I utilize a quick, reliable methodology for diagnosing vehicle charging system, starting system and drivability issues. It typically takes just 2 minutes and is based on proving that the ground system is functioning properly on every car or truck before moving on to more extensive diagnosis. I perform this test on almost every car when it comes in… and every car before I finish.
Why start with the ground system? If the ground connections in the engine compartment are bad, you can chase problems across a variety of vehicle systems for hours and still not properly diagnose the root cause. Ensuring proper ground across critical engine connections reduces the potential for confusion. Drivability, you might ask? Yes – if the ground system is not perfect, it can result in multiple DTCs for both the engine and the transmission.
After confirming that the battery is at or near full charge, start the process with a voltage drop test. With the engine running and headlights on, set DVOM to 20 Volts DC, and make the following connections:
- Positive (+) battery post to the Negative (–) battery post. Reading should be 14.1 to 14.6V.
- Negative (–) battery post to the Engine block. 0.04VDC volts max is expected.
- Negative (-) battery post to the body. 0.02VDC max is expected.
- Engine block to the Body. 0.02VDC max is expected.
If you get 0.00VDC on tests 2, 3, 4, go back and test with your DVOM set to 2 Volts DC. Only vehicles with a very short negative battery cables will read close to 0.00 volts on the last 3 tests. If you have more than the 0.04VDC or 0.02VDC (respectively) on the last 3 tests, it is time to dig in and clean the ground connections around the engine compartment.
What you are measuring is the connection of the various ground cables. As electrons flow from negative to positive, if you don’t have good grounds, you will not have enough electrons to properly power the devices on the circuit.
If the grounds are bad, you may have TOO MANY electrons on something like the engine block, which with components, such as the alternator, to burn out. Why? The alternator puts out current on the negative side into the engine block where it has to go up the negative cable to the battery.
If you have bad grounds and since some computer sensors and solenoids use the engine ground to supply electrons as half the circuit, you will just have issues.
I recommend the following additional checks to further confirm proper ground:
- Alternator output to the positive (+) battery terminal. Because of the amount of current flowing, it’s going to be more than the 0.04VDC on the negative side, but it’s going to be below 0.10VDC (or very close to it).
- Oxygen sensor ground wire to the engine block. This should be 0.02VDC or less.
- Oxygen sensor heater ground wire. This should be 0.02VDC or less. If you ever get an oxygen sensor output voltage over 1.0VDC, look for a bad ground on the heater circuit. This usually is found on 3-wire types of oxygen sensor rather than a 4-wire, obd2 O2 sensor.
Los Angeles, CA