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  1. #1
    FEP Power Member LTDScott's Avatar
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    Default Fuel gauge on the fritz

    The gas gauge on my LTD LX has been intermittently not working correctly. Sometimes when I turn on the key, the gauge doesn't move and just stays pegged at the bottom, but after a while of driving it comes back to life.

    Other times, the gauge will be fine, but then while driving it decides to stop working and the needle starts to sink, then it starts working again.

    Any idea if this is most likely to be the sender in the tank, the gauge in the instrument cluster itself, or something else? I'll have to pull out the old EVTM and start testing stuff, but just wondering where to start.

    I searched here and didn't find much info, but I did find this handy post with sources for a fuel level sender resistor if that's what needs to be replaced:

    http://vb.foureyedpride.com/showthread.php?t=109272

    Of course my tank is completely full of gas, so dropping the tank will have to wait!
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  2. #2

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    Mine will read full when full. The rest of the time it wonders til the car gets to a 1/4 of a tank.
    1985 Mustang GT Mild 331 4bbl 5 spd, 1985 SEFI LTD LX AOD.
    Cardomain: http://www.cardomain.com/ride/565542
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  3. #3
    FEP Power Member LTDScott's Avatar
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    Mine has always "wandered" a bit while driving, but as I said now it frequently refuses to work at all.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  4. #4
    FEP Senior Member FuturaGuy's Avatar
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    The gas gauge, instead of being a voltmeter, is a bi-metal coil that winds or unwinds depending on the amount of current going thru it. When the tank is empty, the resistance from the sender is high, about 85 ohms. When the tank is full, the resistance is low, maybe about 15 ohms. (I should go check my shop manual for the exact numbers but it's in the ballpark.) A build-up of crud and corrosion on the terminals will cause the gauge to read empty all the time.

    If it's any consolation, I'm having the same problem on my '72 Mustang and it's been too cold to do a major project like draining and dropping the tank.
    Last edited by FuturaGuy; 03-28-2013 at 04:14 PM.

  5. #5
    FEP Super Member TWR2003's Avatar
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    Quote Originally Posted by LTDScott View Post
    The gas gauge on my LTD LX has been intermittently not working correctly. Sometimes when I turn on the key, the gauge doesn't move and just stays pegged at the bottom, but after a while of driving it comes back to life.

    Other times, the gauge will be fine, but then while driving it decides to stop working and the needle starts to sink, then it starts working again.

    Any idea if this is most likely to be the sender in the tank, the gauge in the instrument cluster itself, or something else? I'll have to pull out the old EVTM and start testing stuff, but just wondering where to start.

    I searched here and didn't find much info, but I did find this handy post with sources for a fuel level sender resistor if that's what needs to be replaced:

    http://vb.foureyedpride.com/showthread.php?t=109272

    Of course my tank is completely full of gas, so dropping the tank will have to wait!
    Scott,

    Gauge, IVR, sender, wiring

    Disconnect the little pull-off wire connector from the sender. Connect the wire to good chassis ground using a piece of jumper wire. If the gauge pegs high when the ignition switch is turned to ON the gauge and the wiring from the gauge to the sender is good. Then we would suspect the sender or IVR.

    Since your cluster's only gauge is the fuel gauge there is no other way to tell if its the IVR (a bad IVR in a cluster with multiple gauges would affect all the gauges) other than replacing it. But check all other items before replacing it.
    Last edited by TWR2003; 03-29-2013 at 10:27 AM.
    MF: Shoot pool Fast Eddie.
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  6. #6
    FEP Power Member LTDScott's Avatar
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    My car has a factory tach, and it works fine. I don't know instrument cluster stuff, so would a bad IVR affect the tach as well?

    Gotta wait until I get the tank empty before I drop it and attempt to check the wiring and/or sender. Thanks for the advice on how to troubleshoot it!
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  7. #7
    FEP Super Member TWR2003's Avatar
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    Quote Originally Posted by LTDScott View Post
    My car has a factory tach, and it works fine. I don't know instrument cluster stuff, so would a bad IVR affect the tach as well?

    Gotta wait until I get the tank empty before I drop it and attempt to check the wiring and/or sender. Thanks for the advice on how to troubleshoot it!
    No. IVR is used by gauges only - fuel, temp, oil. When all the gauges act funny (all high or all low, etc) thats how you can easily tell its the IVR vs a sender issue since they all use different senders. But since your car's only gauge is the fuel you need to further isolate by testing as we outlined.

