dome light problem

[slepysal]

I have been working out some electical bugs on my 60, I got the high beams, turn signals, dash, parking lights,and horn all work !! But I cant figuer out the dome light.
both wires to it have power , if I run a ground jumper from the plug that conects the lead off of the dome light ( by the bottom of the seat) and it lights up. New head light switch, same problem ANY ideas At this point I am tempted to add an inline toggle switch.
I know this is a simple wire set up, but it is kicking my butt! HELP

[markeb01]

The way these dome lights work is by controlling the ground. Only one of the wires going to the light should have power, and is normally hot all the time. The other wire goes to the rheostat connector on the frame of the headlight switch. Twisting the knob/shaft varies the amount of ground through the rheostat, until it reaches the end of the cycle which provides full ground and full bulb brightness. It took me a while to figure out. I had to read up on it since I'd never seen anything wired in reverse like this. Normally the switch always controls the hot side.

[61TWUK]

Originally Posted by markeb01

The way these dome lights work is by controlling the ground. Only one of the wires going to the light should have power, and is normally hot all the time. The other wire goes to the rheostat connector on the frame of the headlight switch. Twisting the knob/shaft varies the amount of ground through the rheostat, until it reaches the end of the cycle which provides full ground and full bulb brightness. It took me a while to figure out. I had to read up on it since I'd never seen anything wired in reverse like this. Normally the switch always controls the hot side.

man I was going to say that about the ground.

[joe231]

Markeb01 is correct, it is the ground side that does the switching.
ground side switching is very common in many years of automobiles.....

there is a separate contact in the headlight switch that completes the dome light circuit. It is a white wire. unplug your dome light connector and use an Ohm meter to check the resistance of the white wire to ground with the headlight switch in the dome light "on" position (while you have it unplugged check the condition of the contacts in the switch, look for corrosion). It should be close to zero ohms.
Is the switch mounted in the dash? I believe the switch gets the ground from the mounting in the dash.
If you don't have a good ground at the plug, then check at the switch itself, if you have a good ground n the white wire at the switch and not at the plug you probably have a bad wire between the 2.

[slepysal]

Ok I understand how it works,but if the switch grounds the dome light, then both wires should be hot ,and when you turn the switch it breaks the loop and grounds causing the light to go on, right?
The headlight switch has good ground, and like I mentioned if I ground the white wire the dome light works.
I wll check to see if there is a broken wire ,,Strange

[joe231]

Yes both wires will read hot to a seperate ground at that plug as long as the circuit is open, the hot will feed through the bulb. It is all about potential difference. When you turn the switch and it puts the white wire to groung, it actually completes the circuit (or loop, as you put it).
If the wire coming out of your headlight is a good ground, then your problem is certainly the white wire between the headlight switch and the plug by your seat.

Be sure to check the contacts in the plug. You can also use your meter set on ohms. Put one lead on the white wire at the headlight switch plug, and the other on the white wire at the plug by your seat (with the plug unplugged). You should have extremely low to no resistance.

You can do a voltage drop test on it also, I can explain that if needed.
I don't know your electrical experience, or tools you have.

[slepysal]

thanks Joe! Ill hook up my meter tomorow

[joe231]

No problem Sal

[slepysal]

it was a faulty wire I just replaced the whole wire and it works!
Whats funny is that I hardly ever use the dome light, but I cant stand it when I cant get something to work. Thanks too all

[LILRED66]

After reading this entire thread, I suddenly realized I am mentally incapable of understanding electricity. Perhaps I need a correspondence course on automotive electrical systems.

Thanks for sharing your infinite wisdom.

[slepysal]

no wisdom, just luck !!!

[19GMC63]

Originally Posted by joe231

You can do a voltage drop test on it also, I can explain that if needed.
I don't know your electrical experience, or tools you have.

Could you explain this Joe?
Sounds like something I should know.

[777ralph]

Mine was the same way - same solution too. I'm considering putting grounding pins in the doorframes. Imagine the luxury of having a light come on automatically when you open the door!
Ralph

[joe231]

Originally Posted by 19GMC63

Could you explain this Joe?
Sounds like something I should know.

