Fuel Gauge and Sender

[allanrbowman]

My fuel gauges began becoming erratic with the left tank gauge reading empty when actually full, then the right gauge doing the same thing. Randomly each might or might or might not become operational. I checked the plus 12volt leads to sensors and gauges and this is OK, gauges still don't work.

I have not yet checked the grounding but will do this shortly.

Does anyone have a schematic of a gauge and sender that they can forward to me? I know the basics but there are some resistors mounted on the gauges and I would appreciate knowing exactly how these work and are wired.

Thanks, Allan

[JohnW]

This sounds like a ground issue, for sure. The coiled wire "rheostat" inside the sender doesn't "work intermittently". When it dies, its dead. Once you have "established power to the system", the next troubleshooting step is to suspect "ground", THEN sender and/or instrument malfunction. Sometimes, such intermittent CRAP will stem from the Fuel Gauge CBs themselves. There is also [what has been called a] "shunt" located at the gauge connections, but when one of those quits, it doesn't affect BOTH SIDES.

There should be a Piper wiring diagram in the back of your TriPacer Owners Handbook. There are DEFINITELY wiring diagrams on the SWPC Drawings CD, and this is the single most valuable piece of equipment you can own, as a ShortWing Owner/Operator. One question you may sooner or later think to ask will be "What's the resistance in the sender coils?" Answer: "Zero-30 ohms, peg to peg". ("Good" gauges more usually digital V-O-M out at more like "2.0-33.5 ohms", but the "official story" is nonetheless 0-30). Most "modern" fuel senders that can be found are set up for 0-240 ohm operation.

Steve(s)...have you thought of the possibility of becoming a "licensed distributor" for reselling the SWPC Drawings CDs (or AT LEAST linking directly to them)???

[Gilbert Pierce]

SWPN November/December 2008 issue page 55. Tells how to trouble shoot the gage and sending unit and has a wiring diagram and schematic of the system.

[smcnutt]

I've not seen it there but it sure would be nice if they archived those back issues on their site. Especially, any technical articles that were printed.

[Steve Pierce]

Short Wing Piper Electric Fuel Quantity System

Over the past several years there have been numerous requests on the Short Wing Piper Maintenance Forums site {http://www.shortwing.org/forum/} for electric fuel gage and sender information and troubleshooting assistance. The system in simple terms is a variable resistance tank sending unit and amp meter gage unit in the instrument panel.

SENDING UNIT

The sending unit is installed in the fuel tank and consists of a float and arm that is attached to variable resistor. When the tank is empty the float sits on the bottom of the tank. The arm moves the wiper on the variable resistor to zero ohms or a short circuit. When the tank is full the float is at the top of the tank. The arm attached to the float moves the wiper on the variable resistor to the maximum position, 30 ohms.

The sending unit is attached to the inboard side of the fuel tank with five screws and a gasket to seal the interface between the sender housing and the opening in the tank. The sending unit and/or gasket can be replaced with the fuel tank in the wing but the bottom screw can be difficult to get to. You will probably need an offset screwdriver. The gasket is Univair part-number 462-021. I would recommend putting Fuel Lube, TiteSeal, or Permatex non-hardening Form-a-Gasket #2 on the gasket. Be sure to use a little of the Permatex on the screw threads to insure you get no fuel seepage past the screws. Do not over tighten the screws or you will force the gasket out from under the sending unit flange. One of the attachment screws should have a ground wire and terminal attached. The wire should return to your central ground point on the instrument panel. Grounding it to the wing spar can cause erroneous readings.

GAGE

The gage unit is mounted behind the instrument panel with a clamp that is held to the gage by the two gage terminals. The holes in clamp have insulating bushings to prevent the terminals from shorting out on the clamp. The gage has an internal ground to the case. The case must be adequately grounded thru contact with the instrument panel to prevent fluctuating readings.

The gage itself is moving vane amp meter. Inside are two coils. One coil pulls the needle to the full side and the other pulls the needle to empty side. Springs dampen the needle to prevent vibration and short-term fluctuation. The sending unit is connected in parallel across the full side coil. When the tank is full the sending unit resistance is maximum causing maximum current flow thru the full side coil. When the tank is empty, the sending unit grounds the other end of the full side coil so there is not current flow thru it. Now the needle is fully deflected to empty side coil by the increased current flowing thru it. With the fuel level at something other than full or empty, the needle is displaced by the relative strength of the magnetic fields in the full and empty coils caused by the current flow thru them. More current in the full coil causes more magnetic flux pulling the needle toward the full side. As the fuel level drops, the sending unit resistance gets less. This causes the current flow in the full coil to decrease causing the needle to move toward the empty coil.


The gage also has a wire wound 30-ohm resistor attached to the back of it. One end of the resistor is attached to one of the gage terminals. The other end has an independent terminal not attached to the gage. Aircraft 12 volts is connected at this point. This is basically a current limiting resistor or allows a 6-volt gage to be used with a 12-volt system. In normal operation this resistor will get very warm. The other terminal on the back of the gage is attached directly to the center terminal of the sending unit.


