We had lab ossiliscopes back 30 years ago when I was going to school for a third year automotive diagnostic class in the 80's. All we dealt with is automotive electronic controls. Ford's EEC I-V, Chrysler, Bosch fuel injection, GM. I've got the factory manuals for all those Systems at home.
We'd spend a lot of time watching voltage patterns of injectors, baro sensors, knock sensors etc learning what each one did, how it worked and why it will react how it does.
All automotive control systems actually control grounds. All it does. Garbage in, garbage out. So troubleshooting always starts with bad battery connection and bad grounds. Regular maintenance should be cleaning terminals and inspect cables for frays and internal corrosion. That stuff will increase cable resistance exponentially. At the first sign of darkening, it's not a matter of repairs, it's too late. It's replacement time.
Splicing shit in doesn't work. Each splice increases resistance. Probably not enough for most to care about, but it's there.
When your control systems are working off 5 volts and maybe 15-50 mA (milliamps), now that little bit of resistance is a killer to the system.
Facebook group was talking how he was only seeing 0.2 to 4.8 volts on his throttle position sensor on a truck. That's all you'll ever see. Just how the system is designed. You can't go off the ends of the sweep or you'll freak out the computer. Yeah, the system voltage is 5 volt, but you have to have a continuous signal from 0-5 volts. So when you get to the extreme ends of the sensor, you'll still see voltages. 0.2 or 4.8. it's got to be there.
To have it drop off creates spikes. Milliamp spikes that cause major issues from the sensor side of the ecm to the driver side and outputs of the ecm.
Grounds cause similar problems. If you suspect a bad ground, hell, just use a jumper wire between a frame and chassis. Frame and engine. Engine and chassis. It's amazing how you can change the operation.
How do batteries cause this problem? My understanding is that the plates start to short out and arc across themselves. It will actually create signals inside the voltage that sets up magnetic fields. Self-inductance. Those signals create slight and subtle changes to either the input or output of the ecm and nothing gets done correctly. Suddenly you've got weak signals, then strong, then weak. All from a changing power supply.
One trick we'd do is put a small capacitor across a load to see if it would change operation of a sensor or injector. Amazing what that would do.
Imagine how a plumbing fixture works for dealing with water hammer. A capacitor works the same way. It absorbs voltage spikes.
Bad batteries will just jack you around into having so many Gremlins you don't know what to fix. ABS faults, inverter failure, voltage regulators, ecm spikes, injectors not operating correctly.
But they catch you off guard because you're normally not expecting them.