Check6

Put a fuse in the courtesy light receptacle and blows instantly. Looks like a dead short. Where should a I start looking first?
Thanks,
Fred

RetroRanger

Hey Fred

What else is on the circuit? Dome light? Map light? Instrument lights? In general I would follow the wires to any affected light check where they passthru vehicle framework and check the switches too

Lady72nRob71

Check the prongs in the cigar lighter to see if they got mashed when plugging something in incorrectly. Lighter and clock and trunk light are on that circuit

Check each bulb to see if they are the correct ones. Putting a one contact bulb in a double contact socket will cause a dead short and happens often because many owners are sold the wrong bulbs...



BTW welcome to the site!

Check6

Guys, thanks for the quick response, I will check these out and let you know what I find. This is my first OLds. I have owned 10 classic Mustangs, 3 Chevelles, and a Nova through the years with a Corvette and a little Red Express truck thrown in. LOL
Here are some pics.
Thanks for the help.
Fred

1970cs

Is this a new problem or did you just notice that the courtesy lights were not working and stuffed a fuse in? Looking at your prior posts you have had this car for a while?

Pat

Rickman48

Just be real careful - that circuit is live all the time!

Check6

I just bought this car yesterday.Noticed the lights weren't working and that the fuse was missing. Of course now I know why the fuse was missing. LOL
Another question. Are there two wires going to the lights, lighter, etc. or is there just one with the light grounded in the receptacle? Nothing seems to be hacked in the wiring looks like all stock wiring harness.
Thanks,
Fred

MDchanic

Just like on the Chevelles, the Nova, and the Corvette, the orange wire, which is hot all the time, goes to the dome, map, glove compartment, and courtesy lights, and to the cigarette lighter and the clock.

The clock and the cigarette lighter have black ground wires, and/or are grounded to the local metal components.

The lights have a white wire on their other sides, which connects to the dome light (or map light) switches, which switch to ground - you open the door, the switch grounds, completes the circuit, and the lights go on.

You need to look primarily at the cigarette lighter, which is a classic short point, and at the clock and map lights, and anywhere that the orange wire passes near metal.
Because the cigarette lighters short out so often, the insulation of the orange wire is often melted, either out in the open, or inside the harness, and the short may be anywhere along the way if it is.

- Eric

Fun71

Does your car have a rearview mirror maplight? There is a plastic bushing on the top of the mirror that insulates the hot wire, and that can break and allow the wire to contact the metal mirror housing (ground).

jaunty75

My guess is that you have a short in one of the hot leads leading to one or more of the components served by that fuse. You need to work backwards from each one to the fuse panel, tracing the wiring along the way.


By the way, is there a difference between a "dead short" and plain old ordinary "short?"

Check6

Dead short is a figure of speech no difference. I'll check out all the suggestions. I'm leaning towards the lighter as a starting point. It seems that anything you push in to heat a coil could cause a problem just by design. I'll let you know how I make out.
Thanks,
Fred

MDchanic

That light should be connected to the IGN circuit, not to the CTSY circuit.

- Eric

Lady72nRob71

A 'dead short' is one that costs you more money.
I always thought of it as one with as close to 0 Ohm as could be.
Anyway, here a short is a short.
Did you check the light bulbs for proper bases?

Check6

Haven't checked the bulbs yet but hopefully will be looking at all the suggestions. Bulbs should be an easy check. I know the dome light is OK I pulled the light, long cylindrical with contacs on both ends. Checked the bulb on a spare battery and it works fine. At least I know it's not burned out. There are two more lights located on the rear sail panels. I think these pop out.
Thanks,
Fred

oldstata

You could wire in a breaker witch turns on and off and trace the harness with a compass and when it goes crazy you found it

Lady72nRob71

Just make sure you compare the number of contacts in the sockets to the contacts on the bulbs. This is easy to overlook.

Check6

OK I pulled the orange wire from the cig lighter and the clock. They look fine no cuts or melting, left them disconnected. Pulled the under dash light receptacles and replaced the bulbs which were correct. Checked the wiring in the trunk, looks fine. Put a fuse in, instant blown fuse. Any other ideas?? Could there be something going on behind the fuse box?
Thanks,
Fred

MDchanic

Have you also checked the glove compartment light?

I would disconnect and isolate all of the ends of the wire, then test each end for continuity to ground, to see whether the wire is broken anywhere - if there is a short but the wire is otherwise intact, all ends will show zero ohms to ground.
This includes the dome light, and be sure that a dome light mounting screw isn't going through the wire.

At that point, the best bet is that the short is inside the harness, and, sorry to say, the only way to find it is to fillet the harness and pull the wire out. If you wanted to do this, I'd open it in the middle somewhere and just reach in to see whether the wire seems melted there.

The other alternative is to run another wire to all of the places where it goes, and just leave the original wire sitting.

These kinds of shorts can be very tough, but, if the short persists with ALL of the ends disconnected my money is still on a melted wire inside the harness.

- Eric

oldstata

You can wire Ina 30 amp breaker it turns on and off as it gets hot. like 10 bucks to make use a compass to trace along harness it will spike when your near the short open harness and check wires in that area it will pin point a area works great

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MDchanic

You could also just put a light bulb in series where the fuse goes -
bigger bulb = more current = easier to follow, so use a headlight bulb.

