69 charger fuel gauge is goofy

Heres what its doing...

This car has a Tanks Inc EFI Tank, and its got the correct fuel sender in the tank. I grounded the sender to the chassis,

Now. Key on, with roughly 5 gallons in the tank it reads just over 1/4 tank. As soon as I turn the Dash lights on, Headlights on, and thumb wheel is turned on for back lights. The fuel gauge will go to zero. I have put a solid state voltage limiter in the cluster.

Heres what I have done to correct the issue.

Cleaned every ground. made several extra straps for ground. Ran a ground from the battery to the sender, tried OE sender, has new fuel gauge in the cluster, cleaned circuit board contacts. tested board, made a ground from cluster to dash frame, cleaned ground from frame to column, tried different thumb wheel dimmer switch. all have same issue, as soon as back lights in cluster come on the fuel gauge drops to zero.

I tried a different voltage limiter and with that limiter it will peg to full when back lights are on. I can wiggle the limiter and get no change. Ive tried to check power behind the dash but its very difficult to get my scrawney ass back there to test, but Im going to grab some alligator clamps for my DVOM and test again. I thought maybe the battery was low, so I ran the test with 13 volts from my charger on it and it did exactly the same thing........

ANY POINTERS, will be greatly appreciated. New dash harness, New engine harness, new forward lighting harness.....


Thanks in advance

change the printed circuit board on the back of the gauge cluster it comes with a reg..that should wake up all of your gauges

That is odd you get a different result from a different voltage reg.
is it the back up VR solid state as well or the old style?

Can you measure the output of the vr with and without the lights on? see if it is cutting out?

It isn't the sending unit, so it must be the dash.

Can you add a ground directly from the battery to the frame? or even the cluster. The bolts that hold it to the frame should be the ground.

If that solves it, then it is just bad grounds. I would see if the column got insulated some how and isn't grounding as expected.

I have added a ground strap from the cluster to the dash frame. I have 3 solid state regulators, and it acts the same with them all. I have a different board with the VL built into the dash coming for a different dash im working on for a friend. I might swap that and try it.

And I have used 3 standard Voltage Limiters, and honestly got different readings on them all, but they could easily have been crappy as they were in my box of spare junk.....

I am thinking you may have a ground issue somewhere in the lighting circuit as things appear fine until you turn the lights on. The lights may be grounding through the instrument cluster thereby causing your problem.

You can try to identify which circuit is at fault by disconnecting the body harness at the Kick panel area or forward lamp harness along the fender well. It may also be the instrument lights themselves not getting a good ground, Pull the fuse.
Try the above 3 things and see if one makes the problem go away. You will then know if it is the lighting circuit and if so which one let us know

Just to clarify, it does it when the dash lights are turned on via thumb wheel dimmer switch. It wont do it with the lights on. I can turn the lights on, have the thumb wheel to off, and it all works fine, as soon as you move the thumb wheel and theres just a very light glow of back lighting in the cluster the gauge goes to dead empty. Kinda why I thought I had a bad switch. but nope on that one....

That would be a pretty strong indicator that the ground for the instrument lights is the issue and it is most likely robbing the ground from the voltage limiter /gauges.

You can read the output voltage of the IVR at either the oil pressure sender or the temperature sender. Try that in both a headlights on and headlights off state.

Interesting.
I'm battling a similar deal but not a gas gauge, it's related to the thumbwheel dimmer/IP lights.
Mess with the thumbwheel trying to make the IP lighting work, POP goes the parklamp & brake light fuse.
Jumping the IP fuse from another accy feed at the fuse box, same thing.
Dome light works fine, everything else fine.
Those dimmer thumbwheels are notorious, tempted to replace it with a rebuilt OEM unit, but whatta PITA.
Just occurred to me to try a 2nd ground at the tail lamps...

If your blowing the fuse either the switch is shorted or you have a short somewhere in the circuit.
Take the switch out and clean it thoroughly, then test with and Ohm meter.

One could also bypass the switch at the plug with a jumper and see if the fuse still blows. If it does, its not the switch

so a couple of things.
first, I thought you said in the original post that one of the VR cause it to peg when you turned the lights on.
IS that not the case?

Second, it could still be the ground, because you are 2 or 3 times removed and each could have paint insulating it.

Circuit board, to cluster, cluster to dash, dash to column or frame.

Put in a new ground directly from the battery to the cluster and see if it still does it.
This eliminates plain ground as the issue and then points towards circuit board or wiring.

Well I fixed it last night!

It was the bolts that hold the board to the frame of the cluster. They were tight, but not clean. I removed them, cleaned and put back on, and the gauges all work as they should!

gald you got it handled and Thanks for the update

grounds will haunt you at the best chance they get ! they are notorious for pointing you in all the wrong directions.
glad you got it sorted ! easy after all, but how much hair did you lose in the process ? if you are like me, lots.

Agreed, What my Omaha partner in crime said, ground issues are the worst of the worst!

YUP YUP!!

Super stoked, and other than the holes in my skin, scalp, shoulders, elbows from screws and sharp edges im fine!


That damn Nebraska flooding took more hair from me....

I also hope you checked the range on the fuel sender, I went through about 4 from Tanks Inc. before I got one "close enough".