FORDification.com
   (powered by Google)


  HOME

  Technical Articles

  Factory Literature

  Discussion Forums

  Photo Galleries

  The BumpWiki

  Service Bulletins (TSBs)

  Decoding Your VIN

  Diagrams & Schematics

  Links & Resources

  In the Movies

  Downloads
  Member Meet & Greets

  In the Media

  Site Index

 MISC / PERSONAL

  My Truck Projects

  My Heinz 57 '67

  I've Been Censored!

  Contact



You are here: Home Tech Articles & Tutorials Interior / Electrical Fuel Tank Sending Unit Tech
Back to Technical Index

Fuel Tank Sending Unit Tech

 

 

Whether you're troubleshooting your truck's fuel system to find out why the fuel gauge isn't working (or not working accurately) or simply need to know how to check a junkyard part, knowing what to do can save you a lot of headaches and money in the long run. On this page I hope to give you the information you need to know to make an accurate diagnosis of your parts. I'm only going to be covering factory setups on this page, however, since the vast array of aftermarket auxiliary fuel tank setups would obviously make this tutorial an impossible task.

FYI: The most common fuel tank used in the '67-'72 era of Ford trucks is the in-cab tank. The standard version holds 19 gallons, those equipped with evap. controls hold 18.5 gallons, and a 21.5-gallon version was also available.


Fig. 1  -  These are the two most common sending units you'll run across. On the left is the 25-gallon factory in-frame fuel tank sending unit. On the right is the standard 19-gallon in-cab fuel tank sending unit.


Fig. 2

Fig. 3

Fig. 4

Here are some general close-ups of the in-cab fuel tank sending unit. Fig. 3 and 4 show two slightly different styles of heads, but although slightly different in appearance, they both work the same and have the same electrical resistance values.


THE BASICS

 

Even though it probably doesn't seem like it to someone who's having problems with their fuel level gauge, these are actually very simply electrical devices. Understanding how they work makes it much easier to diagnose problems. Here's how the fuel gauge works:

The fuel gage circuit consists of the sending unit in the fuel tank, the gauge in the dash, and the wiring and dash wiring (or circuit board) between them. As with most sensors, the tank sending unit is simply a variable resistor. Its function is to resist the flow of current from the ground to the gauge depending on the position of the float within the tank. The higher the fuel level, the higher the float raises, which decreases the resistance between gauge and ground. The more current that flows through the meter, the higher the gauge needle moves.


Fig. 5

The sending unit operates off a 5-volt pulse supplied by the instrument panel voltage regulator (IPVR, Fig. 5), so just running a 12V hot wire to the unit won't work, and in fact will fry the sending unit. See Fig. 6 for the typical location on the backside of the instrument panel.



Fig. 6


Fig. 7 - '67-'69 vs. '70-'72 style regulator. The later style was for use with a printed circuit board. (click to enlarge)

You can test the IPVR without pulling the instrument cluster. Tilt the seat forward and pull off the orange wire from the fuel tank sending unit. When you probe the connector, your test light should flash on and off. You can also do the same test under the hood. Pull off the sender wire from the temperature sender and/or the oil pressure sender and probe those connectors...they should flash too.


TROUBLESHOOTING

 
PROBLEM:  The fuel gauge does not work at all...no needle movement.

With the ignition switch on, remove the wire from the fuel tank sending unit and ground it to the unit housing. This removes the resistance supplied by the sending unit, and should cause the gauge to swing over to the 'Full' position. Does this happen?

Yes

No

The gauge and wiring are good. The problem is most likely with the sending unit. The sending unit must be removed for further testing. (1) Ground the sending unit wiring to a good chassis ground (instead of the sending unit housing) and watch the gauge. Does it now peg out to the 'Full' side?
     

Yes

No

The problem is with the sending unit ground. Either the unit itself is bad or the float/arm is binding/missing. The sending unit must be removed for further testing.

The power supply to the gauge has been interrupted, (check fuses), the gauge is defective, or the wire from the gauge to the sending unit is open or has a break in it.

Testing the Sending Unit

 

After the sending unit has been removed from the tank, check to make sure the float is present,  intact and still floats and that there is no binding with the float arm. Reconnect the gauge wiring to the unit and ground the unit. Move the float arm up and down. If the gauge reads full in the full up position and empty in the full down position, then the sending unit is good. Verify by hooking up an ohmmeter to it and making sure it reads 8-12 ohms full and 70-73 ohms empty. If not, the sending unit needs repaired or replaced. You can easily disassemble the head of the sending unit to clean and/or repair...see the photos below.


TROUBLESHOOTING NOTES

(1) If you have dual tanks and a non-operative or sporadic fuel gauge, and grounding the sending unit wire on each tank pegs the gauge, you probably don't have problems with both sending units. Take a look at your fuel tank selector switch as a culprit. Test the switch for continuity. You might find you'll need to carefully disassemble and clean the switch internally and lubricate it with dielectric grease.
 


