CraigL

1993 928 GTS 5sp Manual

This is long. In my last post, my A/C odyssey started with a seized compressor clutch. So far I’ve done the following:

1) Replaced compressor which had seized
2) Flushed all lines, condenser and evaporators
3) Replaced 0-rings and both evaporator valves
4) Relocated low side valve to more convenient position
5) Added ester oil
6) Recharged with R12

There was no joy. The compressor clutch refused to engage

7) Pulled the control unit apart. Relay on printed circuit board was fried
8) Replaced the original relay with an external (Radio Shack) heavy duty relay
9) Compressor clutch worked. Air was cool but not cold. I figured this was an R12 charging issue that could be adjusted later on.

Took wife out for a ride. A/C stopped working after an hour.

10) Chased all of the circuits. No issues with low pressure switch or freeze switch or control wire from compressor clutch to the climate control unit edge connector.
13) Pulled control unit apart again. One of the tracings on the printed circuit board was broken (melted).

This happened after installing the Radio Shack relay. I know this because the compressor worked for a few hours. The damage occurred at the thinnest part of the trace. Maybe it got hot and melted during the original A/C clutch seizure? Maybe it was damaged by me during the relay repair? So I jumped the break with a wire and ...

14) Installed a remote power relay to drive the A/C clutch. Now the control unit and external Radio Shack relay only have to activate the remote power relay (dept of redundancy dept)
15) A/C compressor clutch now engages and there is no smoke (yet) !
16) Still getting warm air in the cabin. Rear unit is cool (64F). Front unit is warm (80F). Ambient temp was 95F. High side pressure = 210. Low side = 40.

I know the air should be colder, but there should be little difference front-rear. Right? So this could be an HVAC problem. So I ...

17) Started chasing the vacuum lines with Mighty Vac
18) Heater valve appears to work (Lever moves. Who knows if the unit is leaking?)
19) Applied vacuum to the black line – big leak with ignition ON
20) Pulled the console apart.
21) Pulled the vacuum manifold-solenoid switch out of center console
22) Orange line (center comb flap) leaks like a sieve. All others appear to hold vacuum well.
23) Verified that the blue line activates the recirculation flap in the passenger footwell. At full vacuum, I was unable to manually force the recirculation flap closed any further.

At this point I’ve got rebuild kits for several actuators in the mail. I will also replace the heater valve just eliminate the issue of that unit leaking. And I've ordered an extra long 8mm wrench to use on that hard to reach bolt on the right side of the console (which is why mechanics and engineers need to work together).

Questions:

Could a leaky center comb flap actuator by itself cause a temperature differential front-rear?

Could a bad leak in the orange line (center comb flap) affect the other actuators … especially the recirculation actuator (blue line)?

Besides adjusting the R12 charge and repairing the leaky actuator, does anyone have further suggestions?

Thanks in advance.

Craig

dprantl

A large vacuum leak anywhere in the HVAC system will affect all the actuators, especially at part-throttle when engine vacuum is lower. That includes the heater valve. How much ester oil did you add (why not mineral oil, since you are using R12)? And out of curiosity, where did you relocate the low charge port to?

Dan
'91 928GT S/C 475hp/460lb.ft

CraigL

My suppliers didn't have R12 mineral oil. Everyone recommended using Ester oil since it is compatible with both R12 and R134a. I don't have the volumes handy right now ... notes are in the garage. The low side port was relocated 8" higher and adjacent to the strut brace using a 1/4" flare fitting, copper tubing and a new Schrader valve fitting. All of these things were silver soldered together, the old Schrader valve removed and the flare fitting screwed onto the old fitting. Viola, no more burned hands.

Tails

Craig,
Have you checked the discharge and suction pressures/temperatures of the compressor with gauges to ensure that you have the correct charge quantity and the compressor is working correctly,
Have you checked the receiver dryer for bubbles in the liquid,
Did you change the receiver/dryer, as any moisture can freeze on the outlet of the expansion valve and restrict the flow of R12 -liquid and gas within the system,
Have you checked the vapour return line from the evaporator to the suction of the compressor, it should be cold to the touch especially where it exits from the fire wall,
Have you checked the cleaniness of the evaporator or the pollen filter if fitted. This can be done through the air inlet ducting from the fan by removing the rubber boot. If blocked it can restrict the air flow from the front vents,
Have you checked the setting of the 2 two control flaps fitted prior to the heat exchanger in the HVAC box, which is driven by the electric stepper motor?