    You dont need to wait or empty the tank to do the test. And based on the results of the test you may not need to drop the tank. If you cant reach the connector with the tank installed you can still ground the sender wire (should be YEL/WHT) anywhere along its length using a safety pin and a jumper wire. The sender connects the wire to ground thru a variable resistor. Minimum resistance is when the tank is full so when you ground the wire the gauge should peg at full.
    Last edited by TWR2003; 03-29-2013 at 08:40 PM.
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  8. #8
    FEP Senior Member cottonbear's Avatar
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    I got one for ya, my gauge reads properly when ignition is on or in the accessory position, but when the engine is running it reads about an 1/8 low...

    Ahh the joys of driving old vehicles!
    Current buggies

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  9. #9

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    arent the tach and speedo technically both gauges?
    1984 LTD Wagon - S3AHAUS
    1988 Mustang LX 5.0 Hatchback - NOBIMMR
    1997 BMW 318ti - TinyBimmer
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  10. #10
    FEP Power Member LTDScott's Avatar
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    Quote Originally Posted by frink84 View Post
    arent the tach and speedo technically both gauges?
    Yeah, that was my thought too, heh.

    For what it's worth, I drove the LTD yesterday and for the first five minutes the gauge was completely dead. Then it came back to life. Mine has always read anywhere from 1/8 to 1/4 low.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  11. #11
    FEP Senior Member OX1's Avatar
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    Quote Originally Posted by TWR2003 View Post
    No. IVR is used by gauges only - fuel, temp, oil. When all the gauges act funny (all high or all low, etc) thats how you can easily tell its the IVR vs a sender issue since they all use different senders. But since your car's only gauge is the fuel you need to further isolate by testing as we outlined.
    Don't suppose the IVR from an LTD LX might be the same as
    a std LTD IVR (of which my sad has several I'm sure)?
    His guage hasn't read correct since we put in the LX cluster.
    86 Capri, 5.0, 5Spd, A9L QH/BE, 47 lb Inj PMAS 3" MAF, Single T44 Turbo, Front Mount IC, TW170,
    Stock Cam, Explr Intake/TB, 1.7 Rockers, CF dual friction clutch, 3" DP, 2.5" full Exh, 3.27, 11.932 @ 115.78
    84 LTD, 331-10:1, TW170/Exprl Intake, 47 lbs inj/80 mm LMAF, Full Duals, Quarterhorse, Vortech 7PSI, Lentech AOD, 5 lug Mk VII brakes/rear, Eibach Sway bars, Cobra HB (dads ride, but I fix it )

  12. #12
    FEP Senior Member OX1's Avatar
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    Quote Originally Posted by frink84 View Post
    arent the tach and speedo technically both gauges?
    Well, the speedo is mechanical and I thought the tach used full 12v.

    Here is the wiring. Is the IVR tapped off red/yellow and only used
    by the gas guage? I mean most aftermarket tach's get 12v's, but what do they do with them internally?

    Last edited by OX1; 04-04-2013 at 11:42 AM.
    86 Capri, 5.0, 5Spd, A9L QH/BE, 47 lb Inj PMAS 3" MAF, Single T44 Turbo, Front Mount IC, TW170,
    Stock Cam, Explr Intake/TB, 1.7 Rockers, CF dual friction clutch, 3" DP, 2.5" full Exh, 3.27, 11.932 @ 115.78
    84 LTD, 331-10:1, TW170/Exprl Intake, 47 lbs inj/80 mm LMAF, Full Duals, Quarterhorse, Vortech 7PSI, Lentech AOD, 5 lug Mk VII brakes/rear, Eibach Sway bars, Cobra HB (dads ride, but I fix it )

  13. #13
    FEP Super Member TWR2003's Avatar
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    Ford fuel, temp, and oil pressure gauges from mid '60s to mid '80s (thru '86 on Fox cars) use an IVR that outputs ~5vdc from a 12vdc nominal input.

    The IVR used on all 78-86 Fox cars was basically the same.

    Ford tachometers run on full 12V and are not connected to the IVR.

    Ford speedometers were mechanical until the mid/late '90s (model dependent).



    'tack' as in tackometer. Thats funny (or is it 'tacky'?).
    Last edited by TWR2003; 04-04-2013 at 05:38 PM.
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  14. #14
    FEP Power Member LTDScott's Avatar
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    Quote Originally Posted by TWR2003 View Post
    Ford fuel, temp, and oil pressure gauges from mid '60s to mid '80s (thru '86 on Fox cars) use an IVR that outputs ~5vdc from a 12vdc nominal input.

    The IVR used on all 78-86 Fox cars was basically the same.

    Ford tachometers run on full 12V and are not connected to the IVR.


    Ah, that's what I figured, but didn't know for sure. Won't have time to mess around with this for a while. Thankfully this isn't a daily driver.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  15. #15
    FEP Power Member LTDScott's Avatar
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    Resurrecting this from the dead. I finally got under the car and grounded the yellow wire for the fuel level sender to test functionality. The gauge pegged to full, but it didn't smoothly. The needle kind of surged/pulsated to full. I understand the original style EVR "pulses" to limit voltage to 5 volts (almost like a turn signal flasher) - would that explain the pulsing? Based on the results, does it mean the sender is good and the IVR is likely bad? The LTD cluster doesn't have any other gauges that run off the IVR, so I can't check based on that.

