Air Conditioning Gurus please help...

[Iron Chef]

OK...I know this system isn't in a Charger...it's a 1970 Challenger R/T, but the guts are basically the same, so I'm hoping someone can offer me some assistance.  For starters, it's all stock, not converted to R134a.  I'm running Freeze12 in it at this time.

When the engine is first fired up, I can turn on the A/C and it blows nice and cold for about 10 minutes, then cuts off.  When I open the hood, I can see the compressor clutch cut in for a few seconds, then go off.  The system was filled with four cans of Freeze12.  We put in two cans, drove it for awhile, then two more as we feared the system didn't have enough coolant.  If the compressor stays on, all is well, but it keeps cycling off.

Anyone have any ideas?  Help is greatly appreciated.  Also...if anyone knows of a good A/C specialist in North Texas who can work on these sort of older systems, please post up.

Thanks in advance for any help. 

[nascarxx29]

AC not my speciality but I do have a 70 B Body with AC and got it working with help of my AC friend .Isnt there a low pressure switch on top of the black cylinder on pass side.With 2 blue wires designed to save compressor from damage by cutting out .Maybe switch bad or connections going to it.I used to live In Houston I know AC is a must have

[Iron Chef]

AC not my speciality but I do have a 70 B Body with AC and got it working with help of my AC friend .Isnt there a low pressure switch on top of the black cylinder on pass side.With 2 blue wires designed to save compressor from damage by cutting out .Maybe switch bad or connections going to it.I used to live In Houston I know AC is a must have

Thanks!  Yes on my car the canister (receiver-drier) is mounted on the pass side fender and it does have that switch.  I'll check it out.

Anyone ever heard of the pressure cutting out because it was too high?

[nascarxx29]

When the compressor cuts out.If you jump those 2 wires with a cotter pin.For just a quick touch and compressor kicks on .As you will damge compressor doing it long term.You will see if its low pressure condition or a bad switch

[Iron Chef]

When the compressor cuts out.If you jump those 2 wires with a cotter pin.For just a quick touch and compressor kicks on .As you will damge compressor doing it long term.You will see if its low pressure condition or a bad switch

Thanks, but here's a follow up: I know that if I jump the wires, I'm essentially bypassing the switch.  But how would I know if the switch isn't actually doing its job by cutting off the compressor?

[nascarxx29]

I wouldnt suggest bypassing the switch .Maybe you can ohmeter each switch connection with wires removed.See if you get any reading from the switch.

[gtx6970]

put some gauges on it.  that will give you more info .

If the pressures are good and the compressor cuts off, it's a bad electriclal source. ( possibile cut off switch on the drier )

if the pressures are low then the switch is doing it's job by shutting the compressor off

another way is to ohm meter the switch , if it has continunity, it's good ( compressor on or off makes no difference )

I don''t remember the switch also being a high pressure cutout switch as well, but Actually don't think it is )

[Iron Chef]

Thanks guys, I really appreciate it.  I'll try these fixes!  We're on our 4th straight 100 degree plus day here in N. Texas, and cars aren't driveable without air!

[elacruze]

Newer vehicles have a Trinary switch, which acts as both a low pressure cutout to protect the compressor and also an electric fan turn-on when pressure gets high enough. These old cars have only a low-pressure protection switch.

On my '68 with factory air, the temp switch in the water valve also controls the compressor clutch circuit. You need to have a set of gauges on your system to know what's happening. If the compressor cuts out when the low side pressure gets below 20psi, you have a low charge. If the compressor cuts out when the pressures are still normal, then your water valve is probably breaking the circuit...refer to the factory service manual for diagnostics.

[John_Kunkel]

put some gauges on it.  that will give you more info .

Yep, anything else is just guessing. Four cans (48 oz.) sounds like too much, it's probably overpressuring and tripping the pressure switch. Some of the so-called "drop in" R-12 replacements are a blend of other refrigerants that make the head pressure run higher.

[Iron Chef]

Thanks guys.  I'm gonna get together with a buddy who has a set of gauges so hopefully we can untangle this.

By the way...I checked the factory manual and it says 3.2 - 3.6 lbs is an acceptable charge.  Of course, we're talking about Freon and not Freeze12 or 134a.  It also gave the proper procedure to test the low pressure switch!

[Bob]

I overfilled mine once and it would go on and off.

