Cross post from
Glasair.org
From: "Reggie Smith"
<mooney37v@juno.com>
This is the second in a
series of informational articles on the Vision Microsystems EPI-800 and VM1000
Engine Management Systems. As a
reminder, I’m not a certified avionics technician but an experimental aircraft
owner and enthusiast like many of you.
My intention with these posts is to pass along the information I’ve
learned over the years to help keep your EPI-800 or VM1000 system alive and
well, giving back to the community that has given so much to me. This writing will cover aspects of the
manifold pressure transducer “care and feeding” to help you avoid the “MPT
Blues.”
The VMS engine management
system receives manifold pressure (MP) information from a manifold pressure
transducer (MPT), VMS part number 3010015. The MPT receives 5VDC input from the
VMS data processing unit (DPU) and returns a low level signal proportionate to
the pressure differential between ambient and sensed intake manifold
pressure. The DPU processes this
signal to provide the cockpit indication of MP in inches of mercury. If properly installed and maintained,
the MPT is a robust and reliable unit giving decades of trouble free
service.
The MPT consists of an
electrical sensing unit soldered to a PC board and this assembly attached to a
machined aluminum block. The
first generation units had a tan PC board with red-green-white-black wires;
later units had a green PC board with four blade connectors. The sensing unit is a 0-15 psi differential sensor with two sensing ports. In VMS installations, one port remains
open to ambient pressure while the other is connected to a brass nipple in the
mounting block, a snubber fitting to dampen pressure
transients, and to the aircraft manifold via owner installed
fittings.
For longest service life,
guidelines in the VMS installation manual must be followed. Specifically, it is essential that the
MPT be mounted in a position vertically higher than the manifold pressure port
on the engine. If the MPT is
mounted level with the engine MP port, or worse yet lower than the port, it is
possible for liquid fuel and/or heavy fuel vapors to condense and accumulate
in the MPT assembly. Over time,
the blue dye present in 100LL fuel accumulates and congeals contaminating the
snubber, sensing tube, and eventually the sensor
itself. Picture 1 below shows a
MPT that came to me for repair with about 15 years time in service, the owner
complaint was sluggish and inaccurate indications. The owner had also attempted to
dismantle the unit himself breaking both the plastic sensor pressure ports in
the process. Note the yellow
pressure tube contaminated on both ends with blue dye. Also note the dye residue around the
brass nipple and sensor ports.
Picture 2 shows the bottom of the aluminum block where the snubber assembly and aircraft MP port are connected. Note the excessive amount of congealed
blue dye inside the cavity. After
consulting with the owner, I found that his installation was in the engine
compartment and below the level of the engine MP port, the worst possible
location; it is a wonder the sensor lasted as long as it did. On the test bench, as received the MPT
provided and indicated MP of 24.1 inches when actual pressure was 30.15, in
excess of 20% error. Makes one
wonder how long an engine would hold up if it is consistently overpowered in
cruise by 6+ inches MP?
The MPT has two basic
failure modes, gradual and sudden.
Sudden failure results in a cockpit indication of 00.0 MP with the
engine running. This is usually
caused by wiring issues such as a grounded signal wire, breaks or shorts,
loose wires at the DPU terminal, etc.
It can also be caused by complete failure of the sensing unit, although
this is rare. Gradual failure is
the more common mode. Gradual
failure is characterized by slowly decreasing indicated MP over a period of
time for a given throttle setting.
It can be very insidious occurring over a period of weeks or
months. Eventually the cockpit
indication may be near zero or even negative MP at idle. The telltale way to diagnose this
failure is to check the indicated MP on your VMS display against the local
altimeter setting. The two
numbers should agree within 0.2 in Hg, or about 0.5%
if the MPT is functioning correctly; outside this range and you have an
inaccurate sensor that needs to be repaired.
Now back to our poor old
MPT that had to endure a life of hardship in the engine compartment. I did a thorough cleaning of the unit
after dismantling all the parts.
I removed and discarded the old sensor unit and replaced with new. FYI, you can’t use your “Radio Shack”
soldering iron on the sensor unit.
It’s very heat sensitive, the tech specs allow no more than 2 seconds
contact at 482F requiring a highly accurate variable temperature soldering
iron, precision tip, and special low temp/no acid solder. I know of owners who have shelled out
$85+ for a brand new sensor, “glopped” on hardware
store solder to get it to stick to the new sensor and PC board, then reinstall
only to find it does not work; the sensor was internally destroyed by
excessive heat and incorrect assembly procedures. Now back to the repair job: after I replaced the old sensor, I
reassembled the MPT and replaced the old yellow tube with a clear tube making
it easier to detect any blue dye contamination (see pictures 3 & 4
below). On the test bench this
now “like new” unit is spot on to the local altimeter setting and the owner is
happy to know with his new MPT and corrected installation, he should get at
least 20 or more years of reliable service and accurate cockpit MP
indications.
So, what can you do to
help your MPT give you long and reliable service life? First, avoid the “MPT Blues” by making
sure your installation is IAW the VMS VM1000 or EPI-800 installation manual
guidelines. Never install your
MPT in the engine compartment and never install lower than the engine MP
port. Some owners have gone so
far as to install a small lawn mower style clear plastic/paper element filter
between the engine MP port and the line going to the VMS MPT, I don’t see this
as necessary plus it introduces another point of possible vacuum leak, but
others swear by it. During your
annual condition inspection, take a look at the plastic sensor tube for
indications of dye contamination.
If present, check your MP line for any areas where residual fuel or
vapors can condense and accumulate, correct your installation as
necessary.
Next, remove the MPT for
cleaning—be sure to discharge yourself to ground to avoid static electric
damage to the sensor before starting work. After removing the MPT from the
aircraft, carefully pull the plastic sensor tube from the brass nipple only,
DO NOT try to remove it from the plastic sensor port or you will likely break
it off. If the tube has become
too hard/brittle to remove, use an exacto knife, score the tube longitudinally up the brass
nipple, and pull it off the nipple.
Remove the 4 screws holding the electric sensor/PC board assembly and
set it to the side. You now are
able to clean the brass nipple port and the MP port connection on the back
side. Use a mild solvent such as
mineral spirits, denatured alcohol, or acetone to dissolve the congealed blue
dye. You can use a dropper to
fill the pressure port, wait a few moments, and drain onto a paper towel, when
it comes out clear, your port is clean.
Try to clean both directions, i.e. pressure port toward snubber fitting, and snubber
fitting toward pressure port. If
you suspect excessive contamination inside the block, then disassemble the
snubber and clean separately from the block. Gently blow out the port, allow to dry thoroughly, and reassemble. If the tube had to be cut, or if you
cannot clean out the dye contamination, replace the plastic sensor tube with
new. To remove it from the
plastic sensor port, score the tube with an exacto
knife as before and carefully remove.
Finally, get in the habit
of comparing your altimeter setting to the indicated MP before engine start
making it part of your power-on pre-start checklist. This will preclude you flying with an
inaccurate MP setting and possibly overpowering your engine in cruise. If you are out in the middle of
nowhere and notice a bad indication, you’ll at least have an idea of the
magnitude of the error and be able to power conservatively until you can get
somewhere for repairs.
Hopefully these tips will
keep you and your MPT be happy for years to come, no “MPT Blues” is a good
thing! If I can help you with a
manifold pressure transducer repair or any other Vision Microsystems issues,
please don’t hesitate to contact me at
mooney37v@juno.com
Reggie
Glasair I-RG
Flying!
Mooney M-20E
SOLD
Grumann Yankee
SOLD
>