Don't Knock Until You Try It
GMC Suburban A/C Compressor Knock

"It sounds like the noise is coming from the A/C compressor, but the A/C still seems to work fine," she said. "I just started using the air conditioner last week and that's when the noise started."

"That is an important observation," I said. "I'll note that on the repair order. The technicians like to have as much information as possible to help diagnose these kinds of problems."

As she was waiting for our shuttle to give her a ride home she asked, "Have you ever had sweet potato ice cream?" "No, and that sounds sort of weird," I replied. "Don't knock it until you try it,"
She said. "I make the best, award winning sweet potato ice cream anyone's ever had. In fact, I'm making some today for this year's county fair exhibit. I'll bring some back for you," she related. "That would be great," I said. "I'll try to keep an open mind."

When we got the vehicle into the shop, the technician said he thought he remembered seeing a technical service bulletin that addressed this problem. He was right. A quick check in our ALLDATA automotive information system revealed a bulletin that listed the same symptoms for 1994 and 1995 GMC® Suburban® models built before the vehicle identification number breakpoint of SJ701253 and equipped with Rear A/C only or Rear Heater and A/C systems.

The bulletin went on to describe the cause of the problem. When the rear A/C system is shut off, a refrigerant flood back condition may occur through the rear A/C system. This flooding degreases the internal parts of the compressor resulting in rapid slider block wear and the resulting loud knocking noise. A poor contact between the Thermal Expansion Valve (TXV), capillary tube and the rear evaporator outlet tube can allow the TXV to remain open when the rear system is not in use. The open TXV may allow liquid refrigerant to flood back through the rear system (liquid line, TXV, evaporator, rear suction line) and subsequently flood the compressor.

After listening to the compressor to verify the complaint, I called her to get the okay to do the repairs. She agreed and we went to work. Here is the service procedure the bulletin described, which involved replacing the A/C compressor, the TXV, and adding an in-line filter.

Service Procedure
1. Recover the R-134a refrigerant charge.
2. Replace the compressor and balance the lubricant in the system.
3. Install an in-line filter in the liquid line after the condenser and before the "Y" in the line separating the front and rear systems.
4. Remove, inspect and clean the orifice tube for the front system. It is located in the liquid line after the "Y" joint.
5. Replace the orifice tube.
6. Disconnect the seat belt and remove rear bench seat.
7. Remove the right second passenger seat shoulder belt retainer from the right side C-pillar.
8. Remove the (5) screws from the right side C-pillar trim.
9. Remove the (2) screws securing the right side lower trim panel to the C-pillar.
10. Remove the (5) screws securing the D-pillar covers.

See Figure 1.

11. Lift the right side lower trim panel and roll forward to remove, this exposes the rear HVAC evaporator case module.
12. Using a template (See Figure 4), mark cutting lines on the upper evaporator case using a china marker or equivalent. Do not remove the locating tabs from the templates; they are needed to position the cut area for the access doors (See Figure 1).
13. Using a template (See Figure 5), mark cutting lines on the lower evaporator case using a china marker or equivalent (See Figure 1).
14. Cut through the plastic upper evaporator case and the lower evaporator case following the marked outlines of the templates to create two access doors (See Figure 1). Do not cut rear edge of either access door (See Figure 4 and Figure 5). Use a hot knife or a small (1/2" dia.) rotary abrasive cutting wheel.

Notice: Cut through plastic case material only. Aluminum tubes are located approximately 1/8" behind the case wall (See Figure 2 - view 1).

15. Using a heat gun to soften the plastic case, pull back the access door on the upper evaporator case carefully to prevent breaking the case. Reach in carefully and remove the holding clamp securing the capillary tube to the evaporator outlet tube. Be careful not to damage the capillary tube. Discard the clamp.
16. Using a heat gun to soften the plastic case, pull back the access door on the lower evaporator case carefully to prevent breaking the case (See Figure 2). Reach in carefully with two small adjustable wrenches and loosen the fitting attaching the TXV to the evaporator inlet tube. It will require a 7/8" crow's foot extension to loosen the TXV outlet joint fitting hidden behind the TXV itself. Remove and discard the TXV.
17. Remove original O-rings from the evaporator tubes and replace with new O-rings that have been oiled with 525 viscosity refrigerant mineral oil. DO NOT USE PAG LUBRICANT.
18. Install the new TXV to the evaporator tubes being careful not to damage the O-rings. Finger-tighten the joints and then torque the joints, using a backup wrench to hold the TXV in position to:

Inlet: 20-35 Nm (14-25 ft. lbs.)
Outlet: 15-22 Nm (11-16 ft. lbs.)

19. Pull back the access door on the upper evaporator case carefully to prevent breaking the case (See Figure 1). Align the TXV capillary against the evaporator outlet tube being sure not to damage the capillary line. Place the first holding clamp so it is located 1/4" or less below the crimp in the capillary tube (See Figure 3). Install the second clamp 1/4" or less below the first clamp. Be sure the clamps are fully seated on the tube and that the capillary is retained in the formed seat of each clamp (See Figure 3).

Notice: After all components are installed, evacuate and charge the A/C system. Leak test all joints that were opened.

20. Using a heat gun to soften the plastic case, close both access doors and align the edges of the plastic. Using a soldering gun, melt both edges of the case together along the full length of the cuts, as smoothly as possible.
21. Cover the soldered closures with duct tape to prevent air leakage.
22. Reinstall the right side trim panel, the D-pillar trim, the C-pillar trim, the C-pillar seat belt retainer and the rear bench seat.

We completed the repairs described in the technical service bulletin. And by the time we were done, the afternoon temperatures had climbed to 90 degrees. Jeanette came to pick up her Suburban with a half a gallon of her sweet potato ice cream in tow. "You just have to try this," she said.

I'm not sure if it was the heat or curiosity that prompted me to agree to a taste test, but I'm sure glad I did. She was right. It was some of the best tasting ice cream I ever had - sweet potato or otherwise. I called the technicians into the office to share in my good fortune. Five minutes later, I was standing at the counter with an empty ice cream carton. I handed the keys to Jeanette, my technicians' new hero and she was on her way. We're all counting the days until her next scheduled service.