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Topic: Heat & Air Vacuum Pump (Read 1009 times) previous topic - next topic

Heat & Air Vacuum Pump

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This question concerns operation of the vacuum pump that controls the air/heat doors. Air and heat work fine and the cooled or heated air is directed to the appropriate vents according to the dash control button. The pump operates as it should except when the temperature selection lever is moved all the way to the left. It's a detent position. At this position the pump runs continuously. The pump is pretty new (thanks, Jeff) and all the vacuum lines are new. I've set the control cable so that the far left position on the dash moves the heater control valve to the full closed position. Can't figure this one out. Any ideas? Thanks. (photos show the pump, temp control valve in the heater hose and the temp selection position)



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Re: Heat & Air Vacuum Pump

Reply #1 –
Quote
93 225

This question concerns operation of the vacuum pump that controls the air/heat doors. Air and heat work fine and the cooled or heated air is directed to the appropriate vents according to the dash control button. The pump operates as it should except when the temperature selection lever is moved all the way to the left. It's a detent position. At this position the pump runs continuously. The pump is pretty new (thanks, Jeff) and all the vacuum lines are new. I've set the control cable so that the far left position on the dash moves the heater control valve to the full closed position. Can't figure this one out. Any ideas?

Jor,
That leftmost detent position on the temp control slider is Fresh/Recirculate control.  I don't think the temperature is any cooler in this position than just outside the detent.  Pop the dash cover and look down on the control assembly and you will see that  sheetmetal part pushing down on a little plunger vacuum valve when you move it into the detent position (which is recirculate).  My guess is somewhere in that circuit from that valve, through the line and to the Fresh/Recirculate bellow is a major leak.  I don't think it would be harmful but you might try a drop of oil on that valve plunger to see if it improves - then you've found your leak.
John Fitzgerald
1991 U300 (SAI) Side Aisle Island Bed 40'
Detroit 6V92 with Allison Retarder
Meridian (Boise), Idaho

Re: Heat & Air Vacuum Pump

Reply #2 –
Jor,

Had exactly the same problem. I would be driving down the road and the pump would come on for a few seconds. Got worse until I got tired listening to it. I clamped all the lines starting at the pump and working out from there.

I have a cold and not in a good mood so will skip to the fix.

Nothing wrong with the pump, lines, etc. The problem is in the control slider and assembly it operates (the slider in your photo with the arrow pointing to it). When you move the slider just slightly to the right, everything works as it should. You could drive yourself crazy and replace the unit or make a stop so the slider won't go all the way to the left or do like I did and just remember to leave the slider a quarter inch to the right of the left stop and everything will be fine.

This fix should be in the tech section under vacuum pumps or something as we are neither the first with this problem nor the last. Took me almost an hour to figure this one out.

Pierce and Gaylie
93' U300/36

Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Heat & Air Vacuum Pump

Reply #3 –
John and Pierce,
  As one of my kids would say, you guys rock! I see it now. Don't know how I missed that detent action. Anyhow, I'll try John's drop of oil recommendation and if that doesn't work, move onto the Pierce's (which is what my wife suggested in the first place). Thanks.
jor

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Re: Heat & Air Vacuum Pump

Reply #4 –
Quote from: Pierce & Gaylie Stewart –

This fix should be in the tech section under vacuum pumps or something as we are neither the first with this problem nor the last. Took me almost an hour to figure this one out.


Ask and ye shall receive - I moved the topic to Tech Talk

-M
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320