63' Dome Light and Gas Tank questions

[ABX131]

We made a lot of progress on the 63' this summer. It is now a drivable truck and my son and I are loving it. Question about the dome light. How does it turn on? There are no door pins to trigger it. Does it turn on by turning the headlight switch? Also regarding the fuel tank, It has been repainted and has some kind of padding under the tie down straps. Do the straps act as a ground to the mounts? If so then the paint and/or padding could be preventing a solid ground? When I turn on the key, the guage goes off the right end past the "F" and does not giv an accurate reading.

[Greg63]

The dome light is activated by the headlight switch only.
As for the ground on the gas tank, mine had a seperate wire that ran from the sending unit to a screw above the tank on the rear cab wall.

Greg.

[markeb01]

The dome light is designed to have power at the light all the time. Illumination is controlled by the ground side of the circuit. The headlight switch has a tab on the frame which is engaged at the left end (counterclockwise rotation) of the knob.

[ABX131]

When did they change the Ohm level in the fuel tank sending unit? We had a second tank come with the truck that we switched out because the original tank leaked...Not sure of the year of the replacement tank but it was a direct swap...sender unit worked fine until after the switch...ground tab to the cab is installed, contacts are free of paint.

[luvbowties]

Originally Posted by ABX131

When did they change the Ohm level in the fuel tank sending unit? We had a second tank come with the truck that we switched out because the original tank leaked...Not sure of the year of the replacement tank but it was a direct swap...sender unit worked fine until after the switch...ground tab to the cab is installed, contacts are free of paint.

Since it's easy to get to, why not swap sending units? And, no, straps do not affect ground--just for anti-rust, anti-rub, anti-squeak...

[Greg63]

I am not 100% sure but I believe '60-'63 were 30 ohm and '64-'66 were 90 ohm.
Greg.

[Z POPPY]

0-30 Ohms most pre-1965 GM cars and trucks. 0-90 Ohms most GM cars and trucks 1965-up. Most aftermarket stuff is 0-90 Ohms. Also check and recheck all grounds. Lose wires and grounds are the main causes of the fuel guage problem. I have had floats go bad by sinking(empty reading) and just worn out on the float parts themselves.

[luvbowties]

Originally Posted by ABX131

We made a lot of progress on the 63' this summer. It is now a drivable truck and my son and I are loving it. Question about the dome light. How does it turn on? There are no door pins to trigger it. Does it turn on by turning the headlight switch? Also regarding the fuel tank, It has been repainted and has some kind of padding under the tie down straps. Do the straps act as a ground to the mounts? If so then the paint and/or padding could be preventing a solid ground? When I turn on the key, the guage goes off the right end past the "F" and does not giv an accurate reading.

Greg may be correct, but my memory tells me they changed tank resistors AFTER 1966. [[See: http://www.classicparts.com/1960-66-...info/23-527/]] Yours may now have a 90-ohm in it now, which is a quite common one--my '66 does, and it goes to "F" regardless of level of fuel**. I put it in on purpose when moving tank to under back 'cause I have new gauges for dash(which I've not found time to put in yet) & new fuel gauge requires a 90-ohm. The sending unit is a variable resistor AND serves as the ground for the gauge; gauge is fused and receives full 12 volts when ign is switched on. When tank is full, float is at its highest, is where it has MOST resistance(say approx 30 ohms), which is where it allows MINIMUM current to flow thru gauge (and on thru to ground provided by sender), and which causes gauge to move its MINIMUM amount of distance to left of "F". As gas gets lower, it lowers the gauge, which decreases resistance, which allows more current to go thru gauge(and on thru sender to ground), which pulls gauge further to left of "F" and closer to "E". When gas level approaches empty, float approaches lowest level, resistance goes to its minimum(approx 0-ohms), current thru gauge goes to maximum, and needle is pulled its maximum distance to left of "F", which is at "E". I know, it sounds backwards but you and I didn't design it..........BTW: when the wire from sender is 'cut', the resistance in the circuit is like 'mult-max', or infinity. And this huge resistance lets NO current go thru gauge, which means needle is not pulled away from "F" at all--it goes to "F" any time ign is on and stays.

