Strange noise from under dash

Well we were driving into town yesterday and had the heat going and every time I let off the gas pedal there was a whistle type of sound that came from under the dash somewhere. As soon as I put it in park or stepped on the gas pedal it went away. Only time it would do it was when I was slowing down or at a stop sign. Anybody know what this could be? Was very annoying driving with it.

Thanks,
Ben

Jeepguy91 said:

Well we were driving into town yesterday and had the heat going and every time I let off the gas pedal there was a whistle type of sound that came from under the dash somewhere. As soon as I put it in park or stepped on the gas pedal it went away. Only time it would do it was when I was slowing down or at a stop sign. Anybody know what this could be? Was very annoying driving with it.

Thanks,
Ben

Click to expand...

Sounds like a vacuum leak. Most of the heater control is managed by vacuum lines and you will find 3 vacuum actuators on the interior. 1 is down by the passengers feet and determines if you are pulling air from inside the car or from outside. The other two determine the air flow and are located just above the drivers right knee. The first is easy to get to but the other two will likely need you to remove the lower dash.

I'm not sure if they changed this on the 1991 model (assuming that is your year) but the 1990 also had a vacuum connection to a brake switch for the cruise control. It is located high up on the brake pedal.

HTH
Todd

Sorry forgot to put that this is on a 2000 xj limited.

Guess i will look for a leaking vacuum line then.

Jeepguy91 said:

Sorry forgot to put that this is on a 2000 xj limited.

Guess i will look for a leaking vacuum line then.

Click to expand...

See pic below.

If you place the mode selector on the AC/Heat control panel to Defrost (#5 position), vacuum to all door actuators is vented (vacuum is stopped at the mode selector). This may help isolate if a door actuator vacuum tube is leaking under the dash. If the whistling sound still exists while in the Defrost position the vacuum leak may be in the vacuum feed tube (black) to the mode selector.

If the whisling does stop, try selecting different selections on the mode selector to activate the doors and see if the whisling comes back at any of these selections. Note that in all but Defrost selection (#5) and Mix (#6), the defrost actuator has vacuum applied to both sides of the Defrost actuator to keep it closed.

#6, Mix - Defrost Actuator moves to mid position and Floor actuator opens.

#7, Floor - Defrost actuator closes, Floor actuator opens.

#8, Panel - Defrost actuator closes, Panel actuator opens.

#9, AC Bi-Level - Defrost actuator closes, panel actuator opens, Floor actuator opens..

#10, AC Outside Panel - Defrost actuator closes, panel actuator opens.

#11, MAX AC/Recirc - Defrost actuator closes, Recirculation actuator opens, Panel Actuator opens.

OFF - Defrost actuator closes, Recirculation actuator opens.

Ok still trying to track this down. I started looking at it a little bit this past weekend. When driving it sounds like it is coming from the driver side pedal area. Started messing with it last night and it doesn't really seem to change or go away when i change the controls on the HVAC. Is there a main line that comes into the cabin for vacuum? Just trying to get this fixed cause it is driving me crazy.

If it's coming from the pedal area...
With a flashlight, stand on your head under the steering wheel and look up at the brake pedal. There is the brake switch (square with 8 or so wires coming out of it). Just to the side of that there is a white switch with a vacuum line running to it.

That switch breaks the vacuum to the cruise control servo when you hit the brake. That vac line is likely broken or disconnected.

ParadiseXJ said:

If it's coming from the pedal area...
With a flashlight, stand on your head under the steering wheel and look up at the brake pedal. There is the brake switch (square with 8 or so wires coming out of it). Just to the side of that there is a white switch with a vacuum line running to it.

That switch breaks the vacuum to the cruise control servo when you hit the brake. That vac line is likely broken or disconnected.

Click to expand...

Not installed on a 2000 XJ.

Is this gonna be complete dash removal to find where this is coming from?

Jeepguy91 said:

Ok still trying to track this down. I started looking at it a little bit this past weekend. When driving it sounds like it is coming from the driver side pedal area. Started messing with it last night and it doesn't really seem to change or go away when i change the controls on the HVAC. Is there a main line that comes into the cabin for vacuum? Just trying to get this fixed cause it is driving me crazy.

Click to expand...

The main vacuum feed to the Mode Selecttor is a Black tube. It originates at the HVAC vacuum tee at the firewall, goes through the firewall to the Mode Selector. It goes over the top of the HVAC unit.



The pic below shows the HVAC quick disconnect (white connector/colored tubing), from the Mode Selector to the door actuators. You don't have to remove the glove box (like in the pic) just roll down the glove box door by removing the rubber stoppers. Verify the connector is secure. Leave the glove box rolled down while driving and have a person riding shotgun listen for the leak. Have the shotgun wiggle the plastic vacuum tubing and see if the noise changes.



You can acess a portion of the HVAC door actuators near the gas pedal by removing the steering column trim panel and the knee blocker. If you look up towards the radio area you can see some of the tubing. This may get you nothng but it may be worth a look.



Leave the trim panel and knee blocker off while driving and see if the noise is louder, which may point to the actuator

Thanks. Will start tracking all that stuff down and see if i can find a leak or something.

What does it mean if it makes this squealing noise when the hvac system is turned off? Is it the main vacuum ine coming into thhe cab then?

Jeepguy91 said:

What does it mean if it makes this squealing noise when the hvac system is turned off? Is it the main vacuum ine coming into thhe cab then?

Click to expand...

With the engine running and the Mode Selector is turned to the OFF position vacuum is still applied to the Recirculation Air Door Actuator and the Defrost Door Actuator (see HVAC Circuit Diagram below).

When the Mode Selector is turned to the Defrost setting (#5) all vacuum is cut off to all actuators. In this mode the main vacuum source from the engine (small black tube) is still available to the Mode Selector.

If you select Defrost (#5) and you still hear the noise I would suspect that there's a leak in the main source tube to the Mode Selector.

If you can get a vacuum hand pump, like the Harbor Freight Brake Bleeder hand pump, you can disconnect the small tube at the HVAC Check Valve in the engine bay (see pic above) and connect to the small tube, Select Defrost on the Mode Selector, apply vacuum to the small tube going to the Mode Selector, and see if it will hold vacuum. If not, there's a leak in the tube or the Mode Selector, or at the vacuum tubes quick disconnect (see pic above).

If the noise goes away when you select Defrost then it can be assumed the main source (black tube) is intact, the Mode Selector connection is tight, and the quick disconnect is tight. If this is the case, select OFF. If the noise returns suspect a loose connect at the Recirc Door Actuator or one of the two connections at the Defrost Door Actuator (see pic of the tubing at the Defrost Door Actuator below - Yellow and Blue tubes). You may be able to reach these tubes with the trim panel and knee blocker removed from below the steering column. Wiggle check.

Access to the Recirc Air Door Actuator would require removing the kick panel under the dash and below the glove box.

Defrost Door Actuator tubing:



HVAC Circuit Diagram: