surging while at small throttle and A/C on

[Mike Stitzer]

Recently I noticed on my 95 MTX that while at a steady cruise with the defrost on (a/c compressor on) the car will surge like I'm gently giving it gas for a second then letting off, repeatedly. The compressor seems to be kicking on and off at this low throttle setting. It doesn't matter what temp I set the climate control at. It does it when it's on DEF or AC. I haven't tried it in the AUTO yet.
I don't notice it at full throttle. The car has 107K miles and as far as I know the compressor is original. Could this be the a/c clutch?

Mike

 

[sdpatt]

You are feeling the parasitic drag of the A/C compressor cycling and yes, it does feel as if the engine is surging. The only settings on the climate control that don't allow the compressor to engage are OFF, VENT and FLOOR. The EEC is responsible for adding a bit of extra air flow to compensate for this added drag by opening the IAC control valve when the compressor engages. You may want to try the idle speed reset procedure.

 

[Rob94]

Replace your Clutch Cycling Switch. If that doesn't solve it, you may have a refrigerant leak somewhere.

 

[projectSHO89]

Rob94:
Replace your Clutch Cycling Switch. If that doesn't solve it, you may have a refrigerant leak somewhere.

Cycling switch is working although the switching pressure is unknown.

The original poster made no reference as to the performance of the AC system...

If the system is not cooling well and the system is short-cycling, then the charge needs to be checked before anything else is done.

However, the drag of the compressor and the surge of the engine is, to a degree, somewhat normal.

Steve

 

[Mike Stitzer]

Perhaps I need to elaborate a little. I've had the car 3 years and yes I do know what it feels like when the AC cyles on and off. But now at light throttle (just enough pedal to maintain speed) the AC cylces on and off about once per second. That's not normal. However, the AC works and blows very cold air, always has.
This constant cycling started after I replaced the leaky original radiator. I did disconnect the battery to do this so the first thing I will do is the idle reset procedure.
Maybe the refrigerant is low? I don't know how much refrigerant is in it but the system is orignal. Is there a way I can test this at home? The car has 107K miles and is a 95 MTX. But why does this only happen under light load and not at idle or while accelerating? What do you all think?

Mike

 

[Rob94]

I still say change the cycling switch. I ran into this same problem several years ago, and it was the cycling switch. I believe it is a $30 part, but it is common to most Fords. If you know anyone with a Ford that has the same switch, change them out long enough to see if it is the problem.

 

[jelloslug]

While I was recharging my system the compressor was cycling as I put the first can in, as I got more refrigerant in it stopped.

 

[sdpatt]

Mike Stitzer:
But now at light throttle (just enough pedal to maintain speed) the AC cylces on and off about once per second. That's not normal. Mike

That is the primary indication of a low refrigerant pressure - short clutch engagement cycles. Get the refrigerant pressure properly chagrged. Remember that too much pressure can reduce A/C system effectiveness just as well as low pressure.

 

[rangerj]

Mike,

You can borrow a set of R-134a gauges from Auto Zone for a hefty deposit (fully refundable as long as you return the gauges ).

The blue goes on the low pressure connection, and the red goes on the high pressure connection. The connections are two different sizes.

The low pressure connection is back by the firewall on the passenger side. The high pressure connection is up by the battery.

The low pressure switch takes about 20 lbs to be activated. It sounds like you have a low pressure situation, and the switch is not getting enough pressure to hold it in the on position. The other posibility is a defective low pressure switch.

Check at all of the A/C connections for a black oily substance. This would indicate a leak of refrigerant and oil. The oil is very fine and attracts dirt. Thus you get the oily black dirt at the site of any leak.

Also look for wet oily black spots on the condenser (in front of the radiator). Then look behind the A/C pulley for the same black oily stuff. This would indicate a compressor shaft seal leak.

The leak may be minor enough that a charge of R134a may hold up for a long time. So try this first. rangerj