1994 Suburban K2500 AC

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Sawlty

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Getting started rebuilding the AC in my 1994 Suburban K2500. I’ve owned it roughly a month. Something isn’t right with the AC. I think there is a blockage somewhere. I don’t know what the previous owners did to the system. So, I’m going thru the entire system. Will post updates here.

Can only work on it an hour or so each day, so gonna be a slow process.

A bit of organizing the tool room, gathering tools I’m going to need, and pulled the trim this week!

Next steps…

Drain coolant. Going to replace the rear coolant lines, install rear heater core shut-offs (in case the rear heater core ever develops a leak, I can get it home). Replace rear heater core.

Get the passenger side rear on jack stands and remove the wheel/tire so I’ll have room to work underneath.

Evacuate Freon.

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1998_K1500_Sub

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Going to replace the rear coolant lines, install rear heater core shut-offs (in case the rear heater core ever develops a leak, I can get it home). Replace rear heater core.

Lots of details in this thread and, hopefully, some tips you can use regarding replacing the hoses.

 

Sawlty

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Replacing rear expansion valve.
Unable to get the high and low pressure fittings under the passengers side rear to let go. Disconnected both heater hoses. This gave some play with the air box inside. Was able to remove all screws to separate bottom from top of air box. This exposed the expansion valve. Could no reach the expansion valve fittings without cutting an access hole in the side of the air box. Fitting came apart for expansion valve relatively easily. Will cover and seal this hole once expansion valve is replaced.





Flushed high and low side lines running from engine compartment to the rear unit, as well as rear evaporator. Waiting on new. expansion valve to be delivered, so covered all open line ends.
 

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Sawlty

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Does anyone know??

When I turn the AC on at the front unit, but it is off at the back…is there still Freon cycling thru the entire unit? Is Freon cycling thru both the front and rear systems even though I am not utilizing the rear AC for cold air??

I us d to own a Peterbilt that….

When you turned off the rear air, it closed like a gate valve that stopped Freon flow to the rear unit.

I haven’t seen such a valve on the suburban, but am curious.

Thanks.
 

1998_K1500_Sub

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Does anyone know??

When I turn the AC on at the front unit, but it is off at the back…is there still Freon cycling thru the entire unit? Is Freon cycling thru both the front and rear systems even though I am not utilizing the rear AC for cold air??

I us d to own a Peterbilt that….

When you turned off the rear air, it closed like a gate valve that stopped Freon flow to the rear unit.

I haven’t seen such a valve on the suburban, but am curious.

There's not a hard shut-off either front or rear. There is sort-of a soft shut-off, i.e., if one turns off the front fan, the control head also de-energizes the compressor clutch (on my 1998 anyway), which in effect turns off the AC everywhere.

Assume now the front fan is blowing, so (1) the front evaporator is removing heat, (2) R-134a is evaporating within, and (3) consequently refrigerant and oil are moving through the front system in typical fashion, and (4) the compressor is cycling to keep the suction pressure b/t roughly 25-44psi.

If the rear fan's been running and is then turned off, the rear evaporator temp drops b/c of no airflow across it and the evaporation of some residual liquid R-134a within it. The sensing bulb on the TXV, affixed to the evaporator's outlet (suction), then throttles the R-134a flow to as low a level as necessary to keep the sensing bulb at operating temp... or at least, the TXV throttles the flow to the extent possible. Obviously, very little R-134a flow is needed as there's no source of heat (airflow) to warm the evaporator and, by association, the sensing bulb.

So the rear high-pressure line is full of liquid R-134a all the way to the TXV as is typical, but the TXV's flow will be quite low b/c the TXV's sensing bulb is cold, as is the evaporator by association. Consequently, the vapor return flow in the rear suction line will be very low.

I've left out many of the subtle details, but that's the gist.
 
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Sawlty

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Began moving things out of the way to start the AC rebuild at the front under the hood. Soaked heater hose quick connects with PB blaster to loosen things up. Breaking lines apart. Previous owner rounded off the high pressure line at the front evaporator. So, since I have to replace the orifice tube at the front near the Y, it really doesn’t need to be broken apart at the front evaporator for flushing purposes.

