banner graphic
Airport Webcams
Who's Online
23 registered (Kansas Zephyr, harley6133, Showboatsix, Ross Muir, flyer921, cyendrey, apenney, rans7, turninggrey, nath, Flyer770, Pilot110, 0iceman9, Redbeard, Pilawt, Red.00, Jdragon, BettyWhite, Macsym, michaelj, Kinhop, Kentucky Captain, Dave W), 297 Guests and 0 Spiders online.
Key: Admin, Global Mod, Mod
Newest Members
ronnietang, Russ S., AdnanCessna, skintback, echobase
9132 Registered Users
Top Posters
Bargain Bob 162820
vettdvr 7710
Rusty Rudder 6725
Cessna Dude 6628
California Flyer 5568
Willie 4580
Joeman434 3869
Clyde Cessna 3827
Don Tedrow 3197
Peedie Montgomery 2963
wrecksum 2704
Ward Holbrook 2436
XP Driver 2395
November X-ray 2230
ytodd 1960
oilwell1415 1906
Jim_1 1883
Nightowl 1826
Nintendo Pilot 1730
Bigdoggh 1710
Glenn Darr 1708
Pilawt 1667
RodneyHooverCFI 1645
KevinMcP 1637
EdW 1435
Pilot110 1405
Showboatsix 1336
Shuswap 1304
JD Casteel 1228
Henry M. 1213
Top Posters (30 Days)
Bargain Bob 2518
Joeman434 59
Kentucky Captain 48
Willie 44
Rusty Rudder 42
Viper_96 38
Don Tedrow 34
Skyhawk28F 33
Pilot110 24
vettdvr 22
cyendrey 20
N5479R 19
Peedie Montgomery 19
California Flyer 19
Clyde Cessna 18
Newpilotoldplane 16
turninggrey 15
Showboatsix 15
DeputydogK9 14
Nothwoods 13
cadcap 12
Kejidog 12
Kansas Zephyr 10
Kinhop 10
windsock 10
phillipny 9
Curious1 9
Mark Y. 9
JD Casteel 9
jpoppinmoneyunit 8
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.

#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: 7710
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: 1435
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)
OK, I got my drone license...when does the season open, and what's the bag limit?

#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: 6725
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

#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
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,

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

Registered: 10/10/12
Posts: 890
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


Airspeed Switch


Recording Electronic Tach


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)

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

Registered: 05/25/15
Posts: 53
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
Check out my Youtube channel at AusTexAviator

#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.

#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.


Cessna172 Tribute
Special thanks to:
Thank you for making this site great
Fredericksburg TX '15 Countdown
September 25-27, 2015 T82
Today's Birthdays
No Birthdays
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