Question About Automotive Ground

Tonyr1084

Joined Sep 24, 2015
5,694
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.
Short to ground is not what I was describing - with extreme resistance - approaching an open circuit - the connection was bad. Not a bad ground, a bad connection. You said in part "If the ground itself was bad, it couldn't be fixed". The chassis is the ground.

Granted but this is old school. A lot more is tied to the ignition now through the electronics for a vehicle to start.
True. But not as true as you suggest. These three points are still required in order to start a car. As far as the solenoid control wire (yellow in my experience) is controlled by the computer. Of course the computer also controls some lighting in the vehicle, such as dimming to extinguish interior lighting - I believe the computer controls that. It's possible the computer is going bad. But before I run out and replace the computer I'd check the battery connections at both ends and the solenoid control wire. If those are not the problem then further diagnostics is required.

Same neighbor had a car that wouldn't start. So we jumped the starter and forced the engine to turn over. That didn't work. Not unless he held the key in the start position. As SOON as he let go of the key the engine would shut down. The computer just would't accept an attempt at running the car without the proper key. And the key had a chip (resistor) in it. And that was the entire reason why the car wouldn't start - the cable that made the connection to the key had failed. The solution in that case was to find that cable and bypass it with a resistor of the same value as the key resistor. Once that was bypassed the car could be started at leisure. Probably could have been jumped at the starter with the key in the on position too. But the car - as old as it was - was not a target for thieves. Just goes to show - fully diagnose the problem before you start throwing parts at it. In this case checking and cleaning battery terminals and connections is NEVER a bad idea or starting place.

Stay healthy.
 

Thread Starter

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Joined May 22, 2019
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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.
Wow, not at all what I've dealt with for years. Corroded contacts or connections for one. They create a lot of resistance, suck up power and get hot. I've seen more that one car catch on fire because of a short.
 

Tonyr1084

Joined Sep 24, 2015
5,694
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
A weak connection will still carry the full voltage. But when called upon to carry the amperage it can fail. Or cause other odd things to happen such as my headlight story. You could test a circuit and find it's in perfect working condition for the load a DVM would place on it. However, powering a lamp or starter - the circuit might not be robust enough to deal with it.
 

Thread Starter

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Joined May 22, 2019
106
Short to ground is not what I was describing - with extreme resistance - approaching an open circuit - the connection was bad. Not a bad ground, a bad connection. You said in part "If the ground itself was bad, it couldn't be fixed". The chassis is the ground.


True. But not as true as you suggest. These three points are still required in order to start a car. As far as the solenoid control wire (yellow in my experience) is controlled by the computer. Of course the computer also controls some lighting in the vehicle, such as dimming to extinguish interior lighting - I believe the computer controls that. It's possible the computer is going bad. But before I run out and replace the computer I'd check the battery connections at both ends and the solenoid control wire. If those are not the problem then further diagnostics is required.

Same neighbor had a car that wouldn't start. So we jumped the starter and forced the engine to turn over. That didn't work. Not unless he held the key in the start position. As SOON as he let go of the key the engine would shut down. The computer just would't accept an attempt at running the car without the proper key. And the key had a chip (resistor) in it. And that was the entire reason why the car wouldn't start - the cable that made the connection to the key had failed. The solution in that case was to find that cable and bypass it with a resistor of the same value as the key resistor. Once that was bypassed the car could be started at leisure. Probably could have been jumped at the starter with the key in the on position too. But the car - as old as it was - was not a target for thieves. Just goes to show - fully diagnose the problem before you start throwing parts at it. In this case checking and cleaning battery terminals and connections is NEVER a bad idea or starting place.

Stay healthy.
Yes, you're right but there are many computers in a car. Not just the ECU. Transmission module, body module, fuel control module to name a few and they are all separate from the ECU\PCM. The CAN Bus system requires a lot of protocols to be met before a vehicle will start. I can't disagree though that proper maintenance isn't necessary.

This is all getting away from my question though. Can a circuit be completed through another circuits ground? Can a circuit piggy back on another and carry voltage through ground to complete a circuit?
 

Dodgydave

Joined Jun 22, 2012
9,911
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.
When you say it won't start, is it turning over the engine ok but not firing,

OR is it just not turning over at all?
 

