Electrical Help Please

[67SWT]

Recently my 72E developed a short which would drain the batteries very quickly. I am not that great with wiring but bought a multimeter and was able to figure out the drain is on the fuse that runs the clock, interior light, glove box light, and the luggage compartment light. I also discovered that the draw of power ranged between .065 volts and .25 volts and that when the luggage light switch was in the off position the draw increased the longer the light was off. Currently the clock works but there is no glove box or interior light working (but not sure if they ever worked as I don't usually drive at night). Can anybody give me a starting place or places? Thanks

[orange911]

Recently my 72E developed a short which would drain the batteries very quickly. I am not that great with wiring but bought a multimeter and was able to figure out the drain is on the fuse that runs the clock, interior light, glove box light, and the luggage compartment light. I also discovered that the draw of power ranged between .065 volts and .25 volts and that when the luggage light switch was in the off position the draw increased the longer the light was off. Currently the clock works but there is no glove box or interior light working (but not sure if they ever worked as I don't usually drive at night). Can anybody give me a starting place or places? Thanks

I suspect it is your clock. There is an brief electric charge from the battery every 15 seconds or so to the spring points. This is used to wind the spring. If the points are stuck, then you have battery suppling 12v constantly to the clock. Check with a volt meter and see if you have an intermittent discharge every 15 seconds. If not, then I say its the clock. Clean/check points using clock rebuild posts. Pretty easy to check out.

[67SWT]

I took the clock out and actually the amp draw increased a little. When I use the multimeter there is definitely a draw on one of the three red wires that connects to the positive battery terminal. It is the red wire that ultimately feeds the clock, interior light, glove box light, and luggage compartment light. I also noticed that the draw worsens if I leave the luggage compartment light off. Maybe this has something to do with resistance (what ever that is)?

Another observation that I don't understand on the voltage side deals with this same red wire. When the positive terminal is disconnected from the battery and if I separate the red wires and two large black wires when I jump between the battery terminals to the and the single red wire with the multimeter I get 12.3 volts. When I jump between the positive battery terminal and and the other red or large black wires I get 6.7 volts. Is this the way it should be?

Tonight my plan is just to pull the fuse to the clock, luggage compartment light, interior light and see if my battery is dead in the morning. Any other ideas?

[orange911]

First way to eliminate a gremlin is to measure the amp draw on the battery and pull one fuse out at a time. Measure amps after pulling each fuse. Your bad circuit is the one where the amps go down.

OK, this is just me thinking out loud. I've never even played an electrician on TV. But since you have mentioned luggage lights twice, I will add a commentary on that.

If luggage lights are like license plate lights, they are polarity sensitive. If you (or anyone before you) has ever removed that light and plugged it in backwards, it should blow a fuse after a brief spark. But, if the ground is not great because of paint or rust, then maybe there is not enough resistance to draw the amperage required to blow the fuse.

You can tell which side is ground by looking for the plug that is not insulated from the screw hole flange (grounded with body after screw is tightened).

Olin

[docrodg]

OK... Electrical 101 seems to be in order here, as understanding this is important to troubleshooting. It seems you are measuring voltage to determine if a draw exists, this is not correct. You need to measure amps. The basics are this: Voltage is the potential between two points. Basically it can be equated to a big tank of water with a pipe on the bottom and a valve. It is always there, whether the valve is open or not, open the valve and it will flow water at a specific pressure. Amps (Current) is the flow of electrons. Equate current to the water flow, it is the measure of how many electrons flow past a point in the circuit in a given time. Resistance is measured in Ohms and is just that, resistance to electricity flow. Think of it as a partial obstruction in the pipe... the greater the resistance the lesser the flow. These 3 things make up a fundamental relationship: Voltage = Current times Resistance. Watts is power, which is Voltage times Amps. What happens if you take a wire and just connect it between the two battery terminals? You create a short, a path of nearly no resistance (many decimal points before the number) and thus a huge amount of current (amps) that causes sparks or a blown fuse. A corroded connection will have resistance and can create the same situation, or if corroded enough a high resistance and thus a small draw in Amps.

