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75- Gas gauge drops to empty when parking and headlights are turned on.

14K views 20 replies 10 participants last post by  Beige80  
#1 ·
Just bought a clean no visible rust 75 to replace my older rust bucket 75 and noticed that when the headlights are pulled on or even the parking lights, the gas gauge drops to empty. I'm scratching my head seem to remember from a ways back that this can be caused by a bad ground in the dash. Does that sound right to you all?
 
#3 ·
maybe thats when you use the most gas and it doesn't get very good mileage, j/k my 86 does the same thing but all of the gauges drop to 0 I had it fixed before or at least I thought I did , there was a ground wire under the dash under the light switch that fixed it for me last time but not this time , so maybe that didn't fix maybe I moved something else sorry im not more help
 
#4 ·
I would suggest checking the ground at the fuel tank sending unit. If somehow someway it is making connection with your rear lights, the signal would backfeed up through the wiring to the fuel gauge, etc... causing the gauge to read empty when the lights are on.
 
#6 ·
Thanks gang! I thought it could be a ground but didn't even think of the ground at the gas tank or a short across the rear light wiring and gas tank. I'll send my skinny son under the dash to check it out first. It also partially goes down when the turn signal is activated.
 
#8 · (Edited)
The only ground in the gauge system is at the tank sending unit, which could be at the unit itself, or the tank itself may not be grounded good due to some rust etc. (?)
There is no connection between the tail light wiring and the fuel gauge wiring, in the diagram, so it's hard to understand how the lights affect the gauge. So somehow, the grounds have to be the culprit. The grounds for the tail lights are easily seen when removing the side marker light assemblies. Each side has it's own ground behind the marker light, which is easy to remove for a look. The screw in those grounds can be replaced with a stainless steel sheet metal screws in place of the originasl, to insure a good connection.

And on the left inner side of the rear quarter panel under the car, is the plug which connects all the rear lights etc to the front harness (5 pins in that connector) Which is also easy to get at, to check it out.
 
#9 ·
"There is no connection between the tail light wiring and the fuel gauge wiring, in the diagram, so it's hard to understand how the lights affect the gauge."

Bill this made my head start hurting in that I had to think, How can that be? Main body ground is the only answer my P brain can come up with that would be a shared connection.
I would check the grounds on the core support and the engine to firewall. When things go crazy it always seems to be ground issues. You can never have enough ground connections. If you don't see them make some and put them on. Battery negative to core support, engine to firewall, transmission to crossmember, I have seen two cars burn down that grounded through the throttle linkage and set the firewall insulation on fire. Both had recently had engine jobs and the grounds were not connected back because some of them are a pain to get to.
 
#10 ·
Steelybill, I am aware that the ground from the tank sending unit isn't supposed to be in contact with the light wiring harness. But after basically fixing all the electrical systems in my 81, I believe anything is possible. The two systems in this car being affected were the fuel gauge and the lights, so the first thing I would do is make sure that somehow, someway, the ground for the fuel tank sending unit isn't getting some kind of signal from the lights, because even though the two aren't wired together, they are in close proximity.
 
#11 · (Edited)
Hey guys, this is getting interesting!!:smileyb: I think we can agree that grounds are the problem for many electrical malfunctions.

Bonding straps from the firewall to the engine are not always put back on when engine changes are done, and even though they were usually meant to cancel radio noise, they can help the grounding.

I think we all would like to know the answer to this particular problem.

Sorry about the headache 464elky........

.

.
 
#13 ·
I would like to add my 2 cents,but Im not sure what that is , its late, but you all seem to agree its a missing ground, I would take an ohm meter or even a test lamp,maybe easier, with some LLOOONGG cables, I have used a recently bx cable cause thats what was close by, the story of fires is scary......................maybe thats only 1 cent
 
#14 ·
Today, I crawled inder the Elco with the new fuel sender in hand, to see where it gets grounded. The new sender has a ground wire attached to it. Looked around on the frame etc, but didn't find where it'd supposed to be grounded. The eye on the ground wire is big enough for a 1/4" grounding bolt, but no luck finding the hole for it. I can ground it anywhere, but wanted to locate where it was before, to pass it on to help with the problem on here.
It may be necessary to crawl around under the tank and locate the ground wire where it's bolted to the frame....
 
#16 · (Edited)
Thanks I'll have to look again. The hole may have enough paint in it to make it hard to find.
That's a separate ground from the lights though, which makes the gauge problem something else.

But......I see (finally) that 464elkie nailed it!! There is a major ground connection, which is for panel lights, cigar lighter, fuel gauge panel unit, and other stuff. I don't know where its located though. I'm just getting the wiring sorted out in my '75, so maybe I'll find the ground.
 
#17 ·
I know this is a old thread however I have this same issue. So far, I have pulled the dash cluster. Changed the headlight switch. Checked all the bulbs and sockets. The issue persists. I crawled under the rear and found the pink wire has a scotch lock connector tappped into the ground for the rear taillights. Could this be the culprit??
Thanks in advance..