Parking and Interior Light fuse melting

[boosd]

I have been sitting/ignoring an electrical problem for some time now, and as of today figured I would take another run at it.

About a year ago my combo switch was overheating and melted a wire or two and the plastic connector, seeing as rewiring was out of my range I took it in and had a new combo switch/wiring put in.

Shortly after(if not immediately) I noticed that my parking and interior lights were out. It was a blown fuse.

Currently: The parking light fuse, after about 15 seconds gets red hot and burns out once the combo switch is turned on to parking lights. I disconnected all the parking and tail lights(and headlights) and flipped the parking lights on the combo switch, the fuse continues to over heat.

What I noticed was the dimmer switch, does not turn on the dash lights unless turned all the way up, and then the fuse begins to glow red preparing to burn out after a few seconds. If i turn the dimmer switch down just a tiny bit, the dash lights don't dim, they just go completely out, and the fuse does not over heat.

Also if i unplug the dimmer switch, the parking and tail lights stay on and the fuse does not seem to over heat, I just have no dash lights. I also tried ordering another dimmer switch and replacing it but it is the exact same problem.

With the steering wheel housing cover off I am also seeing that the green and white wire leading from the combo switch to the 9 pin plastic connector is very hot and started melting the insulation and has slightly warped the 9 pin connector.

Any thoughts?

[boosd]

Also, I believe the combo switch that is on my Z now came from a 240z, would there be any problem with this? When I was looking at the green/white wire to the combo switch it seems thinner than the green/white wire going away from the combo switch(on the other side of the 9 pin connector)

Here are some pictures

(I dont know if the dimple came from the over heating or if it was already there, how would I replace this one wire?)

[Captain Obvious]

The problem is in the dash illumination light section, but it's not the dimmer switch, it's somewhere else. Something in your illumination lamp section went almost short and is hogging current. It's not a dead short, but almost.

It's not the dimmer itself.

It's not the parking lights.

Maybe one of your illumination bulbs in one of your gauges failed short? Could also be the illumination bulb in the hazard switch or the heater face. Maybe ash tray lamp (if you have one - not sure when they added that).

I'm thinking that either one of your small illumination bulbs went short or you pinched or melted a wire somewhere behind the dash.

Was everything working fine before you burned up (and then replaced) the first combo switch?

[djwarner]

One of your side marker lights or turn signal lights may have the ground and power leads reversed causing a short through the light housing to the chassis ground. These wires are often dirty, or painted over hiding their original color.

I had this happen after having body work down and the mechanic reversed the leads on re-assembly.

[Captain Obvious]

I don't think it has anything to do with the parking lights. He said he disconnected all of them and the problem persisted. He also said that he can crank the dimmer all the way down and the fuse doesn't pop.

It's got to be something powered by the dimmer rheostat.

[boosd]

The problem is in the dash illumination light section, but it's not the dimmer switch, it's somewhere else. Something in your illumination lamp section went almost short and is hogging current. It's not a dead short, but almost.

It's not the dimmer itself.

It's not the parking lights.

Maybe one of your illumination bulbs in one of your gauges failed short? Could also be the illumination bulb in the hazard switch or the heater face. Maybe ash tray lamp (if you have one - not sure when they added that).

I'm thinking that either one of your small illumination bulbs went short or you pinched or melted a wire somewhere behind the dash.

Was everything working fine before you burned up (and then replaced) the first combo switch?

So I disconnected the dash lamp and fasten safety belt light, problem continued, there is no bulb in the hazard switch, just in the choke and defogger and those seem to be fine.

Is there a way to disconnect the dash lights individually, i pulled the heater face out and cant see how to really get to them at all

[boosd]

**I should also add...

I dont recall there ever being a dash light problem before this, If I recall correctly the head lights went out. There is a possibility though that this could have been the real problem the whole time, and that the overheating of the combo switch 9 pin connector could have been what affected my headlights as well and the parking/tails may have initially gone unnoticed.

The rheostat was replaced because the mechanic working on the car thought it was bad for this reason:

Initially when turning the rheostat instead of controlling the dash lights, it would turn up and down the parking lights.

When pushing on the rheostat we were able to get the dash lights to flicker on and stay on while pushing against it.

