Hobbs wiring

[Bud Davidson]

For the Rotax 912ul, I suspect many of you use the Hobbs meter to keep track of engine run time. Is there a way to wire it to actually track engine run? I initially thought I would just hook it up to the master switch. But in working on the panel gauge installations, I am getting time on the Hobbs even when not running the engine. Perhaps after the aircraft is operable this will not be an issue. For now I have disconnected the Hobbs. But, could it be wired to just monitor electrical output when the engine is running? Anyone confront this?
Bud

[PapuaPilot]

Hobbs meters are usually activated by an oil pressure sensing switch. Typically they close the circuit at 4-10 PSI. Check out the link below.

http://www.aircraftspruce.com/catalog/inpages/hobbsswitch.php?clickkey=3011215

[mr bill]

A better way, is to add a N.O. pressure switch that is on the oil pressure line. When the engine starts and you get oil pressure, the switch closes and power is applied to the Hobbs meter. On my firewall is mounted a tee, one end is the oil pressure sender and another end is the N.O. switch and the third end is the oil pressure line from the engine. This is how it is done by Piper and Cessna.

[Dave S]

Bud,

If you happen to have the 2 1/4" UMA tachometer designed for the Rotax 912, I believe there is a hobbs output terminal on the back of the tach which records on the hobbs only when the engine is running. I used the UMA tach and honeywell quartz hobbs this way on our kitfox.

Added a photo to show what the UMA tach looks like - hobbs below the tach.

[Bud Davidson]

Dave S,
My tach is identical to what your panel shows. But, there are only three pins, positive, negative and from the engine. I looked at a wiring depiction on internet that showed a number 4 position in the exact center of the back of the tach but examination of my tach shows no attachment there. Was too good to work out. Not sure I want to mess with the oil pressure lines but those suggestions make good sense. Not sure what to do.
Bid

[avidflyer]

There are hour meters that run off of vibration also. Here is one, I'm sure there are others. JImChuk
http://www.partdeal.com/enm-vibration-hourmeter-t56c1.html?zmam=74973193&zmas=1&zmac=4&zmap=77196377

[Esser]

Could you wire it to something you would seldom have on unless the engine was running like comms etc.? Vibration one sounds interesting

[Flybyjim]

Has anyone used the vibration tack, looks very interesting?

[Dick B in KY]

Dave S's suggestion is correct if using the UMA tach. It also records one for one if running at approx. 5500rpm, lower rpm - lower hobbs time.

Dick B