Fuel Gauge Wiring

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  • dreddub
    Low Range
    • May 2012
    • 24

    Fuel Gauge Wiring

    Ok Gentlemen,

    What I really need to do is pay someone to rip out the 'half and half' wiring job that the last owner put into this thing and rewire with a new harness, but that's down the road once I get this thing road worthy and decide that I like driving it enough to keep it.

    Now, to the issue, like I said, the PO did a bunch of rewiring and splicing etc. I'll try to take some pics, but I think I can explain. Behind the dash is spaghetti of new and old wiring spliced with no real color co-ordination, etc. Now, there are a bunch of loose wires with taped labels that say things like 'temp gauge' but I don't even have one. Basically, I have a bastardized harness that works for most things, but the fuel gauge doesn't work. I've got the early series IIa with the 'arrow' guage' There are 3 connections on the back of my gauge, all the pics I've seen online only have two. One side is a 'double' and the other is a single. There is a paper like connection between them, with a hole, and the paper is cracked on either side of the hole. I cannot imagine that this is conductive or that it would matter that it is cracked trying to push the pieces together seems to make no difference. Now for wiring, there is a hot solid green wire that reads ~10 -11 volts. There is only one wire coming from my fuel sender it is green with a black line. The previous owner had the hot solid green hooked up to one side of the double, and the green/black hooked up to the other side of the double. The single spade across the gauge had a wire attached that said 'temp gauge' attached to it by him. Obviously, this isn't right as it isn't working. I would assume that the hot (solid green) goes on the side with the double and that the sender wire (green/black) on the single side? I don't seem to have a voltage stabilizer anywhere. Finally, I would think that the double spade might be to send hot voltage to a different gauge with a jumper. Now I've hooked it up that way but it just reads full (which its not). I've hooked it up in reverse and the same thing happens. When hooked up the way the previous owner had it, it reads empty. I get voltage at the sender of about 7V so I assume its working to cut the voltage from 10 to 7?

    To make matters worse, there are two more wires floating around inside the panel that say "fuel gauge" but one of the ones that says "temperature gauge" (which I don't even have one) was connected to one side of the fuel gauge and one of the 'fuel gauge wires was just hanging loose.

    Might just have to get gas every I go out until I can get front and rear seatbelts in this thing and drive it enough to see if I want to keep it.

    If you can make any sense of this rambling and have any ideas. Please let me know.

    Thanks,

    Dreddub
  • disco2hse
    4th Gear
    • Jul 2010
    • 451

    #2
    Positive or negative earth?

    Here are wiring diagrams for both
    Negative: http://www.lrfaq.org/Series/FAQ.S.elec_SIIA-VE.html item 36 (note the voltage stabiliser)
    Positive: http://www.lrfaq.org/Series/FAQ.S.elec_SIIA+VE.html item 21 (note, no voltage stabiliser)

    If it has been converted to negative earth, it may be that the job is half done.
    Alan

    109 Stage 1 V8 ex-army FFR
    2005 Disco 2 HSE

    http://www.youtube.com/user/alalit

    Comment

    • dreddub
      Low Range
      • May 2012
      • 24

      #3
      It is negative earth with an alternator. It certainly seems it is half done/baked. First, looking at the wiring diagram, my fuel gauge has 3 spade connectors, not 2. And I certainly don't see a voltage stabilizer anywhere. So do I just have to buy and install one, or am I in for a lot more?

      Dreddub.

      Comment

      • dreddub
        Low Range
        • May 2012
        • 24

        #4
        Ok, looking at the positive earth wiring diagram, I see where the fuel gauge has a jumper that comes off the first spade connector (this is certainly the gauge that I have) and then goes on to the "screen wiper, plug and socket" So I would think that I could just leave that off and be fine since my wipers are somehow wired and (weakly) work...

        Do I need a new gauge? Or just a voltage stabilizer? Shouldn't the gauge work without the stabilizer, just erratically?

        dreddub

        Comment

        • disco2hse
          4th Gear
          • Jul 2010
          • 451

          #5
          Hard question to answer.

          Looking at the negative earth wiring diagram, the rheostat that is the sender unit on the fuel tank varies the resistance to the earth, and in that sense, the gauge acts as an ammeter of sorts. In which case, the stabiliser is necessary so that the current is also controlled. That is, the stabiliser acts as a limiter from the alternator's output and without it, the amount of current is too high and on your installation regardless of where the sender is set, the gauge is always reading full.

          Oh dear, trying to remember my physics classes... hopefully, someone with more knowledge will confirm or deny my assumptions.

          The other thing, if the dynamo was replaced with an alternator and the regulator was not upgraded at the same time (the alternator may output more current and voltage than the previous unit), then the amount of current in the circuit may also to be too high. Same result too.