- Eric

Check6

Thanks for the tips. I will next remove the dome housing, light housing in the sail panels, and the switches in the door jams looking for pinched or broken wires or screws hitting the wires.
I'll keep you posted.
Thanks,
Fred

Check6

OK , I removed the dome light bezel and found the white wire squashed against the roof of the car. Should i cut this bad piece out and solder in a new piece? Pulled the two bezels located in the rear sail panels and they look fine. I was checking the pole switch in the door jam and noticed it has two wire tabs, white wire is plugged in on one side and I found a black wire dangling in the dash connected to nothing. It was mated to the white wire should I plug this in the other side of the switch? Only bad wire I found so far is the white one in the dome .
Thanks,
Fred

hookem horns

I don't think the dome light is your root cause but can't hurt to fix it. A crushed white wire would ground the light to the body so it would stay on full time. Crushed orange wire would be a short.

Check6

What about the black wire I found dangling? Does it go to the door switch? There is only the white wire going to the passenger door switch.
Thanks,
Fred

MDchanic

That's probably the wire for your annoying key buzzer. Don't worry about it.

- Eric

Lady72nRob71

Does the car have map lights? One in the dash by the radio and one in the mirror? Those feed from that circuit. Any aftermarket stuff like new radios, alarm, etc?

Rickman48

I'd tape the dome wire and try another fuse

Check6

Yes to all 3. Has a map light near the lighter and ash tray, mirror light, and an aftermarket radio

MDchanic

The mirror light should be fed from the IGN circuit (or the ACCY circuit), not the CTSY circuit.

- Eric

Check6

I disconnected the mirror, the map light switch and the radio. Instant blown fuse. Very frustrating. Could there be something going on in the fuse box? The wiring harness looks like it's never been hacked, looks factory fresh.
Thanks,
Fred

MDchanic

There's really nothing in the fuse box to "go on."

Wires connect to fuse terminals - that's about it, just like in the schematic in the Chassis Service Manual.

- Eric

Miles71

Hi Check6
Check6, do you have a digital voltmeter with Audible Ohm meter readings?
You will need a set of alligator clips too.
A good meter will save you many Fuses.
Here are some measurements from my 71 CS
Disconnect the Battery. Unplug the fuse. Set meter to Low ohms scale. Measure
resistance between the Cold Clip (Right Clip) and Ground. With just the driver door open
you should see approximately 0.9 to 0.7 ohms. Push in the Driver Door light pin switch.
The Resistance should change to 3-4 Ohms. If you do not see 3-4 ohms with all door closed
and the courtesy light **** Off, then you still have a short.

I also see approximately 4 Amp load at 12.5 volts, when all 5 inside lights are on (measured at the ctsy fuse).

As you disconnect stuff, watch the meter.
Try to get your resistance to a similar rang as my car:
1 Door open (0.9 - 0.7)
Doors closed switches off 3-4 ohms. All values are approximate. Your actual values may vary.

Repair the offending socket or short and reassemble.
Keep the meter in place during the reassembly, just in case you have other issues.
Do not stick a fuse in again until you see some normal ranges as listed above.

Ohms law, at 12 volts.
A 1 Ohm load will attempt to draw 12 Amps
A 0.5 Ohm load will attempt to draw2 4 Amps.
A 0.25 Ohm load will try to draw 52 Amps and will melt wires quickly.
Note, as Incandescent filaments heat up, their resistance may increase.
This is why my car has a 0.7 ohm resistance (no current ) and is only using 4 amps when power is applied.

Fun71

Must be the ACCY circuit as the IGN circuit is active only with the key on. The rearview mirror map light stays on when the key is turned off - I once ran the battery down and cracked a rearview mirror due to that light staying on overnight.

sweet70oldz

Number disconnect your neg cable to battery before working on these. I would check the sail panels or dome light. Make sure the wire leads are not contacting the body which would be a problem. The wire connectors should be securely mounted into the assembly but over time the plastic degrades and breaks which can be troublesome. It was for me in my Cutlass with sail panel lights,. Good luck, Sir.

MDchanic

Ah. My point exactly.

The '70 Cutlass Convertible I had definitely did not have the mirror light connected to the hot feed.
My recollection is that it was to the IGN line, but it could have been the ACCY line - either way, the light would only go on with the key on.
Presumably, this was because it is absolutely impossible to see that little light from outside the car unless it's really dark out, which makes it very easy to leave it on and kill your battery.

I no longer have that Cutlass, so I can't check it, but the accessory plugs that plug into the fuse block taps usually have little ridges on them - there's one ridge position for ACCY, one for IGN, and one for BAT - so that you've got to work to plug them into the wrong circuits, and I assume that the mirror light plug has one as well.
If you could take a look at yours and let us know, it would put this to rest - maybe my car was the one that was hooked up wrong (in which case I'm glad, because it saved my battery a couple of times).

- Eric

oldsman72

Watching this thread closely.
I have the exact same problem with my '70 442. When I got the car, I know the dome light worked. Now it does not, along with all other interior lights and radio. 25A fuse blows immediately. Thanks to advice from this thread, I know to tackle the lighter next then trace orange wires.

Check6: Do your exterior lights/turn signals work? What about your hazards?

Check6

All exterior lights, backup lights, turn signals, 4 way hazard work perfectly.
Thanks,
Fred

Check6

Is the center console light part of this circuit? I haven't checked that yet.
Thanks,
Fred

Lady72nRob71

Yes - check it also.