Circuits have things in parallel (like a ladder, battery wires for the uprights, components as the rungs). Most circuits in a car are basically parallel circuits, so each device in the circuit will have the same voltage but the current will be split between them proportionally based upon their resistance value. This makes sure that if one thing fails (burned out light bulb) the rest still work.

There are also series circuits where the components are "Daisy chained" together. These circuits split the voltage between each component and the amps is the same at all points (Voltage is divided by total resistance of all components added together)... problem with this is that one component failure (burned light bulb) will cause all electricity flow to stop in the whole circuit. This is the reason some of your wires read half voltage, they are feeding two things that are in series.

Thus a circuit that runs an interior light and the trunk light may have a bulb for the interior light that is brighter, at say 20W, and a trunk light at 10W. Knowing our car is 12V lets us figure out what these bulbs draw in amps and their resistance values. The watts divided by volts will give us amps. Thus the interior light is (20W/12V) = 1.6 amps; the trunk light is (10W/12V) = .83 amps. Thus the interior lamp has a resistance of (12/1.6) = 7.2 Ohms and for the trunk light is (12/.83) = 14.45 Ohms. Since they are in parallel the total amps for the circuit (measured at the battery) would be (1.6+.83) = 2.43 amps when both are lit. The voltage would read 12 volts anywhere you measured it. Turn off the trunk light and the voltage would measure 12 Volts and the Amps at the battery 1.6 Amps.

Now... apply this to your problem:

You have a drain on the battery, so there is something using electricity in the circuit. If you were to disconnect each component from the circuit one by one your should be able to find the culprit by seeing the amp measurement go to 0 when the bad component was disconnected. What if there are two or more bad components? The reading would go down when you disconnect the first bad component, but would not change if you unplugged something that was not drawing current at the time.

So here is the how: Go out with your multimeter and get ready to play detective. Plug the black wire for the meter into the common terminal on the meter and the red wire into the 10A terminal. Now turn the selector switch to the 10 Amp setting. Disconnect the positive battery terminal and put the black wire on the battery post and the red wire on the positive battery cable. Look for a reading and if there is one write it down.

Now, put the meter aside for a moment and reconnect the battery. Make sure that the car doors are shut, lights off, etc. Go to the fuse panel and disconnect the wire leading off of the fuse to the rest of the car, if there is more than one wire that is ok. Be careful and make sure the circuit is "off" (ie: headlights not on) if the circuit is rated for more than 10 amps (you can tell by fuse size in the fuse block) as this will blow the fuse in your meter or fry your meter if it exceeds 10 Amps. Take your meter and place the black wire on the terminal of the fuse panel where you just removed the wires and then touch the wires that were connected to the fuse one by one. If you read a current draw you have a circuit with something going on. write down the reading and the circuit (fuse number, wire color). Compare the current reading to the one from the battery, if it is less than the battery one it is only one of two or more bad circuits, if it is the same then it is the only one. Now check your wiring diagram and find out what that wire feeds. Make sure your meter is connected to that circuit again (same way at the fuse box as you did to find it) and is where you can see it while playing with each component in the circuit. Go to the first component and disconnect it, look a the reading and if no change the component is ok, if the reading goes down then it is bad. Work your way through the circuit and you should find the bad components.

In your circuit the clock is always running, so you disconnected it and the draw lessened, we know that is ok, but there is still a bad component. So the first one you go to next is the glove box (pick whichever you like) and you open the door and find the switch that operates the door. Now with the door open the light is on and your current draw goes up, so push in on the switch and the light goes out and the draw goes down. Did the draw go down to your last bad reading or to a lower or zero reading? If to a lower or zero reading the switch is not positioned properly and the lamp is always on whether the door is open or closed. If it went back to the original reading there was no effect and the glove box light and switch are good.

Now assume that the glove box light was bad (the reading went down). Still not done yet, as there is either a bad switch or a bad socket causing the draw. So disconnect the switch from the circuit completely and grab your meter (disconnect the meter from the circuit). Unplug the red meter wire from the 10A terminal and into the Volt-Ohm Terminal. Set the meter for Ohms (resistance) and put one probe wire on each of the switch terminals. With the switch pushed in (glove box lid closed) you should get an infinite resistance, with it let out (like the lid was open) you should get 0 ohms or darn close to it (no resistance). If you get a resistance other than infinite when the switch is pushed in then the switch is bad. Do the same for the socket without the bulb in it. A reading across the two terminals should be infinite (no connection), anything less than infinite is a bad socket.