Thread Starter

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Joined May 22, 2019
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AAAHHHHH! IT'S NOT A SHORT!!! IT'S A BAD CONNECTION! And they don't suck up power. They BLOCK it.
I wasn't referring to corrosion as a short and not to the point that it's an open. Apples and oranges. I was just referring to heat here. And as I said, I've seen shorts cause fires. Heat.
 

Thread Starter

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When you say it won't start, is it turning over the engine ok but not firing,

OR is it just not turning over at all?
No, it is starting. That's the point of why I say it's not a bad ground issue which others are saying it is. Because the dome light has to be on to start, it's a bad ground. I say no, its a short.
 

wayneh

Joined Sep 9, 2010
16,775
Because the dome light has to be on to start...
The dome light is one of very few things that will come on without the key and, I think, stay on even while cranking. Is there an alway-on accessory power, ie. cigarette lighter? It'd be interesting to see if there could take the place of the dome light. I'm also curious if the car starts if the dome light bulb is removed.
 

Thread Starter

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Interesting question about the bulb. I don't know. I did ask it if matters which door would have to be open if turning the dome light on this way mattered. He said no. Any door open will work. Also, turning the dome light at the switch with the doors closed will also work.

The dome light coming on with an open door is part of the courtesy lamp circuit controlled through the GEM module or CTM module. Which module depends on the options the vehicle has. Both modules have the timer circuit for how long the light(s) stay on after the door is closed and locked. It's also the same timing control for how long the headlights stay on after the vehicle is shut off and locked up. The wiring manual just shows module to relay to light to ground. What's going on inside of the modules is not given in the manuals.
 

drc_567

Joined Dec 29, 2008
1,062
... Looking at an old wiring diagram ... There is a 'Shorting Pin' that is situated just prior to a junction node, with one wire going to the interior Dome light, and another wire going to the ECU module. The purpose of the pin may be to prevent any battery current drain while the vehicle is being shipped or is in storage. So, poor conduction through this pin could conceivably cause a problem similar to that described above. The shorting pin is located in one of the main fuse boxes, in the engine compartment, and has a label saying 'Shorting Pin'. If the car in question has a pin like this, it could possibly be what is causing the unusual symptoms.
 

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... Looking at an old wiring diagram ... There is a 'Shorting Pin' that is situated just prior to a junction node, with one wire going to the interior Dome light, and another wire going to the ECU module. The purpose of the pin may be to prevent any battery current drain while the vehicle is being shipped or is in storage. So, poor conduction through this pin could conceivably cause a problem similar to that described above. The shorting pin is located in one of the main fuse boxes, in the engine compartment, and has a label saying 'Shorting Pin'. If the car in question has a pin like this, it could possibly be what is causing the unusual symptoms.
Nothing similar to the shorting pin for this vehicle. Some of what is tied to the main light switch though is surprising with no explanation. For example the automatic transmission clutch relay and the AC power required for the PWM (Pulse Width Modulation) used for circuits like the fuel injectors where PWM controls how long the injectors are open. The Main Light Switch is divided up in to three sections though with each section having its own connector and there doesn't appear to be anyway that the three sections are tied together. The switch appears to be segregated in to its different functions. It is the switch though that controls the manual operation of the dome light.
 

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I did just a little while ago get a email from the company and their instructions are to perform the component test for the Main Light Switch related to the dome and Autolamp features. The tests identify what the reading should be at various pins with the switches in their various positions and the ohm readings with the dimmer switch. Next was to reinstall the switch, disconnect the battery and redo the same tests by probing the wires for the pins. The readings should be the same. If everything tests normal the problem is not with a ground. If the tests are not as expected then the problem may or may not be a ground problem. The problem may also be a short between different switch functions. There is no ground at the switch except for the AC, PWM but this is isolated from circuits related to the dome light. If this ground was somehow an issue the component tests for the switch would not have passed.
 

narkeleptk

Joined Mar 11, 2019
479
I'd like to know the year/model of the car.
Not many cars have any kind of link between a interior dome light and the starting circuit. With out knowing the year/model I would say its a most likely coincidence.
 

Thread Starter

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I wasn't sure this could be said but it's a 2001, Ford F150.

Edit: Since the advent of the clock spring for air bags and how everything on the steering column goes through clock spring it's not unusual now for these unexpected relationships to take place. You'll find the same thing with GM's.
 