Sometimes you can get a wire that might have many broken strands in it. You can check the resistance, and it will be ok, but once the circuit is under load the lack of continuous strands will create a current blockage, or corrosion in the connectors can cause the same type of issue. The way to check for a voltage drop is to put one lead of your meter on an end of the wire and the other lead at the other end of the wire, meter on DC voltage (working on a DC vehicle system), put the circuit under load (it must be under load to show the weakness). The reading on the meter tells you the potential difference. It should be close to zero, a volt or less will still operate.
Basically you are measuring how much voltage is making it through the conductor (wire)
say you are working with a 12 volt DC starter circuit, the starter will not turn the engine over, you suspect the battery cables are not up to par. so you would put your meter on the DC voltage setting, place one lead on the positive cable end at the battery, and the other lead on the positive battery cable at the starter solenoid. Then either have some one turn the key to start, or set the meter where you can see it while you turn the key to start. Say the meter reads 10.95VDC, but your battery shows 12.45VDC. then somewhere between the battery and starter you are losing 10.95VDC (you are measuring the potential difference between the meter leads, a perfect cable and connections is going to read 0VDC, no potential difference). that means that only 1.5VDC is actually making it to the starter and you either have a bad cable or a bad connection. you can do this on both the positive and negative cables as well as any other conductor you may suspect as not up to snuff.....
anytime i have a no crank condition, first check is the battery voltage, second check is the voltage drop of the cables.

Make sense Andrew?

Tony, i dunno if it will help you or not, but I was told years ago to think of electricity as water in a system of pipes, the water (electron flow) will take the path of least resistance. Many electrical components can be compared to water control pieces. A switch is like a water valve, a blocking diode is like a one way check valve, a power supply (or battery) is like a pump, a resistor is like a flow control device, and on and on. the pump (battery) can not pump water (electron flow) anywhere unless the circuit is complete.

Check out howstuffworks.com

there is generally some diagrams and animations to help visualize it.

[markeb01]

Thanks Joe, your input is always valuable. I like to learn something new every day.

On the door switch topic, here's a link showing an easy way to do it:

http://www.persh.org/pickup/DomeLight.htm

[19GMC63]

Originally Posted by joe231

Sometimes you can get a wire that might have many broken strands in it. You can check the resistance, and it will be ok, but once the circuit is under load the lack of continuous strands will create a current blockage, or corrosion in the connectors can cause the same type of issue. The way to check fir a voltage drop is to put one lead of your meter on an end of the wire and the other lead at the other end of the wire, meter on DC voltage (working on a DC vehicle system), put the circuit under load. The reading on the meter tells you the potential difference. It should be close to zero, a volt or less will still operate.
Basically you are measuring how much voltage is making it through the conductor (wire)
say you are working with a 12 volt DC starter circuit, the starter will not turn the engine over, you suspect the battery cables are not up to par.

Thanks Joe! This will definitely be usefull... tryin' to chase some gremlins in my corvair.

Thanks for the link Mark... pretty neat idea.

[joe231]

Originally Posted by markeb01

Thanks Joe, your input is always valuable. I like to learn something new every day.

On the door switch topic, here's a link showing an easy way to do it:

http://www.persh.org/pickup/DomeLight.htm

Thanks Mark...I'm assuming you name is Mark?

Originally Posted by 19GMC63

Thanks Joe! This will definitely be usefull... tryin' to chase some gremlins in my corvair.

Thanks for the link Mark... pretty neat idea.

Sorry, i posted in the middle of my explanation, I typed half of it on my iPhone then though, what am i doing? and moved to a real keyboard at the computer

[Ham-er]

Next time you find a light circuit with both wires hot, Pull the bulb and re-check.

[joe231]

Originally Posted by Ham-er

Next time you find a light circuit with both wires hot, Pull the bulb and re-check.

to show you what?
That you have no ground. If the ground is good, there won't be voltage on the ground wire. We have already established that the hot was feeding through the light bulb, which also tells you the bulb is good.

[markeb01]

[QUOTE=joe231;3595292]Thanks Mark...I'm assuming you name is Mark?

Yep, that's what I usually answer to - unless it's something worse.

[777ralph]

Thanks for digging up the door switch post Mark.
http://www.persh.org/pickup/DomeLight.htm
Doesn't get much simpler than that!
Ralph