TROUBLE SHOOTING

If your needle appears to float around and give fluctuating readings you probably have a bad ground on either the sending unit or the gage. Use an OHM Meter read between the flange on the sending unit and the back of the instrument panel. You should read a short circuit, zero ohms. If you have any resistance you have a bad ground. Make sure you have a ground wire from one of the sending unit mounting screws to your airframe central ground point and that all of the terminals are tight. Read the resistance from gage mounting clamp to the back of the instrument panel. Again it should read zero ohms or less then 1. If not, try wiggling the clamp and see if the resistance goes away. You may need to tighten the clamp a little. Do not over tighten as you may damage the electrical terminals.

After checking and insuring you have good grounds, disconnect the wire from the sending unit to the gage either at the gage or the sending unit. With the fuel tank empty, you should read zero ohms from the center terminal of the sender to the flange. With the fuel tank full that same reading should be 30 ohms. If the tank is empty or less than full you can use a stiff piece of wire thru the fuel filler cap and raise the float arm until the float hits the top of the tank. At that point you should read 30 ohms.

Typically what happens is your tank does not read full when it is full and drops rapidly toward empty. With the tank full you read only 27 ohms. This usually indicates your float arm is not bent properly to allow full-scale deflection of the arm and does not allow the resistor wiper to reach full resistance. Remove the sending unit from the tank and bend the wire arm so that the resistor reads zero when the float is on the bottom of the tank and not before and reads 30 ohms when the arm is against the top of the tank and not before. This will solve the problem. If the sending unit checks out bad, Keystone Instruments can usually repair them.

If your sending unit checks out good, the arm is properly adjusted and you still don’t have a fairly accurate gage reading, an instrument shop can calibrate the gage.

I have experience with Keystone Instruments, 320 Proctor Street, Lock Haven, PA 17745. 570.748.7083. They can calibrate/repair your gage and make it look like new.

I am sure there are others but I have had good success with Keystone.

If everything checks good so far you can read the resistance of the coils inside the gage and the resistance of the wire wound resistor. With no power on the airplane, read from one end of the current limiting resistor to the other. It should read 30 ohms. If it is bad it will read open, infinity or high resistance.

With the center wire still disconnected from the sender, read the resistance from that wire to the gage case. You are reading the resistance of the full coil to ground. It will read between 28 and 30 ohms. Now read the resistance between the gage case to the where the current limiting resistor connects to the gage terminal. That should also read 28 t o30 ohms. (See that attached drawing). If either of these read open or shorted your gage is bad.

Note: You are reading small resistances, 30 ohms or less. Be sure you have your Ohmmeter on the appropriate scale. If you have it on the 100 Meg ohm scale, 30 ohms will look like a short. Use the smallest range you have that is greater than 30 ohms.

Review:

1. Good ground on both the gage and sending unit.
2. Sending unit reads 30 ohms from center conductor to case flange with center wire disconnected and tank full.
3. Sending unit reads zero ohms from center conductor to case flange with center wire disconnected and tank empty
4. Current limiting wire wound resistor reads 30 ohms
5. Gage sender connection to ground reads 30 ohms with sender wire removed
6. Gage sender terminal to current limiting resistor connection at sender reads 30 ohms



I hope this helps explain the fuel quantity system and helps with the trouble shooting.

Gilbert Pierce


Gotta thank my Dad for putting this all in an easy to understand article. One of the benefits of belonging to an organization of people who want to help each other keep their airplanes flying.

[Andrew K]

Wow, thank you for the info guys! I too have noticed my right gauge starting act a little funny so this will definitely help when it comes time to take a closer look at it.
Thanks again,
Andrew

[allanrbowman]

Gilbert et al. Thanks very much - this is exactly what I needed. As I'm relatively new to the aircraft and you have referenced a short wing CD, where can I purchase this CD? I've done a Google and it did not find it.

These forums are exceptionally valuable.

Allan Bowman

[Steve Pierce]

Here is a link to the order form for the two drawing CD available from the Short Wing Piper Club. http://www.shortwing.org/shortwing_cd.php There is also a Vagabond CD which contains a lot of the small parts that has been invaluable to me.

[Jinkers]

With the fuel tank full that same reading should be 30 ohms.

Most "modern" fuel senders that can be found are set up for 0-240 ohm operation.

I think this explains why the right tank on the Pacer I just purchased reads full until there are just a few gallons left :idea:

While we are talking fuel tanks... Is there a dual tank replacement available for the single tank meter?

[JohnW]

Yeah, quite likely, someone "fixed" a problem on yours at tome point in the airplane's life. Nice to know it had "good maintenance", eh?

All Piper did was add another gauge and "individualize" the wiring, eliminating the toggle switch. There is no available TSO'd "two-in-one" gauge that you can "drop in" to a Piper panel, of which I am aware. But there are SEVERAL options available to install on a Field Approval. Pick one out of the catalogues and take the idea to your IA. I have actually "downgraded" several two-gauge "later" systems to what you have... by retrofitting the single gauge/toggle switch arrangement when I needed to "open a hole" to put in a suction gauge for an engine driven vacuum pump. Darn if panel space isn't at that kind of a premium, on these airplanes!!!