m20j oil pressure guage

[rorythedog]

last nite wifey and i flew to pittsburg, ks to have dinner at "chicken mary's"(of "food network" fame). when we were ready to depart rnwy 32 into the very cold, very black skys, i noticed that my oil pressure guage was in the high yellow zone and when i ran up the engine it moved to the red. after pondering this, i toggled the landing light switch off, and the guage returned to normal...on again and it read high.

i think that the oil pump is mechanical...am i right?  is this aberant reading with the light on a problem? 

thanks for any advice.

p.s. this is a wonderfully friendly and accomodating fbo and is a fun little stop for food and fuel if you're in the area. pittsburg is also the home of "anywhere map" products.  

[Jeff_S]

Can't tell for sure but from your photo it looks like you've got a later-model J, like mine. Those original gauges are electrically driven by transducers that take the analog pressure reading from the oil, fuel etc. and convert it to a signal that drives the gauge.

The point is, anything that may be causing havoc in your electrical system can make them do weird things. It's usually some bad ground somewhere, and since you say the problem only shows up when you have the landing light on, this would be a likely suspect.

I know this because my fuel pressure gauge has a mind of it's own. Sometimes it works fine, sometimes it just goes dead, but a gentle pressure on the screw holding it in usually makes it come alive. And I flew with my A/P last week for a different reason but he noted that simply keying the mic often makes it go live if it's dead. So we have a grounding issue somewhere in the system, which can cost a whole lotta' money to ferret out. Since I'm comfortable that my JPI is giving me good readings, I haven't worried about it too much.

[rorythedog]

thanks for the advice, jeff. i think you're right about the ground....my mooney is actually older... a '78.  it looks newer as it has "round windows" and a 252 paint scheme

[richardheitzman]

a good place to start looking for a bad ground is on the negative side of the aircraft battery, make sure there is no corrosion and that it is clean and protected.

If your landing light is causing an issue, take a look at the lightbulb connections, there are two. Make sure they are clean and corrosion free. (I don't remember if the M20J has wing lights, I think they are. HEY IT'S MONDAY GIVE ME A BREAK). Check the connector at the wing disconnect (inside cabin left/right side behind side panel close to the floor boards). The wing lights draw a lot of power so corrosion will cause a large amount of resistance in the circuit. You can take a look at your Moritze cluster guage as well, on the back side there are a number of connections that can be cleaned up. Have your A&P take off your glare sheild and take a look around. A large amount of the grounds are located behind the panel on the lower lip. Hard to get to to clean up, but usually ok. You might have some that are fraying or fretting and that can lead to partial grounds.

[rorythedog]

thank you richard...i will start at the cluster guage connections as i have recently had the glareshield off to install a panel mount for the garmin 396....i also put a new com radio in just last week.

[DaV8or]

Do you have a voltage meter installed in your plane? If not, you may want to test for voltage drop when you turn on your landing light. I'm just speculating, but if your oil pressure gauge works by measuring resistance from the transducer, then a significant drop in voltage would cause a high reading. If you do have a big voltage drop, it means your charging system isn't keeping up with the load of the landing light. A voltage meter attached to the panel buss should tell you.

[bnicolette]

Hi all,

This morning I took a short hop in the Mooney.  I noticed on taxi out that the oil pressure gauge was reading at the top of the green but the JPI unit was right on the money at about 50 -58 during taxi.  At takeoff power the Mooney gauge went into the yellow and the JPI was right at 65 and steady.  After landing I noticed that the Mooney gauge was now back to normal and the JPI was still normal?

We recently put another washer in the thing-a-ma-jig to bring up the oil pressure a tad as it was running at the bottom of the green and on the JPI was about 58-60 during cruise flight.  The JPI now show the pressure right about 65 during cruise which is right where we wanted it so that is okay, just now the mechanical gauge, at least on this flight, was acting funky till the end.

I do turn on my landing light when cleared to land and I read earlier posts about electrical loads sometimes playing havoc with this gauge?  When clear of the runway I turned off the landing light and it is shortly after that, that I noticed the gauge was back to normal?

Does anybody have any thoughts on this?  I have yet to talk to my mechanic about this issue if it is an issue.  I am going to try and get out tomorrow and see if it happens again.  I was just curious as to why the JPI unit was different than the gauge if it was an actual oil pressure problem?

 

[jetdriven]

the factory guages use resistance to generate a signal, them biases this against system voltage.  Our partner had an alternator failure and the CHT kept climbing into the red.   Because the system voltage was falling.  So, unless your system voltage is stable and consistent, the factory guages wont be accurate at all.  That JPI uses a different transducer I would bet, and is unaffected by voltage.