Road Lamp Electrical Gremlins

[usraptor]

I recently noticed that the left road lamp on my '70 'Cuda was not working.  I disconnected the electrical bayonet power connection and probed it with my test light and it showed I had power.  Assuming the bulb was bad I bought a new bulb.  I installed the new bulb today and it still didn't come on.  While I had it loose outside the housing I probed the two back electrical connections with my test light where the road lamp harness connects to the bulb and both connections had power.  I then took out the driver's side road lamp bulb, that I knew worked, and connected it to the left side road lamp harness and it didn't work either.  I connected both the old and new bulbs from the left side into the right side harness and they both worked.  So I checked the ground wire for the left side and it's tight. I disconnected it anyway, cleaned it and reconnected.  It still won't work. Plus I assume if I'm getting power across the back of both connections at the back of the bulb the ground must be good otherwise if the ground was bad I wouldn't be getting power across both connections, correct?  So I've got power to the bulb but it still won't come on. So does anybody have any idea why good bulbs still won't come on when they've got power on the left side but will come on on the right side??     I'm stumped, which I realize coming from me isn't anything new.

[B5fourspeed]

I would take the left road lamp remove it from bumper  and plug it into the right harness.

[usraptor]

Okay, I'll try that although I'm not sure what it would prove.   

[73_Cuda_4_Me]

Sounds like a bad ground wire on the left road lamp... the reason you have power on the test light on the other side of the bulb is because the 12 volt comes into bulb, through filament, and out to ground terminal... without a ground connection, the 12 volts just goes to where the circuit is interrupted and stops... no current can flow without a ground, so no filament can light. Because the test light is grounded, it lights, and if it passed more current, so would the road lamp bulb. The test light doesn't pass enough current to cause the road lamp bulb to light, though...

If you probe the harness that goes to that side, you will probably find either a break or pinched wire to wherever it grounds... maybe even a bad ground connection terminal...

If the probe has a needle tip, you can probe through the insulation along the wire until the probe doesn't light up, that is on the other side of the break, and should be grounded...

On my 73, the front parking/turn lamps have a separate ground wire connected to a pigtail coming out of the harness. Both sides go to the radiator core support with ring terminal and a bolt that grounds the leads out, on the driver's side of the radiator core sheet metal...

[usraptor]

Thanks 73 Cuda 4 Me.  That makes a lot of sense.  I'll check the ground again tomorrow. 

[fc7cuda]

I agree, it sounds like not grounding. 

Sometimes these gremlins can be wiring decay, is your road lamp wire original or reproduction?

Tom

[dodj]

Not having a 'cuda I'm not sure what kind of bulb it uses, but you definitely have a poor ground on the non working side. If you have a light connector with two wires, you have an issue with the grounding wire. If you have one wire, you have an issue with the mounting of the light housing.

[73_Cuda_4_Me]

You should have a ground wire coming off the road lamp that goes directly to ground...

Here is the schematic from another post here...

[usraptor]

Thanks everybody for your suggestions and you were correct, it was a ground wire issue.      Dodj to answer your question it is the two wire load lamp.  Also, the road lamps and wires coming off them are reproduction. (The  person I bought the car from had taken the road lamps off and misplaced them and he never could find them.) However, the road lamp wire harness itself is original.

I owe 73 cuda 4 me a beer or coke, your preference, if we ever meet. The ground wire had a bad spot which ended up being right at the connector to the bulb.  I snipped the connection off about 1" back,  put on a new connector and all is well; the lamp lights up as it should.  Thanks again.  I'm not the brightest bulb around when it comes to electrical issues, (pun intended) and just assumed since I had power to both terminals on the back of the bulb I had a good ground.  I learned something new thanks to 73 cuda 4 me.  Once again the great people on this forum come through for it's less knowledgeable members.   

Brad

[73_Cuda_4_Me]

Brad,

Glad you got it figured out! We'll definitely have to share beers together sometime!

As far as helping out, I'm only trying to pay back this forum's members who have consistently been eager to help fellow members here, and have helped me through the years!

I learn new things every day about different aspects of our Mopars, and do a lot of drooling over some of the rides that members here have... seems everyone here likes to expand on our love of these e-bodies by helping in any way they can...

Big shoes to fill, to try to do the same!