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Mystery Switch on Dash

January 06, 2014, 05:15:03 pm

As I continue to learn my "new" coach, I am finding wires that are disconnected and switches that I have no idea what they do, as I am fairly competent at troubleshooting most I have been able to figure out... but...
On the dash, directly under the radio I have a LED lighted switch (it doesn't light).  As my dash is laid out I have the Ignition, the at 12v socket, then I have this mystery switch.  I was able to track the wires through the front bulkhead to a vacuum pump?  However I have no idea what this pump should be running.... any ideas? 

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Re: Mystery Switch on Dash

Reply #1 – January 06, 2014, 05:26:39 pm

Vacuum pump is there for the dash vent/heater/AC damper controls.  It comes on with the ignition.  When enough vacuum is achieved a pressure (actually vacuum) switch automatically turns off the pump until the level drops again.  PO (previous owner) might have added this dash switch when a vacuum leak developed and the pump wouldn't shut off.
It's my theory that many vacuum pumps get burned up/worn out by undetected leaks and operator not noticing pump runs all the time.
Common areas where leaks develop are the hose ends on the exterior hoses going into the vacuum reservoir, pressure switch, or to vacuum pump.  Usually just need to snip off a 1/2 inch and reinstall.

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Re: Mystery Switch on Dash

Reply #2 – January 06, 2014, 05:39:32 pm

Humm... so  with the key in the on position I should have power to the pump (provided the switch is on)... I will wait a little for the deep freeze to pass, and then go check that out!  Thank you, this switch was driving me nuts... it just didn't make sense!

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Re: Mystery Switch on Dash

Reply #3 – January 06, 2014, 07:20:58 pm

The switch might be to allow you to power the radio from the house battery when the ignition is off.

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Re: Mystery Switch on Dash

Reply #4 – January 06, 2014, 08:34:03 pm

Lisa,

Some have put in a switch to control the heating/cooling vacuum pump because it started running a lot of the time. The problem lies in the Ford heating/cooling control unit on the instrument panel face. If the slide control is all the way to the left, it may allow a vacuum leak and the pump will run frequently. Either install a small stop so the lever can't go all the way to the left or just remember to keep it a quarter of an inch from the end. Ez to do. No need to replace the control unit.

Should have added that the pump together with the dash mounted Ford temp/heater/AC control operate the vents in the heating and cooling system up front.

This was the problem on our 1993 U300. Several others have PMed me after my previous post several years ago so I know it is a fairly common problem. 

Pierce

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Re: Mystery Switch on Dash

Reply #5 – January 08, 2014, 11:04:02 am

Lisa, did you ever figure out what the switch was for?

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