How Many Grounds?

KitCarlson said
I will try to explain. A ground loop is an undesired current, in a conductor that is connecting two points in a circuit that are assumed to be at ground potential.

For those that want to escape the explanation, a ground loop is unlikely to cause a problem in a stock A-body. Why?. Because stock A-bodies do not have an ECU, or smart ignition module. Some call the 4 or 5 pin ignition box an ECU, but it is not.

If EFI, an ignition with smarts, AFR gauge, or an engine management system is installed, then there are great concerns for how for how connections to ground are made. I will call the prior, smart devices, that have a micro-controller. The smart devices make measurements and control things based on those measurements. Measurements are often made on sensors that are referenced to engine ground. Examples are temperature sensors, O2 sensor, knock sensors, and others. Some more modern sensors are two- wire isolated, and are isolated from the engine to avoid ground loops.

While adding or tying grounds together is a good thing, it is is also important to insure that the smart device share the same ground as the engine sensors. If it does not, sensor readings will be incorrect, resulting undesired control behavior under starting conditions.

Exactly. This is why most modern vehicles have a separate sensor ground or sensor return that is wired directly back to the control unit. That sensor return is isolated internally in the control unit and is typically approximately 1/2 volt higher than chassis ground. This is to prevent electrical noise on the ground plane from inductive electrical devices such as ignition coils, a/c compressor clutch coils, fuel injectors, etc. from interfering with the sensor signals.

A starter motor draws significant current. It can be as high as a few hundred amps in cold weather. While the battery cable that connects the battery to the engine block is large, it still has a voltage drop. The drop may be a few tenth of a volt, or even a volt or two. If a smart device is grounded at the battery, then there is a ground loop. The ground loop path, is the smart device supply ground connection and the sensor measurement grounds at the engine. The ground loop results an undesired current and voltage in the ground connections that parallel the battery ground cable. The undesired current, results in incorrect measurements, and possibly damage to smart device components, including burned printed circuit foils. By moving the smart device ground to the engine ground, the ground loop is eliminated.

If you are curious about voltage drop on the negative battery cable, use a voltmeter while cranking, black lead at battery terminal, red lead on engine ground.