Paul Bakker

Hi.

When I put my car to a stop with the AC ON I somtimes hear a relais sound ticking very quickly as if there is some kind of faulty sensor sending a very quick ON/OFF signal.
When I turn the AC OFF the sound disappears instant.
When I try it a minute later with the car rolling again nothing strange happens. Also this ticking sound is very rare. I drove the car 3000 km now and I've heard it maybe 5 times.

Any ideas ?

Thanks.
Paul.

AO

If the ticking sound is coming from the HVAC control uint in the center console, it's most likely the relay in the head unit. It is prone to failure. There are many writeups here to help you fix it.

joejoe

There is a relay inside the control unit for a/c control unit. They are known to be weak for the car and sounds like yours is on way out. There is an upgrade by lister here that is more stout and can take care of problem, I am sure someone else will post details.

SQLGuy

They are prone to failure, but this is not the typical failure mode. Does the AC continue working when you're hearing the ticking?

Normally, when relays fail, the issue is that the contacts have burned/oxidized and no-longer made good contact. When this happens, the coil is still good, so the relay still energizes and deenergizes normally (one click), but he AC doesn't turn on. It sounds like you're seeing a problem with with a switch contact or connector that feeds the relay coil. If one of the connectors to the AC head is loose or has dirty contacts, the relay could start chattering... such chattering could also quickly burn up the contacts, though, causing the relay itself to fail as well.

Alan

I suspect what you are hearing is just the comb flap actuating... is this accompanied by a change in airflow strength through the console vents...?

Alan

WallyP

I agree with Alan - it probably isn't a relay, but a vacuum-operated actuator. When the rubber diaphragm splits in a certain way, it will leak a little until applied vacuum pulls the diaphragm out enough to open the split. The spring snaps the diaphragm back, sealing the leak enough for the vacuum to pull it out again, and the cycle repeats over and over.


You can test the vacuum actuators. Remove the console side panel on the driver's (USA) side. Look just by the driver's right ankle, and find a bundle of colored plastic lines. These lines run to/from the vacuum system valve manifold, which is just visible mounted in front of the radio.

Using a sharp knife, cut the orange line that feeds the central vent actuator (this is the most common failure point in the system). Using a MityVac or similar tool (or suck on it if you don't have anything else!), make certain that the line that runs up to the actuator will hold vacuum. If it does, use a rubber tube to reconnect the cut ends of the orange line. If the line to the dash doesn't hold vacuum, plug the line that comes from the manifold - no need to plug the end that runs up in the dash.

Repeat on the yellow (footwell), green (defrost) and blue (recirculate/inlet) lines.

The white line feeds the heater valve - you don't want to just plug it, you want to have the heater valve working. If the white line doesn't hold vacuum, remove the air cleaner housing and check the heater valve. If it doesn't hold vacuum, replace it - it's cheap. The black side goes toward the block.

The blue line to the recirculate/inlet flap is another critical device. If this actuator is bad, you can't shut off the air flow, and you get no recirculate on the air conditioning. Before you pull the blower motor to replace this actuator, check from below (over the passenger's feet) to make sure that the flap isn't just sticking.

Keep an eye out for crushed, collapsed, or broken vacuum lines as you work on various areas of the car.

When you finish, and have no vacuum leaks, the HVAC system should work much more efficiently.

When you get ambitious, you can order new vacuum actuators to replace the leakers, and start the major disassembly needed to replace them.

Paul Bakker

Update!
Tonight I remebered I once had a situation where the car was already stopped and put off (key was in my pocket) and I also heard the sound coming from somewhere around the AC unit.
I didn't know what to do so I put the ignitionkey in the slot and started the car. Then I stopped the car and removed the key. Problem solved.
I don't recall if I did anything with the AC ON/OFF button....

Hope this helps to determaine if this is a relay or other problem.

The stop I mentioned was after a long ride so it is possible the cooling fan in enginebay was turning due to temperature. This I mention because as you know these fans keep running after the car is shut and the key is out the ignition.

Thanks.
Paul

WallyP

"Update!
Tonight I remebered I once had a situation where the car was already stopped and put off (key was in my pocket) and I also heard the sound coming from somewhere around the AC unit.
I didn't know what to do so I put the ignitionkey in the slot and started the car. Then I stopped the car and removed the key. Problem solved.
I don't recall if I did anything with the AC ON/OFF button...."

The next time that you get the clicking, step to the front of the car and see if the cooling flaps are opening and closing.

Paul Bakker

My flaps are always open...
The rod has broken of the motor so I used a tie wrap to permanently open the flaps.

WallyP

Then check and see if the motor is running when it clicks...

Paul Bakker

I KNEW you would ask me to do that

Could take a while because I don't use the car often and the problem is'n always there....
Keep you guys informed.

Alan

Well one way to ensure that won't be the cause of any noises - remove the relay for it...

You won't know that was the source of your noise - but if it was it will never happen again.

Not much point having the flap motor run anyway right?

Alan

Paul Bakker

Just drove my car with AC OFF.
Heard the sound again somewere round the center of the dash.
Quickly I put the AC ON but no effect on ticking relay sound.
Also putting AC OFF again, no change.

The sound disapears after several seconds (10 ?).

Like stating above I once heard the sound when the ignition was OFF and the key was OUT....

It sounds like a faulty ground or something like that that causes a relay to make a very quick ticking noise.

Paul

dprantl

When you say you turn the AC off, do you mean just the AC button or you move the position slider all the way to the left? My car made a tapping noise coming from under the center of the dash and it turned out to be a faulty footwell vacuum actuator.

Dan
'91 928GT S/C 475hp/460lb.ft

jeff spahn

It's a dash gnome. Don't put your finger in there!