evacuate for a minimum of 1 hour, then charge with the proper amount of R134a. The cycling switch should ideally cycle the compressor between 22psi and 42psi. The average being 32psi to avoid evaporator ice up. These systems rely heavily on the proper charge to obtain acceptable cooling, (R134a is far less efficient than R12). As little as 2 ounces over or under will cause insufficient cooling and if over charged too much, it will damage the compressor.

Compressor ON time, OFF time, total cycle time, center register outlet temperature, low side pressure and high side pressure all vary depending on ambient temperature, level of humidity, cooling system condition, and the condition of the refrigerant system, (compressor, orfice tube, clutch etc). Each of those are considered when diagnosing problems with the system though. But first of all, you must have the correct charge in the system, (don't forget the oil). If the system is low on refrigerant, it's also low on oil.

[reefgeorge]

If the problem does not involve a compressor burnout then a system flush is presumably not necessary and the system oil level is also unkown. The evacuate, vacuum, measured charge process does not significantly change the oil status How do you guys nail the oil level with an unkown starting point after a leak and repair?

[old_master]

Without catastrophic compressor failure, a flush is not necessary however, it might not be a bad idea to pull the orifice tube and clean/replace as necessary. I usually just replace them, they're cheap, usually under $4. Checking the oil level is a pretty involved process so on a maintenance repair such as you're doing, adding 2 ounces of low viscosity PAG oil is recommended. You can get refrigerant with the oil in it, or you can open a high side line and dump it in, your choice. If the system was rapidly discharged, (blown hose etc) it's best to go through the oil level checking process. With a small leak like you're dealing with, add a couple ounces and call it good.

Another little tid bit about the Harrison HT6 compressor: It is a "split case" design that, over time, will develop a leak at the split commonly called a "belly leak". They usually leak on the underside and requires removal of the compressor to confirm the leak, (discharge is not necessary to inspect). When it leaks, (not if) Sanden makes a replacement compressor that is not a split case design. It's a bolt on design requiring no modifications. It's quieter and lasts longer than the OEM design. http://www.acsource.com/sandenclonef...ompressor.aspx

[reefgeorge]

How do I related the target pressure levels to the cycling process? It appears that the high side at the end of the on cycle is stable and should relate to the 160-228 psi target, but what about low side. How do I confirm low side pressure, which is driven by the low side switch limits. The target value is 28-32 psi and the switch limits deliver a ramp down from 52 to 23 after compressor start up. I assume that you would always get this behavior except in extreme cases of improper charge so how do you confirm "28-32"?

Also, are the low and high side service port valves replaceable or do you have to replace the accumulator and high side hose to get new valves (that are not leaking)?

Thanks.

[old_master]

From post #2: "The cycling switch should ideally cycle the compressor between 22psi and 42psi. The average being 32psi to avoid evaporator ice up."

At the pessures experienced on systems using R134a, refrigerant pressure and temperature are almost equal, (32psi = 32 degrees). When the average low side pressure drops below 32psi, condensation on the evaporator coil freezes and causes the evaporator to turn into a block of ice, there will be no air flow through it if/when this happens. That's why 32psi is the target for average low side pressure. If the average pressure is above 32psi, (yours is 37.5 psi/degrees) insufficient cooling will result, (register outlet temperature will not be as cool as it could/should be).

You can replace the schrader valve cores separately if necessary.

[blazee]

Generally, I like pulling a vacuum before filling them. Most of the time, people don't want to pay for that and just want them topped off. As long as the system hasn't become completely depleted (equalized to outside air pressure and risking introducing moisture to the system), I'll just top it off. Basically, I add slowly until the pressure gets to the point that it is just above low pressure cut off. I then bring the engine up to about 2k - 2.2k RPMS (pretty close to cruising speed on the highway) and then add a little more R134a until it is again just above the low pressure cut off. This ensures that the system provides the coldest temp possible without unnecessary cycling.

This generally gives people sub 40 degree vent temps (normally low 30s). I've even gotten a mini van to consistently blow below 30, and dip as low as 26 degrees. That is colder than "ice cold" air.

EDIT: Old Master, thanks for the info on the Sanden clones. I hadn't seen those.

[reefgeorge]

Old Master:

I take it then that my low pressure switch needs to be replaced because it engages the compressor at 52 psi which is 10 psi higher than the target?

Since you calculate the low side pressure by taking the average of the low side pressure range, I take it that the compressor is designed to cycle all or most of the time, is that true? That implies that charge level is low enough that the system is always heading for a frozen evaporator, only to be interupted by the pressure switch. Do I have that right?

Blazee:

You set the low side pressure by getting it just above the low side cutoff psi which I assume would run the compressor full time in all or most cases. Is that correct? How do I relate that approach to the target low side pressure of 28-32psi? also, how do you get sub 32F vent air with a non-cycling compressor that does not freeze the evaporator?

Do you get to this charge level bypassing the low pressure switch or do you keep adding until the cycling stops?

Thanks,

George

[old_master]

Originally Posted by reefgeorge

I take it then that my low pressure switch needs to be replaced because it engages the compressor at 52 psi which is 10 psi higher than the target? CORRECT.


Since you calculate the low side pressure by taking the average of the low side pressure range, I take it that the compressor is designed to cycle all or most of the time, is that true? Yes, but with high humidity and/or high ambient temperature, compressor ON time increases and cycle time increases to the point where it might not cycle at all. That implies that charge level is low enough that the system is always heading for a frozen evaporator, only to be interupted by the pressure switch. Do I have that right? Spot on perfect.

....

[reefgeorge]

How do replace the low pressure switch? Does the system need to be discharged?

I just looked in DIY alldata and the system descripton was that this is a non-cycling system? It also says that the low pressure switch isnsupposed to cutoff at 30 psi. Whats up with that?

Given that low side pressure is driven by the limits of the low side switch and not the charge level it would seem that a better verification of charge level is the high side pressure at 2.3X ambient?

[old_master]

The cycling switch unscrews from the accumulator. No need to drain the system, there is a schrader valve in the fitting on the accumulator.

Yet another colossal example of worthless literature. All Data, Chilton, Mitchell and Haynes manuals: I suppose each one is good for something, but how do you know what information is correct? If you want correct, complete information and specs, get the factory shop manual for the vehicle you're working on.

As charge level decreases, compressor ON time also decreases. Not sure if 2.3X ambient would work with R134a in a CCOT system, probably not. Both high and low side pressures are constantly changing.

[reefgeorge]

Where can I get a copy of the GM shop manual?

[old_master]

Here's a link to the 2002 GM S&T series "dealer issue" factory shop manual: http://www.helminc.com/helm/Result.a...ected%5Fmedia=

There are over 3,000 pages in this 3 manual set. It is specific to 2002.

[reefgeorge]

OK, manual on order. We will see if this works better.

[old_master]

In the manual, you'll find charts that show compressor ON time, OFF time, register outlet temps and cycle time at various ambient temps and humidity levels. If you're going to be doing repair work on the vehicle, you won't regret having the manual!