M20K Hobbs meter

[Bruce]

Got a hobbs meter problem and looking for some analysis and discussion before throwing more dollars at it.  1979 M20K 231.  The tach does not have hours meter built in to it like most airplanes.  There is a separate Datcon hobbs meter used for that purpose.  Been working fine for the last 10 years, and recently stopped dead in its tracks.  Datcon engineer said to remove it from the airplane put 12 volts on the meter and if it does not run, replace it.  It did not run so it was replaced.  New one installed but still does not run.  Put 12 v on new one, runs fine.  From what I understand from this websource and other sites, M20K model is not tied directly to the master, nor an oil switch (as rental aircraft are), but instead wired to the tach RPM to record "engine time" rather than elapsed time.  Tach RPM ties to JPI 830 engine monitor RPM for the most part.  For the last 280 hours, old hobbs meter was in lockstep to the 10th of an hour with the JPI 830 which I thought recorded elapsed time.  But that doesn't add up if the hobbs meter was designed to replicate tach engine time.  Usually tach time is around 90% of elapsed time on most airplanes.  Voltage going to the hobbs at run-up RPM measures 6-7 or so which is probably not enough to drive the new hobbs meter.  Been test flying it, and still no movement on hobbs at cruise RPM although the cruise voltage to the hobbs was not checked in flight.  The schematic I have does not show it, but I've read there is a Hall Sensor that drives both the tach and the hobbs meter.  I am beginning to conclude that somewhere in its life, this hobbs may have been rewired to an oil pressure switch but not sure.  Looking for suggestions from anyone whose been down this road.  Thanks...

Bruce
N231CH

[co2bruce]

On ours c(M20S) it is wired through air speed pressure switch. Our switch went bad and took the Hobbs with it..

[carusoam]

From the O's POH....

"Hour meter - located on baggage compartment bulkhead and indicates elapsed time while en- gine is running. Location may vary depending on installed systems."

•

The phrase 'elapsed time while engine is running' indicates (to me) a timer that runs based on an oil pressure switch...

Hall effect and air speed switch sorta could do the same... Just why?

The meter / paperwork should be marked with voltage that it prefers. I'd guess, 6 volts isn't it.

Having the hour meter on the back bulkhead is even odder (just sayin')...

COBruce, Does your Eagle's POH have better detail than the O's, like specifically mention 'flight' time being recorded? The O is clearly an 'hour meter'... Somewhat specific to engine but not flight, in real time not engine rpm related...

Best regards,

-a-

[Awful_Charlie]

Bravo Hobbs takes an output from the electronic tach, which in turn has a power feed to the magneto sensor, and a signal from it to calculate the RPM. The MM states that the feed to the Hobbs becomes active at 2000 rpm, but mine seems to trip in a bit below that - nearer 1900 I think, but I haven't put a meter on it so far.

[Bruce]

A few weeks ago I posted the question about a hobbs meter for an M20K 231.  Thanks for all of your input.  Case solved.  Here is the real deal as best as I could gather.  Most aircraft, rented or not, have a hobbs meter that operate off of the ship's 12/24 V electrical system.  These are wired through the master, and unless a rental, are also wired through some secondary switch whether be it a oil pressure sensing switch, gear switch, speed sensing switch, or whatever.  You can find these hobb meters a dime-a-dozen all day long from aviation vendors.  Well actually about $70.  From my reading, I believe that is how most of the Mooney clan is wired.  My 1979 M20K 231 does not have this type of hobbs meter and is not wired that way.  As Charles indicated, it is wired to the tach, which in turn gets it sensing from a Hall sensor unit within the magneto.  Much lower voltage.  As a side note, Hall sensors are not germane to just the aviation community.  Used all over the place.  Thus, they do not get their juice from the ships 12/24 V system, and will continue to work as long as the engine is running above a specific RPM.  My old hobbs was a Datcon model 771, that operated on 4 to 40 volts.  Called Datcon only to find out they no long manufacture any such hobbs that operates on low voltage.  The lowest voltage meter they now manufacture is 10 volts.  The voltage to the meter in my Mooney measures about 6 volts at run-up RPM.  I called around to salvage yards and ended up with a Datcon model 773 from Joe Jenkins Salvage in Dover, Delaware.  The voltage rating on the 773 is also 4-40 volts, identical to the 771, except that the 773 is a 4 screw attachment rather than 3 for the 771.  It was used, but in real good condition, and still work great after I had it installed.  I did make the old Datcon 771 apart.  Extremely simple device as to the operation of it.  Very little to fail, except that the electromagnetic coil windings showed browning and tested open.  Hope this helps other M20K owners.....

