Question About Automotive Ground

Thread Starter

Back to school

Joined May 22, 2019
114
I'm hoping I can get some help with a disagreement concerning "ground" on an automotive forum. My opinion is too many people write off whatever they don't understand is happening with a circuit as a bad ground. In this particular instance the vehicle owner has to turn the interior dome light on to start the vehicle. Most everyone is saying "bad ground," which doesn't make sense to me. If vehicle ground is a "zero voltage reference," then Kirchhoff's Law states the circuit voltage necessary to start the vehicle has to has to be recognized before ground or the zero voltage reference. My opinion is this has to be a short circuit somewhere in the circuit and has nothing to do with ground. Others are adamant that somehow the voltage necessary to start the vehicle is making a "detour" (as they put it) through ground to complete the circuit. It doesn't make sense to me that ground can both be a zero voltage reference and at the same time allow a circuit to find continuity through ground to complete the ignition circuit. This means ground can be both a zero voltage reference and voltage circuit simultaneously. Anyone have any thoughts on this to share? Thank you.
 

drc_567

Joined Dec 29, 2008
815
... Just as a pre!urinary step, check to see if the dome light and something in the ignition circuit are joined in the same fuse. It could be that a fuse element has developed an oxide film or some other laminar resistance requiring a start-up current pulse to begin conduction. If that turns out to be what is occuring, then a spray of electrical contact cleaner or maybe a new fuse should resolve the problem. A review of the wiring diagram would likely clear things up.
 

crutschow

Joined Mar 14, 2008
24,749
It doesn't make sense to me that ground can both be a zero voltage reference and at the same time allow a circuit to find continuity through ground to complete the ignition circuit.
Don't confuse voltage with current.

A "ground" is just a reference point to which all other voltages are measured, it doesn't mean zero current.
Ideally it should have zero impedance so it can be at zero volts and still carry current from the circuits connected to it.
In practice it always has some small impedance.
If the connection to ground is not good and has a higher impedance that causes a significant voltage drop due to that circuit's current, then it is called a "bad ground".
 

Thread Starter

Back to school

Joined May 22, 2019
114
Thanks for the replies. I don't understand replying to multiple quotes here so please don't think I'm ignoring anyone here. I'm not.

This is not a fuse or relay issue. I'm also not confusing voltage with current.

Kirchhoff's Law states the sum of the voltage applied to a circuit is equal to the sum of the voltage drops across the circuit.

If ground or chassis ground in a vehicle is a zero voltage reference than a circuit is not going to send voltage through the ground to find some oddball connection back to continuity for the ignition circuit. For lack of a better way to say it, per Kirchhoff's Law the zero voltage reference is the defining end point of the voltage applied to a circuit. Voltage is not going to be added to a circuit to find some way through ground to complete the circuit. I hope I'm making sense. This is also all DC. There is no alternating current.
 

crutschow

Joined Mar 14, 2008
24,749
Voltage is not going to be added to a circuit to find some way through ground to complete the circuit.
If you have zero impedance, you still can have current (as in a superconducting wire).
It practice all grounds have some finite impedance so there is a small voltage drop (usually negligible) due to any current flowing in the ground path.
So you want to make the ground as low an impedance as practical.
Generally an automotive chassis has a pretty low resistance.
 

Thread Starter

Back to school

Joined May 22, 2019
114
What do you mean by this, especially “voltage circuit”?
I was using Kirchhoff's law and the sum of voltages applied.

Edit: What I'm saying is ground can't be used as part of a circuit. In a vehicle it's a terminus. Also voltage to ground is usually a runaway short with fires being a common result.
 
Last edited:

Thread Starter

Back to school

Joined May 22, 2019
114
If you have zero impedance, you still can have current (as in a superconducting wire).
It practice all grounds have some finite impedance so there is a small voltage drop (usually negligible) due to any current flowing in the ground path.
So you want to make the ground as low an impedance as practical.
Generally an automotive chassis has a pretty low resistance.
Yes. For these vehicles the voltage measure between the battery and ground is to be less than .5 volts. It's more or less ignored if the voltage is within specs.
 

drc_567

Joined Dec 29, 2008
815
Do you have access to the vehicle wiring schematic? On the schematic, there should be a section showing the location of various ground connections. See if any particular ground point is in both the ignition and dome light circuits.
 

