Electrical Isolate Starting System And Engine Sensor Circuits?

DuderMcMerican

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Mar 7, 2016
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Fellas,

I've been chasing a No Spark/Surging Idle/Stall, Buck, Backfire problem for quite some time now. I have worked though most of the tech notes I've found on the online, studied the systems using the Helm manuals, as well as the diagrams by jrichker, tmoss & joel5.0, and Tom Cloud's Technical Notes on the EEC-IV MCU--all GREAT trouble shooting guides/learning materials. I think I am just now beginning to understand how the different systems work together.:nice:

Here's what I'm struggling to understand at the moment:

1. How can higher current circuits be isolated from the circuits that power the sensors? I am particularly confused about isolating grounds.

On one hand, Tom Cloud writes...

"As with any instrumentation and control system, poor or improper grounding of sensors can should be avoided. The TFI module seems to be the most susceptible to problems -- and especially when making a new or modified installation. The largest source of current-induced voltages in the grounding system is the starter current, with the alternator being the next, so every care should be taken to isolate the engine management circuit grounds from the starter current path."

On the other hand, jrichker notes at the end of his tech instructions on grounds...

"The best plan is to have all the grounds meet at one central spot and connect together there. That eliminates any voltage drops from grounds connected at different places. A voltage drop between the computer ground and the alternator power ground will effectively reduce the voltage available to the computer by the amount of the drop."

2. Are Cloud and jrichker talking about different grounding issues? :thinking:

I got to thinking about this, in particular, because I pulled the starter wire off my starter solenoid and it had 0 resistance with other grounds. I did this following solenoid-troubleshooting advice on another forum where the expert said:

"If you are getting power on both sides of the relay withe the ignition wire off and the starter wire disconnected, the relay has failed and will need to be replaced. The wire from the starter should not be grounded. If it is, the starter is bad and should also be replaced. The starter may have caused damage to the relays. Replace the relay and the starter."


3. Is that guy's advice sound? If so, could the fact that my starter wire has 0 resistance with ground (due to a bad starter) also account for my Surging Idle/Stall, Buck, Backfire problems problems, i.e., starter circuit interfering with sensor circuit grounds?
 
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DuderMcMerican

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Mar 7, 2016
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Fellas,

2. Are Cloud and jrichker talking about different grounding issues? :thinking:
I think I get it now after rereading some stuff. I'm thinking that they're essentially saying the same thing because the sensors are all grounded in the eec--pin 46 signal return, so that's what isolates their grounds from the others. If yes, then that pin shouldn't have continuity with other grounds.. otherwise it wouldn't be isolated, right? If that's the case, and supposing it does have continuity, then what's most likely the culprit--a shorted ground wire in the sensor circuit? Still not sure exactly how the grounds are isolated--the mechanism that allows this--but hope I'm on the right track in understanding the basic concept.

Still unclear about the grounded starter wire question.

I really appreciate y'alls' patience with me. This forum is the best stang forum I've joined by far! :flag::flag:
 

jrichker

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The signal ground and power ground both terminate at the battery negative ground cable.

Anytime current flows through a wire, there is voltage drop, even though it may be very small. If there is a lot of current flowing through the wire or it has some high resistance connections, that voltage drop can be significant.

Sensors typically work with much lower voltages than a power circuit, so small differences in the voltage received by the computer can make a big difference.The reason for the isolated signal ground is to keep the voltage drop on the ground signal at a very minimum. There isn't all the current used by the rest of the car's electrical system being carried on it. All it carries is the very small current needed for the sensor to operate. Therefore a small gauge wire is used to provide a separate ground for the sensor with almost zero voltage drop between the sensor and computer input.
 
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DuderMcMerican

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Thanks again, jrichker. This helps a lot! :hail:

The signal ground and power ground both terminate at the battery negative ground cable.
Okay, that's what I originally thought.. I got confused by Cloud's reference to isolating the circuits, and took that to mean that they somehow didn't share that ground. I was slow on the uptake. :doh: Doh!

Anytime current flows through a wire, there is voltage drop, even though it may be very small. If there is a lot of current flowing through the wire or it has some high resistance connections, that voltage drop can be significant.

Sensors typically work with much lower voltages than a power circuit, so small differences in the voltage received by the computer can make a big difference.The reason for the isolated signal ground is to keep the voltage drop on the ground signal at a very minimum. There isn't all the current used by the rest of the car's electrical system being carried on it. All it carries is the very small current needed for the sensor to operate. Therefore a small gauge wire is used to provide a separate ground for the sensor with almost zero voltage drop between the sensor and computer input.
Ahh, okay; so using the proper gauge wire for each system is what isolates the voltage (and voltage drop) for each system. So, if I'm understanding correctly, then using a heavier-than-prescribed gauge wire for a sensor ground will allow it to pick up voltage drop noise from the heavy-gauged power ground wires.. Is that correct?

