Dome light door switch R&R and a question

[mhackney]

The 83 cabs have one interior dome light located above the mirror between the sun visors. Mine has power as it lights when I rotate it to the always on position. But the door switch position has never worked. I thought it might be nice to have a working light for the occasional evening dates with my wife so I investigated today.

First I took the passenger side door switch out and measured resistance across the wire connector and aluminum disk as shown in the photo below. The switch worked properly - open circuit when plunger depressed (the door closed position) and very low resistance when not depressed (the door open position).

So I turned my attention to the driver side and sure enough, I see an open circuit with the switch plunger depressed or not. Here's how I hooked up the leads:

After scrutinizing the switch I learned that the contact is actually the base ring of the "brass" wire connector on the end and base of the threaded shaft as shown at the tips of these rather odd arrows:

I quoted "brass" above because when I first removed the switch, the part was flat black. I didn't realize it was brass until I cleaned off the oxidation. This was preventing good electrical contact. Here is the switch cleaned up and showing a little resistance:


I am about ready to reinstall to test if my dome light works but I also have a question:
If one of the two switches fails - i.e. is always open circuit - shouldn't the other door switch actually activate the dome light? Basically, the switches are wired in parallel as shown here and if either switch closes to ground, the dome light should light up. But there is a diode shown on the right in the diagram. What does that do, where is it located so I can test it?

[mhackney]

I should mention that the other devices on this circuit - clock, glove box light, luggage compartment light - all work. I am curious about the device marked "Key Warning Buzzer Contact". Anyone know what that is?

[mhackney]

Ok, +1 in the win column. After cleaning the contacts on both door switches the dome light now functions properly.

I also found out about the Key Warning Buzzer. It is supposed to buzz when the key is left in the ignition and the door is opened. Apparently mine is not functional. But that is one buzzer I can live without.

[Bob Kontak]

Originally Posted by mhackney

If one of the two switches fails - i.e. is always open circuit - shouldn't the other door switch actually activate the dome light? Basically, the switches are wired in parallel as shown here and if either switch closes to ground, the dome light should light up.

Enjoyed the flat tire thread.

I believe you are right. The door plunger switches provide ground to the circuit. The dome light is always hot. No ground, no light. The manual switch grounds to the ceiling in my targa.

As to the door switch grounds. When you ground there will be zero resistance from the light to the switch. I see a OL in your pic which appears open. And I see 14.5 ohms which is a poor ground. I would like to see .001 ohms on the wimpiest scale.

Your switch is not grounded to the car in your tests. You know this. For this test you are looking for continuity (which is actually the same ohms test) OL and 14.8 are not correct. 0.001 ohms. NO resistance. Easy flow to ground.

Get yourself a wire with alligator clips. Or just a wire with bare ends. Find a good ground on the car. Even the negative post of the battery.

Run it from the wire from the light ground wire where it connects to the door switch to this ground. If it lights up, replace the switches.

If I am misunderstanding something, apologies.

[wrxnofx]

Originally Posted by mhackney

I am about ready to reinstall to test if my dome light works but I also have a question:
If one of the two switches fails - i.e. is always open circuit - shouldn't the other door switch actually activate the dome light? Basically, the switches are wired in parallel as shown here and if either switch closes to ground, the dome light should light up.

You are correct. If one of the two switches grounds, out, both lights should light (i.e. if you open one of the doors and the switch grounds out). As you also found out, if neither of the switches grounds out (either because both doors are closed or because oxidation in the switch prevents it from grounding out), then neither of the lights will light.

[Bob Kontak]

Originally Posted by mhackney

Ok, +1 in the win column. After cleaning the contacts on both door switches the dome light now functions properly.

Damn!

Good news. I type too slow. Timing overlap.

[steely]

+1 - good thread about the tire

The diode is there to let the seat belt relay know if the doors are open or closed (based on door switch position) without zapping it with the +12v from the dome lamp. I don't know where it's located.

[mhackney]

Thanks gents. And I did clean the contacts better and got the resistance waaaay down. So things are good.

[Bob Kontak]

Originally Posted by steely

+1 - good thread about the tire

The diode is there to let the seat belt relay know if the doors are open or closed (based on door switch position) without zapping it with the +12v from the dome lamp. I don't know where it's located.

+1 diodes are like a traffic cop. Juice is allowed to go only one way from what I understand.

[gazzerr]

It will be buried in the dash there somewhere. You'll have to follow the wire to find it I'd bet.