          By the way, it doesn't look on the positive earth diagram that you have a temperature gauge in the circuit. So the wire marked as such may have been the PO's attempt at making the circuit look like the negative earth diagram (just guessing, now)
          Alan

          109 Stage 1 V8 ex-army FFR
          2005 Disco 2 HSE

          http://www.youtube.com/user/alalit

          Comment

          • o2batsea
            Overdrive
            • Oct 2006
            • 1199

            #6
            What year? Early diesels had a low fuel light, so there are three wires on the sender. As you are finding, the wiring is often a mashup of original and later hacks/repairs. A new harness from Britishwiring.com will cure all the problems, but they are not cheap. Better be sure that you want to go all in on the Rover thing as they will bleed you dry.

            Comment

            • dreddub
              Low Range
              • May 2012
              • 24

              #7
              68 petrol. I'm going to read a bit more on voltage stabilizers, but if that doesn't do it, I'm going to button it up and just fill it Up each time I leave the house.

              Comment

              • disco2hse
                4th Gear
                • Jul 2010
                • 451

                #8
                And carry a spare in the rear.

                Of course, one may find such bodges on any old vehicle of uncertain history. That is why we find them so "interesting".
                Alan

                109 Stage 1 V8 ex-army FFR
                2005 Disco 2 HSE

                http://www.youtube.com/user/alalit

                Comment

                • dreddub
                  Low Range
                  • May 2012
                  • 24

                  #9
                  Originally posted by disco2hse
                  And carry a spare in the rear.

                  Of course, one may find such bodges on any old vehicle of uncertain history. That is why we find them so "interesting".
                  Ha! Funny I've got the spare on the bonnet, but no jack or tire iron to change a flat if I really needed to! At least I realized it and will (hopefully) correct that before I really need it. This is just a beach buggy for me, but I bought it in early summer thinking it would actually see the sand this year... Now I'm just hoping for next summer. Still think its way more 'interesting' than a bronco and certainly better than a jeep. =)

                  Comment

                  • disco2hse
                    4th Gear
                    • Jul 2010
                    • 451

                    #10
                    Boy, language....

                    Sorry, I meant a spare can of petrol in the rear

                    PS. I put my spare tyre in the rear too. I like to see where when the front is going when I approach a drop-off.

                    PPS. We ought to classify ourselves as "vehicular archeologists".
                    Alan

                    109 Stage 1 V8 ex-army FFR
                    2005 Disco 2 HSE

                    http://www.youtube.com/user/alalit

                    Comment

                    • SafeAirOne
                      Overdrive
                      • Apr 2008
                      • 3435

                      #11
                      The fuel/temp sender circuits are 2 of the simplest circuits on the vehicle. You'll be able to work out your issue or bypass the mess and wire it up correctly to make the system functional without much difficulty.

                      12v is supplied through a dark green wire (usuallly from a spade connector on the 'downstream' side of the "hot in run" fuse) to either 1) a voltage stabilizer input or 2) directly to the input side of the fuel or temp gauge, then ''piggybacked' on to the input of the other gauge. If a voltage stabilizer is used, then it's the 10v output from the stabilizer that gets 'piggybacked' to the gauge inputs via a light green wire.

                      The output from the gauges (green with black tracer for the fuel gauge, green with blue tracer for the temp gauge) go to their respective senders, where the resistance is varied based on fuel level or temp, then on to ground to complete the circuit.
                      --Mark

                      1973 SIII 109 RHD 2.5NA Diesel

                      0-54mph in just under 11.5 minutes
                      (9.7 minutes now that she's a 3-door).

                      Comment

                      • jvsimmons
                        Low Range
                        • Mar 2013
                        • 54

                        #12
                        I have an 1983 S3 diesel 88 station wagon. I have been attempting to diagnose why my low fuel warning light does not work. I have read many post about the fuel sending unit. I have discovered in my readings that one of the bayonet connectors on top of the sender is marked with a "W" to indicate where the wire for the low fuel light is connected. I assume then that one of the other two remaining connectors is for the fuel gauge. My S3 only has two wires connected to the sender. I am still trying to trace these wires once they enter the bulkhead - having some difficulty because the previous owner installed a new harness but some of the wires ( like the ones connected to the fuel sender) are not the correct color as shown in wiring diagrams. And behind the instrument panel is a plate of spaghetti with wires disappearing in a larger wrapped bundle. Having said all that my question is: should there be three wires ( a ground wire?). The fuel gauge seems to work but have no ideas as to accuracy.

                        Comment

                        • jimrr
                          4th Gear
                          • Dec 2010
                          • 425

                          #13
                          dreddub has this :!, on the wire FROM the tank measure the resistance. also measure the voltage from the vehicle TO the tank. AND OR, you can apply voltage to the wire to the tank with zero or a full tank and measure the resistance. this last tells you if the sender works.

                          Comment

                          • roverp480
                            3rd Gear
                            • Jul 2020
                            • 324

                            #14
                            deleted

                            Comment

                            • jvsimmons
                              Low Range
                              • Mar 2013
                              • 54

                              #15
                              Pulled the fuel sensor, turned ignition switch to first position, gounded the sensor to discover after moving the float up and down the shaft that all is good. Low fuel light works as well as the guage. Also think I solved the other electrical issues. Thanks everyone.

                              Comment

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