    Is the LTD IVR the same as Mustangs? I did some research and it seems tons of different Fords up to '86 use the same style IVR, but I haven't pulled the cluster to confirm. I hope it is the same, since there are solid state aftermarket versions available for the Mustang.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  16. #16
    FEP Power Member LTDScott's Avatar
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    I pulled the cluster out of my car and it is the Mustang style. Mine had part number D1AZ-1084-A on it. The base part number is the same as the similar year Mustangs. I tested mine out but without an oscilloscope or analog voltmeter, all I could do is confirm it was pulsing on and off with a test light. So I think I'll pick up a new solid state one and maybe upgrade to LED lights while I'm at it.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  17. #17

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    I had the same trouble with the gas gauge in my LTD, it worked intermittently then finally died completely. I tried a solid state voltage regulator but it didn't help. Finally found a bad connection where the dash harness plugs into the body harness [under the dash, behind the fuse box]. I seem to remember it being a yellow wire. All the other terminals in the plug looked good but the one for the yellow wire was corroded and loose. I cleaned it and retensioned the terminal then the gas gauge worked again. It still drifts a bit but that is due to the sending unit and it's not bad enough to justify dropping the tank.

  18. #18

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    Scott, yes the IVR pretty much works exactly like a turn signal flasher. The gauges work by heating a length
    of wire inside, which causes the wire to lengthen. The needle is measuring that lengthening of the wire. It's
    a pretty slow process, so you normally don't see the IVR pulses. And since the pulses have such a long time
    constant, it's not easy to directly measure the mean voltage.

    The shop manual specified procedure is to ground the sender wire through a 10Ω resistor, and verify that the
    needle is within the fat line at "F". Then repeat the procedure using a 73Ω resistor, and look for the needle to
    be within the fat line at "E".

    I do suspect you're going to end up in the tank, so to speak.
    Cheers,
    Jeff Cook

    '85 GT Hatch, 5-speed T-Top, Eibachs, Konis, & ARE 5-Spokes ... '85 GT Vert, CFI/AOD, all factory...
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  19. #19
    FEP Power Member LTDScott's Avatar
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    I've been doing a bunch of work on this over the last few days but I'm not any closer to a resolution. Here's what I've done so far, from the beginning.

    -With an estimated 1/3 of a tank of gas (based on gallons added and miles driven) and the original IVR, the gauge displayed a tick under 1/4 full. This is consistent with what I've seen normally - the gauge reads about 1/4 tank low.

    -Grounded fuel sender wire (yellow w/ white stripe) right near the tank which resulted in the gauge pegging full with the key on. This confirms the wiring, gauge, and most likely IVR are good.

    -Removed the IVR from the cluster and bench tested by connecting it to good power and ground, and was able to see the IVR output pulse on and off as it's supposed to. No way for me to measure the average voltage, but at least I can see it's not stuck constantly on or off.

    -Measured the resistance between the fuel sender wire and ground with an estimated 1/3 of a tank of gas in order to see what resistance the fuel level sender (and associated wiring) has. I tried a few times and got varying results between 60-70 ohms. The EVTM says 60-86 ohms = empty and 8-12 ohms = full, so in that case I'd expect to see something like 40-50 ohms with 1/3 tank of fuel.

    -Measured the resistance of the gauge itself. I don't know what the spec is, but in the course of investigating this I found a classic Mustang site that talked about diagnosing the IVR (same exact system as my car has) and they said that all gauges should measure about 13 ohms. Mine was 12.5 ohms, so I believe it's good.

    -At that point I decided to buy a new solid state IVR just to see what happened. First I bench tested it and it output a consistent 5.02 volts, so it appears to be good. I then installed it in the cluster and reconnected the cluster to the car. The gauge needle just barely moved from the pegged at the bottom (way below empty) to just below empty. I thought that was odd, so I measured the output of the IVR while it was connected to the cluster and still got 5.02 volts. Just to eliminate bad ground issues, I connected the case of the IVR to a good ground using alligator clips and the voltage still showed 5.02 volts. So that tells me that the IVR and related wiring to/from it are good.

    I even checked the connector behind the fuse box that mrriggs mentioned above and the pins were clean and tight. I hit them with contact cleaner anyway.

