Here is what I have been dealing with. Posted over on my vehicle specific board, but no one seems to have any idea. Instead of reposting everything hashed over there, I will post the original question and maybe someone can jump over and read my follow up. Any help appreciated.

I am thinking perhaps the relay is momentarily pulling too much from the ign circuit? Thanks again!


"On my 74 SII rebuild. Still doing body work, and run it every time I work on it for a few minutes. Both batteries are charged, tested, and basically brand new. Anyways, I replaced ALL the gauges with after market, tying them all in to the original sending units, if required. The issue I am having is between the horn and the fuel gauge. Since I have a detachable steering wheel, I mounted the horn on the dash via a push button. I have four buss bars tucked behind the dash on the inside firewall- one is solely ground for all gauges and components, the other is solely batt 12 volt for radio, acc, etc. The third is solely ign on. The fourth is solely lights activated on - coming off the light switch - powering gauges/radio that have dim feature.

The push button goes from the 12v batt to the horn relay to the horn and grounds at the horn. The fuel gauge is connected to the original sending unit wire and is working, 12v goes to ign only buss bar, ground buss bar, and light (dim) buss bar.

Never noticing it before today, if you turn the ign key all gauges come on - as supposed to. The fuel gauge reads how much I have, which I believe to be accurate since the percentage has been dropping between filling up, breaking in new engine and running periodically at shop. When I hit the horn button I notice that the fuel gauge (reading the correct amount) "trips" showing zero, then goes back to showing the correct amount. It seems that the action is in conjunction with the horn relay in the engine compartment. It seems to also happen when you let your finger off the push button, especially if you hit the button really quick (before horn can sound)...

I cannot figure out what is happening. Not worth pulling the entire dash - I can live with it, but was wondering what might be going on? No shorts, nothing, everything was meticulously wired, and double checked. Disconnected the ground from the horn, still happens, disconnected the hot going to the horn, still happens, the only thing being actuated is the relay which seems to be "tripping" the gauge momentarily before showing the correct amount.

Thoughts? Thanks."

Here is the electrical schematic for those interested. Thanks.


  • What car is a 74 SII? From the diagram I could deduce it is a old truck with 4 wheel drive, right? Commented May 29, 2016 at 23:05
  • From search around, I deduced this could be a Land Rover Series II, but those were manufactured until 1971. Commented May 29, 2016 at 23:08
  • 1
    Sorry about that. 1974 Scout II (International)
    – Kirk
    Commented May 29, 2016 at 23:18
  • Per OP - Thanks. I looked at it again this weekend and even used a different power source for the horn circuit - I have dual batteries. It tripped the gauge again. It has something to do with the power supply, as I noticed when you pull the horn fuse and honk the horn (obviously will not work) the gauge does not trip... Commented Jun 8, 2016 at 18:50
  • That's not the horn, that's the fuel dump valve.
    – Mark
    Commented Jun 9, 2016 at 1:33

2 Answers 2


Theory 1

From your wiring diagram the horn relay has only 3 contacts; 12v, horn, button. This layout dictates that the horn button should ground the horn relay and not send it 12v. This is because the horn relay does not have it's own ground. Sending the relay power may cause things to behave strangely.

Theory 2

The sending unit is a very old style. There is only a sing wire. The sending unit is then grounded to something else, the tank, the body, something else. The ground between the horn and the tank may be shared. When the initial inrush of the horn happens the ground may develop a voltage drop. This voltage drop can cause the gauge to freak out.

  • Thanks. So how would I go about testing these theory's? ...and how to fix them? Thanks again! Yes, the sending unit is a single wire, grounded to the frame at the back of the vehicle.
    – Kirk
    Commented May 29, 2016 at 23:21
  • @Kirk Theory 1 is easy to check. Find the relay and verify it's wiring. Theory 2 is harder to check. One way would be to use a scope connected between the battery negative and the sender ground. Hit the horn and see if the wave form jumps. A regular multimeter is not good enough. Inspecting the top of the tank may be enough to find the problem.
    – vini_i
    Commented May 29, 2016 at 23:32
  • 1
    Thanks. The only thing I altered on the horn circuit is the push button on the wheel. Installed a small push button on the dash since you can now remove my steering wheel (quick release). Very strange, never had the issues with the old analog gauge. When I installed all digital gauges, this one went weird. The gauge itself does not blank out (like a power cut), it just resets to zero and then goes to what it was prior to hitting the horn push button...
    – Kirk
    Commented May 29, 2016 at 23:36

I did a quick check on the schematic and it appears that the two circuits are completely separated from each other.

I also noted that the fuel sender circuit number is 36 and the horn is 63. You might want to carefully review the schematic.

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