Tails 1990 928S4 Auto

Mrmerlin

once you fix the leaking vacuum lines then the HCV will shut off,
then you will have cold air.
with a leaking vacuum system the HCV wont ever shut so the front AC will be fighting the heat. If its working properly your center vent should be discharging about 30 deg F

JHowell37

I assume you vacuumed the A/C system itself?

CraigL

JHowell37: Yes. I pulled a vacuum using a Robinaire 2 stage pump for 3 hours and let the system stand overnight. There were no leaks. That's when I started to charge the system.

Tails: I installed a new Receiver/Dryer in the beginning. Initially I added 1120 grams of R12. When the center vent temp was hot, I removed some. Right now I have many bubbles in the sight glass and given the pressures (Item 16 in post #1), I am undercharged.

MrMerlin & dprantl: I'm glad you both think that a comb flap actuator leak could cause trouble in other parts of the system. I was thinking about the recirculation flap and forgot about the HCV not closing all the way. While I am waiting for parts to arrive, I think I will plug the orange line and verify that the recirculation flap will completely close under normal engine idle conditions. Probably would be a good time to add more R12 to the system

Tails: There are no diagnostic ports on the compressor. I could measure temps with an IR thermometer. I don't think there is moisture in the system after pulling a vacuum for 3 hours, letting it stand overnight without loss of (negative) pressure. Maybe I'm wrong. And, there is a new drier in the system. Pollen filter. Hmmm. Does the 93 GTS have one? I will have to check this out. And I haven't checked anything with the control flaps attached to the stepper motor. I will put that on the to-do list .

Thanks for all the great suggestions. Feel free to offer others.

Craig

CraigL

Tails: It looks like pollen filters were introduced in 1994.
https://rennlist.com/forums/928-foru...en-filter.html

Thanks for the suggestion.

Tails

Craig,

The temperature is given on the gauges corresponding to the pressure for R12, IIRC and this can be assessed by the enthrophy diagram for R12 to gauge the efficiency of system

The efficiency of the system is also dependent on the ambient air temperature. When the system is working correctly you will only get a certain temperature drop as the air conditioning system is in reality a heat pump, it pumps out heat from the air and when everything is working correct it will only drop the temperature by a predetermined amound depending upon the size of the compressor and the condenser area and air flow over the condenser core etc., (the design of the system).

We used to specify systems to reduce the temperature from 46 degree C to 22 degrees C with 90% humidity, IIRC to cover operation in the tropics. These parameters will give a temperature drop of 24 degrees C at systems most efficient operation, so it cannot be said the you will get a certain fixed air temperature discharge from the vents, as this will vary dependent on the ambient temperature, the percentage of recirculated cooled air and the relative humidity.

The comb flap is designed to better mix the hot and cold air, as a percentage of air goes through the heater core and the rest goes around the heater box, to achieve the preset temperature on the slider as the system is designed to automatically maintain the selected slider temperature.

The recirculating flap when open allows colder air to enter the cabin, as you are recirculating the cooled cabin air in preference to cooling the ambient inlet air through the evaporator. In commercial systems the recirculation air is a set proportion or the inlet air thereby ensuing the a percentage of fresh air is entering the space at all times. If the 928's cabin was airtight, in time you would get a build up of CO2, which would be dangerous.

Efficiency of the system is also dependent on the relative humidity of the ambient air, the higher the relative humidity the less efficient is the cooling of the inlet air, as the air has to cool the water as well as the air until the water condenses out.

There are two control items in the system that control the hot air, they are the vacuum controlled heater valve (open or shut), which fails into a fully open position if there is insufficient vacuum to hold the valve shut and the dual flaps controlled by the stepper electric motor. The workshop manual fully explains how to set these dual flaps to ensure that they fully closed when full cooling is required. When they are fully closed the cooled air flows around the box that holds the heater exchanger core. When you require some heat to raise the cabin air temperature the two flaps open a little (depending on the position of the temperature slider) and allow air to pass throught the heater core and the cool air is mixed with the hot air via the comb flap (mixing flap).

