StratfordShark

My aircon usually blows icy cold, but it must be over a month since last tested it and it was ok then.

Today was first properly warm day of Spring, or at least what passes for warm in this country with 19.5C outside temp showing on dash. The cabin was already hot from sun as I moved off from cold start, but no cold air was delivered after engaging aircon.

Haven't had time to do detailed tests other than verify compressor clutch engaging, and setting motor moving to full cold position.

I'm very puzzled where problem could lie given:

Even if heater valve faulty (I installed new one as PM about 5 years ago), it shouldn't affect aircon from cold start, especially if mixing flap shut.

If there was a leak, then wouldn't LP switch trip and clutch not engage?

So I'm keen to sort this out before weather gets properly hot, but I'm scratching my head even before I start troubleshooting.

Any ideas on possible suspects please?

Thanks a lot

zekgb

Time to get out the gauges and report low and high side pressures.

SQLGuy

If the compressor is running, then I'd suspect the expansion valve. The gauges will tell.

StratfordShark

Thanks both - will slap gauges on and see what they report.

Mrmerlin

simple test turn on the AC and see what the window in the receiver drier looks like,
then feel the lines,
fat line should be cold thin one is hot,
if you see bubbles then the fluid is moving and their is fluid in the lines
feel the expansion valve see if its cold, if not then its plugged.
Then install the gauges after your initial diagnosis

FredR

Are you sure the clutch is engaging and driving the compressor?

dr bob

Went through similar symptoms a summer ago and found vacuum problems. Compressor working, odd pressure readings though. All pointed to a expansion valve obstruction. Tested vacuum system since that's part of the full protocol. In my case the HCV had failed on the vacuum side, causing the symptoms even though it was tied closed. Water side holding. It the leaking diaphragm wreaked havoc on the rest. Plugged the line at the HCV and suddenly all was well. Initial test is easy with the MitiVac on the vac supply line from the booster end.

StratfordShark

Hi Fred yes I'm certain of that, double and triple checked with strong light on compressor nose. I was hoping it would be electrical, as relatively easy fix.

StratfordShark

This is great thanks Stan. Next garage time slot will be weekend so I'll run through this along with vacuum checks. I have set of gauges coming at reasonable cost - will be chance to understand AC system more so will use them whatever.

StratfordShark

Dr Bob I'm often first to point people to vac checks, so this is case of cobbler's children going shoeless!

I can hear all the usual sounds of flaps moving when I move position slider, including thump of recirc flap, but admit posting before getting Mityvac out and checking lines. That will be done next weekend so will proceed from there.

FredR

Hi Adrian,

I raised this point becuase most folks think that because they hear the solenoid go clunk it is actually holding. If you re sure the clutch is holding and the compressor is driving fair enough.

If the compressor is driving then you should be able to feel the cold on the supply pipe at the expansion valve - if you cannot sense this then something like the drier internal mesh has disintegrated and blocked the expansion valve with pieces of dessicant.

If the pipes are cold then Dr Bobs' suggestion is the next obvious thing to look for. Without vacuum the heater valve is permanently open and you then have open warfare between the a/c system and the heating system- again that one is easy to diagnose - just look at the actuator arm on the heater valve- when in a/c mode that arm should be pulled up towards the actuator. If it is in the down position your passive vac system or the valve actuator has taken a dump.

Rgds

Fred

StratfordShark

Ok Fred thanks. Looks like I'll be busy this weekend!

dr bob

I do want to clarify that the heater control valve was tied closed. No hot water was passing through. It was the vacuum actuator in the heater valve that was allowing air to be drawn into the HVAC vacuum system. Ultimately it was some combination of flaps in the correct positions for defrosting that kept normally cold-cold AC air from making it into the cabin. All the other actuator diaphragms were previously replaced. The HCV was last replaced about 15 years prior to this AC adventure, easily the oldest vacuum actuator in the system.

FredR

Mea culpa- must read a bit more carefully- indeed one failed actuator diaphragm is enough to loose the vac reservoir and send the system into chaos- ask me how I know! My recric flap is out, the footwell flap is out, the comb flap went on my late S4 and I do not have a heater valve connected/working [I use mine in the closed position looped back to the expansion tank- no flow to the heater- the actuator is removed and I operate the valve manually when filling the system for venting otherwise it is normally closed to prevent recirc].

Rgds

Fred

StratfordShark

I got to have a quick look at the aircon problem this evening using set of gauges.

Didn't have time to take off air filter and feel the lines, but there is no cooling whatsoever.

Putting on the gauges, with engine off the low and high sides both read about 30psi.

With engine running and aircon engaged, the low side gauge moves down to 0psi, and the high side only goes up to about 50psi!

I would have thought this means I had lost most of the refrigerant through a major leak somewhere, but the compressor clutch is engaging and the compressor running, so I don't understand why the low pressure switch has not shut off power to the clutch.

Does this look like a leak, and if so why is the LP switch not tripping?

Thanks a lot,

Adrian