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narkeleptk

Joined Mar 11, 2019
479
This is from 2001 F250. Its pretty much the same except if the users F150 is chipped (some 01's are, some are not).
I see no way turning on a dome light/courtesy lights would have any effect on the starting unless the central junction box is faulty or there are wiring shorts coming out of the central junction box. In another post you say the user found out by leaving the door open. I would suggest that its not the dome lights but more likely a short in the wiring going through the drivers door when closed. Likely bringing down the GEM module which is causing other modules/sensors to go down.

If the 150 is equipped with a immobilizer then the the starting also includes the Instrument cluster and a key amplifier around ignition before the pcm will allow starting.
starting.jpg
courtesy.jpg
 
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narkeleptk

Joined Mar 11, 2019
479
Edit: Since the advent of the clock spring for air bags and how everything on the steering column goes through clock spring it's not unusual now for these unexpected relationships to take place. You'll find the same thing with GM's.
Not true at all. For most cars (specially pre 2010) unless the controls are on the steering wheel itself then they do not run through the clock spring.
 
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bwilliams60

Joined Nov 18, 2012
1,379
Yes a circuit can find a ground through another circuit if the circumstances are right. "Bad ground" is bad terminology just like everything is a "short circuit". Electrical problems in a vehicle can be classified by open, short to ground, short to power, short to another circuit, excessive resistance and parasitic draws. So which one do you have. If you have a circuit that doesn't work and you suspect the ground, try to measure voltage drop across the ground. If you put your meter on DCV and put your negative lead on the ground "Stud" or "Post" of the battery(not the clamp) and the ground side of the load device, energize the circuit. If you have full voltage, you have an open. If you have <0.3VDC, ground should be good if circuit is complete. If you have >0.3VDC, you have excessive resistance. There are some exceptions to the rule but that is a quick and easy way to figure out if you have a "bad" ground. Classify it as something. If ground is good, check voltage going to the hot side of your cranking circuit. This is the heavy load side. Check voltage to the "S" terminal for your solenoid. This is your control circuit. Those 3 tests will get you going in the right direction. The dome lamp can be diagnosed the same way. Good luck.
 

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This is from 2001 F250. Its pretty much the same except if the users F150 is chipped (some 01's are, some are not).
I see no way turning on a dome light/courtesy lights would have any effect on the starting unless the central junction box is faulty or there are wiring shorts coming out of the central junction box.
It wont have anything to do with grounding.
View attachment 203706
View attachment 203707
It's not as simple as these schematics would lead someone to think. You have to first begin with the Multiplex Communications Network and then Multifunction Electronic Control Modules, Power Distribution, Instrument Cluster, Transmission Controls, etc. Then the Instrument cluster is also known as the HEC, Hybrid Electronic Cluster which controls a lot of the vehicles electronic functions separate from the instrument cluster. Then there's the GEM and Body control module along with the PCM. What the wiring manual gives very little information about is the central control over the CAN Bus communications network. The central junction block you mention is controlled by the GEM or CTM module. For example, your ignition schematic shows the central junction box, the clutch pedal position sensor and the transmission digital range sensor but it doesn't say anything about the necessary relationships needed to start the vehicle. That's in the Powertrain Control Manual with a brief description and pin point testing. In your courtesy lamp circuit follow it back to the Main Light Switch and you will see the AC circuits for PWM comes out of it but there is nothing in the wiring manual about the circuit. I agree with you that on the surface there seems to be no relationship to many of the relationships that exist. Again, your ignition schematic shows nothing about the clock spring but all the electrical connections in the steering column go through it and it can easily prevent anything on the column from working. Circuits have to be followed through from start to finish to see what all is involved. One page won't do it. And even then, you may not have it all. The PATS system for example (anti theft) is almost completely left out of all the manuals. Much of the PATS system was dealer restricted as well. They didn't have access to the information. It was a remote IDS repair or just replaced and reprogrammed for the vehicle.

Now fortunately a lot of this has all changed. At this time most of this type of information was considered proprietary. That's changed thanks to the Europeans. Through what the EU called Blocking Legislation if the US wanted to sell vehicles in Europe that had to open all this up. Back in 2,000 it was considered next to impossible to swap engines between vehicles with different PCM's\ECU's. Everybody is doing it now though.
 
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