Does 1998 tahoe with rear a/c have two expansion valves? I installed a new compressor, rear expansion valve, orifice tube, evaporator and vaccumed the system, and added about 3 cans 134a. rear air 39 degrees front air 48 degrees, two days later low freon, compressor doesn't engage

Acuras don't use orifice tubes. Orifice tubes meter liquid inlet. Expansion valves meter vapor outlet. All Acura systems use expansion valves. Expansion valves are on the evaporator outlet

Depends on the make, model and year of your vehicle. Some use expansion valves and others use orifice tubes with screens on them. Expansion valves usually connect directly to the inlet of the evaporator if your vehicle uses one. Orifice tubes reside in the low-pressure inlet to the evaporator.

Not sure specifically on that model but if you follow the line from the compressor to the condensor then out of the condensor to the evaporator it will be between the condensor and evaporator usually the tube is wraped in plastic but it could be in the heater box itself also if you have rear ac the rear unit uses a thermal expansion valve on the rear evaporator not an orifice tube. What is your concern thats leading you to find the orifice tube if your dont mind me asking

Are your asking about TXV valve thermal expansion valve for A/C system ? If you are ,yours doesn't have one . Has an orifice tube , The orifice tube is located in the liquid line between the condenser and the evaporator !
The orifice tube is the dividing point for the high and the low pressure sides of the A/C system. As the refrigerant passes through the orifice tube, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to vaporize at the orifice tube. The orifice tube also meters the amount of liquid refrigerant that can flow into the evaporator.
Unless you are referring to the auxiliary A/C system for the back of the vehicle , it uses a
The rear TXV is located at the rear evaporator inlet. The TXV is the dividing point for the high and the low pressure sides of the rear A/C system. As the refrigerant passes through the TXV, the pressure on the refrigerant is lowered. Due to the pressure differential on the liquid refrigerant, the refrigerant will begin to boil at the expansion device. The TXV also meters the amount of liquid refrigerant that can flow into the evaporator.

I would check the low and high readings with a gauge set first.
Its possible the compressor has failed internally.
If the orifice is plugged up one of the lines should be cold where the freon gas is concentrated.
The fan clutch is working ok, right ?
I'm showing an orifice tube in the front and an expansion valve in the back, so its doubtful both would fail at the same time.

Grand Am has an expansion valve. Link below has part number and picture of what you are looking for.
http://www.discountacparts.com/addtocart~year~1998~make~Pontiac~model~Grand_AM~descript~A-C_Compressor~part~60-40281.asp

Unfortunately, I do not know precisely. But for the most part auto a/c's are all the same. Below I will describe how to find it. I have also included a aguide to determine system operability that may or may not be helpful. Just trace the system with flow path described below the valve has to be between condensor and evaporator.

Reminder the flow for the a/c system is :
1. From the compressor a high pressure (HP) gas goes to the condenser in front of the radiator.
2. From the condenser a HP liquid goes to an expansion device (orifice tube or expansion valve)
3. From expansion device the low pressure gas enters the firewall to the evaporator.
4. From evaporator the low pressure liquid/gas mixture goes through the dryer back to the compressor suction.

Below is a guide to troubleshoot and check system function. This can be done by most folks.

Look at the front of the ac compressor and have someone turn on the ac from inside the car. The front of the pulley(clutch) will spin when turned on.

If it does not spin, the low pressure switch (LPS) is not satisfied. Meaning there is not enough freon in the system to operate the system. Means low freon capicity because of a leak.

The LPS can be jumpered to cause the compressor to run (clutch to engaged) which will draw the freon in the system.

The LPS is usually a two wire sensor on the suction of the compressor. (between evaporator and compressor) Unplug the sensor and install a jumper, this will engage the clutch. Connect a new can of freon
and fill to system capacity.

The capacity of the ac system will be on a label under the hood, do not overfill. Probably about 2 lbs. Local parts stores (Auto Zone and Advance) will have freon with stop leak. This may help the leak. They also make a freon with a UV dye. I always add a single can with dye so that if stop leak does not work I can find the leak in the future.

The system should be filled until suction pressure is about 30 to 38 psi..

ther is a blockage in the lines somehwere. usual suspect is the a/c orifice tube which is like a mini inline filter. Similar to the orifice in function would be a thermal expansion valve. both have small ports(orifice tube is a mini screen) that can become clogged with metal filings from the compressor as it wears and /or the dessicant material comming loose from the dryer bottle. if there is indeed blockage, the a/c lines will need to be flushed, dryer bottle replaced(recommended), a/c compressor replaced, and the orifice tube or expansion valve( whichever is on the vehicle) to be replaced. vaccumed to 28 inches of mercury and the recharged to proper charge.

the expansion valve is in the rear ac housing and it is bolted directly to the rear evaporator. it is an actual expansion valve and not an orifice tube. the part is about $45. the interior on the passenger side has to come out along with the entire rear ac sytem.

As much as you probably don't want to hear it, I believe it is time to replace the a/c compressor. Be sure to change the orifice tube while you are at it. The compressor should take approximately 8 ounces pf pag 150 and about 32 ounces of freon.

Hi.............
Your suburban Don't use a expansion valve.
It use a ORIFICE TUBE, that made the funtion of the expansion valve.
OK.... If you follow the THIN line coming from the EVAPORATOR to the CONDENSER, you will found.
A JOINT COPLER. just remove and in the part going to the evaporator INSIDE of the line is the ORIFICE TUBE.
Remove and put the new one.

I Know this answear your question.
Thank you for use fixya.