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Vision Microsystems VM1000 & EPI-800: How to aviod the "MPT Blues"

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
1MPTtopcontamination.jpg

PICTURE #2
2MPTbottomcontamination.jpg

PICTURE #3
3MPTgoodasnew.jpg

PICTURE #4
4MPTgoodasnew.jpg
 
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Hi Reggie,

Thank You So Much for your detailed post about the manifold pressure sensor. I knew about the height requirements of the install and have seen two of them at my shop that were installed incorrectly. I really appreciated your in depth description of how the unit works behind the scenes as well as your 'proper care' pointers. I'll be using this info next time I come across another MPT and will print your post for my buds that fly behind one.

Oh yea....great pics too!

Thanks again....you are THE MAN!

And thank you DR for putting it on the front page....I'm sure I may have missed it otherwise :).

Always something to learn here :).
 
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Rick,
I really appreciate the kind words, thank you. I'll echo your comment on 'always something to learn here' ... I've gained a wealth of info from the fiberglass finishing threads and electrical system posts -- what a great forum and a great asset to the RV & experimental community as a whole.
Reggie
 
Reggie,

What are referring to when you say "snubber".

Thanks for all of your help.

George
 
MPT Snubber Fitting

The snubber is a brass fitting with some kind of restrictor built into it. Typically the restrictor is in the form of a porous stainless steel insert (see attached picture). The porosity of the insert allows pressure equalization on either side of the fitting but dampens out pressure spikes or transients resulting in a smoother and more stable readout on the cockpit indicator. For those having issues with the MP indication jumping around, especially during power changes, the snubber fitting will usually solve the problem.

Reggie

snubber-fitting.jpg
 
Faulty MAP Indication

I am having issues with a VM-1000 display unit, that this old thread somewhat references.
This VM-1000 is on a plane that has been sitting for 4 years (hangared) due to the owners health issues. I recently undertook the task of getting the plane back in air. According to the owner, the unit operated nominally prior to the inactivity. The symptoms are as follows:
With the engine off, and the unit powered up, the MP is reading 1.7-5.4" HG in lieu of something close to actual barometric pressure. Also the value is constantly changing. When the engine is running, it seemed that at times the value would be momentarily accurate for existing throttle settings, but always fluctuating.
I have verified that the MPS is mounted in accordance with the instructions (behind the FW, above the hookup to the engine). The clear tube between the brass nipple and the plastic nipple does not appear occluded or stained with fuel. I have "re-racked" the plugs on the wire ribbon that runs from the DPU, through a circuit board, and to the display, with no improvement, or even change. There is no useful troubleshooting info in the manual to speak of.
I called J.P.I. to speak to the tech rep who handles the VM-1000, but he is at AirVenture, and is likely not going to return my call anytime soon!
So as usual, I thought that I would run this by the group here on VAF for ideas. Thanks in advance!
 
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VM1000 MP Indication

Mark,

In this case, i would suspect wiring first, assuming you have a screw terminal type DPU. Likely culprit is poor or intermittant wire connections at the DPU end and/or the MPT end.

However, the indications also appear unusually low which would lend toward failing electronics in the MPT. But, failing electronics is generally characterized by steady indications but known lower than actual pressure.

Reggie
 
Indeed it is the screw terminal type DPU. I will check all the connections of the MPT itself, and at the DPU tomorrow.
Thanks, Reggie.
 
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Inop MPT

After spending most of the morning shoe horned under the panel, I have gleaned the following:

1. The connections to the MPT were not in good shape. I ran known good wire from the DPU to the MPT, and found that the fluctuations in the display ceased. However, the now steady reading was 0" HG.

2. A serviceable MPT (that I conveniently had on my shelf) was jumpered into the system, and the reading was 28.2" HG.

So...Problem likely solved, and my thanks to Reggie for weighing in.
 
Manifold pressure jumping around

Hello!

This is a really great and informative threat... I'm having a similar problem, but neither the "gradual" nor "sudden" failure modes completely describe my problem. Perhaps someone here can help me out?

This morning, I did my preflight, run-up and takeoff - everything looked normal.

Then, pretty much around when I turned cross-wind, my manifold pressure dropped to zero for just a moment, them it started jumping around erratically. The engine sounded fine, but those are pretty uncomfortable indications, so I went ahead and landed one to a full stop. I grabbed a video of the VM1000 manifold pressure display on my taxi back:

http://www.youtube.com/watch?v=LUN-ba29KnY

The way MP jumps in that video is exactly what it looked like in flight.

A new friend and I did some limited troubleshooting - cycled the engine and even went around for another pattern - everything sounded fine, and all the other engine instruments were all normal.

Has anyone seen this? Am I due for a new pressure transducer or perhaps a new or clean 'snubber'?

Thanks in advance for your thoughts and suggestions!
-Anna
 
Jumping MP Indications

Anna,

70% sure you are looking at an intermittent connection at either the MP transducer end or the DPU end. If you have a screw terminal type DPU, then now I'd be 90% sure you have a loose connection at the DPU.

Check each of the connections for the MP transducer, terminal block J3 connections 9, 10, 11, and 12. Give each wire a small tug to ensure security then use a small jeweler's screwdriver to snug up the terminal screws, not too tight, just snug.

You can see my website for more good to know VM1000 information at http://vmsupport.weebly.com/

Happy Flying,
Reggie
 
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