Those pesky Gremlins!

I'm still in the process of "sorting" my car and like most of us once an issue is cured another pops up. So is the case with my GT40. I need input on the following issues with my Smith Gauges and would appreciate any insight before I take the entire car apart to rewire the whole damn thing. :shy: First the speedo. Its a replica of course and electric not mechanical. It has never worked properly. I've replaced the pick/up at the transaxle mount and insured its proper gap from the pick ups. I've rebuilt the very weak connector found at the rear of the car and have noted power getting to the gauge because the lite works and the oddmeter while stuck on 28.3 miles is now showing 28.5 miles even though I've driven the car well over 250 miles since. At times, I'll notice the needle jumps a wee bit when I turn on another electical component but then returns to zero. Ground?

The fuel guage that used to work perfectly is now ONLY working off of one tank. The toggle switch is a 3 position switch. Off, driver side tank (first click) and passenger side tank (second click). When I click the switch for the driver side tank I hear the fuel pump come on but no movement from the needle on the gauge. Everything else on the gauge works. I've made sure there is fuel in the tank, checked the terminals on the guage, checked the leads going to the sending unit and see no problems. I have not removed the sending unit from the tank.....yet. Sending unit.....even though the car only has about 500 total miles it?

Lastly, the lite is out on the tach and I've checked the bulb and it appears to be good. Thanks for whatever you can offer.

I'll admit to not being an expert but multiple electircal issues usually means bad earthing, not usual for multiple things to break at same time, do you have a decent earth point for the Dash portion of the loom?
As for fuel issue, could be broken switch (Can I confirm your setup, 2 tanks, 2 pumps, 3 position switch on dash that not only switches the feed but also the sender output to the single gauge? cool idea, you save space on the dash, means you can fit another gauge to monitor more stuff).

Ant, thanks for your response. The word "ground" has been suggested a number of times so that will be one of the first things I check. You are right about the switch and guage set-up too. So you think it could be the switch itself?

especially since the speedo needle jumps when turning on another electrical component. This really suggests a ground or earthing issue. Make sure that if your chassis is power coated, that the powder coat is cleaned to bare metal for grounding connections, as the powder coat is an insulator.

2 minutes with a continuity tester on the switch terminals. most electronic multimeters have this as a function, the one where it bleeps if the probes are touched together if you don't have one, then a battery hooked up to a bulb and two probes does the trick.

Check switch in multiple positions and wiggle it about in case there are dodgy connections inside. All done with all connections disconnected.

Working on boats, I find that the best way to check all this is first check all the grounds, as noted above, then ring all the suspect wires back- you do this by getting a long piece of insulated wire and isolating every wire and checking its' continuity. It takes a long time but it will tell you if the circuits are good or if they are interrupted somewhere. Note that a three-wire gauge is essentially a device for comparing a modulated voltage to full voltage from the battery. The difference between the two is what regulates the swing of the needle. So the gauge will move even though a wire is off- like the ground wire, but it won't read correctly.