Dome, Cargo, and Courtesy Light Mystery

[green72s]

Hello,

I'm working on my '79 Sierra. I recently completed some small projects on the truck. I repainted the instrument needles, and replaced the following items.

• the clock in the instrument cluster
• instrument light bulbs behind the cluster
• courtesy light bulb under the dash
• dome light bulb
• cargo light bulb

Since adding these new parts and re-installing the instrument cluster there are strange things happening with the dome light, courtesy lights, and cargo light. All the instrument lights work, and the clock worked for a while, that's might be a different issue.

Here's what happens when everything is connected.
Under dash courtesy light
The courtesy light under the dash almost works fine, it turns on when either of the doors is open and mostly turns off when the both doors are closed. What's strange is that when both doors are closed the courtesy light turns on although it is very dim.

Dome light
The dome light does do not work when either of the doors are open. The dome light does not work when the light switch is moved to the dome light on position. The dome light turns on but is very dim when both doors are closed.

Cargo light
Same behavior as dome light, does not turn on at all except when both doors are closed, then it turns on but is very dim.

What happens when I disconnect the dome light and cargo light wiring harness just behind the driver (the orange and white connections)
The under dash courtesy light works exactly as it should, now turning off completely when both doors are closed.

Tests I've done
The dome light bulb is good
The cargo light bulb is good
I cleaned the ground connection between the cargo light housing and the cab.
Was going to clean the ground connection between the dome light housing and the cab but noticed that the dome light housing is plastic, so it must ground some other way.
I cleaned and checked the passenger side door switch for the dome light.
I did a visual inspection on the dome light and cargo light wiring harnes as it runs under the driver sill plate.
With the upper part of the dome light and cargo light wiring harness disconnected (behind the driver), I checked for voltage: the orange wire shows .08 volts with the doors open and with the doors closed, the orange wire shows .01 volts when the doors are open and 12.24 volts when the doors are closed.
I did lots of continuity tests between the beginning of the upper dome light and cargo light wiring harness (behind the drive) and the top of the wiring harness at the dome light and cargo light; everything seems to be OK.
The cargo light switch doesn't work great, you have to finesse it to get it to allow current to flow through it (checked for continuity), so for the time being I bypassed the cargo switch by connecting a wire between the two connectors.

I apologize that there is so much to read here, but wanted to answer as many questions ahead of time.

I'd really like to get this issue ironed out. The truck is so close to having everything working.

Thanks!

[green72s]

I've continued trouble shooting the issue and I have some specific questions if anyone can help.

1. How many volts should I be getting through the white wire with the doors open, and with the doors closed?

2. How many volts should I be getting through the orange wire with the doors open, and with the doors closed?

3. It seems that the cargo light bulb grounds through the dome light assembly and cab. Is that correct?

4. How does the dome light bulb ground, the dome light assembly is plastic?

Thanks

[SunSoaked]

Is your cargo/done light housing chrome plated plastic? I recently had a similar issue after installing new LMC housings. Very elusive until I discovered the chrome plateing on the plastic was conductive and causing all sorts of issues. I used a Dremel and small burr bit to remove the plating around the bulbs clips. Fixed the issue for me.

[green72s]

Thanks SunSoaked, that's a good point you're making. The cargo light is metal and the dome light is plastic. I originally thought the cargo light was supposed to ground through the housing, but now I'm thinking that it is not supposed to. For my most recent tests I have both the housings dangling so that they're not touching any metal.
When I button everything back up, I'll make sure neither are grounding.

There is a new update to the story. I checked the voltage at the fusebox and it's 6.9 to 7.0 volts. That seems like a problem. I'll post a picture of the terminal I'm connecting to that gives me that reading.

[green72s]

The end of the pencil points to the terminal where the orange dome light wire connects. It's reading about 6.8 - 6.9 volts. The terminal below it is about the same, 6.9 volts, it connects to the clock.

Sorry, let me try to rotate the picture.

[SunSoaked]

That fuse tap should be a good 12 volts or better. Does it change if you start the engine?
Also, there should be an orange wire that carries 12 volts to each bulb (cargo, dome, etc). It does not ground thru the fixture itself. It grounds when the door pin switch (white wire) or cargo switch (the reostat in the light switch) is grounded, thus completing the circuit. In my case, that's why the chrome plateing was messing mine up.

[green72s]

That's what I figured, that the BAT terminal at the fuse box should be reading min 12 volts. And now I understand, that the white wire provides the ground when either the door pins are in the open position or when the headlight switch is in the dome light position.

That leaves the cargo light, does that also rely on either a door being open or the headlight switch being in the dome light position? It's worked before, but I guess I never paid that much attention to when and when it doesn't work.

I'm charging the battery right now, it was reading 12.25 volts, but I thought I'd top it off. After it charges for an hour or so I'll start the truck and test the voltage again and lights again.

Thanks!

[phat69]

Check the door switches as well. That is always hot and the switches just ground it making the lights come on.