1999 Jeep Grand Cherokee Limited 4.7, 134a

[urbansound]

Hi,

Thanks so much for the efforts here on the forum. I am a new member, sorry this is a long post but I wanted to be thorough explaining the issues and efforts so far, before asking for help.

I'm helping a friend with his Jeep WJ, 1999 Grand Cherokee 4.7, having insufficient A/C cooling.

The JGC Limited has dual temperature controls and hence dual blend doors. We carved into the plenum behind the glove box and determined both blend door motor drive cogs were stripped. We replaced the blend doors with an aftermarket blend door pair, which is fixed to both flaps, hence the firewall side motor and passenger control are now null. Both blend doors follow the driver's setting. It seemed to work fine mechanically reaching both plenum blocking positions when we were done, closed up the plenums and re-sealed it.

Blend door fix improved cooling some, but not substantially. (more on this below)

We verified all temperature sensors measured and tested within spec for resistance and appear to be working as do the A/C high and low side cut out switches.

We recovered the freon for leak test = 30 minutes holding with no loss at 29.x inches vacuum = Pass. We then continued evacuation for 1 hour to pull any moisture. No signs of desicant contamination during seals replacements and Shrader service we could see, no prior damage to A/C compressor. New Schraders high and low side both with pressure seal caps.

The sticker on the hood called for 24 ounces of R134a which we provided by low side input, via standard gauge manifold set.

During fill, the compressor cycled on and off as the pressures climbed and then went to full time on, (stopped cycling).

We powered down the system and verified we equalized within less than 5 minutes.

At 85 degree outside ambient, 65% relative humidity, near sea level we got the following pressures...

At 700 RPM idle, 43 PSI Low side, 215 High side, 17 degree delta from center louvers panel mode versus outside ambient. The pressures seemed a bit low within spec, but having put the charge in to spec by weight we started looking elsewhere.

With compressor disconnected, system on full demand, we measured temperatures at the louvers to be heating above outside ambient. This called to question new blend door failures, dirty evap or engine compartment hot air getting in past leuvers.

We blocked off the fresh air intake in the cowling above the firewall, set to recirculate max air, considering engine. It helped some, but still no better than 15-17 delta panel louver versus outside ambient.

We then blocked off the hot water inlet to the heat exchanger which did bring the panel louver temps back to outside ambient with compressor disconnected but AGAIN no major benefit to the cooling delta panel versus outside ambient.

We then added 4 more ounces of freon just to read comparison. Pressures rose to 46 psi low side, 230 psi high side at idle, again little benefit to cooling at all. We then ran a road test and noted at 1800 RPM we were seeing 35 PSI low side, 275 high side, after surging to 30 psi / 350 psi briefly before stabilizing back to 35 / 275 at 1800 rpm, 85 degree outside ambient.

We then recovered the added freon back to prior pressure values, to protect the system from over-pressure.

This seems to suggest low on low side, high on high side and makes me suspect the orifice tube is partly blocked even though it equalized appropriately on shut down, 5 minutes to balance fully. No hissing, no squealing, no compressor noises.

At 85 outdoor ambient 65% Rel. Hum. the cabin temperature in the vehicle barely comes below 75 degrees, even with max / re-circulation and full time compressor on.

We have replaced the viscous fan clutch drive new and confirm the solid state relay driving the secondary electric fan is functioning properly as does the fan when called on. Engine operating temperatures are nominal and well behaved. The condensor is in good shape and the stock radiator is 4 years old. Vehicle has been well maintained overall, 3rd owner.

We're intending to ...
1) replace the orifice tube line (integral assembly) if only to assure new component. (pull front clip, uggh).
2) re-open the plenum behind the glove box and verify the blend doors are still sealing to insure all the blower flow is crossing the evap. and not diverting around through the heating plenum.
3) learn if the compressor should cycle under normal conditions or is full time compressor expected on 85-95 degrees outside ambient?

I'd like to ask if I've overlooked anything or missed on diagnostics ? I'm not an A/C expert but have worked on many systems. Seems to me if the compressor can hit 350 psi on surge, the compressor is capable of adequate compression. We're hoping this isn't indicating a dirty blocked up / fouled evaporator considering none of these WJ series ever came with a serviceable screen to keep the incoming air clean.

Thank you again for any suggestions.

Urbansound = Mike, Holland, MI

[JohnHere]

I'm helping a friend with his Jeep WJ, 1999 Grand Cherokee 4.7, having insufficient A/C cooling.

How many miles are on the odometer, and does it have the original compressor?

The JGC Limited has dual temperature controls and hence dual blend doors. We carved into the plenum behind the glove box and determined both blend door motor drive cogs were stripped. We replaced the blend doors with an aftermarket blend door pair, which is fixed to both flaps, hence the firewall side motor and passenger control are now null. Both blend doors follow the driver's setting. It seemed to work fine mechanically reaching both plenum blocking positions when we were done, closed up the plenums and re-sealed it.

Blend door fix improved cooling some, but not substantially. (more on this below)

We verified all temperature sensors measured and tested within spec for resistance and appear to be working as do the A/C high and low side cut out switches.

Looks like you have the blend doors fixed, and the switches/sensors test okay, so I don't think the problem lies with them.

We recovered the freon for leak test = 30 minutes holding with no loss at 29.x inches vacuum = Pass. We then continued evacuation for 1 hour to pull any moisture. No signs of desicant contamination during seals replacements and Shrader service we could see, no prior damage to A/C compressor. New Schraders high and low side both with pressure seal caps.