Would my last question--the one about the starter cable from solenoid being grounded--be relevant here? If not, I'll start another thread explaining what happened.:flag:
 

jrichker

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Thanks again, jrichker. This helps a lot! :hail:

Okay, that's what I originally thought.. I got confused by Cloud's reference to isolating the circuits, and took that to mean that they somehow didn't share that ground. I was slow on the uptake. :doh: Doh!



Ahh, okay; so using the proper gauge wire for each system is what isolates the voltage (and voltage drop) for each system. So, if I'm understanding correctly, then using a heavier-than-prescribed gauge wire for a sensor ground will allow it to pick up voltage drop noise from the heavy-gauged power ground wires.. Is that correct?

Would my last question--the one about the starter cable from solenoid being grounded--be relevant here? If not, I'll start another thread explaining what happened.:flag:
The sensors use a light gauge wire because that is all they need because of the small amount of current that they carry. All the sensor grounds connect to the main power ground inside the computer.

Anytime current flows through a wire, there is voltage drop, even though it may be very small. If there is a lot of current flowing through the wire or it has some high resistance connections, that voltage drop can be significant.

When a heavy electrical load is created by a motor, lighting, or other device, the separate sensor grounds do not carry any of that heavy current load. All that heavy current is carried by the main engine and body grounds. Therefore the resistance of the sensor ground path does not cause a voltage drop that changes what the computer sees coming from the sensor.

A lighter gauge wire does not pick up electrical noise any differently than a heavy gauge wire. Voltage drop and RFI (Radio Frequency Interference) are two very different animals. RFI is electrical energy generated by things like spark plug wires, ignition system and some electric motors. RFI can play havoc on unshielded sensor wiring; that why the aluminum foil shield you bumped into a short while ago is important.

If you ground any of the wires coming off the starter solenoid, there is a good possibility you will make electrical smoke.

The starter motor reads 0 ohms because its internal resistance is very low; that is by design. Volts x amps = power in watts, 746 watts = 1 HP (less a percentage for loss of perfect efficiency). 12 volts x 200 amps (typical starter current draw) = 2400 watts or about 3.2 HP to crank the engine. Now you know why the power cabling to the starter is heavy gauge wire. RFI does not enter the picture here, but resistance (voltage drop) does. See https://en.wikipedia.org/wiki/Ohm's_law for help on Ohm's law.
 
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DuderMcMerican

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The sensors use a light gauge wire because that is all they need because of the small amount of current that they carry. All the sensor grounds connect to the main power ground inside the computer.

Anytime current flows through a wire, there is voltage drop, even though it may be very small. If there is a lot of current flowing through the wire or it has some high resistance connections, that voltage drop can be significant.

When a heavy electrical load is created by a motor, lighting, or other device, the separate sensor grounds do not carry any of that heavy current load. All that heavy current is carried by the main engine and body grounds. Therefore the resistance of the sensor ground path does not cause a voltage drop that changes what the computer sees coming from the sensor.

A lighter gauge wire does not pick up electrical noise any differently than a heavy gauge wire. Voltage drop and RFI (Radio Frequency Interference) are two very different animals. RFI is electrical energy generated by things like spark plug wires, ignition system and some electric motors. RFI can play havoc on unshielded sensor wiring; that why the aluminum foil shield you bumped into a short while ago is important.

If you ground any of the wires coming off the starter solenoid, there is a good possibility you will make electrical smoke.
Gotcha.:nice: I was certainly off the mark re wire gauge and by confounding voltage drop and RFI issues! Sorry for the misunderstanding, and thank you for explaining.
The starter motor reads 0 ohms because its internal resistance is very low; that is by design. Volts x amps = power in watts, 746 watts = 1 HP (less a percentage for loss of perfect efficiency). 12 volts x 200 amps (typical starter current draw) = 2400 watts or about 3.2 HP to crank the engine. Now you know why the power cabling to the starter is heavy gauge wire. RFI does not enter the picture here, but resistance (voltage drop) does. See https://en.wikipedia.org/wiki/Ohm's_law for help on Ohm's law.
Okay--great info. Glad I didn't run out and buy a new starter based on what that other guy said.

Have a great Memorial Day weekend, everyone!:flag: I'll be spending mine boning up on ohms law!:nice:
 
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