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Old 05-14-2006, 08:01 AM
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Fuel gauge help

Looking for a wiring diagram for a 91 GMC sonoma for the fuel tank and gauge . I have a modified dash and a 350 eng with aftermarket gauges , can't get the fuel gauge to work . Can anyone help me out , thanks in advance . Jim

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Old 05-14-2006, 04:15 PM
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Doc here,

Assuming aftermarket Mechanical Readout gauge , The rear of most modern units Is orientated with 3 terminals, "S" , "G", and "I".

You want to verify the following:
  • The "I" terminal goes to 12 Volts Switched.
  • The "S" terminal goes to the Tank Sender unit.
  • The "G" terminal has a hard ground.

If those are correct, Remove the "S" wire from the gauge , Turn the key on and note the reaction of the gauge..It should peg F or E depending on your set up.

Next , Hard ground the "S" terminal, observe the meter deflection, It should be the totally opposite of the open sender..prgging it the other direction.

IF you have that, the gauge is Correctly hooked up to power and ground, AND the gauge is not defective.

Next, go to the Tank, Where the sender wire goes into the fuel pickup / sender egress panel.

Remove the wire (should be a brown wire usually)

Get out your DVOM and set it for OHMS scale, R X 1, Calibrated to 000, and, using a jumper from the sender wire at the gauge (gauge out of circuit) , to the tank wire, take a measurement.

It should read dead on 000, not some resistance or infinite..If it does read resistance or is intermittent or infinite, replace or repair the wire.

If it is 000, then , the wire is good.

Next take a reading at the tank sender, It should read between 0 and no higher than 290 ohms..(depends on the system..0 to 30, 0 to 60, 0 to 90...ect..)

If you get no reading here, add a jumper wire from the Tank ( ground lug at the fuel inlet egress ring) to a hard ground and remeasure. If you get a reading now, Install a tank ground wire.

If still no reading , Or very high resistance , the sender is bad, pull the sender and replace it. While your in there I advocate changing the fuel pickup sock also , so you won't have to do it someday when you least expect it.

On a side note: Since your gauge is aftermarket , BE SURE the gauge matches the Q of the circuit ..If your gauge is 0 to 30 Ohms, and the sender is 0 to 290 Ohms..obviously, It's not going to operate, or be accurate..Check your instructions for the type of sender your gauge requires, and be sure you have that value resistance.

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Old 05-15-2006, 12:27 PM
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Thanks for the reply Doc, The wiring in this truck is different , it has a a two wire feed . One is for the fuel pump , i have put power to this wire and the pump runs . i'm guessing the other wire must be for the gauge . Then a double ground to the frame . How does the sender get power , is it from the fuel pump wire then ? I am not using the electric pump i have a mechanical pump on the motor , any ideas ? Jim
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Old 05-15-2006, 04:08 PM
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Doc here,

ALL YOU NEVER WANTED TO KNOW ABOUT SENDERS:

Senders NEVER have power on them..

It just wouldn't work in 30 gallons of accelerant....One spark and you'd have individual Win tickets in the 1/4 mile for all your parts..

Senders are ALWAYS referenced to ground.

The internal ARM or Float receiver assembly has a variable resistor connected to the mechanical movable arm..(think of a toilet bowl float) ... as the arm moves up or down with the addition or use of fuel, the resistor moves with it.

The Variable Resistor has 3 Connections on it. Terminal 1 is hard ground, terminal 2 or center wiper (movable arm) is the sender, and terminal 3 , no connection, which is Rt (total Resistance drop across the resistor.)

As you fuel the Vehicle, (most systems) , The arm or terminal 2, moves closer to terminal 3..a High resistance value. (Say , 290 ohms) this would be Tantamount to removing the Sender wire, and the gauge raises to "Full" deflection...

As you burn off fuel, The movable contact (terminal 2 ) moves toward terminal 1 (terminal 1 at ground potential or 290 to 0 ohms) The Sender resistor is a linear Taper, (moves in somewhat steady resistive increments) , The resistance to ground becomes more at the sender output. Ergo the Gauge.

The Sender picks up it's ground at the tank or frame rail close by for terminal 1.

Think of the Sender as a Volume control on an old manual control radio or TV.

The Gauge , Does all the power handling for the system via the "I" terminal.

Gauges come in a couple of flavors..

The old traditional gauge we all know and love..It works by power being introduced at the "I" terminal into a Bimetallic Strip inside the gauge..

The ground or "G" at the gauge "Balances " out the heat consumed by the strip at static, (not enough consumed power to coil the strip) And the "S" or sender wire adds a lower resistive value to the strip, which causes it to heat and cool directly proportional to the sender level.

As The Coil , expands or contracts (think of it as a choke heater coil) There is a mechanical connection to the index pointer on the front of the gauge. so it follows suit of the sender into something us humans can deal with..