    Out of all of these tests, the only result that seems wrong is the resistance reading through the fuel level sender, so I think it's bad but that doesn't explain why the gauge showed a bit less than 1/4 full with the original IVR. My best guess is that the sender in the tank is bad and the original IVR is outputting something other than an average of 5 volts? So replacing the IVR fixed one issue, and now the gauge is showing empty based on the incorrect resistance reading from the sender. Thoughts? I really don't want to drop the gas tank but I already bought an NOS fuel pump assembly and everything else checks out, so I think it's the likely cause of problems.
    Last edited by LTDScott; 01-22-2017 at 05:19 PM.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  20. #20
    FEP Power Member LTDScott's Avatar
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    Just to put an end to the story, I ended up dropping the tank (which cracked the hell out of the plastic gas filler "pocket" inside the trunk - had to fix it with epoxy) and pulled the stock sender out. With the sender out of the tank but plugged in, the gauge worked fine. The float on the sender wasn't soaked either - it's plastic of some kind and still floated fine. So I'm not really sure why the gauge stopped working.

    As I mentioned before, I went ahead and bought a NOS Ford pump/sender assembly, so I decided to use it. I tested its operation before I put it in the tank and it was fine, so I put everything back together and the gauge works again - hooray!

    The gauge has always read about 1/8 to 1/4 tank low (meaning when the tank is full the gauge says about 7/8 full), and unfortunately the new sender didn't solve that issue. I know the gauge itself will show completely full when the float on the sender was all the way up, so I have to wonder if maybe I got the wrong sender? There are several different variations of the units depending on sedan vs wagon, 16 vs 20 gallon tank, analog vs digital gauges, and even production date. To determine which unit I needed, I asked the previous owner of the car, a Ford parts counter guy, to look up the OEM part number based on the details of my car, and then ordered that part number from Green Sales. It matched the unit I took out perfectly, however the unit I took out of the car wasn't original (had a 1988 production date on it). I think the previous owner probably replaced the pump when he owned the car (15+ years ago) and he probably relied on the Ford parts computer for part number as well, and I'm thinking maybe it's not the correct one.

    But at this point, at least I do have an operating gauge, so I can live with it reading a little low - at least I know when it reads empty, I have some reserve capacity.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  21. #21

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    This is part of the reason I decided to completely replace the LTD Dash with a Mustang one (other then the Mass Air and better Fuel injection system), but unfortunately it hasn't fixed the Fuel gauge issue, since I had to use an LTD Fuel Tank and sending unit. The Gauge works, but when empty it shows a 1/4 tank. In my case the only fix is to modify the actual gauge in the tank, but I will leave that for another day.
    Funny I thought I would have had other issues with drivability, etc., since I replaced all the electrical, but the gas gauge is really the only gremlin..
    1985 Ford LTD LX
    1989 Mustang 5.0 Coupe
    1994 Ford F150 Lightning

  22. #22

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    LTDScott I've acquired an 84' LTD wagon recently and am in the process of adding an LX gauge cluster. While removing the instrument panel there is this wire with a yellow connector laying behind it that isn't hooked up to anything. My wiring diagram says calls it "nameplate" if I'm reading things correctly. I tested for power with the lights on and it has none. Do you (or anyone) know what it is for? pic attached. Name:  20170301_171902_HDR.jpg
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    1978 Mercury Zephyr Z7 w/87' 5.0/C4 EFI intercooled twin turbo power

    1984 Ford LTD wagon w/87' 5.0/4R70W EFI nitrous outlet X-series



    There is a fine line between a hobby ...and mental illness.

  23. #23
    FEP Power Member LTDScott's Avatar
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    No idea, unfortunately. My car doesn't have that connector. Based on what I saw in the EVTM, the "nameplate" wire is for the passenger side of the dash.
    85 Ford LTD LX: Four eyes and Four doors

    EFI swap, AFR 165 heads, 5-speed conversion, Cobra brakes, etc.

  24. #24

    Default LX fustercluck!

    Quote Originally Posted by DarrenK View Post
    LTDScott I've acquired an 84' LTD wagon recently and am in the process of adding an LX gauge cluster. While removing the instrument panel there is this wire with a yellow connector laying behind it that isn't hooked up to anything. My wiring diagram says calls it "nameplate" if I'm reading things correctly. I tested for power with the lights on and it has none. Do you (or anyone) know what it is for? pic attached. Name:  20170301_171902_HDR.jpg
Views: 412
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    Did you have any luck re-pinning the LX cluster to the LTD dash? I have had no luck. the EVTM doesnt' show the LX cluster with tach as deeply as I have studied it. and the chart above doesnt show the same wires as my 85 basic cluster.

  25. #25

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    I am pretty sure he got it to work. That car has been a feature car inseveral publications. I recall he has police speedo on one side and lx tach on the other. I am similarly re pinning my connector to use the lx cluster, however my ford m-12071-a50 engine harness has a wire from the computer for tachometer that I am going to splice into that pin location.

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