What circuit breaker is the Hobbs meter generally connected to on DA40s?

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impromptu

What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by impromptu » Tue Sep 03, 2019 5:13 am

The Hobbs meter for a DA40 at my club has been perpetually erratic (they bill by tach time and they don't seem to care) but it got me wondering, how is the hobbs meter wired? I don't see anything on the circuit breaker panel that covers it.

Somebody mentioned that the Hobbs is often wired to the fuel pump breaker, is that true?

The only official mention I can find on the topic is this wiring diagram: download/file.php?id=1363

Not sure how to parse that, but it seems like the element labeled "F6" might be a fuse, i.e. the hobbs meter doesn't go through a circuit breaker but has its own fuse? (where is this fuse even located?)

Not sure if that's right — I didn't realize that there could be electrical components that don't go through any circuit breakers at all.
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by Steve » Tue Sep 03, 2019 12:44 pm

The Hobbs meter is supplied through a fuse located in the engine compartment (I suppose, to prevent renters from pulling a breaker and reducing their charged flight time). It is an unnecessarily complex circuit, including a tiny, secret, hidden relay, and the oil pressure switch.
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by BRS » Tue Sep 03, 2019 2:25 pm

Looking at this diagram and recalling a thread about the Hobbs running when the battery gets run down (pilot dome light left on). I'm wondering how that happens. Do I remember incorrectly?
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by impromptu » Tue Sep 03, 2019 2:37 pm

The Hobbs meter is supplied through a fuse located in the engine compartment
Do you know if that's the only thing that doesn't go through a circuit breaker? Seems against core design principles that you can't "turn off" an electrical component of the aircraft. (what do you do if the hobbs meter catches fire!)
Looking at this diagram and recalling a thread about the Hobbs running when the battery gets run down (pilot dome light left on).
I've heard that as well. Something about the relay being activated at low voltage?
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by Steve » Tue Sep 03, 2019 7:21 pm

My recollection is that the relay is always energized through the oil pressure switch (even with the Master off). Oil pressure opens the switch, de-energizing the relay, which closes the contact to run the Hobbs. A low battery (low enough to allow the relay to fall out) or bad connection to the oil pressure switch allows the Hobbs to run as well.

This wiring scheme was illustrated for me during my recent engine removal. I pulled the connector off the oil pressure switch before disconnecting the battery (by 15 minutes) and the Hobbs merrily added about 2 tenths of an hour...

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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by chili4way » Tue Sep 03, 2019 11:23 pm

The Hobbs meter works differently on the DA40NG (Austro diesel). It runs when the Engine Master switch is on, even if the engine is not running.
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by Rich » Wed Sep 04, 2019 2:30 am

There are 2 fuses of interest to this discussion

1. Hobbs/Pilot Map light. The latter is pretty much useless even when not burned out. Particularly true in this age of tablets. My bulb burned out years ago and I leave it that way lest it gets left on, running the battery down and letting the Hobbs run until the battery is toast.
2. Accessory socket.

I've removed the accessory socket and replaced with a Stratus USB powered off the avionics master, with its own breaker.

These are all in the engine compartment, inaccessible in flight except to true daredevils. :shock:
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by BRS » Wed Sep 04, 2019 4:28 am

Rich wrote:
Wed Sep 04, 2019 2:30 am
...
2. Accessory socket.
I've removed the accessory socket and replaced with a Stratus USB powered off the avionics master, with its own breaker.
I'd like to do this one day. I've a USB charger and Stratux that I plug into that socket. Have almost left it plugged in several times. Eventually I'll succeed and wish I hadn't. Just moving that to the main bus would help. Might do that.
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by impromptu » Wed Sep 04, 2019 4:29 pm

1. Hobbs/Pilot Map light. The latter is pretty much useless even when not burned out. Particularly true in this age of tablets. My bulb burned out years ago and I leave it that way lest it gets left on, running the battery down and letting the Hobbs run until the battery is toast.
Ah yes the pilot map light. I guess that's another item that's not wired through any circuit breaker, I wonder if there are more.
These are all in the engine compartment, inaccessible in flight except to true daredevils. :shock:
Where in the engine compartment? I had no idea there was anything other than pilot-accessible circuit breakers.
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Re: What circuit breaker is the Hobbs meter generally connected to on DA40s?

Post by Rich » Wed Sep 11, 2019 2:17 am

impromptu wrote:
Wed Sep 04, 2019 4:29 pm
Where in the engine compartment? I had no idea there was anything other than pilot-accessible circuit breakers.
Remove the top cowling and look on the right side against the firewall right at the top. There is a white cover over some relays and you'll see three black fuse fittings on top with labels marking each. They're the traditional cylindrical buss-type fuses which can be removed by twisting off the covers.
2002 DA40: MT, PF, 530W/430W, KAP140, ext. baggage, 1090 ES out, 2646 MTOW, 40gal.
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