We've discussed this issue at length in the past, but I'll recap briefly: A system that holds vacuum doesn't guarantee that it won't leak under pressure, especially on the high side.

How do you know that the desiccant shows no signs of contamination? Did you cut-open the accumulator, remove the desiccant, have it analyzed in a lab, and then reinstall it? IMHO, that's not worth the required time, effort, and expense when you can buy a new accumulator for less than $20. MVAC (Mobile Vehicle Air Conditioning) Best Practices simply suggest that the Receiver/Dryer (R/D) or accumulator should be replaced every time a system is opened to the atmosphere.

The sticker on the hood called for 24 ounces of R134a which we provided by low side input, via standard gauge manifold set.

You inferred a couple of times that you charged the system by pressures and by weight. The correct way to charge or re-charge a system is by weight.

The process is to place the 12-ounce can or 30-pound cylinder on the refrigerant scale, hooked up to the Manifold Gauge Set, and then "zero" the scale. This way, the weight of the hoses, can, or cylinder are taken out of the equation, and you are weighing only the refrigerant that's being charged into the system. Any other method will give an incorrect amount.

During fill, the compressor cycled on and off as the pressures climbed and then went to full time on, (stopped cycling).
We powered down the system and verified we equalized within less than 5 minutes.
At 85 degree outside ambient, 65% relative humidity, near sea level we got the following pressures...
At 700 RPM idle, 43 PSI Low side, 215 High side, 17 degree delta from center louvers panel mode versus outside ambient. The pressures seemed a bit low within spec, but having put the charge in to spec by weight we started looking elsewhere

Testing pressures and other operating parameters at idle speed will give false readings and is useless because the compressor is pumping next to nothing at that engine speed. Bump-up the engine speed to ~1,800 RPM, windows/doors open, A/C set to Max or Recirc, airflow set to dash vents, and re-test.

You mentioned "17 degree delta," but professional MVAC techs never use that term. Simply record the center vent temperature using an accurate thermometer. Why go through unnecessary mental calculations just to get a center vent temperature, which is normally measured directly?

We're intending to ...
1) replace the orifice tube line (integral assembly) if only to assure new component. (pull front clip, uggh).
2) re-open the plenum behind the glove box and verify the blend doors are still sealing to insure all the blower flow is crossing the evap. and not diverting around through the heating plenum.
3) learn if the compressor should cycle under normal conditions or is full time compressor expected on 85-95 degrees outside ambient?

1. I suggest cutting open the line, removing the OT, and inspecting it. If you find traces of amber oil and little to no debris on the screen, that's a good thing. If the oil is black and you find metallic and plastic particles on the screen, that's an indication that the compressor is disintegrating internally.
2. Not necessary because you've already gone through that process.
3. A compressor will cycle, or not, depending on the temperature of the evaporator, presuming the evaporator temperature sensor (thermistor) is good.

I'd like to ask if I've overlooked anything or missed on diagnostics ? I'm not an A/C expert but have worked on many systems. Seems to me if the compressor can hit 350 psi on surge, the compressor is capable of adequate compression. We're hoping this isn't indicating a dirty blocked up / fouled evaporator considering none of these WJ series ever came with a serviceable screen to keep the incoming air clean.

I would inspect the OT first. If it's clean, then proceed to change the HP line containing a new OT, replace the accumulator, evacuate well, and re-charge the system by weight and according to the specifications on the under-hood decal.

Let's not over-think this. A charge that's "off"—just a little bit—might be the only thing wrong with this system.

[urbansound]

@ JohnHere,

Thanks for your reply. We suspected we'll have to recover the freon again, so we tested increased charge then backed it back down to the same pressures for now, till next weekend or so. On final effort we will do exact tare weight freon charge again, according to hood label = 24 ounces, no additives.

I tender the ambient because most pressure charts indicate relative to the outside ambient heat load reference for general nominal pressure values affecting the delta at the condenser, again this was anticipating further service. I do understand the ultimate goal is to get the panel temps down to a capacity cooling temp, regardless of ambient.

The vehicle has 300,000 miles on it but the engine still has cross hatching in the bores and less than 0.003 ridge at top of bore, i.e. likely the engine was replaced late in it's life as the bores read standard diameter, within 0.0015 factory spec. We put new heads on it recently as it lost head gasket on driver's side, rear most jug. No dropped valve seats but the last mechanic put the wrong springs in it and didn't torque the heads properly, older rockers with poor oil pressure which took out the cam's bearing surface. We cannot know if the compressor was ever changed, but given it can produce 350+ psi I'm hoping that indicates it's still working nominal once the accumulator line and orifice tube are done.

My comment about no descant was simply that as we had the system apart we saw no debris in the system anywhere. When we replace the orifice tube, we'll cut it open and examine it as you indicate for any signs of severe exposure / contamination.

It's good to know we want to evaluate at 1800 RPM, thank you. I can't tell you how many mechanics I've seen overlook this, but it makes perfect sense as the vehicle runs there most of the time or is accelerating surge approaching run speed. Most of my experience is on industrial chillers R-12, R-22 and a few of my own personal cars R-134a, so you've helped us to understand the "correct" process.

I'll reply back after we replace the accumulator and orifice tube. Pretty sure that's gonna prove out, but we're going to verify the blend doors are working right, as I cant imagine why we're getting heat exchanger temps out of the louvers with the compressor disconnected and controls set to full cooling demand.

Thanks again for your input and tips to watch for. I'll update on the results. It's very nice to find this forum with so many professionals helping others do a better job.

Mike