Rolling Hobbs Meter

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Shodan01
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Rolling Hobbs Meter

Post by Shodan01 »

A few weeks ago, my battery died, although it was new 7 months ago. I hadn't flown in months and might have left something with a draw of power on in the cabin. During that period, my Hobbs meter rolled over 300 hours. Since I have flown weekly without issue, and it all seems to line up well.

This week my daughter went for a 1 hour flight and when she came in to log, the hobbs meter rolled another 30 hours.

Any ideas of what could cause this?
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Chris
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Re: Rolling Hobbs Meter

Post by Chris »

The most common cause for this is leaving the pilot's side overhead lamp on.
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Re: Rolling Hobbs Meter

Post by Shodan01 »

Thatll roll the hobbs?
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Re: Rolling Hobbs Meter

Post by Chris »

Not directly, but it will slowly drain the battery and once it gets below a certain voltage the hobbs will start rolling. See also this old thread, among others.

It could be something completely different in this case, of course.
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Re: Rolling Hobbs Meter

Post by Shodan01 »

I "THINK" I might have found it.

I have a USB adapter to charge my Sentry when I fly. I drove back and realized that it has a blue LED light on the Cig adapter, and that may be creating the same draw as the overhead light. Ill watch it!
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Re: Rolling Hobbs Meter

Post by Rich »

Shodan01 wrote: Fri May 07, 2021 1:57 am This week my daughter went for a 1 hour flight and when she came in to log, the hobbs meter rolled another 30 hours.
If I read this right, the plane had plenty of power to start the plane this time and yet the hobbs was running with the engine off. This would suggest an alternative explanation. And yes, there are other possibilities.

Diamond has implement the hobbs circuit in a goofy way. The oil pressure switch is normally closed, as is the contact in the relay to the hobbs. Normally, when there is no oil pressure the pressure switch contacts are closed, which grounds the one side of the relay "coil", opening the contacts and preventing the hobbs from running. (Reference wiring diagram DA4-9231-20-02 in the AMM.) The meter is spring-loaded to run in a number of ways.

Theoretical failures that could cause the hobbs to run inappropriately are numerous. Outright failure of the relay or connections to it would cause the hobbs to run constantly. You could go out there right now and watch it run. Intermittent discontinuity in either of the connections to the relay coil (trace pins 2 and 4 of the relay connector) would cause it to run unpredictably. The same is true for a hard or intermittent short to ground of the wire between the meter itself and the relay (wire marked ...A22)
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Re: Rolling Hobbs Meter

Post by Shodan01 »

Well - today was even more interesting.....

I stopped by last night and thought I had figured it out....and today when my daughter went to fly it had rolled another 0.4 hours...

BUT - it gets even more weird - post a 2 hour IFR X-country training flight with her instructor it only rolled 0.2 hours....

Very confused - sounds like a failing Hobbs????

Would love to get insight if anyone has experience with this.
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Re: Rolling Hobbs Meter

Post by Rich »

Shodan01 wrote: Sat May 08, 2021 1:18 am Well - today was even more interesting.....

I stopped by last night and thought I had figured it out....and today when my daughter went to fly it had rolled another 0.4 hours...

BUT - it gets even more weird - post a 2 hour IFR X-country training flight with her instructor it only rolled 0.2 hours....

Very confused - sounds like a failing Hobbs????

Would love to get insight if anyone has experience with this.
If the hobbs meter itself were failing it's unlikely to have racked up 300 extra hours. It would seem it's now running independently of engine operation - intermittently. Since triggering the meter is all done on the ground side, there are two paths to run down. The circuit (wires and connectors) from the negative side of the meter to the relay and the circuit from the pressure switch to the relay coil. Of course the relay and pressure switch are also under suspicion.
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Re: Rolling Hobbs Meter

Post by Steve »

There was a recent Service Bulletin to replace the pressure switch, but it was because of failures that led to oil leaks, not electrical issues. I did this on my airplane. Not saying that your pressure switch is the issue here, but if you do have to replace it, I would recommend using the upgraded part...
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Re: Rolling Hobbs Meter

Post by Rich »

The pressure switch that activates/inhibits the hour meter does double duty as the input to the "low oil pressure" annunciation light. So if that is working properly, the pressure switch is unlikely to be the problem.

Working properly means that upon turning on the master the low oil pressure lights up until after engine start. And it doesn't give random alerts while the engine is running.

This pressure switch is not the transducer that feeds the oil pressure signal to the engine monitor. But it is the switch Steve refers to as having a service bulletin for replacement.
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