Faulty Rpm Indicator
 

35 HOURS FLYING AFTER INSTALLATION OF BOTH OVERHAULED ENGINES, THE RPM NEEDLES STARTED TO MOVE UP AND DOWN A LITTLE WHILE ON CRUISE. AFTER THAT, WHEN I FIRE UP ONE ENGINE, IT SEEMS TO INDICATE THE CORRECT RPM NUMBER, BUT AFTER I START THE OTHER ENGINE THE NEEDLES START TO COME UP AND DOWN WILDLY ( IT HAPPENS WITH EITHER ENGINE ).THE FREQUENCY OF THE OSCILATION OF THE NEEDLES DEPENDS IF THE ENGINES ARE AT THE SAME OR CLOSE RPM WITH EACH OTHER; LOWER THE FRECUENCY AS CLOSER THE RPM ARE BETWEEN THEM.
THE INSTRUMENT SHOP HAS TRIED OUT EVERYTHING, THEY SAID THEIR PARTS SUPPLIER NO LONGER CAN SHIP THE PARTS NEEDED, AND I HAVE TO SEND THE INDICATOR TO U.S. ( I WILL TRY OTHER SHOP)
ANY SUGGESTION OR APPROACH TO SOLVE THIS? HOW MUCH WOULD BE THE OVERHAUL COST, AND RETURN TIME? WHAT ABOUT DIGITAL INDICATORS INSTEAD?

THANKS FOR ANY HELP,

JOSE L. ICHASO
1969 337D,NORMAL ASPIRATED.

Jose:

Before you send out your instruments, I would check the wiring coming from the tach output on the mag, going to the tach. I have had a similar problem, right after maintenance. The pigtail coming out of the tach has a "quick disconnect" type of spade lug. On my aircraft, one of them was worn and loose, so that vibration (or big mechanics hands) could make it have intermittant contact. On the other engine, I had a broken wire inside the insulation, so it would work intermittantly. Also check inside the mag, where the tach contacts are (they are just a set of points, I think) and make sure they are clean.

Hope this helps, since it is a cheap fix!!

Mark

I second what Mark has said. Those wires get a lot of wear, and it doesn't take much for them to start giving erratic readings. Checking the wires all the way from the magneto through the firewall can save you a lot of money. Mine was worn at one of the connectors or at the mag (can't remember which it was now), and a simple repair by the mechanics made a great deal of difference. As he said, much cheaper than messing around with the tach as an instrument.

And to check, you can always swith the leads feeding the tachs, and see if it still bounces on the other one; this is a temporary way to see if the instrument is OK. If it looks the same on the ohter tach which read fine before you swithced, then you know it's not the instrument.

Jose,

Like Mark and Paul have stated, your problem isn't the indicators... it's the wiring at the pigtail connections near the magnetos.

Gee, isn't it nice that your local shop wants to quickly send your gages off to the U.S. for overhaul, or whatever, at tremendous expense, when all someone needs to do is check the wiring connections? What a bunch of nice people. YUCK!

We had the very same problem you described on the rear engine RPM indicator, and after pulling the rear hatch found the in-line spade connector was very loose and dirty. Cut the old one out, crimped and soldered a new connector in and then laced the wiring so as to provide for movement and everything has been just fine ever since.

Go check the wiring FIRST thing.

SkyKing

Indicator is indicating steady at cruise, intermitent while taxiing, after repair or change one terminal in the "positive" wire to the instrument. My A&P will continue working to resolve it.
Thanks a lot again for your posts!

Intermittant while taxiing is an especially good indication that you have a wiring problem. Much more irregular period vibration on the ground.

Kevin