dprantl

The sight glass and pressure readings are useful when one is just topping up R12 and for diagnosing whether there is the right amount of refrigerant in the system. However, when starting with an empty system, it is 100% accurate to charge based on weight. After all, the manufacturer conducted numerous tests on that specific vehicle A/C system to come up with those weight figures.

Anyway, if you don't have a way of weighing the refrigerant, you should refer to the temperature vs. pressure chart for R12 (sorry, I don't have it handy right now) and adjust the system charge until you see the correct pressures for the ambient temperature at that time. Then, check the rear A/C vent temps since it has none of the mixing problems that the front A/C does and is permanently recirculating.

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

MattCarp

Definitely. I'm skimming this thread hoping to pick some stuff up.

Don't want to hijack, but where is Wally's writeup? I'm looking for the definitive "928 HVAC System Design tutorial".

I'm not sure I've seen a great 928 A/C system - my '84 needed worked and my '94 is OKAY, but I'm not sure if it's operating to spec. If I learn the system, I'll be able to better know what I'm seeing... One thing is for sure, too much noise from the rear a/c fan!

Bill Ball

Wally's 928 AC write-up...

https://www.928gt.com/t-wallyhvac.aspx

MattCarp

Awesome! thanks.

GregBBRD

You seem to be doing everything correct.

Here's the only clues I have for you:

Add a fuse to the wire that runs from the control unit to the de-icing switch. If anything goes wrong with the compressor, etc., the fuse will blow instead of the control unit. We include a fuse with every rebuilt control unit we supply to 928 International. I think Sean is installing fuses, now, also.

There is a "mixing flap" inside the box with the evaporator and the heater core. This is controlled by a "mixing motor" that is under the dash, on the driver's side. If this motor doesn't work, gets stuck, or doesn't shut the mixing flap completely, you will be mixing air that goes past the heater core (which gets hot, regardless if the heater control valve closes or not) with air that goes past the evaporator.

Even if the motor does work, make sure that the mixing flap closes completely (there is an adjustment for it) and the flap actually is working, inside the box. There is a foam seal around this flap and any deterioration of the foam will cause hot air to mix with cold.

Note also that if the control unit is out of calibration, the mixing flap will open at incorrect temperatures. We had one that had the mixing flap opening at 70 degrees. Making that car cool was impossible.

These cars are very sensitive to the amount of oil in the system...a bit too much and they don't blow cold air. It isn't clear what compressor you put on the car, but some of the aftermarket compressors don't use much oil and will push excess out into the system. The pressures will not show this, until the amount of oil is very high. If you get down to the end and it still isn't as cold as you think it should be, recover the freon and pull the two lines that fun to the rear A/C from under the car. If they are full of oil...you've got way too much oil in the system. We find this is the case in a huge percentage of cars we work on.

CraigL

The new compressor was a Denso 10PA20C. I put 140cc of ester oil in the system and distributed it as noted in the WSM and also by Dan (above):
- Compressor 40%
- Evaporator 35%
- Condenser 15%
- Receiver/Drier and lines 10%

Tonight I finally got that last 8mm screw removed from the right side of the console and pulled the whole thing apart. I was able to replace the diaphragms in both the yellow and orange actuators. As expected, the orange one had a tear. The yellow one was replaced because it was accessable. Dwayne's pictorial was invaluable. http://dwaynesgarage.norcal928.org/1...0Procedure.htm

I will check the internal & external temp sensors and also the operation of the mixing flap before starting to close things up.

Greg: What size and type of fuse are you putting in the clutch control line? 3 amp, slow blow? Also, do I still need the fuse after having installed a remote power relay to handle the compressor clutch load?

Bill Ball

I think you've got the fuse issue covered by installing that remote relay, if it is fused. The power source for the clutch should be fused.

Good job on the actuators - you really are getting in there. I apologize if I missed these faults in the inspection before you bought the car. The AC system appeared to work properly then.

The temp sensor operation is probably OK. If one of the sensors is bad, the system will only provide AC when the temp slider is all the way to the cold end, which trips a microswitch there. I.e., the slider loses its temp regulation function, so you get cold or hot but nothing in between.

Do check the operation of the mixing motor, as Greg suggested. You can see it on the driver side of the heater/evaporator box if you can you head down on the floor looking up. There is a cam and arm that should move as you move the temp slider and you will observe the comb move in the center vent. The adjustment of the mixing doors is described in the WSM vol. 4, section 87 page 115, but you seem to be ontop of things so probably already found that.

CraigL

Bill: No apologies needed. I knew these cars were high strung from the get-go. Besides, no one could have predicted that the compressor clutch would have seized.

As for the remote power relay, I used this kit from Painless Performance: http://www.painlessperformance.com/w...rchField=30130
It is weatherproof, comes with a 30 amp fuse and enough wire so I could install it in the right front bumper/fender cavity (next to horns). Right now it is wired to the jump post. It probably should draw from a switched power source. Any convenient sources like that in the right front of the car? Lastly, I used Weatherpack connectors http://www.weatherpack.com/ to splice the power relay into the A/C control line.

ROG100

I am learning something here - so the IR guns are inaccurate? By how much? Surly they can’t be 20F to 30F out can they?
I also found out the matrix you see inside the center vent is the heater and not the evaporator. The evaporator is behind the heater matrix.

I stripped down an HVAC unit and was amazed at the complexity of the ducting and flaps inside. Now I fully realize how important the actuators and stepper motor are.
Found lots of foam seal strips that had disintegrated to dust which could be the cause of drain blockage high in the unit and above the main drain that leads to the drain hose.
I am sure this is why we get the “wet ankle” situation.

Mrmerlin

Greg, The inline fuse sounds like a great idea,
so what size do you suggest??

Bill Ball

They are inaccurate with shiny surfaces. If the vent plastic were dull an IR gun should be accurate.

cobalt

The IR units are not inaccurate as much as they can sometimes produce false readings. When used properly and within their limitations depending on the unit I think you will find they are within +/- 2%. Although sometimes I have found that depending on the surface of what you are measuring they can vary tremendously. A Bi-metal thermocouple can also give false readings if not properly inserted or in the case of some applications isn't inserted into a thermowell. Although within a air flow system like an A/C unit it should be more than accurate enough. The thermocouples can also range on how quickly they read the temperature. The type J thermocouple I use can take upwards of 2 minutes to stabilize.

For example I was installing an In your Face gauge in my turbo. Nice little in tach diagnostic that gives me boost, AFR, intake and exhaust readings along with graphing and retaining high readings for review. After installing the type K thermocouple supplied with the Kit we checked the surface temperature of the bung area using an IR thermometer and we were seeing 200 degrees difference vs what the gauge was reading. When we used the fluke we saw temperatures within 10 degrees of the dash gauge. Not sure if it was the reflective surface or what caused the difference but when trying other comparisons the 2 gauges read similarly.

blown 87

I agree with you on the issue of having too much oil, we also see it all the time on cars.
Seems like some shops will put in 4 oz of oil, and a half a pound of freon just to be safe.
At that point, about all you can do is a system flush.

JHowell37

I was actually wondering about this a few minutes ago and thinking that there are probably more cars with too much oil in the A/C as opposed to having an insufficient amount.

blown 87

I bet that 95%+ of the cars that have had a conversion to 134A done on them are grossly over charged with oil.