Next step is to remove the grill and get to the condenser for replacement.

Question for you guys.

I’m going to replace the switch at the side of the dryer/accumulator. Is this called the high pressure switch?

Also, there is a switch at the back of the compressor. Is the the low pressure switch?

One last question. Is there a switch at the condenser? Though I saw some wires and a switch when looking theu the grill. Will know more once I get the grill off.
 

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1998_K1500_Sub

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I’m going to replace the switch at the side of the dryer/accumulator. Is this called the high pressure switch?

That's the clutch cycling switch. It operates at relatively low pressures (25-44psi in operation).

Also, there is a switch at the back of the compressor. Is the the low pressure switch?

That's the high-pressure cut-out.

One last question. Is there a switch at the condenser? Though I saw some wires and a switch when looking theu the grill. Will know more once I get the grill off.

Not on my 1998. Can't say for certain on yours.

On mine there's a temperature sensor there, in front of the condenser, that's used for a temp gauge that's built-in to my rear view mirror. You may have something similar.
 

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There's not a hard shut-off either front or rear. There is sort-of a soft shut-off, i.e., if one turns off the front fan, the control head also de-energizes the compressor clutch (on my 1998 anyway), which in effect turns off the AC everywhere.

Assume now the front fan is blowing, so (1) the front evaporator is removing heat, (2) R-134a is evaporating within, and (3) consequently refrigerant and oil are moving through the front system in typical fashion, and (4) the compressor is cycling to keep the suction pressure b/t roughly 25-44psi.

If the rear fan's been running and is then turned off, the rear evaporator temp drops b/c of no airflow across it and the evaporation of some residual liquid R-134a within it. The sensing bulb on the TXV, affixed to the evaporator's outlet (suction), then throttles the R-134a flow to as low a level as necessary to keep the sensing bulb at operating temp... or at least, the TXV throttles the flow to the extent possible. Obviously, very little R-134a flow is needed as there's no source of heat (airflow) to warm the evaporator and, by association, the sensing bulb.

So the rear high-pressure line is full of liquid R-134a all the way to the TXV as is typical, but the TXV's flow will be quite low b/c the TXV's sensing bulb is cold, as is the evaporator by association. Consequently, the vapor return flow in the rear suction line will be very low.

I've left out many of the subtle details, but that's the gist.
So, if the TXV is full of metal shavings, it can effect the entire system. Correct?

Thanks for all the info!
 

1998_K1500_Sub

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So, if the TXV is full of metal shavings, it can effect the entire system. Correct?

Well, yes, but...

If the TXV is plugged, then it's functionally akin to blocking-off the rear air... there are block-off kits for the rear AC. Look them up if you're curious.

So the rear air will be quite ineffectual.

The front air should work OK.

There will be some oddities. The rear high-pressure line, being effectively plugged by the TXV, will act like a big accumulator connected to the front's high-pressure line. Ditto for the rear suction line on the low-pressure side. This can alter the cycling behavior of the compressor, but I won't claim it's a detriment. I can go into details if someone's interested.

This "pumping" of refrigerant into and then back out-of the blocked rear HP line may, over time, draw some of the metal shavings back into the front high-pressure line. That's not good.

OK, so I won't go on and on.

Net-net, IMHO if the rear TXV is plugged and someone doesn't want to fix it / abandon the rear air, then the proper thing to do is buy the block-off kit to isolate the rear components from the rest.
 
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Sawlty

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Next question.

I removed the old rear TXV. The in-line screen was full of metal. So, I’m obviously replacing it. I can’t seem to locate a new part with the corresponding part number. So, I ordered one from rock auto. UAC part # EX7420PC.

I received the new part in the mail today. The new part does not have the in line screen. If I use the new part (the one without the inline screen), am I going to run into problems?

Your thoughts!?

Thank you.
 

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