'70 fuel gauge-weird problem

[tom340]

Stumped..My '70 Duster. Here are some details first. All new tank, sender unit, fuel lines. Gauges sent to G car for restoration, and he converted to solid state. Already did the test for the gauge by grounding out, and yes the gauge goes right to full so we know for a fact the gauge works fine. (and it did even before sending to G Car) First sender unit was one of those bad units I told you guys about already-removed and replaced with a unit that is correct. 100% certain no issue with the sender unit!! Oh and also, YES we have the ground strap installed properly.
This is whats happening: When you turn the ignition ON, but dont fire up the motor the gauge reads perfect. But when you start the motor the fuel gauge drops off to Empty. I'm not sure why, but it seems like the ignition switch cuts out the ground when you turn it all the way forward to start the car. Cant figure why the gauge seems to care when the engine is running?
I had been playing around with the ignition switch a bit, what i was doing is turning it forward enough to where you get a fuel gauge reading, hold there a moment, then quickly engage the starter and back off "ever so slightly" without starting up. At this point, the gauge still shows a fuel level. Then I go ahead and start the car, and the sunnofavitch then seems to work properly. Of course, who the hell wants to go through this all the time????
There are no additional lighting or gauge issues. Been there, done that on this resto. We seemed to have chased all the electrical gremlins away except for this dirty bastard.

 

[adriver]

I figure you will wind up contacting the people that converted your gages to “digital”.
Or maybe someone has had this problem with theirs and can give a better idea than me.
This is a WAG. The only difference I can see in your description is voltage drop.
If you aren’t doing a full start load and the gage works.
And then you are going ahead and hitting the starter again and it continues to work, MAYBE the digital has some type of “memory” and you are tricking it by not starting the car on the first hit.
(Do I understand you correctly?)
I think “on” is “on” as far as the starter switch is concerned.
Don’t see any difference there.

 

[grumpuscreature]

If I understand correctly; if you start the car normally your gas gauge doesn't work, but if you back off on the key a tad after starting the gauge works? If that is the case, your ignition switch is worn out and you need to replace it. Sorry 'bout that.:sad:

 

[adriver]

Yes, you obviously need to verfiy that your switch continues to power the circuit before looking to the gage.
I figured if that is the only thing not working properly , it isn't the switch.
That is unless there is a dedicated contact for that one gage on the switch.
Or there is P.F.M. in the digital stuff I don't know about.
I figured "on" is "on". In other words the switch works in a consistant pattern. Not sometimes and sometimes not?
Maybe I don't understand.


Great Horny toads!

 

[tom340]

If I understand correctly; if you start the car normally your gas gauge doesn't work, but if you back off on the key a tad after starting the gauge works? If that is the case, your ignition switch is worn out and you need to replace it. Sorry 'bout that.:sad:

Yep, you understand correctly! Thats it exactly. I was wondering if the ignition switch was THE problem.. There is a bit of sloppiness to it. Im not sure I understand why the ignition switch could be the problem, this had some pretty good people stumped on another MoPar website..

Just a note to the few other post: my gauges werent converted to 'Digital'. G-Car just converted a few of the gauges to solid state which is a pretty common thing to do nowadays. If I remember correctly, it does away with blown fuses..Im sure someone else could explain better. Anyhow, its confirmed, the gauges DO work properly.. Problem is isolated to fuel gauge droping to Empty when the engine is started.

 

[adriver]

An old switch is an excellent suspect.
Keep us posted.
I am not familiar with this conversion so I assumed digital readouts.
Either way I am sure “solid state” involves transistors, diodes and other things.
All I can figure is that if all your gages are feed off the same circuit there must be something different about your fuel gage which stops working when the engine is started.

What seems logical to me is that solid-state devices sometimes require a certain voltage to operate properly (gating things one and off)
And a voltage drop to start the engine would cause a voltage drop.
It is hard enough to trouble shoot when one knows what one is doing and actually can see something.
A bad switch certainly is as reasonable as anything else.
When I read the last part of your post, I understood that the gage worked after a set pattern of bumping your starter, returning to the “run” position and then starting normally without any further switch movement.

I had been playing around with the ignition switch a bit, what I was doing is turning it forward enough to where you get a fuel gauge reading, hold there a moment, then quickly engage the starter and back off "ever so slightly" without starting up. At this point, the gauge still shows a fuel level. Then I go ahead and start the car, and the sunnofavitch then seems to work properly. Of course, who the hell wants to go through this all the time????
There are no additional lighting or gauge issues

 

[grumpuscreature]

The contacts in the ignition switch are getting burned which increases the resistance. This lowers the voltage supplied to the fuel gauge to the point that there isn't enough to run all the way to the sender in the tank and have a signal back at the fuel gauge that will read more than empty. Since the rest of the gauges run off a solid state regulator at the fuel gauge and their wiring runs are lots shorter, they work. By bumping the switch a bit, you are moving to a clean area on the contact and you have full voltage to the fuel gauge and voila, it works again. Ran across this more than once. Let me know how this turns out.

 

[tom340]

The contacts in the ignition switch are getting burned which increases the resistance. This lowers the voltage supplied to the fuel gauge to the point that there isn't enough to run all the way to the sender in the tank and have a signal back at the fuel gauge that will read more than empty. Since the rest of the gauges run off a solid state regulator at the fuel gauge and their wiring runs are lots shorter, they work. By bumping the switch a bit, you are moving to a clean area on the contact and you have full voltage to the fuel gauge and voila, it works again. Ran across this more than once. Let me know how this turns out.

AIGHT!! I still need to order me one of those switches (yahoo- 'mo money) I'll dig this thread up again..