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A/C Question 90 K5

Justin Fleming

Justin Fleming

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1990 k5 350

new ac compressor, accumulator, lines, condenser etc. All new

Installed oil, pulled 30inches of vaccum for 1hr, it held for 1hr, and I charged the system with 134a

Standard system take 3.25lbs of R12 or 52oz. The conversion I used was .88 x 52 = 45.76oz

I have installed 46oz and I am certain of it as I have a Freon scale. For some reason my compressor just keeps cycling in short burst as if it were low on Freon. If I jump the pressure switch it runs constant and makes great A/C


Does anyone have additional help as to how to trouble shoot this, as I am not a A/C expert. It almost seems as if the low pressure switch is bad or something of that sort.

Thanks in advance
 
Sure sounds like its low.
You need a set of gauges.
There are only a few possibilities. The most likely one is that you are a little low.
Or the switch could be bad.
Or else, and this is why you really need gauges, you may have a partial blockage. If you have a partial blockage in the high side, it is starving the low side of pressure causing the switch to cycle.
The danger of that is, that even though the system works when you bypass the switch, your high side pressures are getting very high.
You could try carefully adding some more gas a little at a time until the switch stops cycling, but doing that without a high side gauge makes me nervous.
 
I have gauges, when the compressor is jumped and engaged if I remember correctly I was around 40 to 60 on the low and 200 to 250 on the high.
 
So I just put the gauges on the truck, static pressure was 100/100 high/low. I started the truck and engaged the ac, the low side was pulled down to about 24lbs and the high side builds to about 185 and then the clutch kicks out. When compressor is off, the low side builds to about 43 and the high drops to about 140 and the compressor clutch engages again.

I then borrowed the shutoff switch of another k5 that I have with great A/c and I have the same result.

today is about 85 degrees average humidity for Michigan
 
My good know ac system on my other k5 truck is running at 37-40 psi low side and 190-200 high side not cycling of the clutch....
 
The system acts if it is low on freon, but at 46oz I would certainly think that is close enough to allow the compressor to run constant with out jumping the switch out??

Should I add more?? How many more oz before I should worry about over charge?
 
Seems like it might be a bit low. I'd try to bypass the switch again to get more definite, concrete readings. I think it's good to check it at idle and at a little higher than idle.

40 low 200 high would seem mostly ok, but 60 low and 250 high is off. Sometimes those compressors are garbage out of the box too.

Again, I think it is low. I'd try more, but keep an eye on the max numbers on the chart.
 
If you replaced the condenser, no need for a conversion rate for freon amount, you're not putting 134 into an r-12 system anymore. All the new condensers are 134.
 
Yep, with those numbers, you are low on gas.
Also, more than likely your weight numbers are wrong too. The factory weight is for exact stock equipment. Unless you went back with factory stock, the capacities of the system is going to be different.
Like 4XCrazy said, the new condensers are probably different. Interestingly enough, a friend of mine just replaced the condenser on his big F250 Ford after he hit a bear.
His truck uses 134A from the factory, and he ordered a Motorcraft Ford replacement.
Since I have a Freon scale, we weighed the gas in. However, when we opened the factory box, there was a note and a sticker with the condenser.
It said due to a design change, the new unit held a different amount of gas than the original one, and to change the charge by a certain amount and replace the sticker on the system to reflect the change.
If I remember right, it was 4 or 5 ozs more than stock.

Since your system may not be stock anymore, even if you used stock components, I would put the cylinder back on the scales, hook the gauges back up, and meter in some more gas until the pressures resemble the one that works, always watching the high side for problems.

When the system stops cycling, and blows good cold air with the pressures in the correct range, shut off the gauges and note the amount of Freon you put in.
Add that to the amount you have already put in, and make a note on the sticker so that you will know if you ever have to do it again.

Its a shame that there is not an easy way to put a sight glass in the high side like cars used to come with. I have put many sight glasses in commercial systems when the amount of Freon needed was unknown.
Then I could charge, watching the pressures until all the bubbles were gone, and be pretty close to dead on.
Some systems I would leave a few bubbles when the system was heat loaded, because as soon as they got to temp, the bubbles would go away.
Ice makers were a good example. If you charged them to no bubbles with the plates and water hot, they would be over charged slightly when everything got cooled off.
 
I just remembered, someone told me that they quit putting sight glasses on 134A systems, because the oil does not work the same with that as 12.
Supposedly the oil in a 134A system will tend to cloud the glass and make it hard to see bubbles or be mistaken for bubbles.
Not sure if that is true or not, but I would still like to try one to see.........
 
Thanks for the info guys, I will get back after it today.....

I just did not want to keep adding with out being aware and risk messing some thing up
 
Should I continue to jumper the low pressure switch to meter in more Freon, or should I let the compressor keep cycling pulling it in?
 
I would let the compressor cycle. That way you can see when it starts working correctly
 
So I have added another 30oz of Freon, with the gauges closed and ac clutch engaged on its own 35psi low side and 255psi high side. Today it is 90 degrees out side and my vent temp at idle is 55.7

based on the chart my low and high side number are still low, should I keep adding Freon? I am at a total of 76 oz
 
What kind of fan do you have? Do both pressures drop when you put water on the condenser? How did the A/C work on this truck before it broke? Also, what is your HVAC blower set to during these measurements?

I assume the compressor is running non-stop now. Is the outlet of the evap cold? Are you using a stock orifice tube or a slightly different one for the R-134?
 
it is a mechanical fan, not sure have not poured water over the condenser, A/c was broke when I got the truck, blower set to max/high

compressor runs non stop now, they gave me a tube for 134 i don't remember if it was a red blue or black
 
Fan clutch is tight and shroud is in place? Have you driven it? Some of the mechanical fan setups cool way better when you're driving. The more you cool the condenser, the cooler the evaporator gets. You could try lowering the blower speed to see if Temps come down and the clutch starts cycling. On a hot day they typically only cycle when you're moving.

So....outlet of the evap cold?
 
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