anewbill

I recently had my A/C evaporator core replaced due to a slow leak that eventually turned into a green puddle under the car . I went ahead and had the system converted to R134, replacing the drier and expansion valve while the work was being done.

The good news is, my vent temperature is now down to 38 F with an ambient outdoor temps of 90+ . The bad news is, I now have a intermittent high frequency noise that appears to be coming from the area just above the evaporator behind the dash around where the expansion valve is . The noise seems to change with engine RPM, getting a bit louder as RPMs increase and going away when I take my foot off the gas. It is definitely not the fan motor squealing. My mechanic thinks it sounds like a vacuum hose leaking.

Are there any vacuum lines in that area on a 1991 C2? Does anyone have any idea what might be causing this? Thanks in advance for any ideas.

1991 C2

Heirsh

brake booster is vacuum operated. and sorta in the same area. I don't know how it is routed though.

last toy

Interesting, I just started to hear a high frequency air noise from the same area you described and I also recently converted my R12 to R134. Do you hear the noise with the AC off. I'll check mine. Could be just the nature of the R134 gas going thru a R12 system. Since R134 has a different molecular weight than the freon, I'm not surprised it could cause a high frequency sound transmitted somewhere thru the R12 system. If you are getting 38 degree air, doesn't appear to be a functional problem. If the sound is still there with the AC off, than it is something else.

anewbill

Heirsh,
Thanks for the info about the brake booster. I'll take a look this weekend and see if I can find any vacuum leaks.

Don,
The noise goes away the second the A/C is switched off. Also, it does not seem to be affected by the fan speed. I'm thinking the R134 may just make more noise going through the expansion valve?

Thanks,
Andy

anewbill

I poked around in the area around the front fans and expansion valve over the weekend. Unfortunatly, I was not able to find anything that looked like a vacuum line. The sound definatly seems to be RPM related, as it changes in pitch and loudness as the revs increase. Anyone else out there have an idea of what's going on?

Thanks,
Andy

wf758

i also had my core replaced recently. same noise from same area. I detect a relation to rpm's and the fan speed. ideas?

by the way, i had converted to R134 a couple of years ago (and never experienced this problem)

bwoodry

I also converted and the ac compressor noise seems to resonate through the chassis and causes a sound that does change in direct relation to the engine rpm's . It is very annoying however the mechanic says this is normal. You really cannot hear it when outside the car so I used a screwdriver and placed one end on the compressor and the handle on my ear and I can hear this noise. it is interesting how noise travels through these cars.

wf758

Listened to mine again. Really correlated to fan speed (but sound is very electronic in nature). Took it in to the shop this weekend. Will let you know the outcome.

Wachuko

When they replace the evaporator the whole thing comes out (housing for the evaporator, fans, etc.) What you are most likely hearing is the fans touching the plastic housing. Could be that you need to replace the fans due to wear...

Wachuko

Here are a few photos:

Here is where everything resides:



Cover off and fans off:





Fans:

anewbill

Update...
I dropped by my mechanic's shop last week to have another crack at diagnosing the noise. After investigating the fan housings and listening at different RPMs, the consensus at the shop was that the noise was refrigerant related. The current theory is that at certain pressures, the refrigerant can be heard flowing through the lines and/or expansion valve. Once the sound appears, If you lower the RPM of the engine, it will cease. We tried removing 3 lbs of refrigerant to see if that had any effect. Although the sound did not completely disappear, it seems to be less frequent. My vent temps are still in the same range, so I think I will quit while I am ahead and just turn up the radio.
Thanks to all for you replies and help.