tracing a short

[RRROBB]

I have a 1977 L82. recently I have been doing a complete interior replacement.
The center gauges have a new pc board and new clock. Only the clock works correctly the fuel gauge reads a partial amount no other gauge registers any thing.
after leaving my car overnight I discovered the battery was dead. This was traced back to the cigarette lighter/coutesy lights/ clock circut.
I pulled this fuse and the short is gone.
How do I go about tracing the short from here????
Am I correct in assuming that the short must be beyond the fuse and everything before the fuse is ok??
Please help as I am lost!

 

[jdp6000]

If you pulled the fuse and the problem went away. I'd say you found the circuit with the short. Next thing to do would be to locate everything that fuse feed and eliminate each component.

Which fuse was it?

Jim

 

[RRROBB]

it was the cig lighter// courtesy light// clock circuit.

Also forgot to mention that apparently my overhead light stays on at all times door open or not. I have pulled the bulb but I still get my battery drawn down with the bulb out.

What I am actually seeking is the mechanics of how to trace down the short. What tools to use (voltmeter etc.) And how to hook them up.

 

[Bob Chadwick]

I had the same problem with my overhead light staying on and tracked it down to the dimmer circuit. It is a bear to get to, right between the tach and speedo under the dash pad, and I just pulled all the interior lights to stop the drain until next time I have it out. There are actually three; one overhead and one each under in the passenger and driver side footwells.

I was still having a drain until one night I went out to the car and saw a light at the back of the car. There is a bulb back there under the bumper that runs off the courtesy circuit. Once I pulled that bulb all was good.

 

[jdp6000]

You can use a volt meter or test light. For your purposes a test light would be best. You really don't need to know voltage. with the test light...attach the black to a ground and touch the wires with the red...if there is power the light will go on.

If your dome is staying on check the pins in the door pillar area. when pushed in they should turn of the the interior lights. If not they are bad.

Do you have a dimmer on your linterior lights? The module sometimes goes.

Jim

 

[jdp6000]

Bob,

You can disconnect the dimmer and your lights will still work they just won't dim. But you are right...dimmer does cause these problems when it goes bad.

I see from my diagram that some 77 vettes came with the dimmer and some did not. Some had that spare tire light and some did not. But it looks like all that had the spare tire light also had the dimmer.

Rob, the following appear to be on that circuit, console glove box light, cigarette lighter,dome light,spare tire light,left door and right door light and the main light switch feeds off it. Two fused circuits, 20 amp CTSY/CLK and 20 amp GUAGES FUSE.

jIM

 

[Bob Chadwick]

Bob,

You can disconnect the dimmer and your lights will still work they just won't dim. But you are right...dimmer does cause these problems when it goes bad.

I see from my diagram that some 77 vettes came with the dimmer and some did not. Some had that spare tire light and some did not. But it looks like all that had the spare tire light also had the dimmer.

Jim,

Thanks for the info. I can't get to the dimmer without taking the dash apart. If I could get to it, I think it is still available, and I would replace it. I'm just not taking the dash apart for the courtesy lights.

And your right about the dimmer and the spare tire light. It was part of an interior upgrade as I recall.

Bob

 

[jdp6000]

You sure you need to get the dash off? I used to have a 77 and I thought you could get at it by loosening off the part the holds the tach and speedo and tilting it forward.

Jim

 

[69_coupe]

There is two simple ways to find the source that drains your battery;
- remove the positive connector from the battery and connect a test light between the connector and the battery, if there is a connection to minus in one of the circuits the light will glow.
- connect an ammeter between the same two point and you will read the exact current drawn from the battery.
Remember that interior lights are activated when the doors are open, obvious but easy to forget. Check one circuit at the time until ammeter reading is close to Zero or test light is off.

 

[Bob Chadwick]

You sure you need to get the dash off? I used to have a 77 and I thought you could get at it by loosening off the part the holds the tach and speedo and tilting it forward.

Jim

Possible. I'm just not that concerned about it at this point.

 

[Mr. Chuck]

First rule in finding a short or a mis-wire is to go back and verify all distrubed electrical connections and wire routings are not pinched. Shorts are typically caused by pinching wires between a bracket or other such mounting hardware and ground (frame). Another cause is by connecting wiring to the wrong component; less likely since most connectors in cars have different ships for that very reason. Another cause is old / damage / degraded (greasey dirt packed inside or burnt wiring) connectors. Shorts are either direct or a high resistance noted when a test light lights, but dimly. This tells us that some of the battery voltage is being dropped across a resistance between the battery and the point where the test light is connected. Direct shorts will normally blow the circuit fuse, while the high resistance leaves the fuse intact, but runs down the battery over time. In cases where several circuits are powered by one fuse, you should dis-connect each active component (like clock) so normally correct paths to ground are elininated. A remaining ohm meter reading (insure that the ohm meter is set to the X1 scale, other wise if you hold both sides of the circuit under test in hands, body resistance can give an false reading. A remaining continuity reading is the result of the short.. Actual test method of isolating the circuit with the short is to pull the fuse, connect your ohm meter from the cold side of the fuse to car frame / ground; A reading of zero ohms spells a direct short where as a higher reading is an indication of a high resistance short to ground. With the ohm meter connected and braking the circuit back towards the fuse block, until the reading goes to infinty (far left of the dial). This is a time consuming task, the very reason to start with double checking any wiring you had off in doing you interior up-grading. Good luck! Electrical trouble shooting can be a confusing matter if your electrical experience is limited. Need more detailed help, E-mail me at cddmhr@peoplepc,com

 

[SR'71]

it was the cig lighter// courtesy light// clock circuit.

Also forgot to mention that apparently my overhead light stays on at all times door open or not. I have pulled the bulb but I still get my battery drawn down with the bulb out.

What I am actually seeking is the mechanics of how to trace down the short. What tools to use (voltmeter etc.) And how to hook them up.

Electricty is closer to fluids then mechanics. Voltage is head pressure. Current is the gallons per min. Resistance is the width of the pipe[wire]. Tracing an electrical problem requires you to understand these relationships and use the ones you have available to you, as knowns. Example: You think you have a break in a wire. Use the voltmeter on OHM scale and touch point to point for good connection. No connection=No good. Another way to chk. is volts. TWO points at the same electrical potential[point] will have NO volts between them.If you measure a voltage BETWEEN these places, there's a problem BETWEEN them. Another is from a known + volt point to a known ground. Any OHMs , MEASURED WITH EVERYTHING UNPLUGGED ie radios,cig lighters,etc.in the circuit ,, is a problem.

Voltage = Current X Resistance

Think about it

 

[Mr. Chuck]

My shop manual, pg 8A-31 shows a "Interior Light Delay Timer" which should turn off the Dome , Spare tire, Right & Left Courtesy lights after 20 -40 seconds by removing the ground from these lights. This begs the question, is the Dome light the only one staying on? If yes, then I would check for an ohm reading between the dome light socket housing (white wire) to ground. A reading says you have a short between there and the timer. If other lights are also staying, then the delay timer is suspect. A quick timer check, re-install the dome lamp, verify it is still on, un-plug the delay timer electrical plug, if all named lights go out, check timer. Shop manual shows timer is located behind the glove box. Remove the glove box insert and you should see the timer in the panel wire harness. The manual indicates the timer is an electronic device and may not servicable, especially if it is sealed. My experience with electronic devices in my 81 is they are not sealed and you may get lucky and can open it up, if so check for poor or loose solder joints, broken wires, etc, these types of defects may be repairable with low wattage solder iron. Burnt or other wise damage components are a another story.
Good luck!