Have your checked the operation of the cabin air sensor and fan operation and also the inlet air temperature sensor that is installed in the air cooling piping to the alternator.

If one of these temperature sensors is not working then you have an open circuit and you will loose control of the stepper motor and the flaps will fully open to the heater core and hot air will be blown into the cabin, IIRC.

The inlet air temperature sensor can suffer from a dirty plug connector that is located in the front of the LH side fender forward of the front splash guard again causing an open circuit. Disconnect the two pin plug and check the terminal for cleaniness and when reassembling apply some dialetic grease to keep out the moisture and dirt. Upon reassembly check the continuity of the circuit.

The two temperature sensors have specific resistance reading dependent of the air temperature and these values are given in the WSMs.

The cabin air temperature sensor has cabin air drawn in and blown over the cabin temperature sensor and a check to see if the fan is working is to light a piece of string or cloth and get it smoldering (no flame) and hold it near the inlet on the dash and see whether the smoke is sucked in. You will need the ignition "on" and A/C "on" to check this.

If the fan is working and you still have hot air then you will have to check the cabin air temperature sensor resistance readings. This is a little more difficult to get to and will require removel of the centre console and the radio, temperature slider etc., to gain access to the sensor. There are write ups on this in Rennlist, so use the search function to locate.

When you verify that you have a closed temperature circuit the two flaps should be closed when you set the slider to the lowest temprature and as you move the slider towards the hotest setting the flaps will open. A full description of fault finding/trouble shooting etc., is contained in the WSMs.

If you have bubbles in the sight glass on the liquied receive/dryer then you need more R12, however if you overcharge the system you will get excessive discharge pressure and fall off in efficiency, so to set the system up correctly you need to have the correct ammount of gas, so you will need a set of gauges to do this properly.

What vacuum did you achieve when you pumped the system down? The higher the vacuum the better the boil off of any water within the system and its removal.

In addition to improve the efficiency of the system clean the external fins and passages of the condenser.

I hope that all these suggestions help.

Tails 1990 928S4 Auto

cobalt

Just curious why you are using R12 on a 93 GTS when they came with R 134 from the factory?

CraigL

Tails: Great information! Thanks for spending so much time on this. I'll get on the sensors and the stepping motor right away (while I'm in there). BTW, when I pumped the system down for 3 hours, the gauge was pinned to the lowest value (-30 PSI). It remained at -30 PSI overnight. I didn't have access to gauge that read in microns.

Cobalt: My 93 GTS was actually manufactured in 1992. The 'early' 93 GTS cars were R12. The 'late' ones ... the cars actually manufactured in 1993 ... were R134a. At least that is my understanding.

cobalt

Got ya. Mine is a 93 build. You are correct 92 was the last year of R12, which I actually like better. Far superior for cooling in traffic conditions at lower rpms.

CraigL

Update:

I plugged the leaky orange line, started the car and exercised the controls. In recirculation mode, the recirculation flap (passenger footwell) closes tight as does the heater control valve.

I added 340 grams of R12. The ambient temp measured 2" in front of the condenser was 95F. Air from center vent and the rear unit are both 60F. Low side pressure = 36 and high side = 225. The sight glass still shows bubbles. I probably need to add a bit more.

Parts for the actuator rebuild arrived today as did a new HCV. I hope to get these installed over the weekend.

Thanks for all the great suggestions.

Here's the A/C controller circuit board. Initially the relay fried when the A/C compressor seized. After replacing it, one of the circuit board traces self destructed (blue circle).



And here's a photo of the relocated low side port. No more burned hands!

Bill Ball

Nice job on the port extension.

Are those pressures at 1500 RPM? They should be. The pressures are in the right range for that ambient temp, but they may be falsely elevated if the car has been running for a while due to overheating of the condenser unless you put an external fan blowing on the motor. So, you still could be undercharged, particularly if you are seeing lots of bubbles. Still, you said you orginally added around 1120 gm which is right on, then "removed some". I try to always charge based on weight, but the sight glass should work for R12 too.

You vent temps are better than before but not great - these should be taken at 1500 RPM too.

cobalt

I agree. When I measured mine ambient temp was 95 degrees and I was seeing 52 degrees both front and rear. Although this is with the R134a. I would think you should be seeing similar numbers though.