A/C Blows Hot Only

[bonesy]

So my AC has stumped me, the clutch is not engaging at all. Here is what I have noticed --

Purple Wire from Electronic Automatic Temperature's Clutch Out -- fluctuates between 0 - 12 V (expected)

Red/Yellow Wire after AC Clutch Cycling Pressure Switch -- fluctuates between 0 - 10 V (expected, but not why 12 V, is it because the resistance in the switch?)

DK Green/Orange in the WOT AC Relay Seat -- fluctuates between 0 - 3 V (why such a big drop, resistance in the switches?)

Jump the AC Clutch Cycling Pressure Switch -- no difference to clutch engaging or V @ DK Green/Orange in the WOT AC Relay Seat

Jump WAC Switch -- no difference to clutch engaging -- no difference to clutch engaging or V @ DK Green/Orange in the WOT AC Relay Seat

Connect DK Green/Orange in the WOT AC Relay Seat to 12V Battery -- CLUTCH ENGAGES!

The current low pressure is about ~60 psi, on the gage I have right on the boarder of Green/Red Zones.

Do not have a fitting for the tube from the the WAC Switch, so I do not have the pressure there.

I have rotated the relays in the distribution box, no change.

When I connect the Purple Wire directly to the Relay, there is a voltage difference (?). Meaning, when the purple wire not connected it fluctuates between 0-12V but when it is connected it fluctuates between 0 - 3 V. From good old physics, Volts = Amp X Ohms; aren't volts and ohms fixed in this case, so only amps should change? If so, then why would I be seeing this drop in volts on this direct connection? Is this my problem: once it is connected to the relay, I no longer have the necessary potential to activate the relay whereas if I hook it to the battery I do?

The only thing left in the circuit is the PCM, would this be causing so issue or the drop in voltage of the direct connection?

Thanks for any help, I am getting tired of coming home from work with a sweat drenched shirt even with windows down.

 

[ranger7ltr]

You are making this a lot harder than it probably is...

The a/c system in our trucks require a few items...Voltage when the controls call for it; Refrigerant at the proper level to complete the circuit; Oil to lubricate the rotating parts; rotation of the compressor when the system completes the circuit to turn the compressor on...

You listed 60 psi in the system; If that is on the high side you are WAY undercharged...If it is on the low side, and I expcet that it is, you are looking at the static pressure in the system without the compressor running...

First I would closely inspect the hoses/fittings/seals/orings in the system for any obvious leakage...Then get a proper set of gauges and monitor the high/low sides and then consider either topping off the refrigerant or take the truck to an a/c shop that can evacuate the system, fix the leaks, then properly evacuate and refill the system by weight instead of by guessing...

One primary spot to look is the insulation around the accumulator...If there is rust on the unit, you have a leak...Time to replace that rusty unit...Other leaks will show as an oily/dirty spot on the o-ring/fitting/hose connection junction...Check the connections and body of the condensor for oily spots as well...

When did the a/c system work last?

 

[bonesy]

Thanks for your response.

I did one check with a UV light for any spots but it was pretty hastily done as the engine was still hot. I'll take another look more carefully since I haven't driven at all today.

The AC stopped working only like 4 days ago. It was fine on the commute to work but on the commute home only blew hot.

And yes the pressure measurement is from the Low Side port without the compressor running. Would jumping the compressor to have it run and double check the pressure be a viable test?

The system was completely evacuated with new refrigerant, oil, etc. added last August and I did add 2 cans (24 oz) during all this checking.

I am just puzzled to why if I jump both the WAC Switch and the AC Clutch Cycling Pressure Switch at the same time, and the Elec Temp Control is calling for the clutch, why will the clutch not engage? And why is the voltage in that wire to the relay at 12 V max when not connected to the relay, but only 3 V max when it is (also to mention this measurement is in the same spot before entering the relay)? It doesn't seem that it is the relay, because the behavior is the same with all three relays in the distribution box. Is this voltage drop how the system is suppose to behave?

