electrical oddity to say the very least

[timecapsule]

I've been helping a friend with a few projects on his 1963 Starfire Oldsmobile. As I've mentioned before several times on here, I'm as stupid as a stick when it comes to auto electricity.  But I do know the basics.  I'll mention this first oddity because it may or may not be a clue to the second even weirder oddity.

 

The wire on the hood light broke off, so I decided to repair it. The wire from the receptacle is about a foot long, then there is a male/female connection.  I suspect that is for incase the hood needs to be taken off for some reason.   As I took the light socket apart I discovered that it had a mercury vile in it which activated the light once the hood was opened.  But then my friend told me that he had to put on the parking lights in order to make the hood light come on.  This didn't make any sense to me.  If that's the case, what's the point of the mercury vile? The wiring diagram verifies what he says though.   So then I got curious and checked the female connection going to the hood light with a test light.  Alligator clip on the negative post of the battery, pointy end inside the female connection of the wire going to the hood light.  There was power going to the light, with the parking lights shut off.  Which made no sense either.  Then things got even more mysterious.  My friend has one of those master kill switches interrupting the ground lead going to the battery from the engine block.  He always shuts that switch off,  killing all power to the car.  However the switch was in the off position when I tested the wiring going to the hood light.   I checked the too terminals on the kill switch for power as well as continuity in both positions (on/off) and the kill switch seems to be operating just fine.  I plugged in the hood light to the female connection and the light didn't come on.  However when I tested the hood light, by grounding the light housing to the negative battery post and touching the wire end to the positive post, the light came on when I tilted it to match the angle it would be when the hood was open.   So now with the hood light connected to the female connection on the power wire going to it, and then turning on the parking lights, sure enough the hood light comes on. The car appears to me to be close to 100%  unmolested.  So like I said, I don't see the point of the mercury vile.  But apart from that how on earth is there power to that hood light with that kill switch in the off position???

 

Here's the second oddity, which to me is even more mind boggling. During the testing of the hood light, Once I connected the alligator clip to the battery ground post, I was about to stick the pointy end in the hood wire female end connector.  As I was moving my body to get in a better position the pointy end of the test light brushed against the inner fender and it lit up.  What the !!!  So I tried it intentionally again, same thing, it lit up.  I touched the pointy end to the body of the power steering pump and the test light lit up again.  So then I disconnect the ground terminal wire from the battery.  I then tried the test light again from the bare battery post to the female socket and the test light lit up.   I then got out a multimeter and set it to DC voltage. I touched one lead to the bare ground post and one lead to the inner fender and I got a reading of 12.96 volts.  I got the same reading at the power steering housing as well as other parts like the upper radiator cross member etc etc. 

 

How is that possible???

 

BTW, yes I do know the difference between the positive and negative posts on a battery.

 

My friend likes to keep his car as original as possible so when his battery died awhile back, he ordered a "look a like" AGM battery from some company ( company name escapes my memory right now) that looks identical in shape and size as the acid filled battery that came with the car back in 1963.  It even has the pretend filler caps with Delco written on each yellow cap.   I can't imagine what that might have to do with anything, but it is the only electrical change that has been made to the car.   Like I said I'm not terribly electrically minded but I do have a pretty perceptive mind and I checked all the electrical components and the fuse box and everything looks normal and everything operates normally. 

Any ideas?  

 

 

[JFranklin]

Ask where the backup battery is hiding.

[Frank DuVal]

1. If the battery kill switch is open then the test light connected to the battery negative terminal will act just like you say, since the test light is now in series with the car's entire electrical system.

2. Yes, many GM cars had the hood light only work with the parking/headlamps on. The Mercury switch was to prevent a glow from under the hood while driving. Little did they know later on young people would install lighting to get this glow, only to have police explain that it is illegal....😮

3. I bet you connected the DMM to the battery negative terminal, bypassing the battery kill switch. 😉

 

Please, only troubleshoot an electrical system with EVERYTHING as it should be for the car to operate i.e. no kill switch in use. Best to remove the kill switch for ANY troubleshooting, as they induce their own problems.

Edited May 9 by Frank DuVal (see edit history)

[timecapsule]

10 hours ago, Frank DuVal said:

1. If the battery kill switch is open then the test light connected to the battery negative terminal will act just like you say, since the test light is now in series with the car's entire electrical system.

2. Yes, many GM cars had the hood light only work with the parking/headlamps on. The Mercury switch was to prevent a glow from under the hood while driving. Little did they know later on young people would install lighting to get this glow, only to have police explain that it is illegal....😮

3. I bet you connected the DMM to the battery negative terminal, bypassing the battery kill switch. 😉

 

Please, only troubleshoot an electrical system with EVERYTHING as it should be for the car to operate i.e. no kill switch in use. Best to remove the kill switch for ANY troubleshooting, as they induce their own problems.

