Van's tachometer problem

117 hrs TT and I have developed a tachometer problem I hope someone else may have had and can help me solve.

RV-9A
O-320-D1A Lycoming (Areosport)
carburated
fixed pitch wood prop

Vans 2 1/4" tachometer gauge along with van's transducer arangement (IE VTACHGEN 2) Short one, no vacume pump extension.

Problem: Tach started showing 500-600 rpm's low on a return flight last week. After investigating and tightning the coupling arrangement that afixes the transducer to the accessory case it seemd to be ok for a short test trip around the patch. Today after start-up it seemed to be working just fine untill taxiing towards the runway it just quit and went to zero. Back to the hanger. Checked all connections at the back of the gauge and all was well. Checked all the wire connections at the IE VTACHGEN 2 pigtail, all was well. So I pulled the transducer from the engine and checked the shaft which seemed to be just fine when I notice a seal or o ring (not sure what it was) inside the coupling arrangement is damaged and in two pieces, probably from me overtightening.
I put it back together as best I could for a test run. On start up and idle it seems to be fine but once it gets to 1300 rpm it just returns to zero and will not move no matter what the engine RPM. Shut down and re-start the engine the same thing happens over and over.
Digging into the Lycoming parts book it shows a "seal, Tachometer drive, oil" part #61165 at this or similar location, but it is pretty vague.

My question is do I have a bad transducer or a bad seal of some sort. If its just a seal where can a guy get one. Was this seal or O-Ring part of the coupling arrangement that came with the transducer from vans or was it this Lycoming "seal" off the accessory case that just ended up inside the coupling arrangement after it got squished.

thanks
doug

Doug,

I had a problem using Van's tach sensor attached directly to the engine: The drive pin appears to vary in length, at least on the sensor's I've had (3 of them). I put one on my RV-10 and it worked about 20 hrs. and then died. When I turned the drive pin by hand, it seemed to turn and then jam. I cut open the sensor and found the drive wheel was broken in several pieces. I could see where the drive pin had ground the wheel into the back of the case because it was too long. I attached another tach sensor to the engine without the drive pin and measured how long the pin should be. I then measure the drive pin and it was 1/8" too long, so I ground it down to the proper length. It has been working fine since then.

Kevin Belue
RV-6A flying
RV-10 flying

Mornin' guys.....

....I had basically the same thing happen on a short cross-country. The tach needle jumped left/right and then showed for a few seconds then bam....zero.
I have the standoff cable from Van's driving the tach generator and both ends of the drive cable look good, so am I to assume it's the tach generator and not the tach? How can I check it?

Thanks,

Kevin
You are the man. I orderd a new tach generator along with an extra pin and they came in today. Between the three pins they veried in length from 2 5/8" - 2 7/8" the exsisting one in the old generator that broke was the shortest of the three. I took the old generator apart and it was obvious the pin was pushing the wheel against the back of the case, it had almost worn clear through. It had even caused the back of the case to bulge out and the resistance had caused the inner shaft to seperate and spin freely inside the wheel. I calculated the longest the pin should be to not force the wheel against the back of the case is about 2 1/2". I shortend the closest one down to there...we shall see how long it lasts. If I had used any of the new ones without heeding your advice I'm sure I would have had failure of the new generator in very short order. thanks!

Pierre
Put a small straight edge on the back of the generator and see if it is still flat. The new one was flat and the old one had a high spot in the middle from the pin/wheel riding against it.

FYI the O-ring or seal I had question about in the original post turns out to be a flat rubber washer (kinda like in a garden hose) that Van's provides with the 2" coupling arangement. Could probably get one at an Automotive store.

doug

unreliable tach reading

Forty hours on new prop and engine but the tach reads about three to four hundred rpm low--at full throttle about 2300 rpm while seemingly okay at idle speeds. Anyone with similar issues know where the problem lies? RV-8, O-360

Terry Brennan said:

Forty hours on new prop and engine but the tach reads about three to four hundred rpm low--at full throttle about 2300 rpm while seemingly okay at idle speeds. Anyone with similar issues know where the problem lies? RV-8, O-360

Click to expand...

I don't know where the problem lies, but I have experienced it. Was running about 300 rpm low. Tried everything that has already been suggested to no avail. Fortunately, was planning a panel upgrade anyway and switch to G3X with engine monitoring solved my issue.

unreliable tach readings

Thanks for the reply. Short of installing a G3X or similar, your experience tells me that it could be the tach itself, if you used the original transducer. No?

tachometer ops

Took an electronic rpm measuring unit into the cockpit, mounted on top of the panel, ran the engine up. Vans tach and electronic tach agreed with each other up to about 2500 rpm when the vans tach started receding as the engine rpms increased to 2700. Score: electronics 2700 Vans 2350 or so at full throttle. Anyone have an idea what gives?