Grinding sound from behind the dash

[David Fischer]

I have a 97 Marquis, and my wife noticed a grinding sound coming from behind the dash.  Her hearing is much better than mine, but I took the access cover off and I too noticed the sound.  I checked all the on off switches, and it continued.  I tried tapping on the back side of the tachometer and mph guage and nothing.  I shut the engine off and it stopped, but when I turned the key back to on before starting the engine, it came on again.  My concern is that something may be ready to break and this is just the warning.  After driving for about an hour it seemed to stop.  Any thoughts would be appreciated.

[Joel Ashley]

It may be a dash heat blower fan dragging on its housing.  I don't have experience with that happening on our Beaver, and don't know exactly how things are arranged under the dash, but what you describe did happen on our old Pace Arrow, which had a "standard" automobile blower setup.  The fan motor was worn or somehow out of whack, as best as I can recall, and I just replaced it.  Some configurations have an air or motor driven shutter door that opens and closes with heater activation;  if motor driven and the mechanism has loosened, worn, or otherwise failed, gears may be slipping and grinding.

Those would be the first places I'd check, so obviously does the noise occur when you make sure the dash heat/defrost is off?  If not, cross your fingers it's just loose screws on a motor/fan housing setup.

Another possibility, which also occured on our old Pace Arrow, is slippage in the wiper motor drive(s), but that would be more obvious, happening only with the wiper switch on, unless there was a short circuit.

OK, Guys, what other ignition "On"-activated, under dash motors am I missing?

-Joel

[Edward Buker]

There should not be any movement of the tachometer or speedometer mechanisms with the ignition switch except perhaps the tach coming up to a rest position. These units I believe are primarily electronic and should not be a noise source. I would vote for the blower motor. Perhaps one of those inexpensive mechanics stethoscopes would help sort it out. At least that type unit would have some reach where your ear may not be able to go. Harbor Freight or Northern Tool may offer one.

later Ed

[Dennis Mecklenburg]

I previously owned a 98 Monterey and the dash heat/ac control flapper doors are operated by a small vacuum pump behind the dash (the heater control is an automotive control system, and since there is no vacuum source from the engine, the little pump becomes the only source for vacuum and is pumping into a surge ball type reservoir to operate the doors). The little pump can become noticeable and noisy, but will shut off when the vacuum is at a proper level to operate the heater doors and pump up again at each start up. It is also affected by higher altitude and will often times run continuously when above 5,000 feet altitude. There is an adjustment on the pump to set it for higher altitudes if that is a real concern. I would suspect your noise may be coming from the vacuum pump being worn and rattling a bit. Good luck!

[Gerald Farris]

The vacuum pump that Dennis is referring to is the only thing that I can think of in the dash area that will cycle in the manner David described.

The pump is located on the firewall (between the dash and the front cap) and is accessible through the removable panel in the center of the front cap on earlier model coaches and from under the coach by reaching up between the firewall and front cap on latter models.

The pump will be driven by compressed air from the onboard air system on many of the earlier coaches but all of the pumps were changed to electrically driven pumps somewhere in the 1996 to 1998 era.

Gerald

[Richard And Babs Ames]

Our 1997 Patriot had the air operated pump and when it failed replaced it with the electric pump. The electric is noisier and makes an air pump sound when it runs but not a grinding sound. Have you tried turning off the dash air/heat and seeing if the sound stops? If you isolate the source as mentioned above it will be easy to find what is causing it.