Thanks for helping me understand the principal of the design for the under hood light.  I get it now.  So that when the lights are on, the hood light won't be lit up as you're driving along at night.  But what I don't get is how the idea got off the drawing board.  Let alone how the notion of the idea even made it to the drawing board.  That vile of mercury. as I've called it, which looks much like a tiny fuse, along with the plastic stepped sleeve that it fits in, which in turn slides into the receptacle, which is held in place by a tiny tab. Is far to complex and way too much overkill to accomplish what it does.  It would have made far more economic and manufacturing sense to simply run a wire from a simple light receptacle to the dashboard where there would be a simple switch with a wire running back to the fuse panel.  Mission accomplished at a fraction of the price, and no more inconvenience than turning on the parking lights.

 

Unfortunately I'm not getting that light bulb in my head to turn on when I read the rest of the points that you've made.  At one point I had the negative post of the battery completely isolated.  Nothing connected to it at all.  The multi-meter said I had 12.96 volts between that negative battery post and the inner fender.

[Farmallregular]

I don"t see anything "odd" about what you're describing.  For the case with the test light from the underhood connector to the battery, the circuit is from the positive side of the battery to the light connector, through the test light, and back to the battery.  In this case, it's immaterial whether or not the disconnect switch is open or closed.  If you were to connect the test light lead to a vehicle ground with the disconnect switch open, there would be no complete circuit since the disconnect switch is isolating the battery from the chassis.

 

When you measure across the open disconnect switch from the battery negative to the vehicle chassis with a digital meter, you're measuring voltage through some load connected between the positive side of the battery and the vehicle structure and the battery negative.  Remember, digital meters have very high input impedance, and even a high resistance parasitic load will pass enough current to show voltage on a high impedance meter.  Such a load could be things like a dome light if the doors are open, keep alive memory in the radio if its been updated, etc.

 

In theory, all the metallic parts of the vehicle should be electrically connected, and for a "ground" for a meter or test light it shouldn't matter where you connect it.  In practice, years of corrosion may have introduced some resistance into some of the connections, or somebody has "fixed" something and not properly grounded it.

 

The type of battery would have zero impact on what you're seeing.  Is there some particular issue you're trying to solve?

 

Keith

[timecapsule]

3 minutes ago, Farmallregular said:

I don"t see anything "odd" about what you're describing.  For the case with the test light from the underhood connector to the battery, the circuit is from the positive side of the battery to the light connector, through the test light, and back to the battery.  In this case, it's immaterial whether or not the disconnect switch is open or closed.  If you were to connect the test light lead to a vehicle ground with the disconnect switch open, there would be no complete circuit since the disconnect switch is isolating the battery from the chassis.

 

When you measure across the open disconnect switch from the battery negative to the vehicle chassis with a digital meter, you're measuring voltage through some load connected between the positive side of the battery and the vehicle structure and the battery negative.  Remember, digital meters have very high input impedance, and even a high resistance parasitic load will pass enough current to show voltage on a high impedance meter.  Such a load could be things like a dome light if the doors are open, keep alive memory in the radio if its been updated, etc.

 

In theory, all the metallic parts of the vehicle should be electrically connected, and for a "ground" for a meter or test light it shouldn't matter where you connect it.  In practice, years of corrosion may have introduced some resistance into some of the connections, or somebody has "fixed" something and not properly grounded it.

 

The type of battery would have zero impact on what you're seeing.  Is there some particular issue you're trying to solve?

 

Keith

One thing I'm trying to do is just understand why I would get a voltage reading from ground to ground so to say.  But I'm also trying to understand why there is 12.96 volts at the female end of the wire that is connected to the under hood light, with the kill switch in the off position.  Not to mention with the battery ground lead completely removed from the battery.  Where is that wire getting it's power from? 

 

One would think that by disconnecting the ground lead of a battery that it is safe to work on electrical components of a car.  Am I wrong to assume that?  Should one disconnect the positive battery lead instead?

[Farmallregular]

3 minutes ago, timecapsule said:

One thing I'm trying to do is just understand why I would get a voltage reading from ground to ground so to say.  But I'm also trying to understand why there is 12.96 volts at the female end of the wire that is connected to the under hood light, with the kill switch in the off position.  Not to mention with the battery ground lead completely removed from the battery.  Where is that wire getting it's power from? 

 

One would think that by disconnecting the ground lead of a battery that it is safe to work on electrical components of a car.  Am I wrong to assume that?  Should one disconnect the positive battery lead instead?

