[jntmjt1]

I don't know what I'm doing, but I'm determined to get it right! I have a 1966 Bonneville with the original A/C system. It uses R12 and about every 4 years it need a refill. This time, I decided I'd try to do it myself, and I need some help.

Yesterday, I hooked up my gauges and shot 2 cans of R12 (the real stuff, not Freeze 12) in the system. The outside temp was about 65 degrees, I had the cans of freon in a pan of warm water to give them a boost. The first can, I was reading something like 30 on the low side and 50 or 60 on the high side. After the second can, I was at about 30 on the low side and 100 on the high side. I was getting cool, but not cold air out of the center vent.

Since the high pressure climbed as I added freon, my assumption was I needed more freon (I thought it should be around 160), but I was out of daylight so I called it a night. Today, I was prepared to give it some more freon, and I started the car and hooked up the gauges. I was at 30 on the low side and 25 on the high side, then I noticed a complete frost building up on the aluminum line on the low pressure side of the expansion valve (between the expansion valve and the evaporator). The air was still blowing cool just like yesterday. Then, after I shut the car off, my low side gauge went up to about 70.

So what is going on? Why the big difference in the high side from yesterday, but no real difference at my vent temp? What have I done wrong, and how do I fix it?

Thanks for your time,
John Thomas
jntmjt1@yahoo.com

[gpmodelj70]

It soulds like there may be some moisture in your system and it is freezing up at the expansion valve.
I know you said that you want to do this yourself and you can but some speciallized tools are needed...maybe you can borrow?

Here is what needs to happen...have the R12 recovered...replace all of the o rings...vacuum the system out very well...pressurize with nitrgoen to find the leak...repair it...vacuum and pressurize again...if no leaks then vacuum and leave it for several days...repeat this cycle until it will hold a deep vacuum...fill it with the proper amount of R12 using a bulk tank and scale.

I have restored /repaired four R12 systems so far and all have worked out well.

[jntmjt1]

I have restored /repaired four R12 systems so far and all have worked out well.

Thanks for the tips. Have you, by chance, removed the evaporator from a 66 Bonneville? I need some step by step instructions on how to access that heater & a/c box to remove the evaporator. If I have a leak, that is where it is.

Thanks again,
John Thomas

[lust4speed]

Not questioning results - just confused. If the static pressure is 70 pounds and you fire up the system and the low side pulls down to 30, then how could the high side be lower than the low side? Are you sure your high side gauge is working correctly?

[jntmjt1]

Originally Posted by lust4speed

Not questioning results - just confused. If the static pressure is 70 pounds and you fire up the system and the low side pulls down to 30, then how could the high side be lower than the low side? Are you sure your high side gauge is working correctly?

Thanks for the reply. Assuming my gauges are working correctly, I guess what you are asking is what I'm trying to find out, "how could the high side be lower than the low side?" Could a bad expansion valve or something like that cause the problem?

I'll double check the gauge after work today. I am confident it is reading correctly though, it's a new set of gauges and it's a good brand. When I remove the gauges, they still hold the numbers until I release the pressure from them on a schraeder valve they have built in, so I know they are not leaking.

Also notice, I wrote it climbs to 70 psi after I shut it down (once the car is warmed up) when I first hook up the gauge it is closer to 30, or a similar pressure to what it is when it's running. It is only after I shut the car off that it climbs up to 70. This is all on the low side. I don't know what this means or if it's normal.

[george kujanski]

What happened to me a while back with r12 gauges...the high side was not responding. The culprit was that the rubber seal in the end of the hose, when tightened too much, was closing off the passage so no hi side pressure was getting to the gauge.

George

[ericwy]

Originally Posted by jntmjt1

Thanks for the reply. Assuming my gauges are working correctly, I guess what you are asking is what I'm trying to find out, "how could the high side be lower than the low side?" Could a bad expansion valve or something like that cause the problem?

I'll double check the gauge after work today. I am confident it is reading correctly though, it's a new set of gauges and it's a good brand. When I remove the gauges, they still hold the numbers until I release the pressure from them on a schraeder valve they have built in, so I know they are not leaking.

Also notice, I wrote it climbs to 70 psi after I shut it down (once the car is warmed up) when I first hook up the gauge it is closer to 30, or a similar pressure to what it is when it's running. It is only after I shut the car off that it climbs up to 70. This is all on the low side. I don't know what this means or if it's normal.

Static psi on my R12 system is about 95psi at about 70F degrees. After running a while and shutting it off its about 105psi.

What were your observations prior to adding the 2 cans of freon?
Was there sufficient refrigerant in the system to allow the AC compressor to engage?
What were the gauge readings prior to adding refrigerant?

If the system was substantially low or empty it most likely has air and moisture in the system. Air in the system will cause lower high side pressure as it is compressable and does not condense into a liquid state. If you have a sight glass you should see bubbles.
The moisture in the system could be freezing up inside the expansion valve or orifice as gpmodelj70 said restricting the refrigerant. Icing up on the evaporator side is also typical of a low freon condition which you have. If its simply low adding more will probably get rid of the low side icing. Either way 25psi on the high side does not make sense, its not enough to allow the condenser to liquefy the refrigerant. Without liquefied refrigerant the expansion valve wont work and the system won't cool, much less freeze up. Check your high side fittings/guage/etc...

More seriously is that moisture/air in the system will in time destroy the compressor. It can
also internally corrode the evaporator/condenser eventually causing a leak.
In addition to gpmodelj70's advice to evacuate/flush the system and fix the leak(s) also replace the desiccant/dryer with a new unit.

[gpmodelj70]

Originally Posted by ericwy

...also replace the desiccant/dryer with a new unit.

You are correct....I cant believe that I forgot that...thanks for adding it.

[jntmjt1]

Guys,

Thanks for the tips. I discovered that my brand new red hose that came with my gauges was the problem. It isn't allowing pressure to the gauge. I swapped it out with a spare hose I had and the gauges made a whole lot more sense.

I shot a third can of R12 in it, my high side is up in the 200 range and low is around 40. My car is nice and cool inside now, and no more frost on the low pressure line to the eveaporator. Hopefully I'm good for another 3 or 4 years.

Thanks again,
John Thomas

[ericwy]

Originally Posted by jntmjt1

Guys,

Thanks for the tips. I discovered that my brand new red hose that came with my gauges was the problem. It isn't allowing pressure to the gauge. I swapped it out with a spare hose I had and the gauges made a whole lot more sense.

I shot a third can of R12 in it, my high side is up in the 200 range and low is around 40. My car is nice and cool inside now, and no more frost on the low pressure line to the eveaporator. Hopefully I'm good for another 3 or 4 years.

Thanks again,
John Thomas

That's great! You must have had about 1.5 lbs freon still in the system which was
probably enough to keep any moisture from entering the system. You did not mention
what rpm, ambient temperature, etc... but if those readings were taken at idle you are
probably in pretty good shape. While cruising the in the 2k rpm range your low side will probably pull down a bit which will bring your evaporator just above freezing which is perfect IMO. Just in time for summer cruising!

[jntmjt1]

You did not mention
what rpm, ambient temperature, etc... but if those readings were taken at idle you are
probably in pretty good shape. While cruising the in the 2k rpm range your low side will probably pull down a bit which will bring your evaporator just above freezing which is perfect IMO. Just in time for summer cruising!

Those readings were at idle. The ambient temp was probably around 70 degrees. I hope I'm good to go.

Thanks again,
John Thomas