Hobbs Meter not Running

After having some work done on my panel at an avionics shop, I noticed my Hobbs meter was no longer running. I checked the wiring this morning with a multimeter, and I am getting only 10 volts to the power side of the Hobbs meter when the master switch is turned on. I’m assuming the low voltage is the problem. Any thoughts on why I’m getting only 10? The panel is powering up fine, and the EFIS shows 12.4 volts battery power.

There are two wires to the Hobbs, right ?
A red and a black wire on the hobbs? Black is usually GND.
The first thing I would do is to find out which wire goes to GND.
The wire not going to GND should be powered from the 12 V bus.
Some builders have installed an oil pressure switch that sends 12 V
to the hobbs when it sees oil pressure.

Good luck

I had a Hobbs meter fail at 2,678.4 but I would suspect the oil pressure switch. A Hobbs meter for a boat is very cheap. You might want to buy one to troubleshoot.

Thanks guys. It was the oil pressure switch that was bad.

Besides an oil pressure switch, what other sources are good choices?

Webb said:

Besides an oil pressure switch, what other sources are good choices?

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I'm not sure. I originally thought I was checking the power side of the Hobbs, but it was the ground side. The oil pressure switch is connected to ground. When I re-checked the wires, everything was as it should be: 12.8 volts to the Hobbs and a good ground connection from the Hobbs to the switch. So, the switch appears to be the only thing it could be.

I was thinking of something like the power to one p-mag instead of using oil pressure. I’m going to put a Hobbs in but want to avoid an oil pressure switch so I’m asking the question.

Webb said:

I was thinking of something like the power to one p-mag instead of using oil pressure. I’m going to put a Hobbs in but want to avoid an oil pressure switch so I’m asking the question.

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If your p-mags are wired like most are, they have power to them any time the master is on.

Cross that thought off. Next?

I don't have a different suggestion re where to power it from, but just in case somebody else is reading this thread in the future for troubleshooting advice;

The "standard" way to wire a hobbs meter is upstream of the oil pressure switch. The hot wire starts at the buss, then goes to the hobbs with a nice short run, then out through the firewall to the oil pressure switch and then to ground.

The other way (buss - out to oil pressure -back in to hobbs - ground) takes about twice as much wire to get the same result.

Looks like the OP already solved his issue, but if it's wired like this, they simplest way to trouble shoot is to bypass the oil pressure switch- just jumper the "in" wire to the oil pressure switch to ground. If the hobbs starts working then the switch is bad. if it doesn't, then the hobbs or wiring is bad.

Hopefully it's obvious that if its' wired "wrong", with power going straight from the buss into the oil pressure switch, this is going to have a bad result, so make sure of your wiring architecture first.

As a follow up on this, what do I use to seal the threads on the new oil pressure switch? Is automotive permatex type sealant ok?

Thread sealer

Loctite 565 or 567, applied to the threads on the switch, but not on the first two threads that should be left clean.

PaulvS said:

Loctite 565 or 567, applied to the threads on the switch, but not on the first two threads that should be left clean.

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Thanks, Paul.

Some rental aircraft were wired from the battery bus to the hobbs and then the oil pressure switch. Seems some intrepid renters would turn off the master while flying to save a few dollars when it was wired off of the master bus. So if the engine was running the hobbs would run off of the oil pressure switch.

So many ways to wire a hobbs...