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Old 05-05-2004, 06:03 AM
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39 Chev Coupe Fuel sender Where is it?

Hi Guys
Building a 39 Chev Coupe for my daughter. Car was unfinished project already carpeted probably 20 years ago, Have put a Ford lazer dash in but the fuel gauge works in reverse. Obviuosly a polarity problem but can't find the sender without unglueing all the carpet, It is a 3 seater with the tank behind the seat. Just a clue on where to look would help

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Old 05-05-2004, 08:33 AM
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pull the instrument cluster & just reverse the wire on the gauge if thats the only problem. & if ya want to get to the sending unit in the tank you're going to have to drop the tank....joe
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Old 05-05-2004, 05:34 PM
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Sensor

Hi

Doc here

Look for an open sensor line first... Before Reversing anything!... A D'arsonal Gauge (digital is similar in theory but differs slightly real life) is a Balanced resistance Whetstone Bridge using the gauge load to compensate in a + or - Direction for the imbalance imposed By the Changing resistance to ground caused by the rise and fall of resistance by the Sensor due to Thermo~Mechanical Change...

I.E. In a "Perfect" Solar powered Barbie World... At Static (no Fuel) The Gauge will read zero...Bridge resistance is equal to the Sensor Resistance thus no movement of the meter...It's all good...

As Fuel Capacity increases/decreases, the Resistance at the Sender (in this case nothing more than a rheostat or linear Taper (real Terms:...like a radio volume Control knob) at the end of an arm with a float on it (like a toilet Valve) Increases /decreases poses a load on the bridge..in a + or - direction...The Bridge Will Always Balance the load (within it's limitations) By posing the equal load on the meter...Thus....Fuel...Go , feed or Walk....

In the real world Zero is a matter of contention ..and Calibration ....Ever walk with a gallon Can on a 1/4 tank?.... Or Go for like Days...way below The Big "E"?? By just applying Power and Ground With no Sensor load to the Gauge... It will always drive to zero or well below in order to balance the load on the bridge..And usually in the real world Zero (equal resistance) is somewhere way below where the visual zero is. (Needle Being driven below the lowest increment)

Locate and remove The Sender line at the back of the Gauge, With a DVM or OHM meter It Should have SOME resistance to ground on it...(Your results may vary see EPA standards includes tax lic & Doc fee's...Sigh..)
It SHOULD not be zero ohms to ground or infinite ohms to ground..I.E. 000 reading or an OL reading on the readout (DVM)If this is the Case, Than your Sender line is open/Short..or the Fuel Float sender Has Gone on Vacation...Replacement is a fact...unless you have some spare ping pong balls to throw in the tank to Bounce around and make noise When you approach walk..

To locate the Sender...Follow the fuel lines back to the tank..Where they enter the tank, will (usually) Be a "Ring" With the fuel lines in it and the sensor is (usually)located here too If you have an in tank fuel pump, it'll be here Also. Remove the ring & Gasket, And Replace / repair the Sender...If you do have an in tank pump, you just might want to fabricate A "Slick" looking access Door for future repairs...as we all know Murphy rides in style...

Try that First..

Hope that helps..
Doc
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Old 05-05-2004, 10:05 PM
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Thanks Doc,
I'm confident thet the sender is OK because the original gauge worked OK, The problem now with Ford cluster I fitted is that it now works in reverse so it seems to be a polarity problem with the sender. Can't change this at the cluster because it has a common power lead and would reverse the temp gauge as well.
I need to track down the sender to find how it is wired and can probably change wiring there.
But the tank on these things is mounted on the floor behind the seat and is fully carpeted (glued) and I can't find anything feeling around
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Old 05-06-2004, 03:43 PM
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Sender

Doc again

Normally Senders Are Single wire. However The Capri / Laser Has Got Some Magic Going on with a 2 Wire System...Go to the Site Below, They Have A. Page Dedicated To the Sender, It's Location, And how to Service it...

The Article Shows In the Stock Configuration It Is Located In the Tank, on that ring I was talking about. Since were Talking about a modified System (To fit your Car) The Float unit Has To be located Here...but The wires Could Be Anywhere...

I note the Article Does say, To test the Gauge...Remove and Short the TWO Wires from the sender together The Fuel Gauge Will Indicate A. Full Scale Deflection...So if you locate the Tank sender...You Can Do both..Test for Function, and reverse the wires at that point...

I Would Be very Dubious About Reversing Anything Power and Ground Related In the Panel Unit...

The reason for My caution here.. on like, Standard Stewart Warner Gauges (as well as many others), at the Rear of them, they have 3 terminals, Sensor, (S) Ignition(i) and power Ground (G) but the Ground (G) is common to the metallic Parts of the Case...were you to reverse power and ground here, you'd be applying power to case or metalic ground..Ergo...

Dead short ...unless the ground is Isolated from the rest of the gauge case (or metallic parts). If you decide to try that (not recommended) BE sure Case Ground and Ground ARE not common to each other....

The Site:
http://www.fordcaprilaser.co.uk/sender.htm

Hope it Helps!
Good Luck!
Please Let Me Know how it turns out!

