mprather(at)spro.net
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 Posted: Tue Sep 05, 2006 9:01 am Post subject: Oil Pressure Gauge Problem |
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Hello Listers,
Sorry for the length of this. Just providing background to my
questions...
I have a '58 Cessna 182.. Last year when I bought it (when the
weather was cold) I noticed a sporadic sligthly quivering oil pressure
indication (needle varies +/-5psi at ~2Hz) on the panel gauge. I
asked a couple of mechanics about it, and they said it's probably not
an issue - said it's old gauges. Well, I have never liked it...
This summer I started to noticed that when the oil temp warms (middle
of the indication), the indicated oil pressure comes down from above
50psi at cruise RPM to 30psi at cruise, and sometimes below 25psi at
idle. While this might not be something to be completely alarmed
about, it's right at the bottom limit for operating oil pressure
according to the O-470 specs.
So, I started digging into it. The gauge is an electric style with a
crankcase mounted sending unit. Over the weekend, I plumbed in
another mechanical oil pressure instrument to be able to compare
values with. What I found is that the panel instrument has significan
error - generally showing the OP to be lower than actual. The panel
instrument also seems to have significantly nonlinear (non-monotonic?)
response, unfortunately, right around the middle of the operating
range.. That's a better scenario than having a real oil pressure
problem, but it's kind of annoying.
RPM Panel Pressure External Pressure
520 22 30
650 25 35
700 27 38
1000 32 51
1200 35 55
1300 38 56
1500 38 56
It appears that the oil pressure regulator has enough oil volume to
work with at the test temperature to regulate pressure starting at
about 1200RPM. That seems totally fine.
I thought it likely that I had a bad sending unit (as it's in the
harshest environment). To verify this, I decided to take a look at
the response curve of the panel instrument.. I Googled around and
found that most of the Stewart Warner senders vary resistance between
~30-240ohm over their operating range. So, I disconnected the wire
from the sending unit and reconnected it to one leg of a variable pot.
Using an ohmmeter, I adjusted the pot to various R values, then at
each value I connected the other leg of the pot to ground and then
noted the indication on the panel meter:
Resistance OP Indication
23 108
33 100
43 95
47 93
65 80
75 68
87 60
89 41
93 38
130 20
230 0
I was surprised to find that the notchy response seemed to come from
the panel instrument - the sender may yet be good.. Around 40-60psi,
the instrument responded to a very small change in resistance with a
big jump in pressure indication..
The physical motion of the instrument is quite slow, but smooth. It
doesn't appear that the movement is draggy/jumpy. When the master is
switched on, the instrument ramps up from zero somewhat quickly at
first, but the final value isn't reached for something like 30sec. I
find this behavior interesting given that the needle also quivers
sometimes.
I measured the open circuit voltage at the sending unit to be around
12.5V (close to battery voltage).
Okay, so now for questions:
Is there any reason the instrument would be designed to have this
notchy response?
Is this a common failure mode for this kind of instrument? Has anyone
else seen one do this?
Is there anything in the wiring to the instrument that would cause a
big shift in the operating point such that this notchy behavior is now
right in the middle of the indicating range?
Can anybody suggest a way to get this fixed (or other things to check)?
Now that I have seen the response curve, I'll probably not throw very
much money at it.. The meter seems to have predictable behavior - it
actually does show changing oil pressure, though not particularly
accurately. If the gauge indicates less than 15-20psi while at operating
RPM, I should probably start getting concerned. Otherwise, it seems
nothing to be alarmed about.
Does anyone rebuild these instruments?
This week, I may plot the sending unit resistance vs. independently
measured oil pressure - just to fill in that blank - and prove that
the sending unit is behaving reasonably.
Any other thoughts?
Thanks and regards,
Matt-
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