Glas467
Well Known Member
Vision Microsystems VM1000 & EPI-800: How to avoid the "MPT Blues"
This is the 2nd in a series on the Vision Microsystems EPI-800 & VM1000 Engine Management Systems. As a reminder, I’m not a certified avionics technician but an experimental aircraft owner & enthusiast like you. My intention is to pass along the information I’ve learned over the years to help keep your EPI-800 or VM1000 system alive & well. This post will cover the manifold pressure transducer (MPT) “care & feeding” to help you avoid the “MPT Blues.”
The EMS receives manifold pressure (MP) data from a MPT, VMS p/n 3010015. The MPT receives 5VDC input from the VMS data processing unit (DPU) & returns a low level signal proportionate to intake MP. The DPU processes the signal to provide a cockpit indication. If properly installed & maintained, the MPT is a robust & reliable unit giving decades of trouble free service.
The MPT consists of a sensing unit soldered to a PC board, this assembly is attached to an aluminum block. The 1st gen units had a tan PC board with RED-GRN-BLK-WHT wires; later units had a green PCB with 4 blade connectors. The sensing unit is a 15 psi two port absolute sensor. In VMS installations, one port remains open to cabin pressure while the other is connected to a brass nipple in the mounting block, a snubber fitting to dampen pressure spikes, & to the engine manifold.
For longest service life, follow installation manual guidelines! Mount the MPT in a position vertically higher than the engine MP port. If mounted level with or lower than the engine MP port, it's possible for liquid fuel and/or heavy fuel vapors to condense & accumulate in the MPT assembly. Over time, 100LL blue dye accumulates & congeals contaminating the snubber, sensing tube, & eventually the sensor itself. Picture 1 below shows a MPT that came to me for repair with about 15 years time in service, owner complaint was sluggish & inaccurate indications. The owner had attempted to dismantle the unit himself breaking both 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 & sensor ports. Picture 2 shows the bottom of the block where the snubber assembly (removed in this picture) & 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 & 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, the MPT provided an indicated MP of 24.1 inches when actual pressure was 30.15, over 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 2 basic failure modes, gradual & sudden. Sudden failure results in a cockpit indication of 00.0 MP, usually caused by wiring issues. It can also be caused by complete failure of the sensing unit, but this is rare. 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 0 or even negative MP at idle. Diagnose this failure by checking indicated MP against the local altimeter setting (if at sea level, or the altimeter setting corresponding to "0" feet if at higher elevation). The two numbers should agree within 0.2 in Hg (~0.5%); outside this range you have an inaccurate sensor that needs repair.
Back to the repair: I did a thorough cleaning of the unit after dismantling all the parts, removed/discarded the old sensor unit & replaced with new. FYI, you can’t use your home soldering iron on the sensor--it’s very heat sensitive, tech specs allow no more than 2 seconds contact at 482F requiring a highly accurate variable temp soldering iron, precision tip, & special low temp/no acid solder. I know of owners who have shelled out $85+ for a new sensor, “glopped” on hardware store solder to get it to stick to the new sensor & PC board, then find it does not work; the sensor destroyed by high heat & incorrect assembly procedure. Back to the repair, I reassembled the MPT & replaced the old yellow tube with a clear tube for easy detection of blue dye contamination (pictures 3 & 4 below). On the test bench the now “like new” unit is spot on to the local pressure & the owner is happy to know with his new MPT & corrected installation, he should get at least 20+ years of reliable service & accurate cockpit MP indications.
So, what can you do to help your MPT give you long & reliable service life? 1st, 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 & 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 areas where fuel or vapors can condense & accumulate, correct 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, & 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 (mineral spirits/denatured alcohol/acetone) to dissolve the 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, & snubber toward pressure port. If you suspect excessive contamination inside the block, then disassemble the snubber & clean separately from the block. Gently blow out the port, allow to dry thoroughly, & 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 & carefully remove.
Finally, get in the habit of comparing altimeter setting to MP before engine start, make it part of your pre-start checklist (remember to correct for other than sea level airports). This will preclude you flying with an inaccurate MP setting & possibly overpowering your engine. If you are out on the road & notice a bad indication, you’ll at least have an idea of the magnitude of the error & be able to power conservatively until you can get repairs.