[John_Kunkel]

By the way...I checked the factory manual and it says 3.2 - 3.6 lbs is an acceptable charge.  Of course, we're talking about Freon and not Freeze12 or 134a. 

When converting an R-12 system to R-134 it's recommended to use only 80/85% of the recommended R-12 charge. Since Freeze 12 is 80% R-134 and 20% R-142 you're probably about one pound overcharged.

The high pressure component of the switch is what's kicking the compressor off.

[268RTs4ME]

Next time it does it, check and see if your exansion valve up by the fire wall is freezing up, that is probably your problem, it is restricted, or getting restricted, i wouild change then expansion valve and the dryer, and recharges with R12, about $400.00 right,or now is the time to put the conversion kit on it and put some 134 in it.

[Iron Chef]

Next time it does it, check and see if your exansion valve up by the fire wall is freezing up, that is probably your problem, it is restricted, or getting restricted, i wouild change then expansion valve and the dryer, and recharges with R12, about $400.00 right,or now is the time to put the conversion kit on it and put some 134 in it.

Actually I priced out doing this.  It is my understanding (and...I could be wrong) that in order to convert the system to R134 properly, the compressor must be rebuilt using the proper oil that won't react with R134 and you must also put in a new receiver-drier or have the old one rebuilt.  In my case, because my car is a numbers matching all original model, I'd want to rebuild the drier to preserve the original part.

I called Classic Auto Air and they want about $600 for the process with about a 2 month lead time.  If I go this route, it'll likely be after summer.

[randy73]

OK...I know this system isn't in a Charger...it's a 1970 Challenger R/T, but the guts are basically the same, so I'm hoping someone can offer me some assistance.  For starters, it's all stock, not converted to R134a.  I'm running Freeze12 in it at this time.

When the engine is first fired up, I can turn on the A/C and it blows nice and cold for about 10 minutes, then cuts off.  When I open the hood, I can see the compressor clutch cut in for a few seconds, then go off.  The system was filled with four cans of Freeze12.  We put in two cans, drove it for awhile, then two more as we feared the system didn't have enough coolant.  If the compressor stays on, all is well, but it keeps cycling off.

Anyone have any ideas?  Help is greatly appreciated.  Also...if anyone knows of a good A/C specialist in North Texas who can work on these sort of older systems, please post up.

Thanks in advance for any help. 

You put 4 CANS of freon in, you screwed yourself. That is way to much freon, you have probably blown a seal or two, if you are lucky or you blew a seal in your compressor.
I would put gauges on an check, but compressor cycling on and of is a sign of low freon, bad pressure switch or a compressor leak.

[John_Kunkel]

It is my understanding (and...I could be wrong) that in order to convert the system to R134 properly, the compressor must be rebuilt using the proper oil that won't react with R134 and you must also put in a new receiver-drier or have the old one rebuilt.  In my case, because my car is a numbers matching all original model, I'd want to rebuild the drier to preserve the original part.

It's not necessary to rebuild the compressor, the old oil can be poured out and replaced with ester oil which won't react with the original mineral oil. Replacing the front seal is a good idea though.

The rest of the system should be flushed and the rubber hoses replaced with barrier hose, the R134 molecule is much smaller than the R12 molecule and will leak from places R12 won't.

All this could be avoided by fixing the leaks and recharging with the proper R12 which is much more efficient.

[Iron Chef]

It is my understanding (and...I could be wrong) that in order to convert the system to R134 properly, the compressor must be rebuilt using the proper oil that won't react with R134 and you must also put in a new receiver-drier or have the old one rebuilt.  In my case, because my car is a numbers matching all original model, I'd want to rebuild the drier to preserve the original part.

It's not necessary to rebuild the compressor, the old oil can be poured out and replaced with ester oil which won't react with the original mineral oil. Replacing the front seal is a good idea though.

The rest of the system should be flushed and the rubber hoses replaced with barrier hose, the R134 molecule is much smaller than the R12 molecule and will leak from places R12 won't.

All this could be avoided by fixing the leaks and recharging with the proper R12 which is much more efficient.

When we hooked up a gauge set to it, it held pressure.  There were no leaks present that we could locate.    However, this might be the solution short of getting R12.

Now, to go the other route, the question is where would I get REAL R12?  Open to any suggestions.  