Some totally Electronic ultra modern Gauge circuits use a "Whetstone Bridge " inside the gauge..(or readout) This is nothing more than 4 precision resistors tied end to end in a double tri~angle type of application, they are matched perfectly to one another so they all balance With application of power and ground , but no sender input.

Two of the connection points of the bridge (the farthest two) go to power ("I") and ground ("G") and balance the circuit..the + side of the meter or readout input pin goes to one connection point (the top of the double tri~Angle) and the - side of the meter or readout goes to ground..

Without the sender input , the "Bridge " remains basically balanced with power and ground applied..So the meter does not try to drive..add the sender ( a resistive value to ground) and it "unbalances" the bridge, causing the meter to move by drawing current through the bridge to Balance the resistance across the bridge and "normalize" at it's new given value. (stops driving the meter .)

The Readout can be digital LCD, or D'Arsonal type of meter (like a volt gauge ) and is nothing more than a meter motor. no heat strip..the pointer is part of the mechanical movement to interpret it into a visual reference Or the + side of the meter would be an input to an LCD dividable driver chip.

The last type (Newer Vettes are using this, a real PITA) .. Is a piezo Weight cell mounted in the bottom of the tank..it operates by deducing the weight of the fuel on the sensor and has a computer that calculates it into usable data in gallons at the HUD or DIC..It requires a computer and readout to operate.

YOUR PROBLEM:

Your system, is the first Or traditional type of sender and gauge.

If you are not reading a Resistance or hard ground (depending on fuel level) at the sender connection at the gauge..you either have an open circuit, at the wire between the gauge and sender..OR a bad sender..

Remove the sender wire from the gauge and observe the gauge indications..It should peg one way, ground the sender terminal, it should peg the other way....If you have this, the gauge is hooked up properly and is not defective.

With a jumper installed from the sender wire end , to the gauge wire for the sender, measure with your DVOM, set to Ohms, R X 1 calibrated to 000. It should read 000...If it is infinite..OR has a high (or any ) resistance on it, the wire is bad..replace or repair as needed. If it reads 000, it is good.

With your ohm meter read the resistance at the sender lug to ground..It should read a Value somewhere between a few ohms up to 290 ohms (depending on the sender value and fuel level) If it reads infinite, Install a jumper from the tank ground lug to the frame or other hard ground..measure again..If still infinite replace the sender.

One other ..well, actually two, possibility's DO exsist..the Fuel pump relay could have been handling the sender wire as an extra set of contacts.. (but I doubt it..) and now has nothing to drive it (removed the electric pump system)

OR it has/Had a duel tank system (or was set up for one) and the sender line is switched to a "Blank side" not reporting back to the gauge. (another remote possibility.)

Do that, and you should find the problem.

Doc
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Old 05-18-2006, 08:16 AM
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Doc , Thanks for the help . It may be a week or 2 before i get a chance to get back under the truck to get this problem fixed . Thank you again . Jim
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Old 05-18-2006, 05:54 PM
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Doc here,

No Problem!

let us know how it turns out!

Doc
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Old 05-27-2006, 07:10 AM
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Doc , Tested the system and the sender must be the problem . Only odd thing is the gauge does read , but when i put gas in the gauge goes towards E instead of full . have not put a full tank in yet . Ohms was at 19.8 and gauge was reading allmost full , put in 20.00 $ 6.2 gallons and the gauge moved to just above 3/4 . Any ideas , or is the sender bad .
Jim
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Old 05-27-2006, 03:16 PM
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Doc here,

The possibility Exists That the Float arm is bent past the "Window" of adjustment on the gauge.

You need to pull the sender and Move it slowly by hand to check the linearity of the Movement on the Float Gauge and the Float arm.
  • "E" Should be when the arm is at bottom, or full down.
  • 1/4 should be 1/4 travel on the float leaver.
  • 1/2 should be 1/2 Travel and 3/4 should be 3/4
  • Full anywhere above that.

Movement should be smooth And linear, no jerky movement on the gauge.

Some late model gauges have a "Buffer" or "Sample and Hold" function of the BCM , in the movement of the reporting Device (GM's Cheap Answer to the Fuel tank Baffle) So the movement may only update once every few minutes..you need to beware of that also.

The Float, just like a carb float Should never have fluid inside it, shake it and see if you can hear "Sloshing" as you do, If so replace the float assembly.

The Float can "Hit" and stick on things that have fallen into the tank, Inspect with a pen light when you pull the float.

While your in there replace the Fuel pickup Sock at the same time (never done until the Car quits running and extensive fuel troubleshoot is done) ..Saves you from ending up on a hook someday!

The gauge should NOT report Backwards..Unless the sender line was intermittent or had a short on it..to check, run a secondary wire from the gauge to the sender, If it reports correctly, install it.

That should get you going.

Doc
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Last edited by docvette; 05-27-2006 at 03:18 PM. Reason: I before E .. Except after a 12 pack...
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