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a/c stops working after 15 min of driving?

drhill on Mon February 27, 2012 2:01 AM User is offline

Year: 1973
Make: Rolls Royce
Model: Silver Shadow
Engine Size: 6.75L
Refrigerant Type: converted r134a
Ambient Temp: 85
Pressure Low: 0
Pressure High: 150
Country of Origin: United States

Hi everyone, I have a 73 Rolls Royce project car using a fairly simple ac system (the factory system less the suction throttling valve function which has been eliminated - this was the rolls version of how to control temperature of the ac) so it runs the long compressor that was on all GM vehicles from this era and an expansion valve.

The ac works great and pressures are fairly normal (250 high side, 25-28 low side, 85 degrees ambient) if I switch on the ac and high idle the car with the hood open. Once I drive it 15 minutes or so, the AC stops cooling and will not blow cold again until I stop the car and the engine cools down for a while. The AC may work for a while upon restart but will then stop working again after another drive/warm up.

I noticed that the compressor clutch was struggling to engage at first with a cold engine, but once it engaged the forst time it had no further issues. The compressor stays engaged when the ac stops cooling, the high side drops to 150 and the low side to zero. Also - if i switch off the ac system when this issue is occuring, the hig side stays at 150 and low side very slowly rises to approx 70 or 80 psi.

I'm sure this is something simple - can you guys give me an idea of what may be wrong?

I'm thinking stuck expansion valve?? Seems wierd that it would be affected by underhood heat and be consistient it is failure.

Thanks in advance for any help and direction you can provide!

Edited: Mon February 27, 2012 at 2:25 AM by drhill

mk378 on Mon February 27, 2012 8:50 AM User is offline

Is it just the evaporator frosting up on the outside? Check the evaporator outlet line for ice. There might be a little ice on the inlet line right after the TXV, but should the whole evaporator get covered with ice, the interior airflow will be blocked and there's basically no cooling.

If it is doing that, you need to revisit the control system. POA / STV systems of that era worked really well when working properly, and they can work with R-134a especially with a slight adjustment of the valve. Pressure switch control typical of the "POA Eliminator" kit is not so good. If it's a straight TXV system, ice-up is best controlled by an evaporator temperature switch. (TXV's do not control the evaporator to any particular temperature. They vary the pressure proportional to whatever temperature exists. Thus in a state of freeze-up, the valve will close and the low side drop abnomally low.)

If it's not that, there is a blockage of refrigerant flow, usually the TXV. This also happens when there is moisture in the lines, it will freeze inside the TXV and block it temporarily. Such a system would require a thorough decontamination and of course a new receiver-drier. Was your receiver-drier replaced recently? The original ones from the 1970s, besides being expended by now, cannot handle R-134a. Look for a blue label that says it can be used with R-134a.

Edited: Mon February 27, 2012 at 9:01 AM by mk378

JACK ADAMS on Mon February 27, 2012 3:30 PM User is offline

I would set the gauges up as you drive the car so you can see if it is a head pressure issue or low pressure. With the stock 10mm Tube & Fin condenser, it may not be able to pull the heat out with 134a refrigerant. Do you have a electric fan to help pull air across the condenser? What about the Fan Clutch? More info will be needed I think.

drhill on Mon February 27, 2012 3:40 PM User is offline

Thanks for the input! I'm not used to these older ac systems but they seem pretty simple! There is no icing that I can determine on the outlet line or even inlet...evap inlet from TXV is of course very cold when it's working. I have experience with a freezing evap and this doesn't feel like that...airflow usually reduces when evap freezes, it does not in this case.

I suspect moisture blocking txv might be the case - strange in this issue is that it seems to stay "blocked" with no cold air until I turn off car and come back to it at least 20-30 min later. I guess I could try to put heat to the txv to see if that speeds up the return to working ac by dethawing the blockage?

The receiver/dryer has been painted so I can't tell how old it is...I'm sure it's worth replacing!

drhill on Mon February 27, 2012 3:46 PM User is offline

Ok - just saw Jack's post - watching the gauges when the ac is working, just idleing with engine hot, high side does get pretty high, above 300 psi, hold it at 1500 rpm or so and it dropped to around 250 or so....

here's something I just remebered...put some water on the condenser as a test (hose) dropped the high side real fast (no surprise) and just after that the ac stopped working (no longer cold)...maybe the water across the condenser cooled temps enough to freeze moisture in txv?

Clutch fan does move pretty good is working - no leaks etc. condenser is clean and in good shape - not clogged up etc...