Thanks for helping me sort this problem...as much as I love fixing the truck it seems like I can't avoid the shop on this one

 

[bonesy]

Update:

Completed a more detailed inspection and I could not find any rust, dirt buildup, or UV reflection on any of the lines, connections, compressor, or accumulator.

Checked the Low Pressure with Clutch Engage, at 86 degrees, and 2000 RPMS. Low pressure started at 100 psi (static) and quickly dropped to 45 psi then slowly stabilized at 35 psi. With ac clutch disengages, system when back up to 100 psi.

I monitored the voltage output from the Pressure Switch, it never changes from 12V (my assumption is that when the pressure become low this switch disengages and thus I should see a voltage drop out the output side [?]).

If you couldn't tell, I really don't think it's a low refrigerant level but I could be completely biasing my observations. Is any of this behavior abnormal or does this all seem to be the typical function?

Thanks.

 

[ranger7ltr]

Gotta go on a guess right now...

If I knew the high side pressures @ 86 F I could tell you a lot more about what is happening in the system.. But with the info I DO have, I am gonna guess that the system is overfilled at this point...

With the compressor not running the static pressure is fine but with the compressor running 45 psi on the low side is not going to produce much cooling...I believe the capacity is only 30 oz of refrigerant or so and if 2 cans were added to a low but not empty system you are way over that...

How much over, unfortunately, is anyones guess...If the compressor is not cycling on and off, the evaporator is not cooling to the point of the thermal switch cutout which turns off the compressor when the evaporator gets near the freezing point...And since you didn't find a leak in the engine compartment and the system was losing refrigerant the other obvious source of leakage would be the evaporator...

I wish you could borrow a set of gauges to measure the high side but I believe that you are probably stressing the hell out of your compressor and still not cooling..

Do you have a thermometer in the vents? Or what are the vent temps you are seeing?

 

[bonesy]

FIXED!!! AC Blows Hot, Refrigerant Levels Normal

So I gave in and brought the truck to an Auto Shop, however they were not able to fix the issue but completed the second to last part of the puzzle.

So the pressure while running was 45 psi (low) & 300 psi (high). All switches in typical working condition. When you placed in a 12V power source in where in the circuit the clutch would engage, however in the typical set up the clutch would not engage.

The repair shop suggest replacing the AC Control Assembly for $1090!

I brought the truck home, pulled out the control assembly and unplugged it. I then jumped the purple/white and purple wire -- clutch engaged and cold air came through the vents. This confirmed that something was going on the control assembly.

Pulled the assembly apart and examined the main circuit board. There is was a burnt out little solder. No sure exactly what the piece was it was a large black cube.

So here was the trick - good old physics:
--Voltage = Amp*Resistance

Normal working condition: V=12, Resistance = constant, amp = what is neccessary to run the system.

What I saw:
-- In the disconnected circuit (testing voltage at an unplugged switch for example) -- I gave 12V, very little amps (just enough for the volt meter), and very little resistance.
-- In the connected circuit, I got the resistance and the voltage drop to 3 V. I was not getting enough amps.
Conclusion:
-- The burnout solder how a small enough connection to supply the circuit with some amps, but not enough so in restoring the connection with some new solder, the large connection allowed the appropriate amps and allowed the circuit to have the full 12V potential required.

So is this what happens to these newer (yes I now 14 yrs is not new but its a lot newer than my 1987 Ranger) more electronically controlled systems? It seems it is not so much that parts just wear from too much spin but rather solder joints just end up burning up and the connection dies. This is the second times something of this nature has happened to me.

Well, I hope this saga helps somebody out there. Best of luck!

 

[Glacier991]

I would add that 45 psi on the low side can in fact produce significant cooling. Low side may vary a bit depending on ambient temps.... 35 is generally the cutoff for the compressor. I know of weak compressors that run 45 - 50 on the low side and still produce acceptable cooling.