BTW...thanks again to EVERYONE in this thread.  You all have been most helpful!  This is really a great forum!  

[John_Kunkel]

When you hooked the gauges up what were the running pressures and temperature outside?

R12 is readily available but pricey, a license is required to "legally" purchase it but............

[Iron Chef]

When you hooked the gauges up what were the running pressures and temperature outside?

R12 is readily available but pricey, a license is required to "legally" purchase it but............

I don't recall as I wasn't there when it was done (my engine builder did it at his shop).  I'm going to be with my buddy this weekend who has a set of gauges and we'll re-check.

I noticed R-12 for sale on FleaBay.  Did a little research.  Apparently it's legal to buy as long as you're re-selling it to an A/C tech.  I think I'll go this route as I have no desire to do this myself.  While I'm at it, I purchased a new expansion valve from Classic Auto Air.  As long as I'm going to evacuate the system, I might as well change out this component as several folks have told me it might also be suspect.

[Iron Chef]

Well I got my real R12 and R12 flush solvent as well as a new expansion valve (for $60, why not?).  We'll evacuate the entire system next weekend and start from scratch.  Will report on how things go.

[Topher]

Nothing cools like R-12. Your pressures should be 30 on the low side, and between 175 and 200 on the high side on an 85* day. It'll be higher in higher temps. Look for 40* or lower temp out of the vents at idle. You should be making ice cream going down the road.

[Iron Chef]

Unfortunately, it didn't work.  Put on the gauges, added Freon, changed the expansion valve.  Lowest temp the A/C had was 120.4/ 

Back to the drawing board! 

[John_Kunkel]

How much Freon did you install? Did the clutch remain engaged? What were the gauge readings on both sides? Have you checked to see if the heater valve is shutting off the water flow? (feel both heater hoses)

[nascarxx29]

A cable from heater controls goes to this this to limit water flow

[Iron Chef]

How much Freon did you install? Did the clutch remain engaged? What were the gauge readings on both sides? Have you checked to see if the heater valve is shutting off the water flow? (feel both heater hoses)

We put in 3 cans of freon (14oz per can).  Readings were 200 on the high side and 20 on the low side which are not correct.  We put a thermometer in the A/C vent and it registered no lower that 120 degrees and when we opened the heater valve it shot up to over 150 then we closed it, so I think the heater valve is working (it's also brand new).  The clutch kept cycling on and off, and I'm not sure if it's supposed to.

[y3chargerrt]

20 on the low side is too low. It needs more freon.

[Iron Chef]

20 on the low side is too low. It needs more freon.

You  may be right, but I think 4 cans is too much.  I suspect there is another problem lurking somewhere...likely something simple.  I hope.

[John_Kunkel]

Even with only 2.6 pounds in the system it should cool below the ambient temperature.

[elacruze]

How much Freon did you install? Did the clutch remain engaged? What were the gauge readings on both sides? Have you checked to see if the heater valve is shutting off the water flow? (feel both heater hoses)

We put in 3 cans of freon (14oz per can).  Readings were 200 on the high side and 20 on the low side which are not correct.  We put a thermometer in the A/C vent and it registered no lower that 120 degrees and when we opened the heater valve it shot up to over 150 then we closed it, so I think the heater valve is working (it's also brand new).  The clutch kept cycling on and off, and I'm not sure if it's supposed to.

Compressor clutch will cycle due to the temperature switch, which acts as a freeze switch. Your cycling could also be due to the low pressure switch-on modern stuff it cuts out about 10psi, could be closer to 20 on some. Your differential is too high-you have a blockage somewhere. I had a faulty expansion valve from Classic Air, right out of the box had numbers about 250/10. Replaced and went to 160/35.
If you didn't replace your filter/drier, you need to. If it's plugged you might see the lower half of it icing up; you shouldn't be able to feel a temperature difference anywhere on the filter. You can use an infrared thermometer as well to indicate difference. You also shouldn't be able to see any icing at the expansion valve, the refrigerant should get 6-12" downstream (into the evaporator) before boiling and cooling. Bypass the heater core altogether to eliminate that with certainty. New parts are no guarantee of function.

[John_Kunkel]

The subject car is a '70 E body and, to my knowledge, there is no switch in the electrical system but the low pressure switch on the dryer. It could have been altered with an evaporator switch.