A/C not blowing cold, but pressures are fine

[Ron]

Not sure what to check out next. Ron's suggestion of checking the pressure switches makes sense... Any thoughts?

Sorry, I was suggesting bypassing the LP switch to check out the orifice tube for blockage, RE Dave and Dennis' discussion....The pressures you reported point elsewhere, imo.

You didn't say if the accumulator is getting cold now?

I agree that the cycle times are odd. And posting a vid, including where the pressures are when the clutch cycles OFF, would tell a lot.

The fans should also be on anytime the compressor clutch is engaged.

[Project Vixen]

Off topic, I just want to thank you for the great information on rebuilding the door lock solenoids. I was able to do the same thanks to your information.

Thanks! Wow, that was a long time ago. I just ran across that spool of wire a few days ago. Still have like 5 pounds of it...

[Project Vixen]

I'll try to get a video. Not sure I have enough hands to hold everything, but I'll see what I can do. I might be able to hang the manifold on a ladder.

The pressures I mentioned (30L/185H@85F) were the pressures right when the compressor turned off.

[Project Vixen]

Just in case anyone needs a low pressure switch...

The OEM part number for the switch is:
Part Number:100744, LOW PRESSURE SWITCH

From Houston @ $9.99 it's a good price.

Other -possible- cross referenced part numbers I could find were:
SANTECH STE MT0207
UAC SW 1122C
Global 1711250
AC DELCO 15-5861 and 15-2151
Four Seasons 35751
NAPA TEM 207434

[eight8toy]

I just finished my AC overhaul last week and while I was adding the first can of r134 the compressor cycled like every 2-3 seconds like you're describing. After that and adding the second can it was continuous. I would guess you are low on refrigerant.

[DMC-Ron]

The fans should also be on anytime the compressor clutch is engaged.

A Ron notes, even with the mode selected to max, the fans will only be on when the compressor clutch is engaged (unless you have Dave's fan fail relay which has a delay off feature) . If you are low on refrigerant, the compressor will quickly pull the suction below the low pressure switch setting, turning off both fans and compressor clutch. When the refrigerant is very low, it will take a while for the pressure to build back up, reset the pressure switch, and turn the fans (and compressor) back on. The lower the charge, the longer fans and compressor will be off, to the point where the charge is so low (or completely discharged) and the compressor and fans will not start at all.

I think eight8toy's guess is right.

Ron

[Bitsyncmaster]

Sounds like your orifice is plugged. That would cause the long off cycle short on cycle.

Also your fans won't come on if the low pressure switch keeps the compressor off. The plugged orifice is also causing that problem.

[DMC-81]

Sounds like your orifice is plugged. That would cause the long off cycle short on cycle.

Also your fans won't come on if the low pressure switch keeps the compressor off. The plugged orifice is also causing that problem.

Yes, and is a high pressure of 185 completely normal, or perhaps another symptom of a blockage? I recorded 150 ish at 81 degrees ambient. The instructions I followed said that was normal for a full charge.

Thanks! Wow, that was a long time ago. I just ran across that spool of wire a few days ago. Still have like 5 pounds of it...

Cool. PM me if you want to sell some of it. I still have a spare set of solenoids that I need to rewind.

[Project Vixen]

I'll bypass the 'jump the low pressure switch' step and just replace it. (According to http://projectvixen.com/week33.htm I purchased the Four Seasons 35751 switch at Pep Boys on 06 June 2000). I'm leaning to believe the low pressure switch is the problem. Here's my reasoning:

When the A/C kicks on, it runs to exactly the same pressure each time and shuts off (30L/185H). Now what in the system would cause that to happen?

A blocked orifice tube would cause the pressure to rise -- but that blocked orifice has no way to shut off the electrical power to the compressor clutch. Pressures should begin to rise and keep rising until either the low pressure switch kicks in (due to the backup of refrigerant on the high side with little being returned on the low side, or until the high pressure switch turns it off (or the pressure release valve blows).

From the gauges I can see that the pressures at rest and when the compressor is running are just fine. That also helps rule out a blocked orifice tube, as the pressures should be too high on the high side and/or too low on the low side. It also rules out low refrigerant, as the rest and running pressures could not be correct with low refrigerant levels (verified by manifold gauge set).

The clutch is cycling short. The only things that can cause that electric clutch to cut in and out are electrical. There isn't a great deal in that circuit... there is no relay for the compressor clutch (although IMHO there should be one). If it was the mode switch, it would be unlikely that it would simultaneously fail on both normal and max a/c, and fail in such a way that the compressor would run for 2-3 seconds and then cut out, very predictably and repeatedly. The clutch itself could also be overheating and cutting out, though it's not behaving that way. If that were the case, since it's purely a coil with no electronics inside it would be unlikely that it would 'click' off so quickly. One would expect the resistance to rise and the clutch start to slip before it disengaged... and if it had that much resistance it's highly likely it would blow the fuse.

The only thing left that I can see are the high and low pressure switches. The low pressure switch is easily visible behind the right wheel attached to the accumulator/drier. The high pressure switch is on the '4-way' diamond-shaped hub on the high pressure line nearby. But how can we tell if one of these is at fault? Jumpering each would be the hands-on way, but I'm also looking at it as a classroom exercise. I'll focus on the low pressure switch as from research here it seems the low pressure switch on a D is much more likely to fail than the high pressure switch, but the logic for testing is the same for each switch.

The low pressure switch is designed to shut off the compressor clutch when the system low side drops below ~25 PSI. We know the system is not low on refrigerant (or at least not very low) by verification with the manifold gauge set (both running and at rest).

So what ways can the low pressure switch fail? I can think of three ways:

1. Lax failure -- Failure to cut power to clutch under the low pressure trigger point (low system refrigerant level)
2. Complete/open failure -- the switch is 'dead' and never allows power to the clutch.
3. Eager failure -- the switch cuts off power to the clutch at a trigger point much higher than desired.

(BTW> If it's the high pressure switch at fault, the troubleshooting is similar but the descriptions of #1 and #3 above would be reversed.)

I'm thinking the low pressure switch is in failure mode #3. This switch is designed to cut out at around 25 PSI. We know the clutch is clicking off at the same point repeatedly & predictably -- 30 PSI. If the low pressure switch 'believes' that 30 PSI is its desired cutoff point, it will cut power to the clutch until the low side pressure again reaches a high enough pressure to allow the switch to click the compressor back on.

So that's where my mind's at now.

The principle of 'Occam's Razor' states that the simplest solution to a problem is usually the correct one. NAPA is holding a #207434 for me, so I will grab it after work and hopefully get a chance to try it out tonight. I will keep y'all posted on the results.

[DMC5180]

Here is is some classroom info

Low pressure switch (aka duty cycle switch) cuts out (OFF) at 30 Psi (R12) setting. It cuts in(ON) at 45 psi approximately. This is the only switch cycling the clutch under normal conditions.

location: Low side of System (after orifice tube and evaporator outlet.

High pressure cut-off switch (aka high pressure (SAFETY) switch. This switch is along for the ride and rarely would activate. It has a pressure setting in the 350 psi or higher range and is only there in the event of a blocked Orifice tube and the low pressure switch fails too cut off power to the compressor clutch. It is purely a safety protection switch and had nothing to do with normal clutch duty cycling. Location: High Side just before Orifice tube.

Your long Off period between short ON period. The refrigerant is slow to pass thru the Orifice tube. It is the expanding liquid to gas that raises the low side pressure to the 45 psi too cycle the low pressure switch ON again. The lack of refrigerant on the low side of the Orifice causes rapid pressure reduction to 30 psi (clutch off)



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