Rolling Hobbs Meter

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ahazi
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First Name: Ariel
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Re: Rolling Hobbs Meter

Post by ahazi »

Here is my Rolling Hobbs story...

Picked up my 2005 DA40 G1000 from an extensive and expensive annual and noticed a slight noise, I thought it was the wind moving some papers... It turned out that w/o any power and even w/o any circuit breakers engaged the Hobbs was ticking. It was a weekend, the shop was closed so I figured I would just fly back from KLGB home to KSMO and would figure out what was going on.

I read everything here on this board that I could find about the "Rolling Hobbs", looked at the diagram that had a relay in it and expected the worst. After a few days of accumulating Hobbs hours I decided that I was ready to go and tackle the issue. I was armed with tools, schematic and a determination to fix it without going back to the shop.

I removed the airplane cover, opened the canopy and was happy to find that the (brand new) battery was still alive. I removed the cowling and in about 20 seconds I found the problem. See picture below, can you find it?

It turned out that the normally open (N.O.) oil pressure switch that activates the Hobbs while the engine is running was shorted to ground. Plain and simple, the screw that holds the wire was touching the bolt because a technician was tightening the oil pressure switch until it stopped. A slight release with a 1/2" open wrench took care of it.

My conclusion is that annual inspections many times produce new problems that were not there before. The schematic that is posted on this site for the Hobbs circuit is probably a historic one that was used with a N.C. oil pressure switch, the 2005 model oil switch is connected in series with the Hobbs.

I hope this will help someone if the same problem shows up.

Ariel
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Steve
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Re: Rolling Hobbs Meter

Post by Steve »

Another interesting feature (aside from the circuitry change from NC to NO) is the location of your sensor. It appears to be on a port just in front of the oil filter adapter. Mine is on the right top of the engine, just behind the right upper cylinder baffle. Below is a photo of the location taken when I replaced the sensor with the "new and improved" component:
Upgraded oil pressure switch - RSB 40-089, rev. 1
Upgraded oil pressure switch - RSB 40-089, rev. 1
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