Cessna 172 fuel sender testing… ohm reading?

S

Skip

Pre-takeoff checklist
Joined
Nov 11, 2018
Messages
103
Display Name
Display name:
Monte92
My IA is allowing me to learn a little under his supervision. My 63’ 172 gauges read, but they aren’t correct I know. I’d like to ohm out the senders. In my experience restoring muscle cars… it’s 90% almost always the sender. Rarely the gauge. But I’d like to find out. I’ve looked in the manuals and don’t see an ohm value scale to check them. Anybody have experience with this ? Thanks.
 
Are they Stewart-Warner or Rochester? The original Stewart-Warner senders are ~30 ohms full and ~250 ohms empty. Rochester I can't recall off the top of my head, but unless your 172 had the retrofit (changing the senders and gauges) you probably still have the SW senders.
 
Skip said:
don’t see an ohm value scale to check them
Click to expand...
Some manuals have the values and some don't. But if I recall the 12v system values are 30 - 250 ohms with 30 on the max side regardless of brand.

FYI: as mentioned above, if you do determine the transmitter to be the fault be sure to verify the indicator type as there were several bulletins that swapped Stewart Warner for Rochester components and any mismatch will give you inaccurate readings.
 
I think I measured the SW sender to run between 30 and about 160 ohms. I think. Long time ago. Mostly, those senders wear out and there's little use fooling with them. The float moves the arm which moves a brass arm inside the sender that has a tiny carbon button that runs over a coil of resistance wire, varying the current flow to the gauge. The button wears out, then the brass starts wearing into the coil, which eventually breaks and makes the gauge do stupid stuff. The sender case is steel, and it corrodes with the smallest bit of humidity in the tank. That corrosion hurts the ground path of the brass runner. On the outside, the sender is grounded to the wing frame with a light aluminum strap that oxidizes and can get damaged a bit every time anyone works on the sender. In old airplanes, this all adds up.

The wear in the sender happens constantly if the airplane is tied down outside; the wind moves the airplane a bit, making the fuel in the tanks slosh just a bit, and the float arm moves just a bit and the wear continues. This happens to control surfaces, too, and all their cables and pulleys and fairleads and hinges and rudder bars and control yoke structures. Some low-time old airplanes are really worn out. I've seen it.
 
Got this same problem with my Cherokee. They read, but I don't trust them. My problem is if it is the senders, which it probably is. The leading edge cuffs from the stol kit make fuel tank removal suck that much more.
 
Just put a new sender on MacFarlane do them for 300 dollars more than an equivalent car part.

The sender's can often be 50 years old and on its last legs.

Just replace it and if it doesn't work then start troubleshooting.
 
Last edited:
Jim Carpenter said:
Or, if you get to the point of needing replacements, check out CiES fuel senders.
Home - CiES Inc. (ciescorp.net)
Click to expand...
Anything that gets rid of a potentiometer or rheostat is a good thing, and these do that. Pots and rheostats wear and start giving erroneous readings. We see that in flight simulators and in cars that use electronic throttle controls, taking the signal off potentiometers on the throttle pedal. Devices that use magnetic (Hall-effect) or inductive inputs are far more reliable.
 
Will these sensors work with my existing fuel gauges?

While CiES designed the sender to be able to output to a resistance input analog fuel gauge, there are several issues that make this interface less than ideal.

First - an analog gauge has its own characteristic and that has to be matched to the tank characteristic. This would be, at present, a remove, reprogram and replace proposition until the gauge and tank content correlate.

Second - Analog gauging for fuel applied to aircraft utilized gauge interfaces that were not designed to be accurate. We see gauge hysteresis and variations that make accurate fuel reporting nearly impossible. CiES is committed to accurate fuel gauging in aircraft, and these interfaces do not allow that to easily occur.

Note: if the original aircraft gauge is the only interface that will work for your aircraft, CiES is working on two potential solutions. One will require rebuilt or qualified aircraft gauging, the other replaces the existing pointer driver with a stepper motor.
 
