Just replaced the fuel senders on a C172K, old SW senders always showed FULL with master on, and zero with master OFF. New senders are McFarlane, tested ground strap and ground wire on senders and both ohm 0, when center post wire is removed the gauges go to 0. I’m stumped, helping a friend, anyone have any ideas? To get to the installed gauges will not be fun, I looked up behind the panel and it’s a birds nest!
Cessna 172K New fuel gauges still pinned at FULL
- Thread starter Lndseaair
- Start date
Dan Thomas
Touchdown! Greaser!
- Joined
- Jun 16, 2008
- Messages
- 12,389
- Display Name
- Dan Thomas
I am assuming that the tanks are not full. Right?
It sounds like that airplane has had the old Stewart-Warner gauges replaced with Rochester as per Cessna SBs. SW hasn't been available since 1993 or so. With the SW senders feeding Rochester gauges, you get Full readings down to about a quarter-tank or so.
Check this out:
https://thepilotsplace.com/forum/index.php?threads/fuel-gauge-problem.28212/
From that thread:
The Rochester and SW systems are not compatible. You might have a mixup there. How long has this problem been there in that airplane?
It sounds like that airplane has had the old Stewart-Warner gauges replaced with Rochester as per Cessna SBs. SW hasn't been available since 1993 or so. With the SW senders feeding Rochester gauges, you get Full readings down to about a quarter-tank or so.
Check this out:
https://thepilotsplace.com/forum/index.php?threads/fuel-gauge-problem.28212/
From that thread:
The Rochester and SW systems are not compatible. You might have a mixup there. How long has this problem been there in that airplane?
To add to the above, contact McFarlane tech support. I seem to recall an issue on some Cessnas with this swap, but dont remember the details.
Dan Thomas
Touchdown! Greaser!
- Joined
- Jun 16, 2008
- Messages
- 12,389
- Display Name
- Dan Thomas
Yes, but the problem was there before you changed the senders, which is why I wonder if the SW gauges themselves have been changed in the past to the Rochester gauges. The Rochester gauges don't work properly with SW or MacFarlane senders.
Pinecone
Final Approach
If the gauge show full when the wire is connected and empty when it is removed, it sounds like there is a short between the gauge and the wire. Measure from the wire (removed from gauge) to ground. On my plane, the sender is 30 ohms at full.
Dan Thomas
Touchdown! Greaser!
- Joined
- Jun 16, 2008
- Messages
- 12,389
- Display Name
- Dan Thomas
But on both gauges?
AlphaPilotFlyer
Line Up and Wait
- Joined
- Dec 15, 2020
- Messages
- 510
- Display Name
- AlphaPilotFlyer
the c172k, and most vintage aircraft, stock gauges and senders operate on the SW standard 30-240 Ohm range. the sender is simply a variable resistor moved up and down by the float, varying the resistance/Ohm from the ground wire to gauge 'sense' terminal.
But if you mix OHM ranges of sender and gauge, such as a 20-240 sender to a 0-90 Ohm gauge, it won't work.... again. most are the standard SW range, but not always, especially possible if newer gauges were installed from a salvage yard.
good news. it's easy to trouble shoot.
my suggestion...
1. there are 2 wires at sender...one is common ground other goes to gauge. take the gauge wire and ground it to tank....this sends a signal of less than 30 ohm... this should result in gauge going full....if not, the issue is likely either a open wire to gauge or wrong ohm range gauge, or bad ground wire at sender.
2. if u still have another or old operating sender, use jumper to connect the same wires on installed unit...if the gauge move proportionally when you move float, the wires likely good and likely a mismatched sender vs gauge range. most senders wear out windings near mid level as that's where level usually is (the friction between wiper and winding wears out) so you may not get good gauge reading mid travel, but should be work accurately at empty and full ..
3. to check if sender is working you can simply use a cheap multi meter to display ohms between the sender 2 connections....you should see less than 40 at full position, and more than 200 ohm at empty. compare the readings between both genders to confirm match. if you don't get any gauge movement, recheck wires at gauge .
4. if you have extra wires hanging around... jump the wires at bad tank sender to the terminals at other tank sender (b sure to remove wires at both senders or you will add resistance and get inaccurate readings)...you are simply hooking up opposite sender to gauge ...you should see the same level as before for that tank.
if you need more help, PM me with your tel # if I can help.
But if you mix OHM ranges of sender and gauge, such as a 20-240 sender to a 0-90 Ohm gauge, it won't work.... again. most are the standard SW range, but not always, especially possible if newer gauges were installed from a salvage yard.
good news. it's easy to trouble shoot.
my suggestion...
1. there are 2 wires at sender...one is common ground other goes to gauge. take the gauge wire and ground it to tank....this sends a signal of less than 30 ohm... this should result in gauge going full....if not, the issue is likely either a open wire to gauge or wrong ohm range gauge, or bad ground wire at sender.
2. if u still have another or old operating sender, use jumper to connect the same wires on installed unit...if the gauge move proportionally when you move float, the wires likely good and likely a mismatched sender vs gauge range. most senders wear out windings near mid level as that's where level usually is (the friction between wiper and winding wears out) so you may not get good gauge reading mid travel, but should be work accurately at empty and full ..
3. to check if sender is working you can simply use a cheap multi meter to display ohms between the sender 2 connections....you should see less than 40 at full position, and more than 200 ohm at empty. compare the readings between both genders to confirm match. if you don't get any gauge movement, recheck wires at gauge .
4. if you have extra wires hanging around... jump the wires at bad tank sender to the terminals at other tank sender (b sure to remove wires at both senders or you will add resistance and get inaccurate readings)...you are simply hooking up opposite sender to gauge ...you should see the same level as before for that tank.
if you need more help, PM me with your tel # if I can help.