I attempted to refill my 78 pace cars A/C with freon I pulled a vacumm for about 30 mins. I put in a few cans then fired it up. Compressor runs for 5 seconds, then uncouples. Very repeatable engage, 5 seconds, disengage, Not sure if that is the low pressure or high pressure telling the clutch to disengage.
My problem is after 5 cans the pressure doesn't climb I show 85 PSI on both high and low. What am I doing wrong? the Cars label said I needed many more 6 oz cans to get to as much freon they called for. I had a leak I fixed. No shops will touch it, unless I convert it to R- 134. I am trying to use the R12

 

As standard on a 78 there is no high pressure switch, there is only a low pressure switch (in the liquid line) and a thermostatic switch (which cuts the compressor if the evaporator inlet temp goes below 33F). Of course someone could have changed this over the past 40 odd years. What do your pressure gauges do when you turn on the A/C? Five 6oz cans is much less than mine took.

 

You are using r12? 6 ounce cans?never heard 6 ounce cans.
did you add any oil? What was replaced when it leaked?
I strongly suggest gauges to see where you are at when it cycles.
you should weigh your freon add and watch gauges report back.
you should watch the high pressure and low pressure when it engages and shuts off.
your car should take 3 pounds

 

I have about 80 on both sides, and that ain't right. Comp clutch keeps kicking off ever 5 seconds I can see the high side build up a bit when the clutch engages, but it does not build up due to the Compressor shutting off the clutch. I think the compressor is fine I am tempted to hot wire the Low pressure switch so it can build up the pressure. The other thought was maybe a bad orifice tube?

 

Yes the cans all say 170 gram 6 OZ

 

80 pounds both side not running is almost meaningless
that’s pressure at temp
watch your LOW side when compressor on. If it drops below ~25-30psi…you need more refrigerant..
5 cans x 6 ounces= 30 ounces…minus what you lose clearing lines changing cans…less than 2#..

 

As Interpon asked, is your low pressure side dropping when the compressor is on? Also a photo of your pressure switch would be useful to see if it is still in the original location of the liquid line and whether someone has replaced it with a binary switch. Again as Interpon asked, what have you done to fix the leak?

 

Those pressures are with the A/C on. When the compressor cycles the high side starts to climb low side stays the same. The A/C in this car is completely original Never been fiddled with A/C wise, until me. i may just go and do the conversion. I only put in a few cans because it was not working correctly Why waste Precious Freon. The repair was one of the aluminum tubes to the block at the rear of the compressor had a crack in the weld. Was gushing out so bad, I could feel it blowing on my arms. With the pressures the same on both side, could it be the orifice?

 

Working with r12 can be frustrating as it is expensive…
you said you only added a few cans ..well if it’s empty and you don’t put enough in it will not work.
as mentioned above pics details.
you should have looked at orifice screen prior to filling.
If the system wide open for a long time, you should replace accumulator.
Well you are where you are at…maybe jumper compressor for longer than 5 seconds…maybe 30 seconds and see if lowside starts to drop.
Seems the only way it won’t run is low pressure switch.
or start over.
Inspect orifice screen. If full metal, you will know compressor shot, flush new accumulator, compressor, and i pressure test with compressed air, then pull vacuum for at least hour or more..
add oil and refrigerant weight.
the hardest part of job is changing accumulator…imo.

 

If the both the high and the low side are at 80psi, then the low pressure switch shouldn't cut the power to the compressor clutch. You may have a faulty pressure switch (or a faulty thermostatic switch). I'd try jumping each one in turn to see if you can isolate the problem. Photos might help identify an issue.

 

I was recently charging a r12 system and found my manifold hoses were causing problems. The rubber seal in the end of the service hoses was swelled and restricted the flow of the connection. I took a seal pick and removed the seals and cut a short piece of vacuum hose and made up new ones. Now they work right, I can get the right volume through the connections and they open the schrader valves when connected.

 

R12 comes in 12oz or 14oz cans; or bulk containers. To get 3lbs exactly takes 3x14oz plus 1x 12oz. Ok to use 4x14oz if you don’t want to deal with different sizes. The 3lbs required for an original 78 can be +/- 1/2lb and still work, so a couple oz extra won’t matter. r134 has a tighter tolerance for +/-. You lose 1oz/can plus 2oz for the fill hose when charging. Filling with 6oz cans would take 10 cans and waste 3/4lb of r12 into the atmosphere! R12a (eg Frosty Cool) comes in 6oz cans, but it’s a totally different substance. Leak-stop Freon also comes in smaller cans, but finding the leak and fixing it is the way to go. Real r12 on fleabay is about $30/can ($120 plus oil and dye for a recharge). A little bottle of Nitrogen set to 200psi works great to pressure test for leaks as it purges air from the system and starts the drying process even before pulling a vacuum. Definitely agree with checking the orifice tube before any recharge. For me, minimal replacement parts for an all-original system would be orifice tube, accumulator, compressor(unless low mileage-new seal at the front) and hoses. Either a 78 or 79 service manual give exact and comprehensive info on the entire ac system, including troubleshooting and charging.
Charlie