Tachometer in M20J Indicating Lower RPM Than Actual After Major Work

[MooneyMike280]

Hello All,

I recently had the engine, propeller and propeller governor overhauled in my 1984 M20J. Prior to this work, my tachometer (driven by the breaker points in the magneto?) was very close to actual engine RPM.

The maximum RPM indicated on the tachometer on the first flights indicated ~2450 RPM's. Today we used 2 different handheld tachometers and both read 2700 RPM - exactly where it needs to be. So the propeller / governor combination seem to be working correctly.

Now the question is why is the current tachometer reading off by ~ 250 RPM's, when prior to overhaul / replacements it was almost perfect?

Could there be something different in the magneto setup that drives the impulses for the tachometer? Would a shorted / loss wire or wires - looks like 2 wires from the dual magneto single drive unit goes back to the tachometer?

Any other thoughts on possible causes or methods of adjustment?

Thanks much...

Mike

[DVA]

Do you know what manufacturer and model is of the tachometer? Some have a pot in the back to make fine adjustments to compensate for all kinds of electrical variations.

[MooneyMike280]

Hello Dave,

I am not sure of the manufacturer, but it is the OEM equipment from 1984. I'll see if I can locate the manufacturer on the back of the unit - and also if there is an adjustment pot on the back (which would be great!!!).

Thanks much...

 

[MooneyMike280]

I was able to take a picture of the back of the tachometer - All I can see is the connector going into the back of the case, nothing else.

Any other thoughts or ideas?

Thanks...

Mike

[carusoam]

Have mechanic disconnect, clean and reconnect?

I would be thinking that the guys that took everything apart and put it all back together for you would like to make sure everything is working as you would expect. Simple as a warranty. It has to work at least on the day you pick it up....

What else could have possibly changed? The sensor/pick-up?  Is it the old one remounted or a wrong one installed?

PP ideas, not a mechanic...

Best regards,

-a-

[MooneyMike280]

We will be looking at this issue tomorrow. Will let you know what we find.

Thanks again all...

Regards,

Mike

[aaronk25]

These mechanical tachs are such junk they should have been banned years ago. I've replaced mine 3 times in 8 years due to the same issue your experiencing. The only think I use the tach for is the hour meter, that stays accurate regardless of needle error. When I installed the 830 I switch to using that tach for purpose of engine speed as it is accurate within 10rpm and derives its data from the ignition system not some stupid cable similar to moped speedometer.


Sent from my iPhone using Tapatalk

[mikefox]

We had to replace our tachometer a few years ago - it also started reading low.  Turned out to be a bad tach generator - the part that is driven off an accessory gear on the aft side of the engine case.  Unfortunately one cannot get a new or rebuilt tach generator.

Installed an EI tachometer which, if we didn't want to go for a full engine analyzer, was the easiest way to get back up into the air!

[Guest]

The tachometer is driven by wires connected to the ignition switch, not a cable.  You'll need to remove it to verify the part number.

Clarence 

[flyboy0681]

We still have the original factory tach and according to an optical tachometer that our A&P lent us, it's perfectly accurate after all these decades. 

[Hank]

2 hours ago, flyboy0681 said:

We still have the original factory tach and according to an optical tachometer that our A&P lent us, it's perfectly accurate after all these decades. 

My 1970 tach was just checked last  on the during annual while verifying dynamic balance. It's "off" by 22 RPM. 

[MooneyMike280]

Just installed a new Tachometer - the old Tachometer was the issue!

 

Mike