UMA Electrical Tachometer is Jerky

[jsharkey]

My UMA electric tach (~14 years old) has started to intermittently jerk around a given rpm. It will also revert to rock solid as before. Has anyone else seen this.

I removed the sender from the engine and there appears to be no mechanical wear. Also the only electrical connections are the multi-pin connectors and cable from the sender to the gauge and they seem solid. (I have had an electrical gauge - ammeter - problem before that turned out to be a loose screw on a ring connector.)

Jim Sharkey
RV-6

 

[D-Dubya]

Is this the kind that has mechanical flex cable that goes to an electronic sensor, or is the kind that has a proximity switch pick-up directly on the mag (or wherever it gets its signal)? If it's the former, it's kind of like the speedometer cables that cars used to have. I've lubricated those cables before to cure a jerky speedometer. Not sure if this is the same or not...

 

[Canadian_JOY]

I had one of those 3-wire sensors go bad in the fashion you've described. Started out as small, barely noticeable "bumps" in the needle while at cruise. Then it went to slightly wilder swings, maybe 50-100RPM. Then one day it went from cruise to zero RPM to cruise again... Changed the sensor and also changed its mounting location. Problem never came back.

When I say I changed the mounting location, I had previously mounted the sensor directly to the tach drive port on the engine accessory case. Now it's mounted at the end of a 12" drive cable with the body of the sensor attached to the firewall. It has been like that since 2008 and the tach is rock steady. This leads me to believe the first tach sensor died prematurely from being shaken to death while riding directly on the back of the engine.

 

[jsharkey]

......sounds like it

Thank you....

....yes the "sender" is mounted directly to the tach cable drive and hangs off the back of the engine, with a three wire cable running from it through the firewall to the tachometer.

It has been a great system - although the hours are hard to read - but it sounds like the sender is beginning its death throes.

Jim Sharkey

 

[rmarshall234]

I have the same sender on my biplane, NOS and recently installed but very little time on it. It’s really an electromechanical sending unit, I wonder if it is possible to remove it and get some grease in that driven end.? Might be worth calling UMA and ask them. Last time I called them - with an unrelated question - the owner of the company answered and it was on a weekend. Great cust Svcs.