I purchased a used Robinair 15102-A vac pump last winter (much better
price off season). Now I am ready to use it. I added oil (did not
replace the old oil since I was doing a test evacuation) and hooked
the vac pump up to the vehicle's A/C system. I was able to pull 26 or
27 in of Hg of vacum. As I understand it I will need to pull 29 in of
Hg to properly evacuate the A/C system (after repair).
Will changing the oil have an effect on the pump's performance (pull a
stronger vacuum)? I searched for a manual or an exploded view of the
pump with no success. What other sort of maintanence might I need to
do to the pump? Other than hooking the pump up and letting it run,
are there any other proceedures that I should / need to follow?
Suggestions / links to info very much appreciated.
I do not know much about A/C systems or vacuum pump repair, but I do know
that you can not pull more than the local atmospheric pressure which changes
with altitutde and weather. For example, in Denver it averages 24.8 mm Hg.
On May 11, 8:53 am, email@example.com wrote:
Oil will pick up all sorts of volatiles, I strongly recommend dumping
the old stuff and putting in fresh. Keeping the intake and outlet
sealed between sessions will help, too. You need vacuum pump oil,
too, not just car oil. Just thought I'd mention that, you never know
what some people will do...
Manufacturers sometimes will have rebuild kits, I know Welch pumps
do. Used vac pumps can be a crap-shoot, you never know what kind of
service the thing's been in, once a month, once a week, 24/7, you just
never know. If it's been in the usual repair garage, nobody's changed
the oil since it's been bought new.
Don't be too quick to blame the pump, a car A/C system has lots of
leak potential, until you've replaced all the seals, you won't know
what you've got. Attach the vac gauge directly to the pump and test
it that way. Refrigerant sniffers have come waaay down, that would be
my first choice for tracing leaks. Even HF has one, not that that's
what I have.
I'd not heard that term used. Thanks for explaining.
There was a Dave From Illinois, who used to talk about the
DG (discussion group). Fundy Dave used to post on
alternating havoc usenet list, and rebuke us all as
firstname.lastname@example.org fired this volley in
You _cannot_ determine the degree of vacuum necessary to properly dry and
fully evacuate a system with only mechanical gauges. You need a
thermistor-style vacuum gauge. They aren't expensive.
I'll give you a scenario as to why. Yesterday (yes, actually) one of my
employees asked if I could check out her Toyota's AC (which was blowing
hot). yep. I grabbed the manifold gauges, the 134A adapter fittings,
and the vacuum pump (Robinaire 6cfm). New oil, by the way. Always new
oil -- AFTER every pull-down. Always. Never not. No excuses. ALWAYS...
(did I say, "always, every time, no excuses, ALWAYS? If not -- ALWAYS,
after EVERY pull-down on ANY system of ANY size. Oil is cheap, pumps
aren't and inadequate pull-downs even less-so).
She complained that several people had "charged" her system using one-
hose taps, and it was blowing hot. So I told her to go get two cans of
134A, and let me just pull it down. If it had a leak, we'd know. She
could get the leak fixed professionally (I don't DO auto repairs except
on my own vehicles).
I pumped the goop out of her system (full of leak-check and excess oil),
then hooked up the pump and thermistor gauge, and let 'er run. I was
hoping for 200 microns (mercury column) of vacuum. After three hours, it
wouldn't drop below 400. That's sort-of a sign of a leak, but I had a
clue that said, "no".
By the way, do you know what 400-micron vacuum looks like on a mechanical
gauge? (clue -- about the same as 1000-micron vacuum)
ANyway... the average vacuum wouldn't drop below 400, but the thermistor
gauge would occasionally dip to 275, then suddenly JUMP back to 400.
That's not the way they normally work -- they move _gradually_ from
point-to-point. They never jump -- unless. Unless there were droplets
of water entrained in oil in the system, and they were progressively
exploding into vapor, literally one-at-a-time.
Another clue was that the oil in the pump sight glass was growing cloudy.
