The Cessna 120-140 Association Forum

My old tach has about 1100 hrs on it. Lately it is recording about 20% of Hobbs time, but RPM seems OK. The Hobbes seems right. My A & P will pull the tach this week and get an order out most likely. I think Aircraft Spruce may have the correct recommendation, but he suggested I request advice in this forum out of respect for the advice available here.

Is this a reasonable choice?

Aircraft Spruce and Supply, D1-112-5025, 2300 RPM recording point, and internal markings.

Thanks in advance....

Hobbs read clock time. Tacs read time based on RPM. Cruise RPM should be an hour for an hour. What you are seeing is typical!

My Hobbs time seems to match my actual clock time. I record my Hobbs and Tach times for each flight. Going back about 2 years the Tach time was between 80% and 90% of Hobbes time on most flights. In the last 6 month or more Tach time is about 25% of Hobbes time, even though I fly the same cruise rpm of about 2300. Ex. December 11, (last flight) my Hobbs showed 0.9 Hours while my Tach showed 0.1 hours. Other flights are not quite so far apart. Is it possible that I was cruising at 2,250 rpm and recording nothing because of a 2,300 rpm recording threshold. On August 23 last year I flew .6 hours and recorded 0.0 on the Hobbs. It seems the Tach time is just diverging more.

I always thought if I was "in the green" it was recording, and not recording at idle, or taxi rpms. Maybe I have met the enemy and it is me?

I think if you decide on a new one the p/n is " D1-112-5023 " for 2300 rpm (1 for 1). I've had mixed results with tachs and overhaul shops after swapping about 3-4 of them from ebay, cores, etc. Right now I have a overhauled tach that jumps back and forth 100 rpm on the top end and I've given up as long as the recording numbers stay consistent ( about 1 to 2 tenths lower then hobbs for a 2 hr flight) I would go new if I had to replace next time.

It’ll cost you a little more, but I ordered a UMA electric/analog tach and really love it. It mounts a sensor to the tach drive and runs only wires, not a tach cable into the firewall. Then you have a standard analog tach in the panel. I chose this route because I don’t like digital tachs but my drive seal kept failing and filling my tach with oil.

4203 wrote: ↑Sun Mar 14, 2021 2:48 pmI always thought if I was "in the green" it was recording, and not recording at idle, or taxi rpms. Maybe I have met the enemy and it is me?

No, it is always (supposed to be) recording, even at idle. It is mechanically identical to an old mechanical speedometer/odometer. The faster you go, the faster the miles (time) go by. If it's a 2300 RPM tach it should record one hour for each hour you spend at 2300 RPM. If you reduce power to 1150 RPM (half of 2300) it should record 0.5 hours for every hour.

Something inside of yours must be slipping.

D1-112-5025 is the appropriate tach. Are they going to mark it for you? Mitchel will mark the tach free if you order it that way. I don't know if Spruce ordered them premarked, or if they special order each one. Make sure they know where the red line belongs!

Might be a good idea to replace the tach drive cable and the oil seal?