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Why doesn't my gas gauge work?

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sean View Drop Down
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    Posted: 17 Jan. 2011 at 1:40pm
CJ-2A Fuel Gauge System

Download and read the Auto-Lite "Instrument Circuit" maintenance chapter:

http://www.thecj2apage.com/forums/manuals-and-books_topic15843_post150275.html#150275

The fuel "gauge" consists of 2 parts: the dash gauge and the gas tank level sender.

The general wiring scheme.



**NOTE** :  ALL grounds are achieved by metal-to-metal contact!
  1. the dash gauge case MUST have ground continuity to the dash metal (more about that later)
  2. the sending unit MUST have ground continuity to the gas tank (primarily through the mounting screws)
  3. the gas tank MUST have ground continuity to the body and/or frame (via the tank rear hold down clamp and/or an uninsulated fuel line clip)

GAS TANK SENDER:

The tank sending unit is nothing more than a variable resistor, and can be checked with an ohm-meter. In the "full" (up) position, it should have zero (or very low) resistance. In the "empty" (down) position it should have on the order of 130 +/- ohms resistance.  ( The absolute value is not critical, these are not precision devices, you may find anything from 110-150 ohms). If it loses continuity while swinging up & down, it's getting old and worn out internally.



Internal construction:

The resistance unit is just wire wrapped around a form.  This should have continuity from end-to-end, with total resistance 130 +/- ohms.

If it doesn't have end-to-end continuity, the wire is broken or burned through somewhere.  It could be rewound if you're adventurous!



The wiper is a spring brass contact on the swinging arm.


This spring brass contact can weaken over the years, and lose contact with the resistance winding.  Sometimes it takes nothing more than bending it back out, to make the sender operational again.

A last problem spot: the swing arm bushing.

The swing arm ground depends on good continuity between the swing arm and the bushing it pivots in! (yellow arrow)

If there's any corrosion, dirt, grit, etc in there, clean it all out.



DASH GAUGE:



The gauge only has 2 terminal posts, marked "IGN SW" and "GA" (repro/aftermarket gauges may be marked differently, aftermarket gauges may have more than 2 terminal posts).



"IGN SW" receives positive voltage from the ignition switch ("Run" or "Ignition" terminal).

"GA" is the wire connection to the gas tank sending unit.

HOWEVER ... there is a 3rd "unseen" connection here.  The gauge case needs to be grounded, to the dash!

The gauge mounting bracket has 2 insulated holes (for the terminal posts), and a riveted spring (yellow arrow). The spring provides electrical continuity from the case to the bracket.



Make sure the end of the spring, and contact patch in the center of the case back side, are clean & free of rust, paint, grease, dirt, etc.



The bracket has to make good contact with the dash.  Make sure the 4 points of the bracket, and the dash where they contact, are clean, rust & paint free:



Internal construction:

Internally, the gauge consists of two electromagnetic coils, the needle armature, and a SPRING LOADED GROUND CONTACT!  If this internal ground contact gets corroded, dirty, or loses "springyness", the gauge starts malfunctioning.

Many of these malfunctioning gauges have nothing wrong with them except this internal ground.  If you've eliminated the sending unit, wiring & external grounds as problems, and the gauge is still malfunctioning, and you're adventurous, you can carefully take the gauge apart and restore the internal ground.


The internal ground is a spring brass strip that presses against the inside rear of the case.  It provides the ground path for the "empty" electromagnet coil:





Make sure this brass strip, and the inside of the case where it makes contact, are clean and rust free, and makes positive contact with the case metal.


ELECTRICAL FUNCTION: Schematic representation.

The 2 electromagnet coils inside the gauge are in a constant tug-of-war with each other.

The E(mpty) electromagnet (aka the "Restoring" coil) is directly grounded, so there is always a constant current flowing through it, generating a constant magnetic field strength.  It is always trying to restore the needle over to the "E" mark.

Current flow through the F(ull) coil (aka the "Deflecting" coil) is controlled by the sending unit.  It is always trying to deflect the needle over to the "F" mark.

When the sender is at "full" (0 ohms), the "F" coil produces a stronger magnetic field than the "E" coil, deflecting the needle all the way to the "F" mark.