Now when i turn the rheostat it has no effect on the parking lights or the dash lights until it is turned all the way to max, then the parking lights dim a bit and the dash lights come on and that is when the fuse starts to overheat.

While under the dash I also noticed when i turn on the dash lights that this comes on(seen in picture) I have no idea what it is or why there is a light in it since i can't seem to see it unless i am under the dash, Also i did try unplugging this and the fuse still heats up.

[rossiz]

hard to tell, can't be sure, but is that the little light that illuminates the cigarette lighter when the headlights/dash lights are on?

it pokes into the side of the lighter recess in back, i believe on the side closest to the steering column...

[Zed Head]

The potentiometer would just provide the final path to ground. Each bulb's resistance stops a dead short from happening when the pot. is at zero resistance. So the theory that there's a short in one of your gauge bulb plugs or one of the other bulbs controlled by the pot. seems to fit. It would be a different kind of short, not to ground as usual, but across the intended load. The pot. just completes the circuit.

It's odd though that you've had two potentiometers that are off-on instead of a gradual resistance. Hard to see how a short in the circuit before the pot. would cause that. Since you can remove the plug, a $3 Radio Shack potentiometer might show something. Maybe you got two bad potentiometers.

You didn't mention year of car. I added a 1976 picture, but the early cars had some odd wiring schemes.

Edit - may not change much, but the potentiometer may not actually go to zero resistance. The diagram shows a direct power feed to the pot. itself. So the final resistance in the circuit may be the pot. illumination light itself, if there is one. Or the diagram may be incorrect.

Edit 2 - the electrical guys already knew this but the above (italics) is wrong thinking, That extra red circuit is just the ground for all of the other lights like the AC.heater controls and cigarette lighter. There are a lot of lights on that potentiometer. Any one of them could be the short.

Edited October 16, 2014 by Zed Head

[SteveJ]

Check the sockets on the running lights. They are subject to corrosion.

[Captain Obvious]

The pot doesn't go OFF-ON instead of gradual. He's got a very low resistance connection somewhere between the output of the pot and ground. That connection is sinking away most of the current through the pot and only a small amount goes through the illumination bulbs because their resistance is higher than whatever is shorted. (Think voltage or current divider between the short and everything else.)

So when the pot is cranked to full brightness, there's just enough current through the bulbs to get some light out of them even though the improper connection is sucking down most of it.

But when you start cranking the pot away from full brightness, the current drops and the bulbs go dim. Bulbs are still passing the same percentage of the total current, but it's not enough to glow the filaments.

Even with the pot turned a little off full bright there's still lots of current flowing through the pot (too much in fact), but not enough to glow the filaments or blow the fuse.

At least that's my read... Electrical troubleshooting is very hard from afar!

[SteveJ]

I'm glad the good Captain got me thinking. (The thinking part happens rarely.)

I'm used to diagnosing long distance. The most likely problem is a short around a bulb base in the dash lights. The next likely culprit is in the splices in the dash harness. You could also look at the connectors for the lights on the speedometer and tachometer since the positive and negative pins are next to each other. (Ref page BE-16 of the 260Z FSM)

Bruce hit it perfectly. The short is bypassing the load on one of the branches of the circuit.

To trace this down

1. Get an ohmmeter.

2. Remove the fuse.

3. Disconnect all of the gauges except for the speedometer. Also disconnect the illumination for the heater control panel.

4. Turn the rheostat to the brightest setting.

5. Measure the resistance from the outside clip of the fuse to ground. It should be high. If it is a couple of ohms or less, inspect the connector, wiring and bulb sockets on the speedometer.

6. After eliminating the speedometer as the source of the short, plug in the tachometer and measure the resistance from the outside clip of the fuse to ground. It should be high. If it is a couple of ohms or less, inspect the connector, wiring and bulb sockets on the tachometer.

7. After eliminating the tachometer as the source of the short, plug in the oil pressure/temp gauge and measure the resistance from the outside clip of the fuse to ground. It should be high. If it is a couple of ohms or less, inspect the connector, wiring and bulb sockets on the oil pressure/temp gauge.

Keep doing that for the ammeter/fuel gauge, clock and illumination on the heater control panel.

Edit: I missed that this circuit also can go to the radio illumination and hazard switch illumination. Include unplugging those in your diagnostics. Those are on page BE-17.

Edited October 15, 2014 by SteveJ