It never does that on a healthy system. Only on one with lots of water
BTW... do you know what water looks like in a mechanical gauge? (clue --
about the same as no water, only there's an important distinction
concerning how the system will work with or without it, and the gauges
don't tell you that)
So... we let it run; All of 6 hours plus, actually. It pulled
"droplets" for almost the whole time, gauge slowly moving down to 275,
then jumping up to 400 again suddenly. Then, in the last 30 minutes, the
thermistor gauge started indicating improvements. Eventually, it held 50
microns on pull, and rose to only 200 after being off the pump for 10
minutes. That's a good pull. No air, no water, nothing to interfere.
A quick re-charge was all that was left -- and you already know how to do
Quick answer -- no... you can't tell with mechanical gauges.
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message
interesting stuff snipped ----------
interesting post - I wonder if where you live matters
I have repaired a number of cars where the AC system is empty - mostly 944s
in fact - swap the seal on the pump put in some of the new oil for R134, and
fill - works fine. on one that exploded a hose in 130 deg weather, just
replaced the hose (a few weeks later), and it worked fine. Never saw any
moisture effects. So, why does your experience differ? could it be a drier
climate, or could it be something else? Note, I am not questioning your
advice/report, it makes perfect sense, I'm wondering why I can get away with
not doing all that stuff?
Possibly the systems you worked on were clear of moisture before you opened
them, and you had them open for just a short time, so the amount of moisture
in the system was minimal.
I do commercial HVAC service and all manufacturers recommend pulling a
vacuum of 400 microns or lower to remove moisture from the system. If the
system is new, and relatively clean you can reach 400 microns in minutes. On
the other hand if the system has moisture in it it can take hours, or even
days to pull a system down to 400 microns.
One time I vac'd a system for about 60 hours before it would hold less than
400 microns. The year before another service company replaced the compressor
that failed. I replaced the compressor a second time, ran the vacuum pump
for about 24 hours, changed the oil on my vacuum pump, purged the system
with nitrogen and pumped it for about 24 more hours, changed oil on my
vacuum pump again, purged the system with nitrogen again, them pumped for
about 12 more hours. At this time the vacuum held at about 300 microns. That
second compressor has been running for over 6 years.
useful story snipped
In my particular case, at least one of these cars was open for several weeks
while I sent the compressor out for repair, the one with the exploded hose
was open for at least a week. I think the drier climate, and the fact that
I wasn't operating the compressor with the system open might explain some of
it - still, the very different results surprises me. I have pulled a vac
(no, I don't have one of the "good" gauges) and just let the pump run
overnight on one system - I didn't notice any improvement, but it wasn't
worse either, compared to the ones that didn't get this treatment.
On my 59 cad, the car was empty of freeon for probably 30 years - I added
freon and it worked (but leaked down after a year or so, I haven't recharged
it in a long time) - but it was stored (in very poor shape) near Oildale - a
very very dry area.
I wonder - maybe the careful treatment for moisture applies only when the
humidity is above 5%? 10%?
This particular vehicle had - at one time - a leak on the low pressure
side of the compressor. It had been run like that, recharged, run,
recharged over and over without fixing the leak.
So it was sucking in non-condensables more or less continuously every
time it ran out of refrigerant. In Florida, those consist of both air
and prodigious quantities of humidity. The long pull-down was partly due
to sucking the moisture out of the filter-drier (which can be re-dried
with a long-enough pumpdown).
I had thought filter driers were chemical system. And that
you couldn't restore them. Maybe on cars, you can?
Yes, I easily imagine it drawing the system full of
humidity. And of course, the compressor oil is hydoscopic.
Some are calcium chloride-based, and only baking will dry them. Many
contain "molecular seives". They're ceramic pellets about the size of #6
shot that have precisely-sized pores that will only admit and hold water
molecules, while rejecting or releasing other substances. They can be
"recharged" either by baking or by holding a hard vacuum on them for a
And yes, the oil is quite able to absorb a quantity of water -- in fact,
that's part of its duty in a system. Water not in vapor form, and
entrained like that isn't nearly as harmful to the system as hot vapor in
contact with chloro/fluoro-carbons, or free liquid.
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