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An OHM meter will tell the tale!

Getting back to working on the Super. I haven't had time to do anything with it for several months.

Right now I'm trying to figure out the fuel gauge problem. Gauge never reads anything. There are two posts on the rear of the fuel gauge, and tried grounding the one in the center of the fuel gauge....turns out that just makes sparks at the ground. The upper post is the one that brings the signal from the fuel tank. So I grounded that one out tonight for less than a second. The gauge flew immediately to Full, and went immediately back down to Empty when I quickly released ground. Now it doesn't move at all when grounded anymore.

Did I blow the gauge? The car has been converted to 12V. No idea what sort of protection is present for dash gauges and lights. None of the lights work on the dash and none of the gauges work except for speedo (mechanical), oil pressure (mechanical), and temperature. Everything else (ammeter, fuel gauge, clock, etc) is dead. It's been this way ever since we got the car running again. No idea if they ever worked in the past.

I will still drain and drop the tank (ugh) to check the fuel sender. But I wonder if there is something else going on here. The wiring behind the dash is a nightmare....so many additions that I have no idea what anything does or where it goes. Is there a wiring diagram out there for 38's?

-Kevin

Wire diagram should be in the literature section, probably in there service letters under specifications.

As far as circuitry, it shouldn't be that much different than my 37. The fuel gauge is the only gauge that needed a voltage step down. The others are mechanical and the Amp meter doesn't care.

I'd go through the fuel gauge testing in the service manual. It's important that gauge itself is grounded or it won't read correctly.

I small 12v bench too power supply that I use to check all my electric items and bulbs before they go in the car. I have a 12v-6v runtz installed on the back of the gauge. Which also needed to be grounded.

I always treat unknown wiring and brakes like my arch-enemy last worked on them. Original wiring at this point should be considered past its prime and unsafe.

Thanks Kev, found the wiring diagram for '38. I need to check the Literature Archive more....I've been going to Model Information mostly but it doesn't have all the other documents listed there.

I have been reading through the service manual and ran the tests they said for the gauge, which is why I'm perplexed. The gauge responded the first time I grounded it out and never again after that. Gauge itself should be grounded but I guess I can check that too.

To fix the wiring under the dash all the modern additions will need to be deleted (A/C, air handler, etc). It would be a monumental task and I'm hesitant to even attempt it. But the way it is now I can't make sense of what anything is or where it goes. There are at least 6 additional switches, rheostats, and buttons mounted under the dash that I'm clueless as to what they are for. Tracing the wires is impossible.

-Kevin

According to both the 37-8 (page 92) and 38-9 (page 90) service manuals the gauge is a Motor Meter magnetic type and if the conversion has powered it directly with 12v and you hit it with straight ground, there is a good chance you burned out a coil -- or maybe both coils depending on where the connection was made.

To verify if there is damage you might disconnect the gauge completely and then check the coils with an ohmmeter. Between the battery post and ground there should be continuity for the potential coil (left coil in drawing) and between the sender terminal and battery terminal would check the series coil (right side coil). Between ground and sender terminal will put both coils in series so there should still be a reading but slightly more resistance. Typically coils of that type show very little resistance so depending on several factors and meter sensitivity it might appear to be a dead short when reading thru the coils or maybe a bit of resistance in the leads. If the meter is capable of such small measurements it will show more resistance reading thru both coils in series. An open or no reading at all would indicate a burned out coil.

The way the gauge works in this SM illustration is the full time or potential coil always provides a bias or slight pull on the needle to force it to the empty end of the gauge. The changing resistance in the sender varies the strength in the series coil to work against the always present bias and when tank is full, sender has minimal resistance which makes the series coil stronger so needle moves toward full on the gauge. The varying strength between the two coils will cause needle movement needed to change the reading. Different brand magnetic gauges will work the same way but depending on brand there could be different methodology. One brand might have the series coil be stronger and the sender change weakens it while another does the opposite. One brand might bias the needle always toward empty and another might bias it toward full.