canddmf

Please advise. I have a hissing sound within my dash. It only starts when I first engage the A/C. It is intermittent, in that when I am at regular speed for more than 8 or 10 seconds, it goes away.

Among the symptoms I see:

- Hissing sound when engaging A/C unit using push button
- Goes away after 8-10 seconds at speed, intermittent very light hissing sound thereafter
- Reduced or no cold air coming out while hissing, but decent cold air when no hissing
- When coming off cruising speed, hissing sometimes restarts, sometimes doesn't.

This sounds like some sort of vacuum leak associated with sending air across the evaporator coils, but I'm not familiar with the layout of the air flow to know that.

I've had my CCU rebuilt and I've got the A/C/Heat Servo's up front all working correctly.

If anyone can point me toward a proper diagnosis and suggest how to access the "hissy" part, I'd appreciate it.

Thanks,

Chuck
'89 C4

canddmf

New information...

The A/C gets nice and cold when running in 5th gear and either cruising or decelerating. Almost drops off entirely when accelerating in any gear. Also, my mpg dropped about 2 mpg over the last week (averaging 23 down to 21).

Is there a book or something that might cover this?

Thanks for any info...

Mark Brook

Chuck,

I had the same sort of hissing in my RHD 90 C2 non A/C. It was one of the pipes from the headlight levellers. Not sure if you have those Stateside, but it worked for me !

Cheers,

Mark 90C2.

canddmf

Thanks for the suggestion.

I'm not aware of any headlight levelers on mine, but this sound only starts when I turn on the A/C, so perhaps I'm being blind, but I thought it must be A/C related.

I'll check to see if I even have the levelers.

Thanks

garrett376

Is your refrigerant level correct? The evaporator is basically on the other side of dash firewall, so if you're low on refrigerant that might suggest the erratic operation and/or the hissing. It seem unlikely to be a vacuum issue. If you hear the hissing and turn off your A/C and this hissing goes away, it sounds unrelated to a vacuum line. The only vacuum lines I know of are the headlight levelers on Euro cars (not US) or the brake servo on a C2.

canddmf

Is there a sight glass to check Freon level? I poked around in the engine compartment near the compressor without seeing any sight glass like I've seen on other brands of cars.

Thanks

Chuck

Wachuko

That is because the dryer is under the front fender. Oon the left side of the car under the plastic shroud under the fender... hard to look at it... you will need to hook up gauges to measure...

canddmf

Grrr...

Thanks for the info. Off to an A/C shop I go. And I thought this was "the last of the user maintainable 911''s"

Thanks again

Chuck