Thread Starter

Back to school

Joined May 22, 2019
114
Do you have access to the vehicle wiring schematic? On the schematic, there should be a section showing the location of various ground connections. See if any particular ground point is in both the ignition and dome light circuits.
Yes, they have a common ground connection. I don't see what difference this makes though. I can read the circuit through any of the ground connecting points. That's just the nature of grounds in a vehicle. I don't even need a ground connecting point. I can read it anywhere on the chassis, body or engine. It's also why I'm of the opinion this is a short happening between the circuits irrespective of ground. A bad ground is not making this short happen.

Maybe shouldn't add this but I was always taught there is no such thing as a bad ground. There are only bad connections to ground. The analogy was a baseball bat isn't bad bat because no one can get a hit with it. The bat has no control over someone connecting to the ball with the bat.

Edit, By saying I can read the circuit through any ground I mean anyone can. This is not my vehicle.
 

Tonyr1084

Joined Sep 24, 2015
4,391
Had an old Chevy. Oh, it would start. But the problem was that when you turned the headlights on you had to step on the clutch pedal down to the floor before the headlights would come on. Once they came on they typically stayed on.

One night no matter what I did the headlights wouldn't come on. So as all guys will do - I opened the hood. Saw a bright spot of light on the radiator support bracket. There's no lights there - that shouldn't be glowing. But like a fool I touched it. It was a white hot ground point that was rusted so badly it wouldn't complete the circuit. Why the clutch pedal? Because the engine was grounded (directly connected to the negative terminal of the battery). The clutch hydraulics would press the throw-out bearing against the pressure plate and make a pathway to ground through that point. Not good for the bearing either. Nevertheless, I'd found the issue. A bad ground. In fact, all grounds were bad. This car was gifted to me because it was junk in the eyes of the previous owner. Mine too.

The next day I made several new grounding points and all electrical problems disappeared. Funny how they have a strange way of manifesting themselves. Prior to the ground going so bad that it wouldn't work - establishing a pathway to ground (chassis ground that is) was enough to get current flowing and establish some sort of electrical weld, and the lights would work.

I have noticed that people call all kinds of electrical problems "Shorts". A short is simply a pathway from the power source back to the power source. In this case a battery. It is a short pathway back. Short does not mean all electrical problems. I've seen people call a failed lighting system a short when in fact it was an "Open" circuit. Bad ground is another way of saying an open circuit. In the case of the junk car there was no ground, hence, not a short - but an open.

Another story - Had an apartment with electrical issues. Half the lights and outlets would fail for some strange reason. However, when I turned the electric stove on all outlets would start working again - even with the stove off. The problem??? A loose connection at the meter. One of the two legs (US electrical power) was loose. When switching the stove on - the power draw was so great that the contact would temporarily weld itself and all outlets would work. Took weeks to find the issue. Weak - "Open" - circuit.
 

Tonyr1084

Joined Sep 24, 2015
4,391
Three points of contact are required for the starter to function: Positive battery voltage via a heavy red cable to the starter. Negative battery connection from the battery to the engine block. And a small (probably yellow) wire connected to the starter solenoid. If the starter doesn't have a good connection to ground then the solenoid won't kick in and the starter won't turn over. It has been my experience sometimes the black battery cable to the engine block comes loose. Chassis grounds have to make up for the lacking of good contact. Also dirty battery terminals can play havoc when trying to draw high amounts of current like starting the car whereas smaller loads are so much NOT affected that the owner doesn't notice the problem. Not until they have to touch the radio antenna with their left hand while their right foot touches the back bumper and their left hand has to touch the hub of the wheel. OK, extreme nonsense - but I'd start with cleaning battery terminals and making sure all connections to the engine, starter and solenoid are good and tight. But be careful with the solenoid connection - I've seen people over torque them and break the wire inside. My neighbor did exactly that.
 

Thread Starter

Back to school

Joined May 22, 2019
114
Had an old Chevy. Oh, it would start. But the problem was that when you turned the headlights on you had to step on the clutch pedal down to the floor before the headlights would come on. Once they came on they typically stayed on.

One night no matter what I did the headlights wouldn't come on. So as all guys will do - I opened the hood. Saw a bright spot of light on the radiator support bracket. There's no lights there - that shouldn't be glowing. But like a fool I touched it. It was a white hot ground point that was rusted so badly it wouldn't complete the circuit. Why the clutch pedal? Because the engine was grounded (directly connected to the negative terminal of the battery). The clutch hydraulics would press the throw-out bearing against the pressure plate and make a pathway to ground through that point. Not good for the bearing either. Nevertheless, I'd found the issue. A bad ground. In fact, all grounds were bad. This car was gifted to me because it was junk in the eyes of the previous owner. Mine too.