As the fuel level decreases, the sender resistance increases, reducing current flow through the "F" coil, producing a successively weaker magnetic field, allowing the "E" coil to influence the needle, pulling it towards the "E" side, until it eventually dominates (at "empty", 130 ohms +/-), thus "restoring"  the needle all the way to "E".



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misterberry View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote misterberry Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar. 2011 at 1:25pm
Thanks for this checklist Sean !
Best and concise explanation of a problematic challenge many of us face. I have had mine apart to correct the grounding issue, but I am still running 12v so I am not entirely sure where to place a voltage compensator and what it should really be... exactly
James Berry

'48 2A ser.187593 "Jeep Jeep"

It's a "Lefty"....
Moved north from California-
Now waving the Canadian flag -
A Canuck back home in Edmonton
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sean View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sean Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar. 2011 at 8:26pm
James:
Quote ... but I am still running 12v so I am not entirely sure where to place a voltage compensator and what it should really be... exactly
Reducers go in-line with the +12v feed wire (between ign. switch & gauge input terminal).

Easiest reducer is a simple resistor: about 20 ohms , rated 3 watts or more (suggest 5w min.).

Ideal solution is a solid-state chip voltage regulator, but these require you to run a ground wire.  See the example, at bottom of this page:


In either case, try to mount the unit to the metalwork, to act as heat-sink (or go even bigger than 5w resistor).

Sean


Edited by sean - 25 Oct. 2011 at 5:58pm
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Schimms15 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Schimms15 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov. 2012 at 2:47pm
sean if one of these gauges is around is rebuilding it a possibility or is it a junk piece if broken to be discarded? lll
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Schimms15 Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov. 2012 at 2:48pm
46 2a
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sean Quote  Post ReplyReply Direct Link To This Post Posted: 17 Nov. 2012 at 8:05pm
Originally posted by Schimms15 Schimms15 wrote:

sean if one of these gauges is around is rebuilding it a possibility or is it a junk piece if broken to be discarded? 
They are not "rebuildable" in the usual sense, since none of the internal pieces is available separately (and not designed to be replaceable), but they can be "restored" to functionality as long as the internals are still functional/useable.

There's only 1 moving part: the needle/armature.  As long as it's not bent, broken, frozen, etc., it's probably good to go.

Couple of easy mechanical tests:
  1. hold the gauge vertical (as it would be installed in the dash): needle should rest on the "empty" stop  pin.
  2. slowly tilt it to horizontal (face up) and slightly beyond
  3. slightly beyond horizontal, the needle should flip over to the "full" stop pin w/out hanging.

  1. hold the gauge vertical again
  2. rotate it slowly CCW 'til the needle is approx. horizontal
  3. continue rotating CCW, the needle should "float" at approx. horizontal 'til it hits the "full" stop pin.

Electrically, there's 2 electromagnet coils. As long as they're not open or short circuit, or burned out from 12v operation, probably good to go.

Measure coil resistances:
  • between terminals: 33-35 ohms
  • from voltage feed terminal (original "Ign SW" marked) to gauge case: 45-46 ohms
If resistances are significantly out of line, then the coil(s) may be damaged, or internal connections/ground strap may be corroded.

These gauges are fairly simple devices. Other than internals getting rusted out from moisture, or cooked on 12v, there's not much that can go wrong.

The internal ground strap is a weak point: it gets corroded & adds too much resistance to the case ground, causing erratic & inaccurate readings.

Occasionally there's a broken wire at one of the solder joints:  tough to fix as it's extremely fine wire (.005").

Your gauge w/the broken terminal would be probelmatic: the terminal studs are splined (just like a wheel lug bolt), and pressed into the insulator boards.  Replacing one would be a chore, and probably not worth the effort.

Sean

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Post Options Post Options   Thanks (0) Thanks(0)   Quote NealzWorld Quote  Post ReplyReply Direct Link To This Post Posted: 18 May 2019 at 8:51am
Awesome post there Sean.   Lots of good info.   My gauge was always reading full, found I hadn't grounded the gauge.  I had ordered a new replacement gauge that doesnt ground thru the case.   

So the gauge works now, kind of.   The needle doesnt traverse the entire the range of the gauge.   I ohm'd the sending unit, went from 0 to 115.    But on the gauge the range is from F to the first line next to the F.  Doesnt go all the way down to E.  

Any suggestions?  
1947 CJ2A #88589
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