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1989 M20J with no tach time meter?


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5 minutes ago, Pixleyad said:

I'm finalizing the deal on a 1989 M20J and noticed during pre-buy inspection there is no tach meter. Any recommendations to determine engine time without a tach meter? 

Have you looked in the baggage compartment?  I know Mooney put the hrs meter there for some models (others will probably know the exact models and years).

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17 minutes ago, Pixleyad said:

I'm finalizing the deal on a 1989 M20J and noticed during pre-buy inspection there is no tach meter. Any recommendations to determine engine time without a tach meter? 

Check the panel by the right passenger's left knee. 

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welcome aboard pixelyad.

Wait a sec...

what are you asking about?

We have tachs...

we have tach time, often part of the mechanical tachs...


and we have hour meters in newer planes... not associated with the tach...  magically the hour meter is often built by a company called Hobbs... so it becomes the Hobbs time, from the Hobbs meter...

tach=tachometer...


Engine hours and tach time are all designed to do essentially the same thing...

record how many revolutions the engine has had...

Since it doesn’t record a difference for high power climbing vs. cruise vs. gentle descent... it is close enough to use...

If it didn’t have a method of measuring how long it has been since its last OH... that would be problematic...

There is probably an FAR update that defines this quirk...

Know that old planes mechanically counted engine revolutions  and associated them to equal time...

Newer planes have a clock connected to a switch that turns on when oil pressure or other similar event occurs...

 

For questions like this... it makes sense to have a professional PPI... so they can represent you while you figure out what the plane has or doesn’t have... or what it needs to have...

You are using somebody to do the PPI... why are they not explaining this to you?


PP thoughts only, not a mechanic....

Best regards,

-a- 

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Some of the M20Js of this vintage have an interesting hybrid system. The electric tachometer provides an output signal to the 'Hobbs' meter at a frequency related to the RPM which the meter at the copilots knee records as 'hours' but like a mechanical tach it is actually recording revolutions of the engine(lower RPM==lower time accrual), so this is 'engine time' or 'tach time' and not 'Hobbs time' even though the meter may be made by  'Hobbs' An actual real-time meter is an option and would be in addition to that meter.

Edited by Steve W
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On 3/6/2020 at 11:01 PM, Steve W said:

Some of the M20Js of this vintage have an interesting hybrid system. The electric tachometer provides an output signal to the 'Hobbs' meter at a frequency related to the RPM which the meter at the copilots knee records as 'hours' but like a mechanical tach it is actually recording revolutions of the engine(lower RPM==lower time accrual), so this is 'engine time' or 'tach time' and not 'Hobbs time' even though the meter may be made by  'Hobbs' An actual real-time meter is an option and would be in addition to that meter.

This is timely, I was just studying the electrical diagram for my 1986 M20K. I am removing most of the engine instruments, and could not understand the 3 wire 'Hobbs'.  One wire goes to the tach.  The EDM900 has a 'tach' timer based on a certain number of RPM giving 100% and a Hobbs timer, and I regularly record every flight, so I think it is OK to remove both the timer and tach.

 

Aerodon

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12 hours ago, carusoam said:

Great follow-up pixleyad!

What else do you have?

Ask more questions... :)

Best regards,

-a-

OK, so why are there resistors in the wingtip recognition light circuits  on a 1986 Mooney.  Dropping from 28 to 14, surely they could have found a 28V version?

 

Aerodon

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11 minutes ago, Aerodon said:

OK, so why are there resistors in the wingtip recognition light circuits  on a 1986 Mooney.  Dropping from 28 to 14, surely they could have found a 28V version?

My best guess is that Whelen didn't have a 28v version at the time, they do now, but it's a filmstrip projector bulb that they took a Dremel to. Alternately, they got the first 28v aircraft built, turned on the recognition lights, which promptly blew and realized they probably should fix that. The good news is that if there is a 1 for 1 LED replacement there should be no need to remove it. It won't drop to 14v any more with the lower current but most (decent) LEDs should handle the higher voltage fine. In other news my 1 for 1 LED replacement idea still hasn't gotten any farther along.

Another note for anyone if your bulb blows and you measure the voltage it will be 28V since your meter doesn't present enough of a load for the resistor to drop any voltage, you need to measure it with the bulb in the circuit and working to verify the voltage. I've heard stories about people measuring them, assuming the resistor was bad and buying the 28v version, not that it really matters as it doesn't have any FAA regulations around required brightness.

 

 

Edited by Steve W
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11 minutes ago, Steve W said:

My best guess is that Whelen didn't have a 28v version at the time, they do now, but it's a filmstrip projector bulb that they took a Dremel to. Alternately, they got the first 28v aircraft built, turned on the recognition lights, which promptly blew and realized they probably should fix that. The good news is that if there is a 1 for 1 LED replacement there should be no need to remove it. It won't drop to 14v any more with the lower current but most (decent) LEDs should handle the higher voltage fine. In other news my 1 for 1 LED replacement idea still hasn't gotten any farther along.

Another note for anyone if your bulb blows and you measure the voltage it will be 28V since your meter doesn't present enough of a load for the resistor to drop any voltage, you need to measure it with the bulb in the circuit and working to verify the voltage. I've heard stories about people measuring them, assuming the resistor was bad and buying the 28v version, not that it really matters as it doesn't have any FAA regulations around required brightness.

 

 

I haven't looked yet, do you know where this resistor is?  And on a related note, I have a spare PulseLite controller - is there any reason to ever have the wingtip lights in the on position?  I'm thinking just put the pulselite in with one switch that turns the recognition lite on in the pulse mode?

And while I am it, I don't like the Mooney avionics master relay setup.  Seems to be a single point failure.  What do you think of two 35A circuit breaker switches over on the CB panel for two separate avionics busses?

 

Aerodon

 

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17 minutes ago, Aerodon said:

I haven't looked yet, do you know where this resistor is?  And on a related note, I have a spare PulseLite controller - is there any reason to ever have the wingtip lights in the on position?  I'm thinking just put the pulselite in with one switch that turns the recognition lite on in the pulse mode?

And while I am it, I don't like the Mooney avionics master relay setup.  Seems to be a single point failure.  What do you think of two 35A circuit breaker switches over on the CB panel for two separate avionics busses?

The parts manual is unclear on the resistor location, as I recall on mine it's a few inches back riveted(or screwed) to the last metal rib on the wing, which makes sense as it's gonna get hot and you need it on a good heatsink.  

The only good reason to keep the recognition lights on steady is to make sure you melt the plastic covers if you land without turning them off...Ok, I'm a bit cynical there. Whoever added my pulse controller did have 2 switches one for steady and one for pulse, but I don't see much need for steady. 

The Mooney Avionics master is a 'fail safe-ish' design, if the relay fails it should fail in the on position. Personally I'd not bother with dual busses unless I was going to also go all the way and get a standby alternator added(now, if someone would get one approved that'd be great) and make a true emergency bus. With the hand-held GPS devices today along with the retrofit avionics having their own batteries I'm less worried than maybe I should be.

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5 minutes ago, carusoam said:

Steve add an update to what model you are flying... when able.

There are some Mooneys with two alternators flying around here...

Best regards,

-a-

It's gonna be a while before I can update what I'm flying, it's still in pieces on the floor of my hangar, along with the single alternator.

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