Air conditioner playing tricks on my 1990 S4.

Compressor has been re done and hoses replaced but there's still a leak somewhere. System blows hot after a few weeks then dies. And when it was gassed up warm air blew rthrough vents. Mechanic said the pipe leading into evaporator was 8 degrees when still charged, so he thinks it's an evaporator problem.

Could it be something simpler like the heater bypass valve? Or the heater controls leting in warm air? Anything to save me removing the dash.

It's summer here - Australia - and not the best time for all this.

Cheers,

groppo

 

G'day Groppo

First thing to check is the heater valve - under the air box and easily reached. Can test it with a vac tester.

Second thing to check is the vacuum solenoids in the center console - you can get to them by removing the side plate of the console (1 screw), and possibly the parcel shelf (2x M5 2x M6 acorn nuts).

Then its 2 phillips screws to remove the plastic shield around the vacuum manifold in there, and then a single phillips screw in the centre - then pull the whole lot out and apply a vacuum tester to each coloured vac. line.

Read the HVAC section here: http://members.rennlist.com/pirtle/svc.html

If any of them are leaking, you'll get reduced vacuum to the heater control valve which lets warm water through.

If you don't have a vac pump and are in Sydney, I'm near Frenchs Forest (Northern beaches) and have a mity vac. PM me if you're local and want to pop by - I fixed my last leaking HVAC actuator a couple of weeks ago so its all fresh.

If you're losing refrigerant, then you'll still have to deal with that - I pulled my compressor off yesterday after discovering I lost about half my r134 since last summer. AC shop should have put some dye in there which will show where its leaked.

Hilton.

 

Groopo,

The first thing that has to be done is to locate the leak, as it sounds as though you definately have one.

If it is in the evaporator you A/C technician will need to use a sniffer unit. You have not said whether the A/C is operating on R12 or R134a.

Three methods that come to mind to detect a leak are:
1, Dye injected into the gas and by using a suitable light source to locate the leak, like UV light.
2. Soapy water sprayed aroung the pipe work and joints etc and look for bubbles
3. A sniffer unit. For R12 I used to use a butane flame torch that sucked the air by venturi effect of the butane gas into the flame which turned bright blue and gave of a distinctive odour. I don't know what they use for R134a, but just do a google search.

The inlet into the evaporator before the TX Valve (expansion) valve should be hot and the outlet pipe from the evaporator should be cold when the A/C is working correctly.

The cabin heater is located after the evaporator in the air box and the cabin heater valve is controlled by electrical operated solenoid vacuum valve.

The mixing flap that mixes the cold air from the evaporator and the hot air from the heater is controlled by a stepper motor located on the passenger side of the air box.

To control the cabin temperature there is a comparison circuit between the cabin temperature sensor located in the dash and the outside air temperature located in the cooling air pipe from the passenger side front compartment of the LH front mudguard to the alternator in RHD cars. The setting of the mixing flaps is carried out by the stepper motor described to achieve the cabin set temperature. The quickest way to test the temperature comparison circuit is to put the slider fully to the left, the lowest temperature setting. If you crack the slider off this setting and you get full hot air the comparison circuit is opened circuited.

To stop you getting hot water into your heater in the summer many owners have wired or 'zip' tied the heater control valve closed. I have fitted a spherical shut off valve into the circuit which I shut in the summer months. Other owners put tape over the outlet from the heater outlet just in front of the 'comb' flop located just behind the the centre vent outlets. If you do a 'search' you should find the threads.

The other flaps which direct the air to the various outlet vents are controlled by electric operated solenoid valves.

Just to complicate the circuit a little more your car is fitted with cooling control flaps, however when the A/C is switched on the flaps fully open and the two cooling fans go to full speed. You may wish to check their operation.

To get the full effect of the evaporator cooling the inlet air it is suggested that you clean the air inlet side of the evaporator by gaining access by removing the vent fan from the air box on the passenger side of RHS car. You can gain access by removing the plastic cover over the area behind the engine bay that houses the windscreen whiper motors, the evaporator non freeze controller and the TX thermostatic controlled expansion valve.

There have been numerous threads with photographs showing you how to gain access and how to clean the evaporator. This is probably the best access to take a "sniff" for a leaky evaporator.

There is a full fault finding procedure in the WSM or in Jim's Technical CDs, and as well in the CDs, Wally P has written up a great procedure for the HVAC system.

Tails 1990 928S4 auto

 

groppo,

Lots of good advice so far.

Go to our website at www.928gt.com then click on "928 tips/links", then on "Wally's World", then on "The HVAC System". There you will find a tech paper that will give you info on how the system works.

 

All good advice above, except for this IMHO. Doing this in an enclosed space can kill you, since R12 decomposes into phosgene gas in the presence of an open flame. This was used in WW1 as a chemical warfare agent. If you can smell it you are well over the permissible exposure for an 8-hour work shift(<0.1ppm). While you may not easily get to the threshold of fatal exposure while fooling with your AC system, you can easily reach the threshold where lung damage occurs. While you are googling, look up phosgene.

 

Dave,

Thanks for the safety advice.

The message is possibly showing my age, but I don't go back to WW1.

Last time I used the torch method was back in the early 1970's, mainly in open spaces (no too much OH & S in those days when I worked out of Hong Kong) and I was also exposed to phosgene in an inclosed refrigeration chamber that nearly got 2 of us.

Maybe this is one of the reason for some of the problems I have today.

A good safety message here.

Thanks again.

Tails 1990 928S4 Auto

 

Thanks for all the info and links, the system has the new type of gas and has the fluoro chemical in it too. So finding the leak will be easier. I've previously worked on the blower fan and centre console so I know how to work on these areas. I'll let you know how I go.

groppo

 

Tails, WW1 was waaay before my time too. I was just trying to make the point about the risk. I hope you are well.

 

Dave,

Thanks for the thoughts.

We all suffer from a 'case of terminal life', but the best medicine is to have a positive attitude, count your age backwards, enjoy life, our cars and keep out of trouble as much as possible.

Now I got that off my chest I can enjoy our recession.

Tails 1990 928S4 Auto (licence suspended once - too quick through the mountains)
1974 Honda MT 250 Elsinore (3500km only, recently restored to its now near original condition- gee I will have to start it again soon)
1998 Commodore Calais (Hers)
2004 Winnebago Motor Home on Izusu NPR 350 Auto chassis, turbo charger and intercooled (home away from home) touring speed around 85 to 90 kl/hr - for smelling the roses
2008 Suzuki SR4S iAWD 4X4 for flat towing behind motorhome for sight seeing
1990 Mitsubishi GSR 2 ltr double Overhead Cam with computer controlled suspension (in storage) great car to drive, have to try it again one day
1960 356B Super (sold unfortunately)
1965 912 (sold unfortunatley +1)
1960 Austin Healy Sprite (sold unfortunately) great car for sliding through corners
1967 NSU RO80 with around 4 engines with spare and numerous body parts, sold as spark plugs too expensive. (around 25,000 miles out of a set of rotor seals) one of the last ones still with the original wankle engine when sold
(numerous other bits and bobs, 4X4, yacht, convertable, caravans and so the list go on- now all sold).