So after finally getting around to it… I topped off both tanks. Both gauges read full. About an hour into the flight and the left tank seems to read say, 3/4 or so. The right tank is reading full. After landing, it seems like the right tank reads full or at least way more full than the left. Can the sender read high if the float IS NOT stuck ? I’m familiar with the float, the wire wrapped making resistance… but that’s about the extent of my knowledge of how the actual sender works. I’m just trying to do some research before my AP just starts throwing $1000 in senders at it! Haha. Thanks.
 
Skip said:
So after finally getting around to it… I topped off both tanks. Both gauges read full. About an hour into the flight and the left tank seems to read say, 3/4 or so. The right tank is reading full. After landing, it seems like the right tank reads full or at least way more full than the left. Can the sender read high if the float IS NOT stuck ? I’m familiar with the float, the wire wrapped making resistance… but that’s about the extent of my knowledge of how the actual sender works. I’m just trying to do some research before my AP just starts throwing $1000 in senders at it! Haha. Thanks.
Click to expand...

Wait a second. Did you actually confirm that both tanks had equal fuel? I know you started out the thread looking at bad senders. highly possible. But, your last post sounds like the typical Cessna fuel imbalance, very common for Cessnas to burn out of the left tank first, even with fuel selector on both. If that's the case, you could have a blocked or misaligned fuel vent (on the left side), or blocked/bad cross-tubing between the right and left tanks.
 
Jim Carpenter said:
Wait a second. Did you actually confirm that both tanks had equal fuel? I know you started out the thread looking at bad senders. highly possible. But, your last post sounds like the typical Cessna fuel imbalance, very common for Cessnas to burn out of the left tank first, even with fuel selector on both. If that's the case, you could have a blocked or misaligned fuel vent (on the left side), or blocked/bad cross-tubing between the right and left tanks.
Click to expand...
That Cessna fuel imbalance can have several causes.

1. The vent is into the left tank, and when the tanks are full, the vent crossover line between the top fronts of the tanks is also full of fuel, and the vent pressure forces fuel through that line into the right tank until the fuel level has dropped enough to let air into the line. That makes the right tank more full than the left. The dihedral of the wings keeps that vent crossover line submerged for some time. Fuel IS flowing from the right tank, but the fuel crossing from the left keeps it full.

2. A leaking cap on the right tank will cause the low pressure atop the wing to pull slightly on the fuel in the right tank, slowing its flow. If the leak is aggressive enough, fuel will flow from the left tank through the fuel line to the selector valve (on Both) and up the right tank's line and into the tank, keeping it full. If that cap is really bad, you'll find fuel stains on the wing behind it. And fuel cap gaskets are often cracked and hardened and shot, and the vent check valve in the cap is also often shot. These cause those fuel flow differentials and also let rainwater and snowmelt into the tanks.

3. Cessna had problems with the 172's fuel flow until they installed a modified fuel line/vent interconnect at each tank, to evacuate bubbles that formed in the lines and obstructed the flow. A bubble in the vertical section of line could be maintained in one spot there, forcing fuel to squeeze around it. The bubble's speed of rise and the downgoing fuel flow speed were the same in cruise when both tanks were feeding, and so you saw placards in some of those airplanes telling you to switch to single-tank operations above 5,000 feet. The faster flow with one tank feeding pulled the bubble down and vented it out the carb bowl vent. There's a kit to modify the airplanes built before that system was included on new airplanes at the factory.

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=36949

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=33762

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=30276
 
Dan Thomas said:
That Cessna fuel imbalance can have several causes.

1. The vent is into the left tank, and when the tanks are full, the vent crossover line between the top fronts of the tanks is also full of fuel, and the vent pressure forces fuel through that line into the right tank until the fuel level has dropped enough to let air into the line. That makes the right tank more full than the left. The dihedral of the wings keeps that vent crossover line submerged for some time. Fuel IS flowing from the right tank, but the fuel crossing from the left keeps it full.

2. A leaking cap on the right tank will cause the low pressure atop the wing to pull slightly on the fuel in the right tank, slowing its flow. If the leak is aggressive enough, fuel will flow from the left tank through the fuel line to the selector valve (on Both) and up the right tank's line and into the tank, keeping it full. If that cap is really bad, you'll find fuel stains on the wing behind it. And fuel cap gaskets are often cracked and hardened and shot, and the vent check valve in the cap is also often shot. These cause those fuel flow differentials and also let rainwater and snowmelt into the tanks.

