Automotive Air Conditioning Information Forum

I've converted an '86 Bronco to 134a by installing a new Compressor (delivered with 3 oz oil inside), all new hoses, new condenser, new accumulator, new FOT (red), new low pressure cycling switch (26psi off/47psi on), flushing&drying the evaporator, then adding 5 more oz of PAG 100 oil to the new accumulator (8 oz total, recommended by shop manual).
After evacuating for an hour to 30" vacuum, and holding for an hour (no leakdown), I've charged with 36 oz of 134a, to rock steady pressures of 33/235 at ambient of 90 degrees, with max blower/max A/C/windows open/1200 rpm.
After that, I'm getting 50 degrees air at center vent. Same driving at 70 mph.
Thinking I'd under- or over-filled, I added several ounces of 134a in half ounce increments to pressures of 35/295 at ambient of 95 degrees. Still vent air temp no better - 50 degrees.
Next, I removed several ounces of 134a in one-ounce increments, to pressures of 28/250 at ambient of 92 degrees. Center vent temp best now 49 degrees.
Now, when I roll up the windows, the compressor cycles between 27 and 47 psi, but center vent air temperature stays ~50 degrees... until after awhile, it gets down to 49 degrees.
Next, I clamped off heater hoses completely. No change in center vent temp - still 50 degrees. Still no better on highway.
Next, I checked the vacuum operated air door, and confirmed it's working.
Now, what can I do to drop the center vent air temperature by 10-15 degrees? Should I change the FOT to Orange? Blue? Should I replace the new condenser (Spectra 7-4150 serpentine flow) with a more modern one that has parallel flow microtubes?
Is there some other change that can get my center vent air temperatures down by 10-15 degrees?

At this point, cheapest option to try is the Blue OT.

Condenser is not really showing signs of a heat load issue. So for now I would not replace it.

Are the inlet and outlet on the evap (pipes) the same temp. Good way to tell if it is fully charged.

Way too much hysteresis in these new switches.

Adjust the cycling switch to turn ON at about 35 psi. The evaporator does not need to spend long above 33f to de-ice in a cycle. Ignore the "off" pressure.

Right now you are averaging 37psi evaporator pressure - 40f Blow warm air over that and you get 50f

.

Thanks for the advice!

First, I adjusted the pressure cycling switch 1/4 turn CCW, which improved the cycling such that it cycled between 25-40 psi, but it was still only able to produce 49 degree vent air at 25/215 psi.

Next, I took another 1/2 turn CCW on the pressure switch, and this stopped all cycling, but best pressures were still about 25/215, and vent temp was best 48 degrees.

At this point, both inlet and outlet of evaporator were frosty. Thinking I'd maybe overcharged, I removed refrigerant until my pressures were 21/220, and the accumulator output was just barely not frosting, while the inlet was only slightly frosty.... at this point, the accumulator temp (with IR thermometer) was 29 degrees with heavy frost on it, and the vent air temperature was 45 degrees. I figured it was now slightly undercharged, and the evaporator was superheating...

I didn't think of it at the time, but at this point, maybe I was icing the evaporator, and limiting air flow, causing the relatively high vent air temp?

Anyway, I thought I'd now become undercharged, and decided to try the Blue Orifice tube, thinking I could get a lower low pressure with full charge, and that might remove more heat from the air flowing through the evaporator... With the Blue OT, best results were 22/220 psi, and vent air temp of 48 degrees, with accumulator temp of 34 degrees.

No matter what I've done, I can't get better than 45 degree air - with the blue OT and 21/220 psi.

It seems that, at 21 psi, with evaporator temp around 24 degrees, I ought to see better than 45 degree air at the center vent. What next steps might be appropriate here?

Is all of this testing at idle?

Obviously if you can make the evaporator that cold there is an airflow issue of some sort in the evaporator path. Outside air mixing, bypassing the core etc. You have ruled out the heater core, but somehow with a very cold evaporator you are not getting cold air.

Adjusting the switch is part of wringing out the best performance, but not all of it.

You should do a full load test - doors open, cabin fan on highest speed & engine rpm 2,000 Record the pressures after 60 to 90 seconds while the engine rpm is still at 2,000.

If you were icing the evaporator you should have seen a decrease in airflow - unless the air is bypassing the core. How humid is it where you are at?

You are getting cold. Now to get the air flowing over the cold thing in the dash...

All of these measurements were at 1,200 rpm engine speed, windows open, all vents closed except center vent. These latest were in ambient of 88F; I don't know the humidity, but, based on the condensation and frost observed, I'd say it's fairly high humidity.

