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Topic Options
#17839 - 01/05/11 10:39 AM Hobbs Meter Pressure Switch
mames Offline
New Member

Registered: 12/31/10
Posts: 16
Loc: OK
Just wanted folks to be aware of what recently happened to me and my 1969 172 Lycoming O-320. I experienced a failure of the Hobbs meter pressure switch located at the front top of my O-320. Oil poured out of it and luckily I was on the ground and became aware of the problem before takeoff. There’s not much of a sicker feeling than to see oil coming out of the bottom of the cowling. Luckily, I just had to remove the part, plug it, and add one quart of oil. I got by lucky, but it could have been a lot worse. It's worth keeping an eye on this part, although not being a mechanic I'm not sure what can be done other than regularly checking for any leakage. Of course, one might evaluate whether they even need a Hobbs. When mine failed oil poured out. Also, the part is just above the large hose that takes air to the muffler shroud for cabin heat. I noticed oil also went into this hose and muffler shroud, not a lot but I still removed and cleaned it out. Just wanted folks to be aware of this potential issue.

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#17851 - 01/05/11 12:11 PM Re: Hobbs Meter Pressure Switch [Re: mames]
vettdvr Offline
Club Sponsor
Diamond Pilot

Registered: 06/24/10
Posts: 7580
Loc: Slidell La
I totally agree. The oil sending unit looks like a car application and if the gasket fails on a car it just leaks but IFR, VFR or just wheels off the ground makes it a really bad thing.
_________________________
Vettdvr

Single/Multi/instrument/type/commercial But then I am still learning.

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#17867 - 01/05/11 03:44 PM Re: Hobbs Meter Pressure Switch [Re: vettdvr]
EdW Offline
Club Sponsor
Gold Pilot

Registered: 11/21/10
Posts: 1413
Loc: Just east of Graceland
I recently had to rebuild an O-360 for the same reason. The engine was still running when he passed my hangar, but sounded like a John Deere tractor. After having to have the case and accessory case overhauled by DivCo, he wound up with two connecting rods, the hydraulic pushrod units, and the tubing/accessories off the original engine. That $25 item wound up costing him a bundle!


Edited by EdW (01/05/11 03:45 PM)

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#17878 - 01/05/11 06:14 PM Re: Hobbs Meter Pressure Switch [Re: EdW]
steve71e Offline
Second in Command

Registered: 02/22/10
Posts: 172
Loc: Atlanta, Ga
The fitting on the engine case suppling that line for the pressure switch or even the actual direct reading oil pressure gage should be a "restricted orifice fitting" which means if you do have a switch, hose or gage failure it will make a mess but greatly reduse the amount of oil flowing out of the leak and give you time to get down before you get below the minimum allowable oil amount in the sump (2.75 I believe on an O-320). The fitting should have only a tiny, tiny hole allowing oil to pass through.
_________________________

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#17880 - 01/05/11 06:23 PM Re: Hobbs Meter Pressure Switch [Re: steve71e]
Rusty Rudder Offline
Diamond Pilot

Registered: 02/26/10
Posts: 6559
Loc: NorthEast
On the 1982 P model, that hobbs sending unit is up under the Co-pilots dash panel, and when it leaks the oil goes onto the floor mat on the Co-pilots side. Very noticeable right away...
_________________________
Take-off's are optional, landings are manditory



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#276214 - 01/12/16 03:44 PM Re: Hobbs Meter Pressure Switch [Re: mames]
GarySD Offline
New Member

Registered: 06/14/14
Posts: 2
Loc: CA
Hi:
1978 C172N owner. The fuse to the Hobbs + clock keeps failing. We've disconnected both and it still blows. Anybody know where the pressure switch is located? My suspicion is that the oil pressure switch to the Hobbs has a short to ground. Does anybody know where the pressure switch is? 2 posts say: G model is on the engine and P model is on the co-pilot side of the instrument panel. My model is in between these 2.
Blue Skies,
Gary

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#276220 - 01/12/16 04:55 PM Re: Hobbs Meter Pressure Switch [Re: GarySD]
magman Offline
Silver Pilot

Registered: 10/10/12
Posts: 857
Loc: buffalo NY
Many Pressure Switches are not really designed to be

mounted on a HOT & Vibrating engine.

Engine mounted switches have a no-no & were failing

for over 40 years.

If not for hire there is no reason for it.

Not quite as "accurate" is to run off Master only

or

Airspeed Switch

or

Recording Electronic Tach

or

Use a watch & pencil to note times.


The Press Switches I've seen on N's were under panel

on the back side of Firewall.


Edited by magman (01/12/16 04:59 PM)

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#276281 - 01/12/16 10:17 PM Re: Hobbs Meter Pressure Switch [Re: GarySD]
AusTex Aviator Offline
Safety Pilot

Registered: 05/25/15
Posts: 52
Loc: TX
I have a 172N. My pressure switch is mounted on the co-pilot side on the firewall inside the cockpit. You can follow the oil lines.
_________________________
1979 172N
KRYW
Check out my Youtube channel at AusTexAviator

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#276387 - 01/13/16 05:08 PM Re: Hobbs Meter Pressure Switch [Re: AusTex Aviator]
PPMD Offline
New Member

Registered: 11/03/12
Posts: 5
Loc: KY
I also have 1978 172N. Pressure switch location as above post noted. One of the two wires is hot all the time unless fuse is blown. Remove fuse on firewall before removing or it may spark.

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#277070 - 01/17/16 11:02 PM Re: Hobbs Meter Pressure Switch [Re: PPMD]
GarySD Offline
New Member

Registered: 06/14/14
Posts: 2
Loc: CA
Thanks to all for the advice. I located the pressure switch as described.
Gary

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