Vacuum Pump Running Too Often MAYBE

[ronnie_g]

I have read through many reply's to this question. But I have not found my answer, at least I don't think I have..... We are now living at 5000 ft elevation. Just got the RV back from a local shop where they (hopefully) resolved a no dash ac problem. But returned it with a poor dash heat, low fan volume concern.
My question is: How many inches of vacuum should my pump be pulling.
I have my dash & heater open to the world to try to find a solution.
What I have seen is my pump is running about 25% of the time.
And the vane / flap to shut off the defrost never moves to completely close the defrost. I can see wear indicators on the seal that prove it once moved full travel. But now it is only moving about 3/4 full travel.
I have +- 10" of vacuum measures at the ball size reservoir. Till I get some small rubber hose I am not able to test vacuum at the switch.
I have cut all the lines at the vacuum switch & tested each one with a hand vacuum pump. All hold vacuum for a reasonable time. So I feel none are leaking after the switch.

I did notice that it takes +- 15" suction on Red + White to fully cycle the defrost vane. So going by that either I don't have enough suction or my servo motor is binding.
My first assUmption was that if the pump is cycling it must be pulling the correct vacuum it was designed to pull. But after testing the defrost now I am wondering if the pump is under performing. We are still on Covid concerns so have yet to find hose to patch my lines back after cutting them. Once I have some hose to do further testing I also want to seal off all the lines at the switch to verify I don't have a leak at the switch itself.
As with a small hand pump I was never able to hand pump even 10" and have it stay even a few seconds. Still even with a leaking switch my pump is cycling off at +- 10" and my defrost is in need of +- 15" to close.
Looking for suggestions, would hate to buy a +- $120 pump unless it is needed.
Thanks for your thoughts & suggestions.
Stay Safe!

[Skip426]

Your system uses a vacuum pump in place of intake manifold vacuum ( gas engine application of the same dash air system ) and 15" vacuum wouldn't be possible from a gas engine intake manifold 99% of the time.
I'm thinking the vacuum actuator is binding .
What vacuum is required to fully cycle the other actuators?
But the quick cycling of the pump would indicate there is an issue with leakage.

Freightliner , didn't install the dash or the section of the heater A/C system your working on , the coach builder did .
Have you seen the name of the dash unit anywhere ? Evans, and SCS are common suppliers of dash systems with vacuum controls.

[Vintage Iron]

I just went through some similar issues on my rig. I found some (2) of the vacuum lines were collapsed where they went through the firewall. With a collapsed line, you can get a no-leak reading but the vacuum does not get or is restricted to the actuator. My system operates off of engine vacuum so it is around 15 to 17 at the reservoir. I think I got replacement hose and fittings to test with at Auto Zone in my area.

[Skip426]

Well I'll have to correct myself

I went to find the electric vacuum pump's specs and came across this site .

www.accuratediesel.com/shop/132.html

With a document that lists the operating range of the pump , and other applications ;and what appears to be a good price.
I'd still be interested in what the other actuators require for vacuum to operate , because if the defrost door shows it hasn't been moving full stroke , I'd say that actuator is binding.

[ronnie_g]

Thanks for the info so far, and more food for thought...
Answers: I have a total (as best as I can tell) of 4 vacuum motors.


3 types

(#1) 1 Hot / Engine water H valve

(#2) 2 (2 stage) vent motors.


All above start to open / operate at +- 3" and once fully open start to close and +- 8"

(#3) The last one, the one I have a problem with is called a 3 stage motor.

It has 2 vacuum lines to it, and has 3 functional positions.
0" closed, 6-12" (red) 1/2 open, (red+white) 5-17" fully open (defrost closed)
If I apply vacuum to only the White, nothing happens.
If I apply vacuum to only the red, it opens 1/2 way.
Vacuum on both red & white the vane opens fully to block the defrost.
Acting like the white is a damper for the red.

That being said, it is located in a awful location to even look at it.
I can get a finger on the white connection, but can't even touch the red.
Although both hold vacuum for an extended time frame, no bleed off.

Also it has a good size spring to hold the large air vane under tension.


I eventually found online drawings, and believe my unit is made by Evans

I thought v-8 manifold normal range was 17-22" but that's a guess?
Sun is setting, going down to the teens tonight, will test the vacuum at the output of the reservoir tomorrow after the world warms up. But I am pretty sure I don't get 12" out of it, also if I seal off the suction at the pump it cycles often like it has a poor seal on the suction side inside the pump, although their is nothing to buffer it like that. I would have expected to see the pump running, then shut off for a time then cycle again. Cycles more often w/o a reservoir connected.

[ronnie_g]

Reading info on the replacement unit. In Truck apps says it is also used as part of the cruse control.


This got me thinking, after service my cruse control is acting up.
I can not 'set' a speed, but if I press resume the speed appears to be set about 10 MPH lower than I am going?


Anyone know if this vacuum pump supplies the cruse control also?

[Skip426]

Originally Posted by ronnie_g

Anyone know if this vacuum pump supplies the cruse control also?

It does not . The cruise function on your diesel engine is from the engine ECM .
( Electronic Control Module )

No reaction to set , would indicate the ECM isn't getting the input from the switch . When you hit resume , the ECM will use the last " set " speed in memory .
Does the " accelerate " function work ?
Do you have a " smart wheel "?

[ronnie_g]

Originally Posted by Skip426

It does not . The cruise function on your diesel engine is from the engine ECM .
( Electronic Control Module )

Does the " accelerate " function work ?
Do you have a " smart wheel "?

I don't know / remember if the ACCEL worked.
I was in a state of shock after paying just over $4000 in repairs and still finding stuff not working. Fearful about bringing it back to the same place.

Humor: When I dropped it off, I told the shop I was not in a rush. Finally started to regret those words, they had the RV 6 months.

Yes I have a "smart wheel" Have already pulled the switch unit.
Opened it up & verified that all the switches do clean make/break drew up a matrix of how it works.

BUT: Have not been back on the road, have the dash torn apart trying to resolve the poor heat & why the Defrost vane is not closing fully.

[ronnie_g]

I pulled the 3 position servo off the damper lever.
The damper moves free, and at no point does it bind.
BUT it is held in one extreme by a (IMO) pretty stiff spring.
With the servo off the damper it moves full stroke w/o any stress at +- 8".
But connected to the servo (damper vane) it will no go full stroke even when connected directly to the vacuum pump which is only providing 13" vacuum.

My pump cycles off at +- 13" and back on at +- 12".
That's measured before the reservoir or after the reservoir and with everything (switch & all vacuum motors) connected or removed.

Only way I can get the Defrost to shut 100% is to use a hand pump to get +- 17" of vacuum on both white & red.

I received one link that suggests a replacement pump cycles off at 17" back on at 13". Which would indicate my pump is under preforming. But even at that, those #'s would just barely operate my damper.

Wondering if my being at 5000 ft elevation has any bearing on this?
At sea level, the air presses on a square inch of surface with 14.7 pounds pressure; at 5,000 feet with 12.3 pounds pressure.
I have 3 vacuum gauges, 2 read +- 2" of one another the other appears to read 4" lower that the other 2, so I have stopped relying on that gauge.