1994 Suburban Rear Air Warm, Front OK

Drove my '94 Suburban 1/2 ton, 2WD, R4 compressor dual air today. This is R134a from factory. Front temperatures were fine, rear was warm. I noticed that the switch for heat in the rear was "on", but the switch up front was set to "front control" and that was for heat off in the rear. I didn't have time to look further, will take a better look this weekend, to see if system was actually delivering heat to the rear. I was under the impression that slightly low refrigerant would be the opposite, with the rear AC cold and the front AC warm, and this is the opposite. I'm not expecting a clogged rear expansion valve, any random thoughts as to what it might be? If it does this Friday afternoon, I'll take and report pressures both high and low.

I did replace the AC manifold/lines in January, was OK after that, haven't noticed (but haven't had reason to check) for a leak. Thanks.

What is the low side and high side gauge readings at idle and at 2K rpm with compressor running?

What is the low side and high side gauge readings at idle and at 2K rpm with compressor running?

I don't have time to read the pressures/check out whether rear heat was being mixed until Friday afternoon or Saturday. I'll definitely update what I find out. Since the little cussers are grown up now and we rarely have extra passengers, plus age of the Sub, it may not be worth doing a rear expansion valve if I would need to pay labor. I could do it all myself if I knew exactly where to cut that rear cover (thanks, GM for not having a bolt-on cover) and what to use (Dremel tool?).

Make sure it has a good charge.
And good low and high side prerssure.
Low charge will rob cooling from one end.
Bad expansion valve will mess up pressure reading.
If you can pinch off a rear line and recheck pressure readings.

I don't have time to read the pressures/check out whether rear heat was being mixed until Friday afternoon or Saturday. I'll definitely update what I find out. Since the little cussers are grown up now and we rarely have extra passengers, plus age of the Sub, it may not be worth doing a rear expansion valve if I would need to pay labor. I could do it all myself if I knew exactly where to cut that rear cover (thanks, GM for not having a bolt-on cover) and what to use (Dremel tool?).

I just changed my expansion valve on my 95 and I used a utility knife to open the box. I had the advantage of having the entire interior out of my car and thus I was able to remove the top half of the rear ac/heat unit. Therefore I could see exactly where the valve was located. This fix would be extremely difficult without removing the top piece because of limited access to the expansion valve's wrench holds (even if you cut a hole exposing the valve.) I had to move the evaporator around a bit to get access for the wrench to go on and therefore I do not know if it is possible without having the top removed.

If you want to have a go at it, remove the large plastic trim piece which covers the entire rear right of the vehicle. How? Remove pass middle row seat belt top bolt, then remove rear right pillar trim which has four or five screws, then remove front two 7mm bolts from main plastic trim piece. Then remove top plastic storage bins (two screws each). Then remove the top pieces which held the plastic storage bins. Then remove the rear cover over the rear blower motor area (two screws.) There may be a couple of screws holding the rear of the large main plastic trim piece in but I can't remember. Once all the screws are out it simply pulls straight away from the side of the vehicle towards the top of the piece (it is held by three of those snap in place holders.)

Once exposed, the large black "box" is now available to be worked on. To get the top off you have to remove the black duct work going up to the roof first. Then, remove all screws screwing the black box to the wall of the vehicle. Then, remove all the small gold screws going up in direction all around the perimeter of the black box (there are several and they are smaller than 7mm I think.) To reach the rear ones you have to pull the box away from the wall towards the top. It is not super easy to reach them all but doable. Once the top is off you will see what you need to cut to access it. I cut about a five inch high by four inch wide spot right where the valve is, but again found that having ONLY that access would not allow me to get the valve out due to not having enough space or angle to get a wrench in there and remove it.

By using a utility knife to cut it open I was able to easily re-close it by forcing the piece back into place (took a little time to get it exactly right) and then running a soldering iron tip around the seam melting the piece back together. It took a little pressure to get it to seal back up. If I had used a dremel there may have been a gap and then nothing to melt together.

My ac now works great! (I also changed my compressor, condenser, accumulator, orifice tube, almost all o rings (two spots would simply not come apart so I didn't disturb them.) I sealed all orings and fittings with Nylog. It seems to be back from the dead.

PS If you want to cut the hole first just to see what you can do, you need to cut exactly over (by over I mean on the SIDE of the main black box, not the top) and slightly to the left of where the main ac pipe (the one closest to the driver's side) comes up through the bottom of the vehicle and into the rear evaporator. The expansion valve is located to the inside (meaning towards the driver's side) and slightly forward of the evaporator body. The black box has a natural boundry of where to cut. There is a large surface area where I cut. I could not have cut much bigger without going around bends. Cut about 1/2 inch below the top/bottom line of the box (if I remember correctly,) as far left and right as you can on the large piece in question and also as far down as you can before the curve away from you.

PS II This is a very time consuming and involved job. Expect it to take all day.

PS III If you do use a utility knife, be careful! It is not easy to cut through and you have to use high pressure. Do not do it in a direction that could cut you if/when you slip.

