Question for Paul McBride, the General Aviation News engines expert: I have owned, operated, and maintained my 1978 Piper PA32-300/IO-540K1G5 aircraft since August 1979 to the present. A tach was installed new in 1991 and operated flawlessly until its failure in January 2024.
The tach system consists of only three parts: The cable assembly, angle drive, and the tach itself. Previous requests for help from Piper Aircraft support were ignored. Therefore, I am hoping you can provide insight as to the cause and repair of this failure.
My troubleshoot included the removal of the tach inner cable for inspection, which showed no signs of breakage, wear or distortion. I then disassembled the angle drive (failing to check its rotation prior) and found no signs of gear wear or damage. I stupidly failed to check the tach at this time assuming it’s the problem. I ordered a new Mitchell Aircraft tach and installed.
Now the mystery starts — nothing works.
I returned to the assumed failed tach to check its operation with a drill motor and find there is nothing wrong. I now have two tachs that work perfectly when tested alone which are counter clockwise (CCW) driven.
Next step was to check each tach with an external test cable and drill motor turning CCW. Performance was good. Clockwise (CW) turning fails to operate the tachs. I then inserted the angle drive motor between each tach and external cable turning CCW and nothing works. I then changed the cable rotation to CW and all works. However, the engine wouldn’t turn the cable CW. This is the issue I’m requesting help.
Why for 46 years has this system been working with this original angle drive and now it doesn’t? Do you think the disassembly of the angle drive is the root cause or something else?
As a reminder, the Lycoming IO-540 tach drive (viewed from rear) rotates CW thus the cable’s aft end, which enters the angle drive rotates CCW, which transforms the cable rotation back to CW into the tach, which now fails to operate the tachometer. Possibly could you get your hands on a manufacturer’s drawing of the angle drive?
My future plan is to replace the cable assembly with new, however the angle drive situation needs to be resolved first.
James Ferrando
Paul’s Answer: James, I’d like you to first check to see if the tach shaft is turning in the engine accessory housing.
After you remove the angle drive from the accessory housing and can observe the tach shaft in the accessory housing, have someone turn the prop to see if it turns. If you see it turns, then it’s got to be a problem with the Piper-supplied angle drive adapter.
If the tach does not turn in the accessory housing, then you’ve got some more serious problems, which will require you to remove the engine accessory housing for further inspection.
I reached out to a former Piper Product Support Specialist seeking any information he may recall regarding the angle drive. Here’s what he said: “I seem to remember the adapter having a brass internal piece which had very small ball bearings surrounding it. I have to admit, I’ve not seen one disassembled.
“The adapter had a machined slot for a blade screwdriver (for disassembly). Notice the housing itself is cast aluminum. I don’t know if the roller bearings could have caused a wear situation that is the cause.
“James seems to have done all the right things. The only other thing would be the tach cable routing. As I recall there are a couple of directional changes in the routing, that if disturbed, could cause an issue.”
I have taken things apart for years. some time I can get them to work and some times not. . Over the years I have found out I should never have work on them. I always have learned things. But keeping things simple is always the best. Knowledge is learned by trying. Don’t give up. Keep studying. When it dose not work find someone to get advice. So as for planes make sure you are right before testing it.
I remember reading Continental allowed 10% error for Electric tach readings in dealing with RPM overrun cases.
The story is the most basic mechanical diagnostic and should be left to someone not confused by it. Same problem with my ’86 International dump truck. Took about 10 minutes to find out it was the angle drive and 20 minutes to repair it.
While I applaud the troubleshooting and patients James has shown, I will add that electric tachs do away with all of the nuisances of the mechanical tach plus they are far more accurate and reliable. I haven’t had a mechanical tach in a very long time.