Edited: Mon February 27, 2012 at 3:49 PM by drhill

JACK ADAMS on Mon February 27, 2012 3:54 PM User is offline

Do you know if there is a deicing switch in the air plenum box? Also 300psi at 85 ambient is a bit high, what is the low side reading at the 300 hi-side reading?? How much refrigerant do you have in the system now?

drhill on Mon February 27, 2012 4:32 PM User is offline

I don't think there is a de-icing switch - definately not one listed in the shop manual. If need be can some type of anti-icing control be retrofitted?

The electric motors controlling the hvac on this car are all shot so controlling stv is not really an option any longer. The system has been "simplified" on that side. I have flaps all set for recirculation and just a blower motor control.

Also - if i was having a problem with a faulty coolant water tap not closing off coolant from to the heater core - would that possibly cause this issue?

sitting with low idle in the heat (heat soaked motor and condenser etc) 300 psi high, low was around 40-50psi? I can't quite remeber...

JACK ADAMS on Mon February 27, 2012 6:33 PM User is offline

you will need some sort of cycling switch set in the system. A mix of hot air will cause cooling issues once warmed up. You didn't mention how much refrigerant you added.

drhill on Mon February 27, 2012 6:48 PM User is offline

Don't know how much refrigerant is in the system...this is how I purchased the vehicle a few months ago. I do realize the heat will affect cooling - didn't know if it somehow would affect the TXV feezing up.

I did forget to mention - my TXV has one wire leading to the evap outlet line as a heat sensor and one tube from the TXV to a fitting where the STV was as a pressure sensor...???Don't know if that affects or controls anything...

Also - can I buy some kind of cycling switch aftermarket? Do you sell anything like that?

Edited: Mon February 27, 2012 at 6:56 PM by drhill

mk378 on Tue February 28, 2012 12:09 PM User is offline

That's a common setup of the TXV, it's called an "externally compensated" type. The line over to the outlet of the evaporator is to sense the pressure there. It works slightly better than the "internally compensated" version which measures pressure directly at the TXV, thus at the inlet to the evaporator. At the outlet is where the pressure matters. The principle of a TXV is to flow as much refrigerant as conditions allow, yet keep the evaporator from overfilling and sending liquid refrigerant into the compressor which would ruin it.

At this point you can't know what is flowing in the lines, it could be overcharged, contaminated with air, likely contaminated with water, mixed types of refrigerant (maybe even some HC for good measure), likely different types of oil, maybe too much oil. It would be best to remove all the refrigerant or whatever else is in there and not re-use it. Clean everything up, change the oil, fit a new receiver drier and probably a new TXV. The fan clutch is likely shot, sure the fan still spins but not fast enough. A parallel flow condenser would help a lot especially if you're going to be idling in hot weather.

Edited: Tue February 28, 2012 at 12:13 PM by mk378

drhill on Tue February 28, 2012 7:49 PM User is offline

ok..thanks for help guys. Looks like I need to spend some time and $$ on the system and do it right.

I appreciate the help!!

bohica2xo on Wed February 29, 2012 2:38 PM User is offline

Wow, another train wreck system.

1) A conversion, no mention of flushing. Dryer not changed to 134a compatible type. Unknown lubricant type(s) and quantities.

2) STV removed, no evaporator pressure control.

3) Refrigerant quantity currently in system unknown.

You need to recover the system, and go back to square one.
Remove the dryer, and break the system down for flushing. Inspect the TXV carefully for rust or debris from the improper (and likely waterlogged) dryer. Flush the condensor and evaporator back to bare metal inside. Flush any hard metal or flexible lines that do not have mufflers or "cans" installed.

Put the STV back in to the system. It does not regulate cabin temperature. It controls evaporator pressure to prevent icing. The system was never designed to "cycle". Cabin temperatures are regulated by blending air to the desired temperature.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Thu March 01, 2012 12:04 PM User is offline

Don't recall reading about successful SVT or POA valve R-134a conversions. Old Mitch found an old car, was going to try it, but died instead. These were great systems, recall driving in a 125*F ambient with ice cold air coming out of the vents. And since in the USA, still can get R-12 once the system is returned to the original condition.

You will never do this by converting to a cycling switch system. Also understand these old parts are still available, but kind of getting crazy. Had a mishap with my 88 Supra, rear window broke into a thousand pieces. Contacted seven different vendors, five told me to chase around salvage yards to find one and they would be happy to install it. But one found a new one in California, but had to pay an extra hundred to have it shipped here, so went that route.

Closest salvage yard claiming to have one was 250 miles away, could be a worthless 500 mile trip for nothing, then only paying an extra hundred for a new one and that is shipping cost.

I consider the V-7 a great alternative, but trying to get it to fit and the electronic modifications required. May have to make your own sheave and mounting bracket plus figure out how to install a matching parallel flow condenser. Then you will be fine with R-134a.

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