What might cause a hissing noise on 960?

[Peter Milnes]

Start with where the hissing is heard loudest, (Either under the bonnet or inside the car). That will tell you where to start looking. As a possible clue there are three vacuum lines which feed into the car from the attachment plate on the bulkhead. They pass through metal inline connectors and then through three plastic pipe-runs to the vacuum control relay. I was idly checking over the spare piping from my old 760 today and found that one of the hoses (large white one on right when looking from inside car) had started to split just inboard of the three way rubber connector. This one feeds to the vacuum reservoir inside the air-con box. Just a thought but that would mean almost permanent hissing and not a great deal of response from the vacuum operated valves inside the box. Worth a try but is in a very involved place to get at.

All the best, Peter.

[Mike Arnold]

This might be helpful (or it might be a distraction) but here goes!

Firstly, the vacuum that drives the servos comes from the engine inlet manifold, through a non-return valve into a vacuum reservoir (behind the passenger side dash) and then feeds into the ECC unit that uses electrically operated valves to direct the vacuum to the appropriate servo. This is what the previous posts have already said.

The most powerful vacuum is developed when the engine is at minimum throttle (eg idling or on the overrun). There is no vacuum at all at full throttle. George makes the very good point that the compressor is disengaged at full tilt to maximise engine power to the wheels.

This leads me to surmise that if the noise is constant regardless of throttle (and the non-return valve is working) the leak is in or after the reservoir. If the noise is related to throttle position (big hiss at idle and no hiss at full throttle) the leak is before the non-return valve (assuming it is working).

Finally, there is another vacuum line from the manifold that simply sucks air past the cabin temp sensor which is mounted it the light cluster on the roof. If that tube is split you might get a hiss but you may also get poor cabin temp regulation. I had poor temp regulation on my old 760 and was due to Volvo crushing the tube in the dash when the car was assembled.

Of course, the evaporator behind the dash makes a hissing noise but this will stop within 20sec of turning the aircon off so, from your earlier note where you say that the noise occurs regardless of the aircon being off or on, you will not be mistaking this.

Enough. Stop waffling, Arnold!

[960_Owner]

I am having the same problem with my 1992 960 wagon.

It hisses all of the time, whether running A/C or not.
The only way to stop the hissing is to place the middle
switch somewhere "in-between" one of the positions, thus
apparently suhtting off the ECC system. The A/C and heater
both work fine -- that is, I can make it very hot or cold.

I have not been able to find a bad hose visually.
I will next try to find it aurally by putting some
flexible plastic tubing in my ear and hunting around
with the other end.

Has anybody successfully stopped the hissing sound?
Is there anyplace I can find specific instructions
on how to access and/or replace these hoses? So far,
this forum is the best resouce I've found in several
hours of web searching.

Thanks for any input.

-Kirk (brand new member here)

[ranger]

It sounds like one of the vacuum actuators under the dashboard on the drivers side. the middle one of the three is leaking this is most common and has two vacuum lines to it a blue and yellow. block of the yellow one and see if it stops your hissing noise and the only problem this causes when you block of the yellow line is when you select airflow to floor only you get 10% still going up the screen I had the same problem with mine and doing this eliminated hissing .
I have left mind like this because it is a major job to change the double action vacuum actuator and for 10% still going up the screen was not worth it Hope this helps

[960_Owner]

>It sounds like one of the vacuum actuators under the
>dashboard on the drivers side.

Thanks for the reply. Could you (or anyone) please
point to info on how to access this actuator? Can I
get at it from underneath the steering column? I've
installed a radio, but never had the top part of the
dash off above the steering column. Any pointers would
be much appreciated. I would be very happy with your
fix, as the 10% above during floor position is no problem
to me. Thanks again for your help.

-Kirk

[960_Owner]

I found the vaccum servo, and blocking the yellow line
with a pair of pliers did indeed make the hissing stop.
Hallelujah! Now I just need to figure out how to block
it permanently. Any suggestions? Thanks very much Keith!

-Kirk

[960_Owner]

Well, it's fixed. I used a small U-bolt with a
nylon insert, and I was able to close it off with
finger-tight pressure. It's working great. I had
forgotten how quiet this car was, as I'd been living
with the loud hissing for well over a year. I am very
grateful for this site. I would like to buy some
windshield wipers from one of your sponsors. Thanks
especially to Keith for posting a cheap and effective
solution to the problem.

-Kirk

[ranger]

I am happy that my solution has helped you. Sometimes it's very easy to fix a most annoying problem. This particular vacuum actuator has a diaphragm which is pulled in both directions and splits causing the air leak where the other actuators are just pulled one-way if you ever decide to replace this actuator and spend a day stripping down the car to do it I recommend you do all of the actuators in one go. It's on my to do list for the future but for now you can stay as it is. Happy to help.