The negative terminal is not "ground" unless it's connected to "ground", in this case through the disconnect switch.  See previous post for description of the circuit from the positive battery post to the chassis "ground" that completes the circuit when you measure voltage from the negative battery terminal to the chassis.

 

As I previously pointed out, the test light or meter is competing the circuit from the underhood light connector to the negative post of the battery, irrespective of whether or not the disconnect switch is open.  If you make the same measurement between the connector and vehicle ground with the disconnect switch open, you'll get zero volts.

 

Disconnecting the battery negative lead does make it safe to work on the electrical system.  Better to disconnect the negative, since if you happen to slip and the wrench contacts the surrounding body, nothing will happen.  If you're disconnecting the hot side with the ground connection still in place, there's potential between the wrench and surrounding chassis.

 

Remember, electricity only flows in loops.  An open circuit will show voltage across the open portion, but no current will flow.  If you disconnect the negative battery cable, you've opened any return path back to the source (battery), and no current can flow back to the battery.  The idea of "ground" is a bit of a misnomer, the metallic portions of the vehicle are simply another electrical conductor that's handier than running a return wire to lights, etc.

 

All of this is just basic physics...you might try drawing an equivalent circuit for each of these conditions, would make it easier to understand the source of voltage (electrical pressure) and current (electrical flow).

 

Keith

P.S.  Spent 47 years as a practicing electrical engineer on communications systems, military aircraft, and spacecraft.

 

[jdome]

Below is a simple circuit of a negative ground  battery, switch & bulb.

 

Notice that when you connect a  tester, test light or DC test meter, at the proper polarity points as shown, you will read 12V.  Notice that with switch open, you will read ground (-) on both sides of the bulb because it is now a wire connection from ground to the open side of the switch. Removal of the bulb will open the grond at the switch.   Below are drawings of the 4 possible switch positions leading to the under hood lamp in its original configuration, before the kill switch modification.

 

Depending on whether the Park SW and/or the Mercury SW is open or closed, the associated bulb will either light or will make a ground path as shown. The kill switch modification adds some quirks to the system. Addition of the kill SW between   + battery terminal and park/hood light circuit and other accessories will test as above as long as the kill SW is turned on.  Below is a diagram of how thecircuits will test with the kill SW open.

 

With the kill SW open there is no power to the accessory harness so it does not matter whether the Park & Merc switches are on or off. There is a bulb in each section that grounds both sides of the switches.  Because there are other devices (dome lights, clock, running lights) connected to ground on one side and wired to the harness  or switched on and connected to the harness; the entire harness will test as ground at almost any point, with kill SW open.  There is nothing wrong with adding kill switch. Just be aware of what is happening when testing.  I would suggest that you calculate the total current demand of all bulbs and accessories to make sure the kill switch can handle the total current through the switch at its 12V DC rating. Could present a fire hazard.

[JACK M]

I have seen mysterious stuff happen like this when the vehicle is searching for grounds. 

The hood may not be grounded to the body, the body may not be well grounded to the chassis etc.

An old guy told me once that "99% of all electrical problems are groundless".

[timecapsule]

5 hours ago, jdome said:

Below is a simple circuit of a negative ground  battery, switch & bulb.

 

Notice that when you connect a  tester, test light or DC test meter, at the proper polarity points as shown, you will read 12V.  Notice that with switch open, you will read ground (-) on both sides of the bulb because it is now a wire connection from ground to the open side of the switch. Removal of the bulb will open the grond at the switch.   Below are drawings of the 4 possible switch positions leading to the under hood lamp in its original configuration, before the kill switch modification.

 

Depending on whether the Park SW and/or the Mercury SW is open or closed, the associated bulb will either light or will make a ground path as shown. The kill switch modification adds some quirks to the system. Addition of the kill SW between   + battery terminal and park/hood light circuit and other accessories will test as above as long as the kill SW is turned on.  Below is a diagram of how thecircuits will test with the kill SW open.

 

With the kill SW open there is no power to the accessory harness so it does not matter whether the Park & Merc switches are on or off. There is a bulb in each section that grounds both sides of the switches.  Because there are other devices (dome lights, clock, running lights) connected to ground on one side and wired to the harness  or switched on and connected to the harness; the entire harness will test as ground at almost any point, with kill SW open.  There is nothing wrong with adding kill switch. Just be aware of what is happening when testing.  I would suggest that you calculate the total current demand of all bulbs and accessories to make sure the kill switch can handle the total current through the switch at its 12V DC rating. Could present a fire hazard.

Very cool.  Thanks for all that time and effort making up those drawings.  I'll pass this on to the owner of the car, and assure him that he can go back to getting a good nights sleep.

[Xander Wildeisen]

Sounds like you are back feeding through the element in the light bulb to ground.