Ducman82

so my AC is running nice and cold (for a black car in this LA heat wave at any rate)....

but i noticed at idle it cycles on and off about every 4 seconds. not sure while going dow the road as is it blows cold.

i also noticed the AC fan will come on when i turn on the ignition after it has been sitting a bit.

i do have a Trinary switch controlling the compressor and fan.

compressor off at 29psi and 411psi fan off at 150 psi

compresor on at 33 psi, and 300psi fan on at 190 psi....

so maybe i am getting to much pressure? or just to hot at idle, need the extra air flow?

928 DesMoines

I'm not an AC guru or anything, but could you be low refrigerant charge? The AC clutch kicking on and off is a sympom of low freon. I know this sounds weird, and i don't know the "tech speak", but the outside tempurature affects the systems pressure somehow, so it makes sense that the symptoms go away while driving. The ambient temp while idle would cause the pressure flux and if your idle pressure is already low, it will bounce the clutch on and off as the pressure is moving below the sensor threshold. Check to see if your low side is at around 35-40lbs pressure when running at idle. Stating again, I'm no expert.

Speedtoys

Those pressures sound amazingly high , like 150-250 too high on the high side...low side..about right..but...

I think its your high side safety switch kicking in and out.

Ducman82

ya, i think its the high side too.......... if the refrigerant was low, it would not blow cold at all. it blows nice and cold...... and it does not seem to do it while driving, only when at a stop/idle. and not every single time. i can be driving, pull up to a stop and it stays engaged for a bit....... then goes all on/off'ish.

hmmmm

Speedtoys

This suggests overcharging...a blockage would have high hihg-side pressure too, but I -think- that would also show unusually low low-side, as its being starved.

You're pumping too much ahead of the expansion valve, with a normal valve flow to the low side.

Ducman82

im in the process of getting Robs gauge set in the next day or two.... everything was new.

UGH... i dont even use AC that often, but i can stand a system now working..

Alan

This is an R134a conversion right?

If R134a it is probably insufficient fan cooling for the condenser at idle. This will cause the high pressure to spike, alternatively just get a higher pressure cutoff switch (more like 460 psi) like the stock GTS R134a switch.

Alan

Ducman82

hmmmm, that is a good point. I'm using the stock AC fan.... maybe i need something bigger..

Ducman82

I'm going to get it all warmed up tomorrow, and run water over the condenser and see if it changes the cycle.....

Alan

Yes that's probably a good simple test of it. Spraying with a mister bottle might be even better - you want to promote evaporation - not just wetness. With a spray you get some precooling of the water on the way too. If you can just trigger your fans to 100% (or close) that could be a good test case too.

Alan

Ducman82

i have a nice mist attachment that should work

griffiths

Other than a function of a pressure switch to turn on the AC fan,
the low and high side pressures are a function of the amount of charge and the given ambient (outside pressure of the day) and are not governed by the switches unless
there is an issue.

Stock 928 controls (pressure switches) work just fine with either R12 or R134a.
I've run all my sharks (1978, 1984, 1989) with R134a with stock pressure switches and never had any issues.

Your low and high side 'on' and 'off' pressure switch thresholds should have nothing to do with controlling a 'normal' system. The low side cut off is suppose to prevent the compressor clutch from engaging in case the system is too low on refrigerant; to prevent compressor damage due to low refrigerant/oil flow. The high side cut off is suppose to prevent the compressor clutch from engaging when pressures exceed the "norm" for given ambient, again to protect the system, to prevent blow outs and overheating the compressor.

If the system is properly evacuated (no air or moisture), your TEV (exp valve) is good, your condenser fan is good, all you need to do is properly charge the system and the compressor will cycle on and off as needed based on the thermostat setting and freeze out circuit in the evaporator core.

If you are using R134a simply put in 85% by weight of the original factory R12 charge weight (85-90% is a rule of thumb), and review your high side pressures based on the ambient temperature using a P&T (pressures and temperatures chart) for R134a and adjust the charge amount accordingly; typically the amount of R134a will be 85% (nominal; +/- some).

Help Pages:
Wally's
Nichol's

Alan

Porsche significantly changed the system controls between R12 and R134a systems. They clearly did this for a reason.

2 things: the compressor doesn't cycle based on the thermostat setting AND if you have insufficient cooling of the condenser you will see very high system pressures in an R134a system - that is why the higher high side compressor cutoff was added. Jake has an after-market fan controller system, so fan 'working' doesn't mean its getting enough airflow at idle...

Alan

griffiths

Alan, OK, thermostat does not cycle compressor. Please advise what will (I only have half of the shop manual).

However, if the system is charged properly with R134a the high side should not be seeing pressures anywhere near 411 psi unless the ambient is 175F, nor should he be seeing 300 psi unless its 125F outside.

You do not have to change pressure switches to use R134a in any Shark.
The stock system will handle it easily.

Maybe Jake can advise:
1) Why he changed the stock PS configuration,
2) How much refrigerant he put in the system,
3) What is the ambient temperature when the compressor is cycling on and off and what is the high side pressure when it happens.

Alan

The only thing that should cause the compressor to cycle is the freeze switch, or the pressure switch - otherwise when AC is selected it runs continuously.

No this is quite wrong. My GTS is a stock factory R134a system. If just one cooling fan fails in Phoenix in the summer (>100F) the system will overpressure and cutout. The engine loading is very significant to achieve these pressures - the compressor belt will slip and squeal under these conditions - there is no question what is happening... fix the fan (and change the Ac belt) and all is good again...

The ambient temperature only counts for much if you have sufficient airflow through the heat exchanger to take advantage of it (so a parallel flow condenser would help here. If one or both fans fail or if they are just set to come on at too low a flow rate then you will have cutout issues at least at idle.

I don't think so - not under all plausible (incl. failure) conditions. The stock R134a pressure switch changed: the high side pressure was added. An additional integrated switch was also added to kick the fans to higher speed when the pressure rises above 18Bar. Pre-R134a systems have a simple low pressure switch or a high temp switch (one or other)

Would it seem to work most of the time - yes. Would it be working correctly across the whole pressure range and able to handle extreme cooling requirements - probably not. Could you easily tell the difference - most of the time probably not... Porsche spent money to change things for this, at the time Porsche was in financial trouble they were economizing on the 928 development for the new model developments. R134a conversion was a mandated requirement - they had to do it and probably wanted to do just the minimum changes to be in compliance. However here they also changed the loom, pressure switch, pressure sensor and fan controller configuration. They clearly thought they needed to do these things - even under the significant pressure to spend less...

Alan