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Vacuum pump update

Yahoo Message Number: 8356
For those of you who have had some problems with the vacuum pump system that operates the dash air, here's an update on my fix.

There are basically three things that can go wrong here--you can have a vacuum leak, the pump can go bad, or the control switch to the pump can go bad.

The first is diagnosed by determining if the system will hold vacuum for a reasonable period of time. Assuming it can, you can then check the vacuum pump separately for "suck". Covering the inlet with your finger will NOT cause the pump to stop running as it is designed to reach rated suction and then "coast" at that level. The Gast pump in my rig was rated for 24 inches water but you must also take into account altitude, which can reduce it to 20 inches.

The pump is readily repairable, the drive is very simple and strong. There is an easily replaced diaphram on top of the piston.
Also, the leaf valves can go bad and there is an inexpensive repair kit available for this. I was able to obtain parts from the local distributor listed by Gast on their website. Fixing the pump is a 10 minute job that almost anyone but the most mechanically challenged can manage. So before you spend $200+ for a new pump, see if you can fix the one you have.
The pressure switch in my rig was located behind the instruments and just under the big padded cover over the dash. It has two spade terminals and a hose nipple for the vacuum hose. On the top, there are two screws, one sets the low level start, the other sets the high level cutout. The way to adjust these is to listen to the pump and set the high level below the point where the pump "unloads".
You'll hear this because the pump speeds up at that point.

The low level is set about mid-point and at a level high enough to keep everything working properly. I tinker with this by turning the zone controls to use vaccum and trigger the pump. It should take about 3 flips of the zone control to trigger the low-level start and the high level should cut in before the pump unloads.
Feel free to email me if you have other questions about this system.

Dan

1998 Allure #30180

Re: Vacuum pump update

Reply #1 –
Yahoo Message Number: 8372
Thanks for the very definitive update. I will look into again finding the switch and maybe getting a pump repair kit. A couple years ago someone else told me about the adjustable switch behind the dash but I couldn't be sure it was the one I saw.

"For those of you who have had some problems with the vacuum pump system that operates the dash air, here's an update on my fix."

Sincerely

George W Becker

gwb36@... 1999 Country Coach Intrigue #10700
 

Re: Vacuum pump update

Reply #2 –
Yahoo Message Number: 8379
George, take the big access cap off (the padded one that covers the entire instrument cluster) and look for the switch screwed to the underside of the solid portion of the dash on the right hand side as you're facing it. The switch will have two red wires connected to it and one rubber hose. You'll see two little adjust holes on top with small screws at the bottom and there are little directional arrows painted on the face of the switch.

Most likely your pump does NOT require rebuilding. Follow the procedure I outlined to first check the vacuum tightness of the system, then check the pump is actually developing a nice "suck", and then find the switch and fiddle with the high level adjustment screw.
Dan

Quote from: George Becker
> Thanks for the very definitive update. I will look into again

finding the

Quote
switch and maybe getting a pump repair kit. A couple years ago

someone else

Quote
told me about the adjustable switch behind the dash but I couldn't

be sure