1999 Chevy Tahoe will not cool properly

[hekg]

Hi all, I've been going crazy with my dual a/c truck and a system that refuses to cool properly. I'm currently getting about 68 degrees out of the front vents on max cool and about 62 out of the rear vents.
The main issue driving me crazy is that I have changed literally everything in the system except the rear evaporator and rear lines. Just to outline what I've done, I put in a new parallel flow condenser, new sanden 4440 compressor, new front evaporator, new front lines, new orifice tube in y pipe and new rear txv valve. Plus I also removed the heater core so that there is no heat at all going into hvac box.

I added 11 ounces of oil, vacuumed the system down and let it sit for two hours and it held just fine then I charged it with 64 ounces of r-134a.

Now, since almost everything is brand new, what would prevent it from cooling properly and why does the rear ac cool much more than the front air? Please help me because I'm at my wits end with this truck.
Thank you!

One more thing, the other day as I sat in the truck with both front and rear air on max cool I decided to turn off the rear air to see if it would help the front but it actually made the front air get even hotter. :/

[Cusser]

Attach a real service gauge set and post both high side and low side pressures at 2000rpm after a couple minutes of operation.

Are you positive that the front system has just ONE orifice tube?

[JohnHere]

In addition to what Cusser said, bear in mind that a properly charged system with front and rear A/C will show pressures that are somewhat higher than vehicles with front-only A/C.

What color was the OT that you installed? It should be yellow (or beige) and white—0.062" orifice—with a single black or green o-ring.

Did you by chance add 11 fluid ounces of oil to the several ounces of oil already in the replacement Sanden compressor? If you did, that would also be a reason for the system's poor cooling performance.

[GM Tech]

Most common mistake is two orifice tubes in system.. A dual system does NOT use one in condenser outlet, only just beyond the "Y" where liquid line splits to front and rear system feeds. New condensers typically come with an OT in condenser outlet. Did you check and remove one if in there? They don't cool good with two OTs. Seen it many times.

[Carguychris85]

Verify it has one orifice tube only after the Y as mentioned. However the Sanden compressor is a HUGE downgrade. Get a 10 cylinder 88947 upgrade from Four Seasons. Had the same problem with my Express van using the garbage Sanden 4261. The 4261 would only cool even decently at higher engine rpm. Swapped the 88947 in place and instantly 10-15F colder vent temps and cooled well even at idle. The rear system has a TXV and the front is fixed orifice. The rear system is cooling better because the TXV is wide open due to the insufficient refrigerant flow and because of that your front evaporator is suffering. The 4261 does not even match a single GM evaporator well. Some people defend that Sanden HT6 replacement garbage compressor but the best place for it is the closest trash can. It is unfortunate Sanden did not build a larger displacement, higher BTU, more suitable compressor for the application. My 4261 was a $$$ Genuine Sanden piece as well shipped from their facility in Wylie, TX not some knock off unit either.

This was testing after installing the 88947 in place of the 4261 and no other changes on a fairly humid 109F Texas summer day. At highway speeds it dips in the upper 20s center vent temps before the compressor cycles.

https://youtube.com/shorts/d_wkF47r5Kk? ... NzBw2ZOJwD

This was in 100F
https://youtube.com/shorts/msL17b8nppw? ... oOWFJ9RulG

The other thing I have run across on this era of GM is a failed LPCO switch. They will often start short cycling the compressor at as much as 70 psi on the low side. Results in a big lack of cooling. I got tired of that failure prone design on both my 97 Express and 99 Tahoe. Swapped both to an earlier adjustable LPCO switch for a 1993 G-van. Has the R134a thread but is the older, more reliable (larger electrical contact points internally) adjustable switch. The connector has to be swapped but readily available as well. Same switch works well on GMT800 trucks as well.

[Carguychris85]

I also had a Sanden on my 1987 G20 van that I swapped to a Denso 10S20F this past summer. HUGE improvement in cooling on it as well. 4261 struggled to cool it and overall inadequate in BTU capacity as well. This was after the 10S20F swap.

https://youtu.be/8UuKKrHdz9A?si=Zf9zResxZNi1H-sW

[Mark86]

Most common mistake is two orifice tubes in system.. A dual system does NOT use one in condenser outlet, only just beyond the "Y" where liquid line splits to front and rear system feeds. New condensers typically come with an OT in condenser outlet. Did you check and remove one if in there? They don't cool good with two OTs. Seen it many times.

Try reducing the charge to 44 oz. Did you look that charge weight up on the internet or did you read it off of the service tag under the hood? My Yukon calls for 44 oz 134A. Every place I looked said 64 oz, that is for the Suburban.

I discovered this on my 99 Yukon in another thread. There was a single OT in the condenser outlet, which is normal for a front only system. The OT belongs in the "Y" pipe which services the front only. There is a fitting in the Y pipe to service the OT. In my case there wasn't one there, just in the condenser. In my scenario I'm sure the front worked somewhat, but not sure how the rear would have worked being fed expanded gas to the expansion valve. Two OT in the system will explain your problems.

Many condenser suppliers fit an orifice tube in the condenser, and in a two system vehicle you have to remove it,

Secondly, where did you get your condenser and how thick is it? I ordered 3 from different manufacturers and all claimed to be for my dual system but all were about a half inch thick (same as single systems). The OEM condenser in my Yukon, same as your Tahoe is 1.20" thick. I wanted to replace it, why not, they're cheap and I'm in there. But I wasn't going to cut my condenser capacity in half.

[JohnHere]

The specifications that I have for your 1999 Chevrolet Tahoe with rear A/C are as follows: 64.0 ounces net weight of R-134a, and 11.0 fluid ounces of oil—PAG-150 for an OEM Harrison compressor, or PAG-46 for an OEM Denso compressor. If the under-hood decal differs, the decal always takes precedence.