A/C Problems. '90 D-150 w/5.9

[Manfred318]

Here goes. My compressor went bad about 2 weeks ago. I bought a reman one from the local parts store as well as a new expansion valve. Anywho we took the truck to a guy dad works with who is supposed to be a "mechanic"(and I use that term loosly). He got the compressor and exp. valve replaced and converted the system to 134. Here's the problem. The compressor kicks on and off like there is not enough freon in it. The system is fully charged. Its not blowing very cold out of the vents. Its cool but not COLD. It will cycle about every 90 seconds under load and maybe 2 minutes at idle. He cant seem to figure out the problem. I have no real clue myself as I'm not too familiar with AC systems. I do know if the temp sensor is removed it will not cycle off and on. Could the sensor be bad? Its getting really frustrating, so if any of you guys out there could shed some insight it would be greatly appreciated.

[John_Kunkel]

How do you know "the system is fully charged"? Have you had gauges on it? What are the pressures?

[Tilar]

Sounds to me like it doesn't have enough freon.

[Manfred318]

How do you know "the system is fully charged"? Have you had gauges on it? What are the pressures?

At idle its running around 30lbs of pressure.

[Tilar]

I know it's been a long time since I've messed with A/C, but isn't it supposed to be 35-40 on the low side?

[John_Kunkel]

At idle its running around 30lbs of pressure.

On the high side or the low side?

[Manfred318]

At idle its running around 30lbs of pressure.

On the high side or the low side?

Low side.

[John_Kunkel]

Jeez, ya gotta drag an answer outa some people...what is the high side pressure?

[Manfred318]

Jeez, ya gotta drag an answer outa some people...what is the high side pressure?

Sorry man Im not sure on the high side.

[John_Kunkel]

The reason I ask is because low presssure on the high side will trip the low-pressure switch and disengage the clutch. Without both gauge readings it's hard to tell if it's fully charged or overcharged which is possible with R134 conversions. Is the sight glass clear?

Find the expansion valve capillary bulb where it plugs into the suction line and pull it a ways out of the line (or all the way out) and see if that makes the clutch stay engaged longer.

[Manfred318]

The sight glass looks a little milky. I think he put it 4 or maybe 5 cans (12oz) plus the oil charge. According to him it was "fully charged". I should have just forked out the extra cash and bought the R-12. If I pull the probe out of the line the clutch will not cycle off and on. The line will eventually freeze up. As soon as its put back in (maybe 20-30 seconds) the clutch will kick off.

[Cooter]

Depending on how hot it is outside, the high side pressure should be somewhere in the 250-300 LB range. FYI, the compressor should "Kick out"(Cycle) about every 2 min. at idle to keep from freezing up..That is why the probe in there. Sounds like there's either an overcharge/undercharge in the system, a blockage somewhere, or a blend door problem under the dash.

Remember, a 40 degree drop is typical for an A/C system, so if it's 100 Degrees out, 60 degree air is considered "Normal"...I've found with today's public, they tend to want to expect to much from Vehicle A/C systems, and want ice cubes hitting them in the face when they get in that 250 degree car first thing.

[Tilar]

The sight glass looks a little milky. I think he put it 4 or maybe 5 cans (12oz) plus the oil charge.

Did he pull a vaccum on the system? I'm guessing no.

[John_Kunkel]

The sight glass looks a little milky.

Usually means it's low on charge, you need to hook up a gauge set and check the pressure on both sides. 

I think he put it 4 or maybe 5 cans (12oz) plus the oil charge.

That's way too much. The factory R-12 charge is 44 oz. and you're supposed to use 80% of that (35 oz.) when going to R-134. If the system had leaks before the conversion, R-134 will leak out at a faster rate because it has smaller molecules. Even though it was overcharged it might now be low due to leakage.

I should have just forked out the extra cash and bought the R-12.

Yep.

If I pull the probe out of the line the clutch will not cycle off and on. The line will eventually freeze up. As soon as its put back in (maybe 20-30 seconds) the clutch will kick off.

Before the line freezes is it putting good cold air out of the registers?

[Manfred318]

The sight glass looks a little milky. I think he put it 4 or maybe 5 cans (12oz) plus the oil charge.

Did he pull a vaccum on the system? I'm guessing no.

He was supposed to have. He said he did anyways.

Yes before it freezes up it puts out good cold air.