Thanks, I'll check this out Friday afternoon or on the weekend, and post what I find.
I noticed two weeks ago that the main suction line from the accumulator was loose, saw a tiny oil residue there, apparently I hadn't tightened that last fitting fully when I replaced the AC manifold in January. So if a tiny bit of oil escaped, then so did some R134a. How much, I'll need to check pressures Friday when I feel the lines going to and coming from the rear system. So it may now be a little low on R134a.

OK, here's what I found. 95*F temperature outside, relatively cool here for June, Suburban had been parked under the carport in shade.
Initially today (all readings at 2000 rpm):
static = 110 psi
Low = 25 psi High = 210 psi
Front vent "cool", rear vent about 89*F. Rear blower fan operates fine.

Since there had been a loose connection, and a tiny amount of oil residue was visible two weeks ago (no more residue since then) I added R134a. After adding 22 ounces R134a:
Low = 25 psi High = 240 psi (240 psi agrees with 2.2 times ambient, does this sound high enough?)
Front vent temperature drops to 44.2*F, rear vent temperature about 78*F while stationary.
Low pressure line from the rear is not cool at all at the compressor or in engine compartment, both AC lines at the rear near right rear wheel are hot.
This Suburban always (since buying it in 2000) cooled significantly better while moving than when stationary. I might just take the cautious approach and try it for a few weeks to see how the comfort factor is. If the comfort's OK, and we rarely have rear passengers except dogs these days, I may decide to just run it "as is", due to value of the vehicle, its age, and cost of the repair if I can't figure out how to crack that rear case open.

Does anyone think that I'm still a little low on refrigerant?

OK, here's what I found. 95*F temperature outside, relatively cool here for June, Suburban had been parked under the carport in shade.
Initially today (all readings at 2000 rpm):
static = 110 psi
Low = 25 psi High = 210 psi
Front vent "cool", rear vent about 89*F. Rear blower fan operates fine.

Since there had been a loose connection, and a tiny amount of oil residue was visible two weeks ago (no more residue since then) I added R134a. After adding 22 ounces R134a:
Low = 25 psi High = 240 psi (240 psi agrees with 2.2 times ambient, does this sound high enough?)
Front vent temperature drops to 44.2*F, rear vent temperature about 78*F while stationary.
Low pressure line from the rear is not cool at all at the compressor or in engine compartment, both AC lines at the rear near right rear wheel are hot.
This Suburban always (since buying it in 2000) cooled significantly better while moving than when stationary. I might just take the cautious approach and try it for a few weeks to see how the comfort factor is. If the comfort's OK, and we rarely have rear passengers except dogs these days, I may decide to just run it "as is", due to value of the vehicle, its age, and cost of the repair if I can't figure out how to crack that rear case open.

Does anyone think that I'm still a little low on refrigerant?

With a vent temp of 44, it is generally cold enough and likely has close to enough r134a. Therefore, you need to either leave it as is and deal with limited cooling in the rear or replace your expansion valve.

25-30 low side is good.
150-250 high side is good.
The high side will vary to outside tempt and rad condenser cooling.

A over charge will build up high pressure on high side like 350-450.
Also a bad rear expansion valve can mess up pressure.
I like to plug or block off rear line to check for bad expansion valve that is messing with pressure.
Good luck

I've taken the trim covers off, the rear AC box is exposed. I think I can get the top rear of the housing off, but think I still need to cut a flap in the lower housing to replace the rear TXV.

Wonder how many flat rate hours a shop would charge to do this job, anybody know?

I've taken the trim covers off, the rear AC box is exposed. I think I can get the top rear of the housing off, but think I still need to cut a flap in the lower housing to replace the rear TXV.

Wonder how many flat rate hours a shop would charge to do this job, anybody know?

I don't know if a shop would do it this way. They might simply remove the rear evaporator which then exposes the expansion valve for removal. The real problem with that is that on the two vehicles I have played on, the rear evaporator fittings are FROZEN in place and can't be removed.

But yes, you must cut a hole to get it done, and also probably need to remove the top as well.

I don't know if a shop would do it this way. They might simply remove the rear evaporator which then exposes the expansion valve for removal. .

See http://www.alldata.com/service_provider/techrx/2001/20010703f.html

From the GM Tech who wrote the TSB: "I wrote the original TSB for all '94 and 95 suburbans rear TXV changes- they were mounted wrong- had the sensing section not touching the pipe correctly- the TSB was to attach the sensing tube with two clamps all along the suction pipe- you cut the hole in the side of the plastic case with a soldering iron with cutter attachment- very easy- no bolts on case to remove- I did dozens in '94, and later

It is not engineered to require cutting-- the TXV's are serviceable (not supposed to fail anyway) by removing the entire rear HVAC unit- which is to unhook all heater hoses, a/c lines, all bolts and take the whole unit out and sit it on your bench and take it apart to gain access. Removing entire HVAC units is very common across all rear systems on most all makes-- heck, since around the year 2000, you have to remove the entire front HVAC module on most every car to access their evaporators and some of their TXVs.. it is the way things are designed now.

The cutting came about as a shortcut- so as to save about 2 hours labor- you can still take it out- or you can cut it- it is your choice, but on an older truck like yours, thoses rear lines probably will fight you- why disconnect them if you don't have to."