**If resistance is just enough too great(90- vs 30-ohms is too great enough), far too little current flows thru gauge and sender, and it does not pull the needle off "F" at all. There is a way to splice a resistor into sending unit wire and on to a solid ground (in parallel with sender's resistance) which would decrease total resistance to 30 ohms. If you wanna know the value of such a resistor and how to wire it into system & where to buy it, contact me & I'll go thru it w/you. But you'd need to know the resis of your sender--need to take it out & check w/ohmmeter--lotsa trouble, but it'll allow you to use whatever sender you have.

[dthela]

run a ground from tank to frame. ground is wired to bolt holding sending unit in but you can use a screw in edge of flange. no ground = full guage

[dthela]

I bought a switch and installed in dash. Glued old knob onto shaft to match ( you can use a wiper switch etc.)

[ABX131]

I think I still have the old sending unit which I think used to work just fine. My son tried to get started on the swap without me and in an attempt to loosen the threaded fuel line connection twisted the solid fuel line like a piece of licorice...

[luvbowties]

Originally Posted by ABX131

I think I still have the old sending unit which I think used to work just fine. My son tried to get started on the swap without me and in an attempt to loosen the threaded fuel line connection twisted the solid fuel line like a piece of licorice...

If you can find and use it, fine. Be sure to do as dthela suggested: run an extra ground from tank to body/frame. But don't go out & buy a new sender just to get the 30-ohm. Resistors are dirt cheap, and you'd need for a good** 90-ohm sender to get a 45-ohm resistor from like Radio Shack for about $2 for a pkg of 5--1/2 watt rating, or anything close should do just fine.

The 90's are most popular and I'd guess 270-ohm's are used quite a lot too. For it, you'll need to use a 34-ohm, which may be hard to find. Techie at R-Shack should be able to help you come up with a combo of 2 or 3 resistors to get pretty close. He should be able to show you how to wire them to get what you want--series(end-to-end), or parallel(side-by-side: with a pair of their leads together to form one lead, and other pair together for 2nd lead), etc. BTW: resistors look like these in picture "IMG 5660b.jpg".

**Be sure your sender IS WORKING. Put one end of ohmmeter on wire lead that goes to gauge. Other end goes on 'far' end of the resistor in the sender(looks different from these in the pic). Move float from one extreme to other, and it should move meter from close to 'zero' up to around '30' or '90' or '270'.

[ABX131]

I really appreciate the help but I need it dumbed down a bit...My old 30 OHM sending unit worked but is damaged..my new one does not and I believe it is a 90 OHM unit...all wires are in good order. Can I get one of those items pictured above and just splice it in between the tank and lead wire? If I understand right I need to reduce the resistance from 90 to 30 so I need a 60...What exactly do I ask the guy at Radio Shack for?

[luvbowties]

Originally Posted by ABX131

I really appreciate the help but I need it dumbed down a bit...My old 30 OHM sending unit worked but is damaged..my new one does not and I believe it is a 90 OHM unit...all wires are in good order. Can I get one of those items pictured above and just splice it in between the tank and lead wire? If I understand right I need to reduce the resistance from 90 to 30 so I need a 60...What exactly do I ask the guy at Radio Shack for?

OK, I'm assuming you have a good 90-ohm sender; we need to get a 45-ohm resistor to correct it. We need to join (solder?) 1 end of that res. onto the wire running from the sender to the gauge. Do not cut the wire; just strip off some insulation to where you can connect one resistor-end to it. Next we need to join the other end to a ground; it will be easier to do this by soldering a piece of wire onto this 2nd end and then connecting the free end of that wire to the frame or body so it is grounded. (The end of the resistor is sorta stiff & harder to deal with than a piece of wire soldered to its end. And this soldering can be done on your bench, not cramped up in the cab. You may desire to treat the 1st end the same way.) That circuit will produce a total resistance of 30-ohms which is what we ultimately need....job now completed.

Should you wish to see the math and electronic interrelationships that make these 2 resistors combine to make an "equivalent resistance" of 30-ohms, we'll do that in another discussion. But we don't really need to go thru that. Just try & trust me--it works.

[ABX131]

Just got back from Radio Shack and brought this thread with me for the guy to read so we were on the same page...fortunately I had an employee who also knew about this stuff and hooked me up. I will let you guys know how it turned out....LUVBOWTIES, thanks for your help..