Doc
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Old 05-06-2004, 05:18 PM
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Doc.
You are spot on in what you are saying, in fact it was a bit of a nightmare working out the wiring in the first place. and in the testing I tracked down the sender wire by grounding it and the gauge operated correctly, The original only had a power wire and another which I then assumed must go to the sender, This appears to be correct as the gauge operates , except in reverse, I don't know whether possibly the original system operated by sending power to the gauge instead of earthing it. I think it takes me back to the original problem of finding the sensor and it could be rewired to suit, I imagine it would not be difficult to take it to earth instead of through the circuit if that is the case, but if the tank has to be removed I have to destroy the Glued in carpet,
I am now wondering whether the tank itself may be under a bolt in section but still have to destroy the carpet. Only other way maybe if I can pick up the wiring somewhere further up and rewire from there but would I be correct in saying if it worked originally on a system of powering up the gauge, there would have to be 3 wires
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Old 05-06-2004, 06:28 PM
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Sensor

Doc Here

I can't recall any "In Tank" sensor that operates off + DC (that explosive thing..) but Indeed, yes, you would be correct in the 3 wire theory.

I guess I should have inquired about this first off...(shiny objects .. Pretty women...headlights on the horizon...oft Distract..me) But are you using the Stock 39 Chev Tank and sender?

If so herein lies the rub...It's one wire...with hard (Case ground where the sender actually goes into the tank) The gauge may be looking for two sources of ground ...reference...and ground.

If you are using the Ford Tank and sender, you should be all good and have the wires...at any point in the wire run that you can identify those two, you can snip them and reattach them opposingly to cure the problem. (Hopefully)

And a Last resort:
If you REALLY want to try this:
And if it applies...

You mentioned the Gauge Shares a Ground with another instrument in the cluster...and therefore cannot be "swapped " at that point Without effecting (Reversing)the other... Is this also on a laminate Circuit card also?

I can give you a trick that I did on my Corvette to put in an after-market gauge that worked on laminate ...(if your handy with soldering iron..)

Cut both Traces from the gauge sender posts on the card with a razor (carefully and leave enough to reverse the procedure if you find it won't work..) Burnish all four points and solder in jumpers in an "X" (.. Left trace to right post..visa~Versa)...pattern.. insulate the joints (paint or nail polish) and see if it goes in the right direction...That should work...effectively you are reversing the wires..only at the cluster...If you find the other gauge reversing (which it shouldn't), you can install a 5 amp 50 PIV Diode between the two Gauge grounds (allowing ground flow of current to go in one direction only, effectively isolating the 2 gauges from each other but both common to ground) Again, last resort, but perferable to ruining the carpet...

Tell me more about the Gauge Configuration (Number of posts) and if it's laminate card mounted or not...and what tank/sender your using...maybe I can figure a little more out for you...Kinda going on the" Basically how it works" premise here...Which doesn't apply to all apps.

Doc
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Old 05-06-2004, 07:29 PM
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Doc
Yes it is the original 39 Chev sender, Not sure what you mean by it could be looking for two sources of earth, Could the Chev originally have been a positive earth system and the previous owner somehow have isolated the gauge system to still run that way.
The dash is a laminated circuit and I think that there is two small bolts which hold the gauge in and these are actually terminals so it may be feasible to just break the circuit and go directly to these,
Bit hard to have a look at the moment as I had surgery yesterday and a little bit incapacitated for a few days
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Old 05-07-2004, 04:01 PM
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Sender

Hi Woody,

Doc again

First, So sorry To hear about your Surgery and temporary Disability... Hope you Recover soon... With positive aftereffects.

OK, On the 39 Chev Sender, there should only be a single wire leading away from it. The Ground is provided by the Case of the sender, where it bolts directly into the tank. They Also provided, As I remember a Small ground wire from the tank (sender) to the frame or other ground to insure good ground bonding.

My inference to two sources of ground, was, Reference: that ground which is measured THROUGH the sender and Ground: Hard (frame)Ground: Zero resistance from which the gauge / sender would use to offset/Compare with the other resistance indications.

In that type of application, two wires WOULD be used, the sender would be Series to the Gauge..I.E. required to complete the bridge circuit (or the 4 Th. resistance value) no hard ground at the sensor, it being isolated from ground and part of the Gauge circuit (needed To complete The Circuit) The Completion of the Gauge Circuit includes the connection to the gauge to even move the pointer up or down.

The other, and most common, Parallel, is usually one wire, Frame Grounded At the sender (internally) Supplies a single varied Ground resistance to the Gauge (depending on where the float / rheostat is located during a given sample)

This resistance (from the sender) implies an "unbalance" To the Balanced Bridge inside the Gauge, The Bridge, being in theory no more than 4 resistors in a Diamond shape and all inputs "Balanced" to Zero at static...(no sender load) Samples The rise and fall of varied Resistance from the sender and Balances the current load by "offloading" (so to speak) the difference in current / resistance on the meter movement..Hence...The meter moves up And / or down depending on the amount of resistance applied / unapplied to the bridge.

This Should be (I'm pretty Sure) The Sender you have, Typically used in most pre ~ high tech vehicles....again, internally grounded at the tank, and a single wire Carrying a varied resistance back up to the gauge sensor input. And that's all good.

I Think from what you have described, all you need do is reverse The leads (traces) At the laminate card and it will be all good! What to try is: Remove the gauge, and using alligator Clips and jumper wires for the gauge temporarily "jumper" it in and see if it gives the proper indications prior to modifying the card...If not we'll work on a plan "B"...

Mend Soon!

Doc
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Old 05-07-2004, 04:29 PM
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Thanks Doc
When I'm back in action I will go and play, Helps now that I know the sender is standard fare
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