Hopefully these tips will keep you & your MPT happy for years to come, having no “MPT Blues” is a good thing! If I can help you with a MP transducer repair or any other Vision Microsystems issue, please don’t hesitate to contact me at [email protected]
Happy Flying,
Reggie
PICTURE #1
PICTURE #2
PICTURE #3
PICTURE #4
This is the 2nd in a series on the Vision Microsystems EPI-800 & VM1000 Engine Management Systems. As a reminder, I’m not a certified avionics technician but an experimental aircraft owner & enthusiast like you. My intention is to pass along the information I’ve learned over the years to help keep your EPI-800 or VM1000 system alive & well. This post will cover the manifold pressure transducer (MPT) “care & feeding” to help you avoid the “MPT Blues.”
The EMS receives manifold pressure (MP) data from a MPT, VMS p/n 3010015. The MPT receives 5VDC input from the VMS data processing unit (DPU) & returns a low level signal proportionate to intake MP. The DPU processes the signal to provide a cockpit indication. If properly installed & maintained, the MPT is a robust & reliable unit giving decades of trouble free service.
The MPT consists of a sensing unit soldered to a PC board, this assembly is attached to an aluminum block. The 1st gen units had a tan PC board with RED-GRN-BLK-WHT wires; later units had a green PCB with 4 blade connectors. The sensing unit is a 15 psi two port absolute sensor. In VMS installations, one port remains open to cabin pressure while the other is connected to a brass nipple in the mounting block, a snubber fitting to dampen pressure spikes, & to the engine manifold.
For longest service life, follow installation manual guidelines! Mount the MPT in a position vertically higher than the engine MP port. If mounted level with or lower than the engine MP port, it's possible for liquid fuel and/or heavy fuel vapors to condense & accumulate in the MPT assembly. Over time, 100LL blue dye accumulates & congeals contaminating the snubber, sensing tube, & eventually the sensor itself. Picture 1 below shows a MPT that came to me for repair with about 15 years time in service, owner complaint was sluggish & inaccurate indications. The owner had attempted to dismantle the unit himself breaking both 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 & sensor ports. Picture 2 shows the bottom of the block where the snubber assembly (removed in this picture) & 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 & 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, the MPT provided an indicated MP of 24.1 inches when actual pressure was 30.15, over 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 2 basic failure modes, gradual & sudden. Sudden failure results in a cockpit indication of 00.0 MP, usually caused by wiring issues. It can also be caused by complete failure of the sensing unit, but this is rare. 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 0 or even negative MP at idle. Diagnose this failure by checking indicated MP against the local altimeter setting (if at sea level, or the altimeter setting corresponding to "0" feet if at higher elevation). The two numbers should agree within 0.2 in Hg (~0.5%); outside this range you have an inaccurate sensor that needs repair.
Back to the repair: I did a thorough cleaning of the unit after dismantling all the parts, removed/discarded the old sensor unit & replaced with new. FYI, you can’t use your home soldering iron on the sensor--it’s very heat sensitive, tech specs allow no more than 2 seconds contact at 482F requiring a highly accurate variable temp soldering iron, precision tip, & special low temp/no acid solder. I know of owners who have shelled out $85+ for a new sensor, “glopped” on hardware store solder to get it to stick to the new sensor & PC board, then find it does not work; the sensor destroyed by high heat & incorrect assembly procedure. Back to the repair, I reassembled the MPT & replaced the old yellow tube with a clear tube for easy detection of blue dye contamination (pictures 3 & 4 below). On the test bench the now “like new” unit is spot on to the local pressure & the owner is happy to know with his new MPT & corrected installation, he should get at least 20+ years of reliable service & accurate cockpit MP indications.
So, what can you do to help your MPT give you long & reliable service life? 1st, 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 & 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 areas where fuel or vapors can condense & accumulate, correct 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, & 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 (mineral spirits/denatured alcohol/acetone) to dissolve the 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, & snubber toward pressure port. If you suspect excessive contamination inside the block, then disassemble the snubber & clean separately from the block. Gently blow out the port, allow to dry thoroughly, & 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 & carefully remove.
Finally, get in the habit of comparing altimeter setting to MP before engine start, make it part of your pre-start checklist (remember to correct for other than sea level airports). This will preclude you flying with an inaccurate MP setting & possibly overpowering your engine. If you are out on the road & notice a bad indication, you’ll at least have an idea of the magnitude of the error & be able to power conservatively until you can get repairs.
Hopefully these tips will keep you & your MPT happy for years to come, having no “MPT Blues” is a good thing! If I can help you with a MP transducer repair or any other Vision Microsystems issue, please don’t hesitate to contact me at [email protected]
Happy Flying,
Reggie
PICTURE #1
PICTURE #2
PICTURE #3
PICTURE #4
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