Random air conditioning blows hot air

EricC

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#16
Did you hook up your gauges and see what your static line pressure is? Most systems require ~50 psi before the clutch will energize.
 

jigfeett

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#17
Yes but I have a Robinair 34788ni and rechecked the readings when I got home. It is no big deal for me to recover and recharge at will.
When I jump 30 to 87 at the relay it will engage and blow cold air.
Thanks
 

jd

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#18
30 is usually the "Hot" side of a "Bosch Relay" (those cubes, now actually produced by TYCO, who therefore now makes the best ones, most are cheap knockoffs) and 87 is the Normally Open contact, meaning circuit isn't made up till the coil closes 30 to 87.

The other two terminals (unless your particular relay also includes a Normally Closed 87A terminal) power a coil and have to get +12V and -12V (Ground) to close 30 to 87. Those coil terminals can work by being powered, by being grounded, or both, but they need both + and - to make up 30 and 87.

So when you connect 30 and 87, and the Compressor runs, you might be bypassing a bad relay. You might be bypassing a bad control, like a low pressure cutoff (LPCO) switch, or a temperature control device. OR... your system may be correctly NOT telling that Relay to run the Compressor. Like say the refrigerant charge is below low limit.

Haven't worked auto HVAC for a long time Thank GOD, but LPCO was in the low 20-psi range with R134A. If charge is inadequate, it'll exceed that till the compressor starts, then immediately fall below, opening the switch and stopping the compressor.

Trick with Refrigerants... They'll have "Static Pressure" (meaning the system is not running) whether fully charged or nearly empty. That's why putting gauges on a system that's NOT running is meaningless unless they tell you it's completely empty. It should show roughly ambient temp at static. Running, should be at a low side pressure that translates to an Evaporator temperature (right on the blue low side gauge dial) say 35-45-degrees F. About 35-45-psi. That'll give a good cold air flow out the vents but can't be below freezing or the evaporator core will freeze over from condensation. If the LPCO cuts off, low 20's, pressure may float up into the 50's before switch closes and compressor pulls it back down to that 35-45-psi. High Side, running, has a spec, but ballbark is 2.2 x ambient, so if it's 100F out, High can be in the low 200's. Hopefully not too much more, but R134A is very sensitive to charge level and air flow over the condenser. Manufacturers added a High Pressure Cutoff (HPCO) switch to keep things from getting too crazy.
 

jigfeett

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#19
Thank you, current update I junk yarded a control unit 2002 year only and when I swapped it out the original unit has scorching on the back case plastic, instead of being nice and white one corner is brown.
I took it apart and found the circuit board scorched also and two of the 100 ohm resisters are not reading correctly.
Now we wait until a hot day should be about Sunday this weekend.
Thanks
 

jigfeett

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#20
Hot day yesterday and the outside temp was 81 but the vents were blowing at a cool 42.
The head unit from the junk yard seems to have fixed the issue. A used $30 part.
Thanks and I hope this helps others.
 

jd

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#21
Good for YOU! It's always good to do a thoughtful visual inspection. You probably don't know how that module controls the compressor. But you COULD tell that it was scorched and maybe badly enough to cause a problem.
Great Catch!
 

jigfeett

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#22
Hold the presses, back to the drawing board It randomly acted up and blew 75˚ when it was 80˚ outside. Yesterday I rechecked it after work and it was 81˚ and the system blew cold air 41˚ so for now I am back to the same state as from the beginning.

Yes I do understand how the control works, it is a simple signal from the control unit to the pci and the pci groundsthe relay which provides 12v to the compressor. There is of course low and high pressure switches as a safety devices.

It is no big deal for me to recover and recharge so I might replace the compressor because I read there could be an internal issue with the coil.

I will update when I have more information.

Thanks
 

jigfeett

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#24
Final update, I hope.
very hot lately and the system is blowing a cool 42°.
Total tally.
New parts
orifice tube
accumulator
condenser
low pressure switch
compressor was the final fix
hvac head unit from junk yard( original unit had scorched plastic and circuit board)
Water pump and thermostat

My opinion is when a vehicle has over 150k systems need total replacement

I am into this less than $500

Dealer wanted $1200 for just the condenser and accumulator which would not have fixed the issue.

Thanks and enjoy.
 

jd

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#25
This oughta do it... Did you find debris on the Orifice Tube Screen? Delco started selling liquid line filters that were supposed to make flushing the system unnecessary. You really can't flush a cross flow Condenser (Evaporator for that matter) so replacing was a good move. Fords used to have "Black Death" compressor failures. Took a "firewall Forward" replacement to resolve. You pretty much did that.
 

jigfeett

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#26
Orifice tube was clean, 91° out and cold inside no worries, I just hope this long thread helps others.
thanks