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Low side pressure pinned guage

Chevelle_nut on Fri August 02, 2013 3:28 PM User is offline

Year: 2002
Make: Ford
Model: Focus
Engine Size: 2.0 SOHC
Refrigerant Type: 134a
Ambient Temp: 87
Pressure Low: 250
Pressure High: 150
Country of Origin: United States

Back story, last year AC stopped working (had bearing failure in clutch). Replaced belt, tensioner and clutch assy. After repair checked pressures and they were good, ~2.5X ambient on high side low side was steady 35 or so and it blew ice cold at the vents.

Lately it has been intermittently dropping out and vent temps go to ambient. Went for a drive today and when we got to destination and looked at clutch and it was not engaged, i tapped the throttle cable and when it revved the clutch pulled in and it got nice and cold at vents. Shut the car off and left it in parking lot. 3 hours later we returned home ( about 15 -20 minute drive) and ac worked at first but then dropped out about 5-10 minutes into drive. Got home left engine running, looked under hood but clutch was not engaged. Put my gauges on to get a look at whats happening and found low side had buried needle to 250psi pin. High side was about 150 or so. I shut it down and checked to see if relay was problem and it was HOT.

I have not gone back with meter to check voltages because I am afraid I may have blown compressor ? I have never seen pressures that high on low side before.

webbch on Fri August 02, 2013 5:10 PM User is offlineView users profile

Were you properly depressing the schrader on the gauge set? Low side being 100 psi higher than the high side doesn't compute for me. Even if there was a blockage, I fail to see how the low side could go that much higher than the high side.

Second problem - If compressor clutch pulls in when you tap it (perhaps the response to the vibration induced when you revved the engine?), then your clutch gap is likely too wide, and a shim needs to be removed to restore about a 0.020" air gap on the clutch

Edited: Fri August 02, 2013 at 5:22 PM by webbch

mk378 on Fri August 02, 2013 5:39 PM User is offline

Most low side gauges are not calibrated above 125 psi. The "250" mark at the end just means it is safe to apply 250 psi to the gauge and not damage it.

Relay getting "hot" could be normal heat from the coil (a good sign, the car is trying to engage the compressor), or it could mean the contacts are burnt. Check that the plastic of the relay socket is not melted. Enough heat to make that happen usually causes the socket contacts to lose their spring temper and you won't ever get good contact even with a new relay.

Most likely though you just need to reduce the clutch gap like the other guy said. You can make it as small as you want as long as it doesn't drag when off. Measuring a used clutch with a feeler gauge doesn't work very well because the surfaces will not be flat.

Edited: Fri August 02, 2013 at 5:46 PM by mk378

Chevelle_nut on Fri August 02, 2013 11:31 PM User is offline

So what could cause the low side to get that high ?

I would think excessive gap would cause slip in drive to compressor and thus lower high side pressure ?

Am I in early stages of a black death failure ?

webbch on Sat August 03, 2013 12:21 AM User is offlineView users profile

This might be a bit of an unusual check, but.....mk378 brings into question whether the low side reading was accurate at such a high pressure. The high side you say was 150 psi. I would not expect the low side to be any higher than this, even under fault conditions. For a quick check, what if you attached the low side service hose to the high side gauge on your gauge set? If the low side pressure is now around 150 psi, then you've verified that you cannot rely on the low side gauge at such a high pressure. If the low side pressure still looks to be around 250 psi, then I'm still stumped.

Edited: Sat August 03, 2013 at 12:22 AM by webbch

Chevelle_nut on Sat August 10, 2013 10:52 AM User is offline

Update: I did check the clutch gap and a .030 feeler slipped in. I pulled a .051 shim and replaced with a .035, so clutch should be tightened up. I can still look down and see daylight between plates when running but off. No drag that I can determine. I replaced the cutout relay because contacts looked burnt.

The high side still creeps over 200 psi but low side is now around 40 psi. @ 80f in front of condenser. Seems like low side drops off really slowly...

Fingers crossed it is all good again. But I have doubts

My gauges go back to 0 on atmospheric so I am hoping they are ok, no Idea what caused the numbers I got.

Chevelle_nut on Sun August 11, 2013 3:10 PM User is offline

2nd Update:

No Bueno, still runs nice for awhile then cuts out after 15-20 minutes. Will toss gauges on it this evening before / after driving...

webbch on Sun August 11, 2013 10:00 PM User is offlineView users profile

Have you tapped on the clutch when it cuts out (since you've reduced the gap) to see if it starts up again?

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