The next day I made several new grounding points and all electrical problems disappeared. Funny how they have a strange way of manifesting themselves. Prior to the ground going so bad that it wouldn't work - establishing a pathway to ground (chassis ground that is) was enough to get current flowing and establish some sort of electrical weld, and the lights would work.

I have noticed that people call all kinds of electrical problems "Shorts". A short is simply a pathway from the power source back to the power source. In this case a battery. It is a short pathway back. Short does not mean all electrical problems. I've seen people call a failed lighting system a short when in fact it was an "Open" circuit. Bad ground is another way of saying an open circuit. In the case of the junk car there was no ground, hence, not a short - but an open.

Another story - Had an apartment with electrical issues. Half the lights and outlets would fail for some strange reason. However, when I turned the electric stove on all outlets would start working again - even with the stove off. The problem??? A loose connection at the meter. One of the two legs (US electrical power) was loose. When switching the stove on - the power draw was so great that the contact would temporarily weld itself and all outlets would work. Took weeks to find the issue. Weak - "Open" - circuit.
I understand what you're getting at. I think what was once all shorts has become all bad grounds due to the electronics in vehicles. In this instance it's not an open or the circuit wouldn't work and it does. What you describe to me is a short to ground with a lot of resistance creating a lot of heat. You had bad ground connections, not a bad ground. If the ground itself was bad, it couldn't be fixed with making better ground connections.
 

Dodgydave

Joined Jun 22, 2012
9,089
Forget Kirchoff rubbish, get a Dvm and start testing Voltages from the ignition key to the starter motor, and any ignition coil, or other firing circuits.
 

Thread Starter

Back to school

Joined May 22, 2019
114
Three points of contact are required for the starter to function: Positive battery voltage via a heavy red cable to the starter. Negative battery connection from the battery to the engine block. And a small (probably yellow) wire connected to the starter solenoid. If the starter doesn't have a good connection to ground then the solenoid won't kick in and the starter won't turn over. It has been my experience sometimes the black battery cable to the engine block comes loose. Chassis grounds have to make up for the lacking of good contact. Also dirty battery terminals can play havoc when trying to draw high amounts of current like starting the car whereas smaller loads are so much NOT affected that the owner doesn't notice the problem. Not until they have to touch the radio antenna with their left hand while their right foot touches the back bumper and their left hand has to touch the hub of the wheel. OK, extreme nonsense - but I'd start with cleaning battery terminals and making sure all connections to the engine, starter and solenoid are good and tight. But be careful with the solenoid connection - I've seen people over torque them and break the wire inside. My neighbor did exactly that.
Granted but this is old school. A lot more is tied to the ignition now through the electronics for a vehicle to start. Even the old neutral safety switch isn't the analog device they use to be.
 

wayneh

Joined Sep 9, 2010
16,384
What you describe to me is a short to ground with a lot of resistance creating a lot of heat.
That's an oxymoron. If you have "lot of resistance", it's not a short, not much current flows and therefore not much heat is generated.

Ground is established at the negative terminal of the battery. Everywhere else on a car will have a measurable (but often trivial) higher voltage. To minimize that voltage, a thick cable goes from the battery to the starter or the engine block. Other parts carry lower currents (compared to the starter) and are grounded by straps or normal wiring. Those straps and wires, and the relays that control them, are where "bad grounds" can arise.
 

Thread Starter

Back to school

Joined May 22, 2019
114
Forget Kirchoff rubbish, get a Dvm and start testing Voltages from the ignition key to the starter motor, and any ignition coil, or other firing circuits.
That was my suggestion exactly. Didn't do it though. It's a lot easier I guess to just blame a bad ground. The process is given in the manuals to do exactly what you suggest starting with the voltage between the battery and ground and then proceeding to going through each fuse to see if there is an unwanted voltage drop across any circuits. One person also suggested a sympathetic inductive transfer of power between circuits and that's how this was working. I understand inductance is possible in DC circuits but it's not happening here. Most just accept that the ignition circuit is being complete through a bad ground connection. I don't see it.
 
Top