 

Bruce

 

 

 

 

 

[carusoam]

Great detail Bruce!

Best regards,

-a-

[KSMooniac]

Do you know if the hours on a used Hobbs meter can be reset to zero or some other arbitrary number?

[Bruce]

These particular Datcon meters can not be opened without destroying them.  Assuming you could get a Datcon meter open and closed again without destroying it, I suppose you could reset, but lot of work for a moot return.  In the logbook entry, just do the arithmetic for time going forward.

 

Bruce

[KSMooniac]

Ok, thanks.  I'd like to install a similar arrangement as I'm removing my recording tach with an EDM-900 upgrade.  I'd like something more "permanent" to record hours, but have them referenced to engine time > some RPM.  This sounds like a perfect solution, so I'm surprised/disappointed there isn't a new replacement.  The vanity in me wants to start at zero since I'm going in with an overhauled engine at the same time.

[carusoam]

It's partially airframe hours....

so starting at zero may represent the engine well, but not the rest of the wear items related to the air frame...

Since it's a Mooney, with no A/F limitations related to hours....

High A/F hours are a badge of honor for some...(right Jim?)

Go figure,

-a-

[Awqward]

I record my Hobbs hours for academic interest, but I use the FAR 1 definition for time in service and record takeoff to landing time manually (actually as captured by Skydemon running on my iPad)....

"Time in service, with respect to maintenance time records, means the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing."

[Bruce]

This is corrected information that what I posted a few years ago on the subject.  I have a 1979 M20K 231 that has a separate engine hour meter, not a hobbs meter, and no tach meter in the tach.  The hour meter failed in 2014 and was replaced with a used serviceable unit.  After fairly extensive research back in 2014, I relayed to this site that the meter received its power from the halls sensor in one of the magnetos.  I recently found out that this was incorrect.  That used serviceable unit that was installed in 2014 recently failed.  After discussing the issue with Mooney technical support to get to the bottom of these failed units, I got the CORRECT information.  The engine hour meter gets its power from the AUX terminal off of the alternator.  As I understand, it is not uncommon for these AUX powered meters to fail.  Past 1979, as I understand Mooneys engine hour meter or hobbs meter were/are 12v meters with the switch being an airspeed switch.  Mooney said a conversion to this set up is possible, but would require another airspeed switch pn 940016-515 and a field approval.  Its on the list, but not towards the top.  Has anyone done this conversion, and if so, what was the cost of the switch?

Bruce

[PeytonM]

I have a 1989 J and I had a Hobbs meter some 20 years ago, with an oil pressure switch used to activate.  It was replaced a couple years ago (stopped working). It has quit again, immediately after an oil change, so I wonder if I broke a wire when I was in the engine compartment. (You already know how accessible everything is!)

Any suggestions? Has anyone got a pic of what the oil pressure switch looks like or where I should be looking? Nothing obvious to the naked, untrained eye  

I did check the meter itself; it is fine. 

[Tommy]

Will oil pressure switch save you any time? 

[jetdriven]

Oil pressure switch is just to the passenger side of the oil filter with a single wire going to it. Google Hobbs oil pressure switch to see what they look like. It is relatively easy to find it and fix the wire that was likely broken whenthet switched the oil filter

[PeytonM]

I thought that was oil temp. (Just curious, where's oil temp?)

[RobertGary1]

I wired mine to a post on the back of the alternator just for this. Runs anytime the alternator is generating power. Mine died so I paid the $10 for another one at the aircraft junk yard. 

 

-Robert