Gas gauge stops working with lights on

[boogiedude]

Title pretty much says it all. A new problem developed last night on the way home. Driving along and all of a sudden the gas gauge goes dark and needle pegs to full, even though I am near empty and the red low fuel light stays on(as it should, since I am almost out of gas). With the headlights off, the gauge goes back to reading the proper level, and the low fuel fuel light remains on. All the connections are on tight, and I can't see any obvious reason for the change in behavior. All fuses look good, and all other gauges function properly.

On a side note, my low oil pressure light stopped working completely a few weeks ago. Doesnt come on at all, even at start up. Changed the bulb, and no change. Any ideas?

[GeorgeT]

I suspect that you have a poor ground. I believe the grounding pod for the instrument panel is a "crown" under the dash. Pull each ground wire off and clean the terminals, apply dielectric grease and then push the ground wires back on. A check of Brad Artigue's wiring diagrams indicated that the fuel gauge and oil pressure light share a common ground wire.

[Zippy]

I agree with George. It looks like a situation where a poor ground would cause this problem. In looking at the drawing I found an error. The light on the fuel guage is labeled the Low Oil Pressure Indicator and the Fuel Low light is shown on the tach. This is opposite of what my 78 Spider is. Brad's drawings are a fantastic resource, especially when my garage is a mess and I can't find my manual.

[Weelan]

Mine did the same thing boogiedude. I'm gonna maybe clean off the grounds and put new connectors on tonight. It's almost certainly the grounds. I have the exact same symptoms.

[bradartigue]

You want to know the sad thing - that diagram is wrong not only in my drawing but also the original FIAT drawing that I used as a reference. You have to realize the process of creating these gets mindless after a while, so you just go with what you have. Looking at it now, I'm like "how stupid."

The fuel lamp stuff is obviously on the fuel gauge.

It is a ground, it is a ground, it is a ground. Same ground probably makes everything read goofy. Dig around, find it, and don't think it is just under the dash, could be the ground in the ass end of the car or even under the hood!

[Zippy]

You want to know the sad thing - that diagram is wrong not only in my drawing but also the original FIAT drawing that I used as a reference. You have to realize the process of creating these gets mindless after a while, so you just go with what you have. Looking at it now, I'm like "how stupid."

The fuel lamp stuff is obviously on the fuel gauge.

It is a ground, it is a ground, it is a ground. Same ground probably makes everything read goofy. Dig around, find it, and don't think it is just under the dash, could be the ground in the ass end of the car or even under the hood!

You shouldn't feel bad about it, I do it all the time when I'm counting light fixtures on drawings. I have to use multiple colored highlighters to make sure I get them all. Of course now I use software that puts a little diamond on every fixture I count, so it's more reliable. I tried to find my manual to see if it was wrong there but I don't know where I put it. I'm not surprised it was wrong there too.

As a side note, I have been driving mine every day since last Friday. Last night when leaving the in laws I turned on the headlights and I had no dash lights. I reached under the dash and wiggled the wires and they came on but then a few miles down the road they went out again. Working under the dash is my least liked task. I would rather be covered in grease with bloody knuckles than be under the dash contorted like a pretzel.

[spidernut]

I feel your pain. Use a good volt meter and ohm meter and start checking to see where the ground or power stops working. It'll save you a ton of time and frustration.

The wiring schematics are good most of the time, but not always. I was working from a factory 80 Spider schematic for my 80 Spider trying to diagnose why my license plate lights wouldn't work. The wiring colors and harnesses didn't match the schematics at all. Thanks to my multimeter, i figured out that the harness between the tail lights was installed in reverse of how it should be wired. The weird part? Only the license plate lights didn't work. Everything else was fine.

When I had a similar problem to yours (not exactly the same), it turned out a ground on the driver's side front fender came loose. I reconnected the ground and everything was up and running properly. I thank members of this forum for helping with that one!!! I'm not sure I would ever have found that one without he help of someone who has been there before.

Tip: Remove the front seat when working under the dash. Sure, it is a pain to remove, but it'll save your neck and back and allow you to somewhat lay down on the floorboard instead of being a contortionist.

[opus10583]

Hi,

When I fished the odo reset knob back through the dash plate I invoked the same problem, when the guys installed a new speedometer cable for me a month later the problem had 'been rectified':

Check and double check all the connections of the dash gauge harness, it's very likely in there somewhere.

Mark

PS It appears my speedo cable has recently disconnected itself, I'll be in my dash tomorrow.