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Vanremog(at)aol.com
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 Posted: Tue Apr 13, 2010 11:24 pm Post subject: VM1000 Oil Pressure |
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All-
In the spirit of documenting problems for posterity here is my story on a recent problem with the Oil Pressure circuit of the VM1000 system. The oil pressure system uses a transducer (50 mV full range 0-100 psig).
During my last flight in cruise, I developed a low oil pressure reading. It was low across the board (both at cruise and idle). Taught to believe my gauges, I aborted the flight and cautiously returned to the field.
I did normal troubleshooting by following the VM documentation and was unable to identify any significant anomalies in the wiring. I removed the oil pressure transducer (Vision Microsystems P/N 3010018, Honeywell M/N MM100PG1QA, Honeywell Data Instruments P/N 9302904) and connected it up on the bench to a 5 VDC supply voltage across pins 2 and 4 and screwed it into a manifold along with a 0-100 psig mechanical (steam) gauge. Playing with compressed air I was able to check the output of the transducer across pins 1 and 3 over the range 10-80 psi and all was smooth and well. This transducer is designed to put out 0.5mV/PSI, so 30 psig=15 mV, 40 psig=20 mV and so on.
I put it all back together and all is now well, so here is my analysis. The low output of this transducer system is okay as long as your terminals (in this case they are all Tin plated) are good and clean. After 12 years of being on the engine side of the firewall they may have oxidized a mite or gotten a little bit of oil on the terminals dropping a little bit of that already low mV. A gas tight connection is a good connection, but these are just slip-on quick-disconnect terminals. The more environmentally resistant Hirshmann connector equipped version of this transducer (change Honeywell M/N suffix from QA to HA) is probably much more resistant to this situation and is probably a good option going forward. I think that the newer VM systems ship with the Hirshmanns. My VM was bought back in 1996. Removal of the terminals burnished the connections removing the voltage drop and all is now functional.
Probably would be a good idea to check and reseat these periodically.
N1GV (RV-6A Flying 936TTAE Silicon Valley, CA)
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gregoryf.flyboy(at)comcas
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| Posted: Wed Apr 14, 2010 2:24 am Post subject: VM1000 Oil Pressure |
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Oxidation is the scurge of connections. Maybe throwing a little dielectric grease into the terminals of the sensor probably wouldn't hurt, either. It might eliminate most upkeep, since it keeps the air off of them. As you may know already, it is considered an insulator, so it should not effect any readings, in fact it will help keep stuff off the terminals that might affect the readings, such as oil, water, etc. For ease of obtaining, it should be able to be found at most automotive outlets, most probably. Just a thought.
Greg Fuchs ..hope to get back to the bird soon, from almost a 6-month break....time seems to fly even faster if not working on her
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After 12 years of being on the engine side of the firewall they may have oxidized a mite or gotten a little bit of oil on the terminals dropping a little bit of that already low mV. A gas tight connection is a good connection, but these are just slip-on quick-disconnect terminals. The more environmentally resistant Hirshmann connector equipped version of this transducer (change Honeywell M/N suffix from QA to HA) is probably much more resistant to this situation and is probably a good option going forward. I think that the newer VM systems ship with the Hirshmanns. My VM was bought back in 1996. Removal of the terminals burnished the connections removing the voltage drop and all is now functional.
Probably would be a good idea to check and reseat these periodically.
N1GV (RV-6A Flying 936TTAE Silicon Valley, CA)
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