banner graphic
Airport Webcams
Who's Online
6 registered (Wayne R, Joeman434, flynal, CJpilot316, Ghost, Ross Muir), 146 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Newest Members
Mickeys, MAM, ScottTomlinson, Nancy Driver, Goody
8044 Registered Users
Top Posters
Bargain Bob 135605
vettdvr 7206
Cessna Dude 6628
Rusty Rudder 6066
California Flyer 5268
Willie 4181
Clyde Cessna 3504
Joeman434 2900
Don Tedrow 2742
wrecksum 2704
Peedie Montgomery 2556
Ward Holbrook 2398
XP Driver 2382
oilwell1415 1906
ytodd 1903
November X-ray 1842
Nightowl 1826
Nintendo Pilot 1722
Glenn Darr 1708
RodneyHooverCFI 1645
Jim_1 1639
Fred R Ali 1633
Bigdoggh 1589
Pilawt 1576
EdW 1375
Shuswap 1247
Pilot110 1198
Showboatsix 1173
Propduster 1105
Henry M. 1062
Top Posters (30 Days)
Bargain Bob 2351
Joeman434 105
Peedie Montgomery 86
November X-ray 78
Willie 65
Don Tedrow 57
vettdvr 57
Skyblazer 56
California Flyer 52
Rusty Rudder 48
N5479R 46
Flubber4.0 45
Ward Holbrook 44
Kinhop 32
BettyWhite 30
Luvrv8 28
Mike492 27
Rjusten 23
cadcap 21
magman 21
Flyboy712 19
JD Casteel 18
Newpilotoldplane 17
combahee 17
XP Driver 16
Clyde Cessna 15
Jim_1 14
Wayne R 13
Renegade 9 13
Topic Options
#17839 - 01/05/11 10:39 AM Hobbs Meter Pressure Switch
mames Offline
New Member

Registered: 12/31/10
Posts: 11
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.

#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: 7206
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.

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

#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: 1375
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)

#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.

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

Registered: 02/26/10
Posts: 6066
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


Cessna172 Tribute
Special thanks to:
Thank you for making this site great
Fredericksburg TX '15 Countdown
September 25-27, 2015 T82
Today's Birthdays
Dav, diego72, eqfan592
Member Map
Where in the world are the Cessna 172 Club Members?

View the map

Helpful Links
100LL Price Finder
AD Directory
Aging GA Aircraft
Aircraft Directory
Airplane Report
Airport Facilities Directory
Aviation Weather
C172 History
Crash Records
DUAT Voyager Planner
Flight Aware
Flying Tools (Files)
Google Earth
Ground Speed Records
National METAR Map
Sky Vector
Uvalde Flight Ctr
All the Webcams
Shout Box

Weather Lookup
Airport code:
(separate multiple codes with spaces)

List of stations