When I did slow the engine to idle speed (~700 RPM), after 4 minutes it stabilized at 27/205 psi and 47F accumulator temperature, and 50 degree air at the vent. This seems odd, because R134a at 27 psi ought to be 31 degrees, but I'm seeing 47F at the accumulator... Any thoughts on that?

Analyzing the data, at coldest accumulator temperature of 29F (1200 rpm, red OT @21/220 PSI), I got 45F vent air, and latest accumulator temp of 47F (700 rpm, blue OT @27/205 PSI) I got 50F vent air. I agree with your observation that, even with an 18F swing in accumulator temperature, I'm only seeing a 5F swing in vent air temperature, which does lead to the conclusion that air flow is a problem, or the air must be heated somewhere in the system...

I'd clamped off both heater hoses to rule out the heater core effect, but maybe that wasn't 100% effective. I think I'll install a shutoff valve in the heater line, and see what difference that makes.

After that, I'll have to disassemble the system and look for airflow problems.... maybe the evaporator has major airflow blockage or something.

Sidebar: I completely disassembled and rebuilt this system a couple of years ago, after a winter fire in the air handler (Ford's Bronco design allowed leaves entering through the cowl grille to get into the fan assembly, where they were spun up into the fan speed's wirewound resistor, causing a fire). At that time, I added 1/4" mesh hardware cloth under the cowl and as a screen at the entry to the fan housing, to prevent leaves entering the system. At that time, I also cleaned everything and confirmed all the doors and flaps were working properly. Now, it looks as if something has changed with the air flow...

BTW, I got lowest accumulator temperatures with the red OT (29F @21/220 PSI), but I was also able to achieve that (albeit fleetingly, with windows closed) with the blue OT (29F @ 23/220 PSI). Since I'll have to open up the system again, which OT do you recommend? I'm leaning toward the red, but maybe there's value in trying orange?

Thanks for your help in diagnosing this. I'll dig into it further today, and I'll come back to report findings here afterward.

I have an '93 F150 - the cabin air handler is very good at inhaling crap - including dirt from the passenger side floor when on recirculate...

If you open all of the vents how does the airflow feel? What does the vent temp do?

I have never run anything but the blue tube - but I converted to a TXV on mine a long time ago. Kept the accumulator, added a receiver & a TXV. Insulated the liquid line past the TXV in the engine bay & put the capillary on the accumulator line.

I'm declaring victory!

I installed a vacuum operated shutoff valve in both coolant lines to the heater core. (Motorcraft F87Z-18495-AA) The result was zero warm coolant to the heater core, except when the heater or defroster is turned on.

Now I've got consistent 41F vent air, with 32F accumulator @ 18/185 PSI, 1200 RPM windows open, max air and blower, Blue OT, (85F ambient).

Only lingering issue is that the cycling switch doesn't cycle at all. I tweaked it for an hour, but the best I could do was to get it to cycle on at about 47 PSI, and that switched off at about 28 PSI, which was where I started the whole troubleshooting affair.

Turns out, though, even without cycling, the accumulator temp hovers right around 32F, and the evaporator inlet is barely frosty, while the outlet is just short of frosty, so I think I'm probably not icing in there.

When I idle it down, it stabilizes at 43F vent air, @ 23/200 PSI... all at ambient today of about 85F.

Do you think with the Red OT I could get the low pressure low enough to activate the cycling switch, so I could drop the average evaporator temperature without icing the core?

Since I had to open the cooling system, I replaced both radiator hoses, the bypass hose, and the fan clutch, while I was at it. Flushed and changed antifreeze too, so I'm ready for winter now.

Tomorrow, we'll see how it does in real driving, but I'm happy with the result so far.

Huge thanks for your help in diagnosing and fixing the preheated air problem! I'm just teaching myself about A/C so I can keep my old jalopy running, here in Hot as Hell Austin, Texas.... where, ironically, this week, the temperature has been well below 100F, after 4 months well above that mark!

Thanks again!

Drive it a few weeks, see how it works. If OK, STOP.

That's FANTASTIC advice, Cusser; THANKS!

I did drive it today, and was pleasantly surprised that it performed even better on the road in actual driving than it did in driveway tweaking! It bottomed out at 35F Vent air, on the highway, and 41F at stoplights... The air feels truly 'chilled' now, which has NEVER been the case in this vehicle before.

The engineer in me still wants to wring out every last drop of performance, but I think you're right about just exploring its current performance fully before making any further changes, and remaining open to the possibility of making NO further changes at all.