Think a corroded ground connection is a culprit? easy to figure out. Take off both battery terminals. Set to read resistance (Ohms) as above place a probe on the connection (terminal or screw that connects wire to chassis) and the other probe to the battery ground cable. The reading should be 0. If it isn't just clean the connection and take a new reading.

Never measure resistance with the battery hooked up to the circuit, it will mess up the reading and possibly your meter.

[67SWT]

I have narrowed it down to the one fuse and hope to have it figured out next weekend. I am guessing that luggage light and or the clock . Olin I really appreciate your input, and have printed up docrodg's response for the current and future problems. I did several searches for this the topic online and found yours to be the most helpful. Thanks a lot to both of you and hope I can pay you back or pay it forward some day on a topic I have experience with.

[HughH]

Dogrodg

great explanation of trouble shooting electrics

it should go in any compilation of technical help

simple and easy to follow and execute from

[docrodg]

Thanks for the compliments! I really appreciate them, especially since I had not finished my first cup of coffee when I wrote that.

67SWT: Your clock can act a little differently than the basic resistance stuff in most car circuits. Because it winds itself there will be a slight increase in draw when it is doing that from when it is just running. Due to this I recommend you just leave it out until done since you already have it out.

[docrodg]

OK... next question may be something like this: "How do I do this for circuits with greater than 10 amps current?" Well, that gets a little more complicated. The best thing to do if you are troubleshooting is to get a current clamp or a meter with a higher range. A current clamp can or probe can be purchased that just plugs into the meter instead of your probes to increase the reading capability of your meter but some can be rather expensive.

The redneck geek way:

You can solve the problem on the cheap by making a shunt using exactly 12" of 10 gauge solid copper wire, it has a resistance of .001 Ohms. Attach a pair of clips (one on each end) and you have a nice little shunt that can handle high power without frying. Take the wire and begin by measuring the resistance and verify it is indeed .001 ohms, if not trim it or cut a new piece or just write this value down on tape and wrap it on the shunt. Now coil the wire around something to make it a nice size that will not interfere with testing and get in your way, a dowel or bolt works fine, just coil the wire around it and then remove the bolt.

Now you can take your wire coil and test a headlight circuit. Find the fuse for the headlights and disconnect the wire leading out from the fuse (leave the fuse in to protect the circuit). Connect one end of your shunt to the fuse and the other to the wire leading out to the relay, just like you would for a multimeter. Now set your multimeter on the millivolts scale, that's right, MILLIVOLTS. Start at a higher setting if your meter has multiple scales for millivolts. Turn on the headlights and then measure the millivolts across the shunt by placing the negative lead on the shunt at the fuse panel and the positive lead on the shunt at the other end. You are now measuring the voltage drop across the wire. Since Volts=Amps X Ohms you can easily convert this to amps. Lets say you read 12.5 millivolts... 0.0125 Volts = X Amps times 0.001 Ohms. Do some easy algebra and make the equation right: Volts divided by Ohms = Amps, and you can figure it out easily. .0125 Volts divided by .001 Ohms = 12.5 Amps (12.5 millivolts reading across the shunt is 12.5 Amps flowing in the circuit). If you had a different resistance for your shunt then substitute that one in for the .001 Ohms. Of course, being lazy, we do this easily by using .001 Ohms as that makes our millivolt reading the actual amps that are flowing. Now you just proceed in the troubleshooting as you did for other circuits.

[orange911]

docrodg,
great write up. I am a bit lazy in my write ups because I make too many assumptions that the person I respond to knows EXACTLY what I am talking about. For instance, measuring amp draw with the battery connected (blk on neg and red on pos) would probably fry the innrds of your meter. I also did not catch that he measured in volts and not amps. Good catch.

Craig, good luck and let us know the updates.