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.
Here is the electrical schematic for those interested. Thanks.