3. Cessna had problems with the 172's fuel flow until they installed a modified fuel line/vent interconnect at each tank, to evacuate bubbles that formed in the lines and obstructed the flow. A bubble in the vertical section of line could be maintained in one spot there, forcing fuel to squeeze around it. The bubble's speed of rise and the downgoing fuel flow speed were the same in cruise when both tanks were feeding, and so you saw placards in some of those airplanes telling you to switch to single-tank operations above 5,000 feet. The faster flow with one tank feeding pulled the bubble down and vented it out the carb bowl vent. There's a kit to modify the airplanes built before that system was included on new airplanes at the factory.

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=36949

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=33762

https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=30276
Click to expand...

Thanks for that write up reply. Right now sitting level, I’m about 5/8 tanks. Right reads way more full than it actually is. Left appears to read pretty correct. This winter I will drain both tanks and fill incrementally and just see what is what. I have a feeling my right sender is erroneous. I don’t rely on these gauges or any fuel gauge but it’s nice to have stuff work correctly you know. Thanks again.
 
Sounds like you might have an SW sender feeding a Rochester gauge.
 
My RH gage is showing empty. Pulled out the transmitter and Its resistance was very high, or intermittently open. Suspected the resistor, but there is no access to it since it is inside a riveted can. Turned the fault was with the contact with the case/ground, not the resistor. This was due to the very poor design, where the leaf spring, which serves as both, pot (variable R) wiper on one end, and ground on the other, had bad ground connection. So I cleaned it by squeezing emery paper between the leaf spring and where it rubs the case, and it sprung back to life, with perfectly stable resistance reading of 35 to 240 ohms. The unfortunate thing was that I also discovered that the float was leaky. So I ordered a new one $$$$. Guess the years of work as an electronics engineer at Fluke pay off again and again.
 
Last edited:
Send to address below. They’ll bench test and repair. Can also exchange for an overhauled (improved) fuel sender for $275. Very pleased the exchange I got a couple months ago.


AIR PARTS OF LOCK HAVEN, INC.
PO BOX 418, 1084 EAST W ATER STREET LOCK HAVEN, PA 17745 570-748-0823
570-748-1786 FAX
W W W .AIRPARTSOFLOCKHAVEN.COM
 
Overhaul the gage, the sender or both?
 
Not the gauge. Just ship the sending unit. It’s possible they can bench test and repair without overhaul. Either way they’ll have the unit there and can do whatever’s appropriate. One of the improvements is a 4” wire properly torqued and torque-sealed to the unit. This way connection is made by a handshake connector on the opposite end. Prevents the common over-torque, crack, leak, problem.

Get the fancy little $20 screw kit from McFarlane for when you go back on with it. Has the seals already made under the heads. Also can get the gasket there for half of what TextronChinaCessna wants.
 
Most of these fuel senders came out of 1950's era cars and have a nominal resistance of 330 ohms. The resistive element is not sealed and suffers badly over the decades. I replaced mine with EI's magnetic units and things work much better. You've still got the problem that you've trying to measure fuel sloshing around in a flat pan, but at least the needles aren't twitching all over the place because the wiper is hitting a bad spot.
 
Overhauled ones are a heck of an improvement over originals. Simple job, inexpensive, and works exactly as it should afterwards.
Here’s a pic of the screws with seals made into the washer. They work well.
 

Attachments

  • 2255AD52-77F3-46DF-B36B-530494378306.png
    2255AD52-77F3-46DF-B36B-530494378306.png
    742.2 KB · Views: 17
Those seals keep the fuel from leaking out around the sender (supposedly), they do little to fix the resistive sensor getting gunky.
 
flyingron said:
Those seals keep the fuel from leaking out around the sender (supposedly), they do little to fix the resistive sensor getting gunky.
Click to expand...

Exactly! They take the place of the original screws and rubber washers to help prevent leaks through the mounting holes. They do absolutely nothing for any sort of sensor gunk!
 
You must log in or register to reply here.
Back
Top