Air con problems
 

Hi all,
I've never had any aircon problems with any other cars, (probably because i like to have the windows down rather than the a/c on) but on my bug, every time i switch it on (the A/C)..a very annoying rattle comes from the engine, you can even feel a bit of vibration on the pedals..i cant really trace it, it also dramatically sucks all the power out of the car and really drags the car down..on really hot days it becomes worse... has any one here ever had the same problem/ or similar symptoms...

cheers

a/c draws 10 to 15 kw from the engine.

its draining alot more than that...what about the rattle/ vibration ?

On hot days the car will make less power (hot compressed air makes less power) anyway.....Running the A/c will have the under bonnet temps over 65 degrees on hot days.

What mods do you have?????

Pod under the bonnet?

yea, i know the under bonnet temps will typically result in lower power out put on any engine, but its the rattle and the vibrations that really bother me, sounds like something is loose (or becoming loose)....its quite annoying as the car runs like new when the a/c is off..

the mods are pretty basic, Pod filter, and turbo back 3" exhaust..(used to have an electronic boost control, but that was disconnected by the guys at subaru when the engine blew one piston, which they replaced, and and new short block as well )...

would these mods affect how the a/c functions?..

[QUOTE=blurex;429026]

the mods are pretty basic, Pod filter...
would these mods affect how the a/c functions?..[/QUOTE]


That is your main problem right there, pod filter sucking hot air constantly, and even worse in summer.

Pod will KILL the power on a hot day.... A/c adds heat load to the engine bay.

Plus a pod with a stock ecu is asking for AFR leaning issues / flat spots.

So YES, running around with the A/c cranked on a 38 degree day, when you engine is being asked to make power from air being drawn through the pod under the bonnet (where the air temp is approaching 70 degrees celcius), will make it feel doughy and unresponsive/ lack power / idle lopy / idle low / surge etc.

Get rid of the pod and buy a process west cold air intake (goes inside drivers side front guard).

Get a tune.

Reckon it will be the aircon compressor giving way. Either because the aircon oil in the compressor is low or that the belt on it is worn and due for a change.

Also do check your air con clutch is not wobbling when the air con is turned on. That might be the cause of the rattling sound

From what you have said, sounds like as said above the aircon compressor is starting to seize. if you bring the revs to about 3000 and turn the ac on and off do the revs drop dramatically? eg more than about 300rpm? if they do then chances are the comp is getting tight.

also i cant see a 4pk belt being able to transfer 10-15kw. no offence ross mate, but that would be a seriously inefficient aircon system. 15kw will cool a large house :)

No offense taken, a house is a completely different scenario though.

Houses generally don't exceed 60+ degrees inside unless they're on fire. :eusa_doh:

Let's compare some interesting numbers..... (sorry op)

Average automotive Hvac system:

20,000 Btu/hr (average)
Outside (fresh) air, Panel Vents
30 deg c Ambient + radiant heat absorbtion
50% relative humidity
= outlet temp 2-3 deg C

Note that the 20,000 BTU/hr works out to just less than 8 hp delivered (do the math), and assuming a reasonable compressor efficiency, this puts you back to the 10-15 kw region for draw from the compressor, not including the additional electrical (alternator) load added by the evap fan (100w), condenser fans (175w each) and the a/c clutch itself (50w).

Compared to a house a/c where a 12000-Btu/hr air conditioner is usually sufficient for approximately 550 to 650 sq. ft.

This is not too surprising really if you think about it for a second, it doesn't take long to pull down a 60 deg C car interior to something more reasonable.

/Infomercial

back on topic :p

If the comp is failing the belt will start to scream/slip as the load goes through the roof.

check the tensioner pulley bearing also, it's only under real load when the a/c clutch is engaged.

Look around the engine bay for belt dust near the a/c belt's path.

your calculations do make a some sense but i honestly dont know exactly many Kw's you would need to cool the volume of air inside a standard house (over a given period of time), but isnt it safe to assume you need considerably lower Kw's to cool the volume in a car?.. however you may be right about the bearing, the vibration/ rattle "feels" like its coming from a moving/or rotating component, I'll check for belt dust as you suggested, and post what i find ...please any more suggestions on what could be wrong.. Cheers guys

It's all to do with heat absorbtion of the cars structure / interior materials.