Fig. 8

For educational purposes I hooked this sending unit up to both a digital and an analog ohmmeter to compare the readings. Both gave similar readings and verified that it was reading within normal parameters.

NOTE: You MUST make sure your ohmmeter is capable of reading the range needed...many inexpensive hardware-store variety units are not designed to measure that fine a range. My little analog ohmmeter only had one range for reading resistance, which was 1-1000Ω. This scale is MUCH too great for reading the slight variations needed here, and didn't even result in the meter's needle moving at all. You need a tester having a 1-100Ω or 1-200Ω range. Having the right equipment makes all the difference!


Fig. 9


Fig. 10

Fig. 11

Measuring the resistance in the 'Full' position of the float gave readings between 9-11 ohms.

Measuring the resistance in the 'Empty' position of the float gave readings between 67-70 ohms.

As mentioned above, a fuel tank sending unit is nothing more than a variable resistor, and the fuel gauge is simply measuring the resistance between the hot wire and the ground. The fuel tank sending units used on all Ford products up through 1986 have a resistance range of approximately 8-12 ohms full and 70-73 ohms empty. (I tested my factory 25-gallon in-frame auxiliary tank's sending unit, however, and got readings of 20Ω full and 85Ω empty.)


Fig. 5 - To open the head, simply straighten out these metal tabs holding the unit closed and gently pull the two halves apart.


Fig. 6 - Once the unit is opened up, you'll see the thin wrapped wire which make up the resistor. The thin metal wipe glides along the wrapped wires, and the resistance is measured between the end of the resistor and the end of the metal tang.


Fig. 7 - Here's another view of the exposed resistor, with a better angle showing the metal wipe inside. This wipe can be carefully bent, if needed, to make better contact with the wound resistor wire. Occasionally the wipe has fallen off the shaft and can be soldered back on. You can also gently clean the internals with fine steel wool, but be VERY careful not to break any of the wires.

 

If you're unable to find an exact factory-style replacement sending unit (or unwilling to pay the prices they're getting these days), there's another option. You can simply go to your auto parts store and tell them you need a sending unit to work with those resistance readings mentioned above. If you can only get one which similar resistance values but not exact, you can usually bend the float rod and/or the tangs on the sender body to alter the resistance to suit. Most sending units have universal flanges that will work in any tank unless it is home made.

If you're going an aftermarket gauge, depending on who you get it from, you might have to use their sending unit. Most aftermarket gauges are not for use with a factory Ford sending unit. However, Autometer does have special gauges which work with the factory sending unit. From the Autometer website:

http://www.autometer.com/tech_faq_answer.aspx?sid=1&qid=56

The short sweep electric fuel gauge manufactured by Auto Meter for the Ford 73-10ohm range is calibrated for the original Ford factory supplied, non-linear, fuel level sender design. If you view the gauge dial itself, you will notice that the halfway point in the gauge is in fact not centered in the field in order for the calibration to match this special sender.

The Ford Fuel Level sensor with a 73-10ohm range operates with the following ohm ranges for Full, Half, Empty, and Below Empty:

Full: 10ohms
Half Tank: 25ohms
Empty: 50ohms
Pointer Width Below E: 73ohms

If you have replaced the original equipment Ford sending unit in your vehicle with an aftermarket sending unit, you may notice that the gauge will match the sending unit at the Full and Empty portions of the scale, but that the readings in between will not match up. The reason for this is that some aftermarket senders operating with the 73-10ohm range now utilize a linear movement instead of the original non-linear scale. The comparative ohm reading difference for Full, Half, and Empty have been supplied below for your reference.

Full: 10ohms
Half Tank: 41ohms
Empty: 73ohms

As you can see these numbers differ radically from those of the factory Ford sending units and have a drastic effect upon the gauge readings.

In the event that you have this situation, we recommend one of two options. The first would be to seek an original equipment fuel level sending unit from Ford directly for your vehicle. The other would be to purchase one of our 240-33ohm scaled gauges and the matching #3262 sending unit, or if this unit will not fit your specific tank type, a similar 240-33ohm fuel level sending unit from your tank manufacturer. We recommend the 240-33ohm units because they are a broad linear scale and provide a very accurate reading of the fuel level in the tank.


 

Ohms/Resistance
(Empty/Full)
Popular Models
0-90 Ohms most GM cars, 1965-up
73-10 Ohms pre-1989 Fords & most Chryslers
240-33.5 Ohms Industry standard, works on many popular cars
0-30 Ohms most pre-1965 GM cars
16-158 Ohms most '89-up Fords
 

 
You are here: Home Tech Articles & Tutorials Interior / Electrical Fuel Tank Sending Unit Tech


Want to link to this site? Please save this banner to your hard drive to place on your webpage.
The correct link to use is http://www.fordification.com


Copyright 1999-2010 FORDification.com unless otherwise noted. All rights reserved.
All brand names and product names used on this website are trade names, service marks or registered trademarks of their respective holders.  
No portion or content of this site may be reproduced or otherwise used without explicit permission.
To report problems or provide comments or suggestions, please click here.