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Pressure reading and evap icing

pomelo on Tue April 06, 2010 11:33 PM User is offline

Year: 94
Make: Nissan
Model: Sentra
Refrigerant Type: 134a
Pressure Low: 32

I can not find my high side quick connect, so I was only able to check the suction pressure. I was getting a reading of 32psig. It was around 80 degrees today. I didn't have someone to rev the engine to 2000rpm, so this was at idle. Does this sound like it could be low? I'm trying to figure out if my evaporator coil is icing up. With residential air conditioners (house a/c), the evaporator coil will actually ice up when it is low on refrigerant. It does this because the saturated temperature drops below 32 degrees. At 28 psig, R134a has a saturated temperature of 32 degrees. This will produce ice on the coil. Do automotive a/c usually ice up due to being low on refrigerant? The reason I ask is that Nissan uses a thermo control amp to prevent the coil from icing. I believe domestic vehicles use a cycling pressure switch. I was thinking my problem could be an iced coil and the TCA device is just doing its job. The only problem is that it stays off for quite some time. I would think that the ice would melt very quickly with the compressor off.

mk378 on Wed April 07, 2010 8:33 AM User is offline

Mobile systems of either type won't ice because of being low on refrigerant. Unlike home systems, they have controls to prevent it. The thermo control is supposed to turn off the compressor before any ice forms, not after. Vent air temperature should stay above 38-40 degrees minimum. Generally if a freeze-up has occurred, the suction line under the hood will have frost on it too.

NickD on Wed April 07, 2010 9:33 AM User is offline

A simple wooden wedge between the TB arm and stop works to set your idle speed. In your TXV Nissan, ice up is common if your thermo switch isn't working, some use a capillary tube that leaks out gas keeping it closed, the evaporator temperature is constantly monitored, should kick off the compressor at 33*F, back on again at about 38*F. Recall some Nissan's that had a semi-Econ position on the mode switch that raised the kick in and out temperatures by about 5*F.

Can always hot wire the compressor with a remote starter switch directly to the clutch coil with gauges attached to verify the mechanical condition of the system, then search into electrical problems. But good to know its operating well mechanically.

I made my own thermal switch for my TXV motorhome, had the parts, knowledge, and was bored. But that was after doing a search for a replacement, couldn't find anything even close. Doubt if you will have that problem with your Nissan. Problem was a leak in the capillary tube, so the contacts remained closed at all times and the evaporator would freeze up. For awhile, could both hear and feel the blower slow down and the air would get a musty odor, so left the blower running and switched off the compressor until the air got up to speed again, then switched the compressor back on again. But I replaced that chore with some diodes and transistors.

Did have a job once holding coats in a restaurant as a kid, but was replaced by a coat rack, that was degrading to be replaced by a coat rack. Decided at that time, better get some more skills that are not so easy to be replaced. Really wasn't that bad manually switching off my compressor, but don't expect to train your wife to do that, when strapped in a seat with nothing else to do except look at that white line on the road, was entertaining, but also a mild challenge to make it automatic.

Chick on Wed April 07, 2010 7:14 PM User is offlineView users profile

Look behind the grill for the high side service port if it's not out in the open.. might be near the drier, you need both pressures and post them here..I'm home so I can't check the ones at the shop right now..

Email: Chick


Freedoms just another word for nothing left to lose

NickD on Thu April 08, 2010 10:00 AM User is offline

I thought the high side was near the compressor, but how can I remember that far back? All I know is, it is there, and sometimes can take up to 30 seconds to find it by tracing through the AC system. Just look for it.

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