Trying to cool an uninsulated tin shed the size of your house would take 3-4 times more cooling than a modern double brick, insulated house needs.

[a/c 101]

Is your car performing any better in the current milder weather?

The car's performance DOES vary with changes in weather as would be expected, even when the a/c if off ( which supports your earlier assumption of the Pod aiding in sucking in more hot/ cool under bonnet air)...Am struggling to come to terms with the fact that the vibration can only be caused by a worn out component or loose fitting (due to wear), and from what you guys say its either a 1. worn clutch, 2. loose/worn belt, 3. Worn bearing..am hoping the problem ends there as fixing an a/c can be quite costly.. I'll check for any other fault signs and let you know...do you know of any good a/c specialists (not necessarily subaru ones)... cheers

Temperature is irrelivent, as the compressor doesnt use any more power if the air it is cooling is hotter.

Now your average 6pk drive belt tensioned correctly can achive about 14kw power transmission. a 3pk belt can achive up to about 3kw of power transmission. The subaru ac comp uses a 4pk drive belt, which can handle a max of 5kw power transmission before slipping.
Something that mechanics are taught at tafe. I take it that you arent a mechanic?

Now even more so. When i did my airconditioning training we set up a compressor on a test bench, with a cars evaporator, which was run by a 1.5hp motor. It achived normal gas pressures and cooled to the same standards as a car aircon, infact it was the same thing on a test bench.

The aircon compressor clutch is a magnetic clutch, which has too friction discs that are about 10mm wide, by 70mm dia. that drives a shaft which is about 8mm diameter. This shaft defiantly couldnt handle 15kw.

seriously how much power do you think a compressor can use?


[QUOTE=Rossco;429221]No offense taken, a house is a completely different scenario though.

Houses generally don't exceed 60+ degrees inside unless they're on fire. :eusa_doh:

Let's compare some interesting numbers..... (sorry op)

Average automotive Hvac system:

20,000 Btu/hr (average)
Outside (fresh) air, Panel Vents
30 deg c Ambient + radiant heat absorbtion
50% relative humidity
= outlet temp 2-3 deg C

Note that the 20,000 BTU/hr works out to just less than 8 hp delivered (do the math), and assuming a reasonable compressor efficiency, this puts you back to the 10-15 kw region for draw from the compressor, not including the additional electrical (alternator) load added by the evap fan (100w), condenser fans (175w each) and the a/c clutch itself (50w).

Compared to a house a/c where a 12000-Btu/hr air conditioner is usually sufficient for approximately 550 to 650 sq. ft.

This is not too surprising really if you think about it for a second, it doesn't take long to pull down a 60 deg C car interior to something more reasonable.

/Infomercial

back on topic :p

If the comp is failing the belt will start to scream/slip as the load goes through the roof.

check the tensioner pulley bearing also, it's only under real load when the a/c clutch is engaged.

Look around the engine bay for belt dust near the a/c belt's path.[/QUOTE]

You are correct (and I never said) "the compressor uses more power to cool hotter air".......The system does how ever need to be sized to take into account it's working environment and performance expectations (pulling the cars interior down from 60+ degrees to around 20 deg or less and maintain it).

Ambient temperature absolutely plays a part in automotive a/c..... A condenser doesn't reject heat at a constant rate regardless of ambient temperature and / or airflow, does it?........It's basic physics.

Your trying to tell me that your bench demo a/c was a 20,000 btu set up running on 1.5 hp?.......Sorry, but I don't think so.

Do you honestly think a 1.5 hp system will cool your car on a 35 degree day?

The rated belt loadings you mention are interesting, seeing as the commode super charger / auxiliary drive belt (8pk) on the supercharged v6 delivers 30-40 kw to drive the factory eaton supercharger when it's producing only 8-9 psi (and up to 70kw when the supercharger is overdriven to produce 10 - 12 psi).........Plus the same single belt also drives the a/c, water pump, power steering & alternator.

Me a mechanic?......I can't even change oil..... :icon_wink

In regard to BTU = watts
20,000 BTU per hour = 5.858 Kw (7.855 Hp) @ an unrealistic 100% efficiency.