Vacuum Conversion Factors: A Test for the Hard Core

Working on my '89 300E. And I know for certain that I have at least one leak somewhere in the vacuum element/hose system for the climate control. And I'm one of the smarty-marties that sprung for the factory CD that by coincidence has a section on trouble shooting this system. Before I get started, though, I have a question. And this question lies in the conversion factor, because I'm going to be using a pump that is marked in PSI, and not mbar, which is the factor the service CD uses.

According to the coversion chart I've seen, the formula to covert PSI to mbar is: mbar = psi x 68.97

If this is correct, then it would only take about 6 psi to get the required 400 mbar that the service CD says I need to test the system. And the reason I'm concerned is that this really isn't much vacuum.

Does this mean that these are really delicate valves/lines and I might suck one to pieces by applying more vacuum than that? Surely the motor pulls more vacuum than that! I just want to make sure I don't break something by applying 10 or even 15 psi just because I get a little excited with my sucking machine! Sorry. I couldn't resist.

And my other concern is that I can use a, and don't laugh too hard when I say this, hand pump instead of an electric pump. I'm asking because I don't know how steady of a vacuum source the system, when working properly, requires.

I knew I shouldn't have posted this. But if you can pick yourself up off the floor long enough, I'd appreciate any serious answer you can give.

As Always.............

[ILUVMILS]

Let me see if I remember this. 1000mbar=1bar~14.7psi. This is equal to atmospheric pressure. If any of you diesel owners out there are ever messing with your vacuum transducers, the vents are usually marked "ATM", for atmosphere. Anyway, anything less than 1000mbar(negative pressure) is vacuum. Are you sure your guage isn't marked in/Hg? Don't worry about applying too much vacuum with a hand pump. In fact it's the perfect tool for the job. What kind of ACC problem are you having anyway?

[MikeTangas]

I don't understand how you can be using a pressure gauge (PSI) to read vacuum, unless you have one that reads in negative numbers. Vacuum should be read in Inches/HG (Mercury), which is something like a barometer (milibars)

Iluv: Makes sense then. Approximately 6 (negative if you must....) psi would be proportionally equal 400 mbar as 14.7 psi is to 1000 mbar. And if anything less than 1K is vacuum, that also make sense. As for the problem with the system, it's that the system is drawing air in from outside the car around the cooling and heating "functions". So in the summer, when the car is at low rpm with the A/C on, the system draws warm outside air in and dilutes the cool air through the vents. And in the winter, and with the heater on, cool air from outside comes in through the vents diluting the warmer air. And this effect is most pronounced at low rpm. I can't think of anything else it would be since the effect is the same (in reverse, of course) in both scenarios.

Mike: OK. Sorry for what could be interepreted as the implied "positive" pump pressure thing. But that begs the question: What would you call negative psi? I dunno. I've used vacuum gauges on domestics for years and it's pretty much universally referred to as "psi" even when there's a negative draw. Guess I'll need to change my way of thinking if you have another term for it.

Wendell:
Anything below 1bar (or 14.7lb/sq.in) is vacuum or negative pressure as opposed to atmospheric pressure.
Your handpump is the tool you need.
IF you ever find the solution to your problem, pls. post it here.
I have been chasing the very same problem on my Benz for months and have spent so far over $1000. in diagnostic to find the cause.
My dealer and independant mechanic both agree there's a problem but neither can find the cause.
I've posted this problem twice on this tread but did not get any result so far.
JackD

[Arthur Dalton]

A bar is actually 14.5 psi [14.5037]
An ATM is 14.7psi @ sea level. [ 14.6959 ]

So, to help those used to using inches of Hg when the given info specs. is in mbar of vac, just multiply the mbar spec. by .0295 and you will have your inch/vac conversion..

Note that a perfect vac [unobtainable. but besides the point] is simply NO air.
Which means that a negitive 1 atm [ Atmosphere pressure-14.7psi] at sea level is equal to 29.92 inches of vac.
Whereas , a 1 bar vac [1000mbar] would only be 29.5 inches of vac. [ the same 1 atm vac in mbar would be 1013 mbar]
I only give this example to help show the difference between ATM and Bar pressure.
Bar is simply another way of psi measurement....but bar/atm are so close a measure that they are often interchanged..

[MikeTangas]

Given your expanded symptoms, it sounds like the actuator for the fresh air damper might be leaking. Not sure if it is one you can test or replace without gutting the dash or not.

Using your hand pump (mighty vac, etc) your system should easily hold well over 15" HG, might even be able to draw down to 20+" if the system is tight.

On a side note, my vacuum gauge (not my might-vac) does have positive pressure values as it doubles as a fuel pressure gauge.

Originally posted by Jackd
.......IF you ever find the solution to your problem, pls. post it here. I have been chasing the very same problem on my Benz for months and have spent so far over $1000. in diagnostic to find the cause. My dealer and independant mechanic both agree there's a problem but neither can find the cause.
I've posted this problem twice on this tread but did not get any result so far.
JackD

Will do! It probably won't be till next week since I'll be going out of town later this week, but I'll let you know how it goes when I do tackle it. I got used to this kind of thing on my old Corvette since their systems (air box, wiper cover, and head lamps) all operate off of vacuum. Hopefully this won't be too bad. Other than tearing the dash apart, of course!

Thanks Wandell.
I had vacuum problems with my Vette too, but everything seems easier to fix on the Vette. When the headlights don't go up, you know it's a vacuum problem and those big vacuum actuators are right there in plain view.
Pls. keep me posted.
jackD

Jack: Ya' worry my, guy. I just noticed the "1995 John Deer tractor lawn mower" in your sig. I sure hope you're not married. At that rate, if you are, you won't be so for long!!! :p

Don't worry. The John Deer is a ''she''. She turns, she vacuums, she pushes, she pulls, she has a mind of her own, she rarely stars at the first try, but never failed me. A real woman.
jackD