Low pressure low side, short-cycling

[whitewolf]

Hello guys.
Im not really into those systems and am unfamiliar with, which is why im looking for some help.. there are not many good mechanics i could trust where i live and i also like doing stuff on my own. I searched the internet but did not quite found anything in particular, and im also getting that feeling when somethings wrong and is not something i could fix, bugs me out :/

Long story short i bought a bmw e36 320i. Turned on the ac and it was not blowing cold. Checked pressures and there were none.. so i charged it correctly and it is now blowing good cold as it should be.
However, the compressor is short-cycling a lot..

I measured low side pressure at idle rpm, and it is dropping to below 1.5 bar (15psi?) which is when both the fan and the compressor turn off for like 5-10 sec and they kick in again.
When it does turn on back again, low side pressure is 2-2.5 bars, which i assume is a good pressure (if it wasnt dropping)
Cycles last like maybe 30 sec on idle and maybe less while im driving on the road

On another hand - before i recharged the system i noticed the fan was not running when i turn on the ac.. eventually found a blown fuse that is for the fan relay. Changed it and fan is working fine now, so i went to check pressure and refill as mentioned above.

Any ideas on what could be causing these drops and short-cycling? What should i look at and how could potential issues be diagnosed?

Thanks a ton!

[tbirdtbird]

"so i charged it correctly"
Please elaborate.

2.5 bar is about 36 psi which is just about where you want to be.
Pressures testing s/b done at 1500 RPM, max fan, max cool, recirc, doors open (full load test), with ambient temp identified.
Report back

[whitewolf]

Charged correctly i mean the amounts mentioned on the oem label.
Used a bosch machine we have at the shop.. so it does its thing

Will check pressures again as you mentioned and will report back.

Thanks

[whitewolf]

Just did a check again..

Outside temp is 23-24 celsius, rpm between 1500-2000, car warm up to temp.

While on full load (max cold max blower recirc and doors open) im reading somewhere between 1.5 and 1.8 bar on low side and like 18 bars on high side. When revving engine high side is slightly moving then returning back to normal when i let off the throttle.
There were no constant changes while holding stable rpm, i could tell pressures were stable.

While performing the check (like 5-10ish mins), compressor did not short-cycle as much (maybe not at all even)
But as soon as i disconnected everything and went for a drive it started again:
Driving on full load ac and i could tell it is cycling, it lasts maybe a minute or two while driving. At red lights it cycles a bit more often. If i turn down the blower to the first step it is short cycling more.

Although it is blowing cold air no matter the cycling

[Cusser]

Outside temp is 23-24 celsius

Although it is blowing cold air no matter the cycling

Wait for a considerable warmer day to assess whether the vent temperatures are cold enough. 23-24 celsius is not that warm.

[whitewolf]

Noticed just now: when it does short cycle, it only stops for a split second. The pulley disengages and engages back in a frame of a second. It is fast so the compressor does not even fully stop spinning. Again - thats on full load.

Low load (low blower speed) it only works for like 20 seconds, turns off for 10 sec, back on for 20 and so on…

When it does disengage the radiator fan also stops

[bohica2xo]

Sounds like the system is working properly.

At 75f ambient, the compressor only needs a short duty cycle to keep up with the evaporator heat load.

Between the low side pressure and the evaporator temperature sensor, the system is trying to keep the evaporator from icing up.