What We Have Established So Far
We have discussed how electron current flows through the ground circuit and how to measure ground electron current. We discussed a few points about the DMM and the importance of grounding the DMM at 0.00 V when measuring ground side voltages. The two locations on a vehicle where this ideal ground voltage is available are -GEN or -BATT. If there is no Vd (voltage drop) between these two electrical points use the easiest one to access for the DMM ground.
On a new vehicle, it is reasonable to expect the voltage drop between -GEN and -BATT to be a true 0.00 V reading and we could use either point for our DMM ground. This can be confirmed by a simple voltage drop test with the engine running as previously illustrated in Part 10, Figure G05V.
As a vehicle develops some mileage, connections and cables begin to develop corrosion that impairs the flow of electrons passing through them. This causes Vd readings to begin to rise and we discussed the increase in Vds in the last segment. The higher the level of corrosion the higher the Vd. We established the fact that a good electrical ground should not be higher than 0.10 V although a number of electrical circuits can operate properly with the ground voltage above 0.10 V. A good computer ground should not be higher than 0.05 V although some computers can continue to operate if the ground voltage exceeds 0.05 V. A small Vd allows for a slight amount of corrosion in the ground cable and connections as a vehicle ages.
Only by experience can it be determined which electrical circuit ground or which computer ground circuit would be adversely affected should the voltage drop exceed these limits. The important point to consider is what the ground side voltage drop should be and ensure that the ground side voltage is as low as possible.
Corrosion is inevitable resulting in higher voltage drops as electron current flows through increasing corrosion. Forget about measuring the resistance of the corrosion in a wire or connector with an ohmmeter. The ohmmeter method is too unreliable. An ohmmeter could indicate very low resistance ground connection when it is bad but the voltmeter would indicate an excessive Vd. The opposite is also true. The ohmmeter could indicate some resistance indicating a corrosive connection yet the Vd would be acceptable. Believe the Vd reading of the DMM to confirm a good cable or connection. Do not rely on an ohmmeter test.
Remember corrosion produces resistance in a circuit. Corrosion begins to reduce the electron current that can flow through the circuit so circuits begin to become less efficient. Measuring electron current only tells you if the electron current is normal or if it’s too high due to component failure or too low due to corrosion developing in the circuit. It doesn’t tell you where a problem is in the circuit. That is why we use voltage measurement to find circuit problems.
How Much Vd between -GEN and -BATT?
For purposes of discussion let’s say we find a Vd of 0.10 V between -GEN and -BATT. This would indicate some corrosion has begun. There are two circuits to check.
- Generator Ground: With the engine running, measure the Vd between the generator housing and the engine block. I would like to see 0.05 V or less. If the Vd is higher it could indicate corrosion where the generator housing connects to the generator support bracket or where the support bracket bolts to the engine block. Check bolts for tightness. If necessary, add star washers to the bolts to improve electrical connection as the bolts are tightened. Verify Vd of 0.05 V or less.
- Battery Ground: Measure the voltage drop between -BATT and the engine block. I would like to see 0.05 V or less. If the Vd is higher it could indicate corrosion at several possibilities. There could be corrosion on the surface of the battery negative post; corrosion on the inside surface of the negative terminal clamp; corrosion where the cable enters the back of the negative terminal clamp; corroded negative cable; corrosion where the negative cable enters the ground terminal on the engine block; or corrosion between the ground lug and the surface of the engine block. Clean connections and verify Vd of 0.05 V or less.
If the Vd between -GEN and -BATT is greater than 0.10 V and has been reduced to less than 0.05 V after cleaning connections, check to see if the electrical problem in the vehicle has been corrected. If the electrical problem remains, at least you have established the main grounds in the electrical system are in good condition and you can safely ground at -GEN or -BATT and rest assured all voltage measurements you take will be a true reflection of the circuit condition where you are testing the circuit with your V/Ω (red) test lead. Do not think this is just a lot of unnecessary extra work. I have seen this attention to detail in the ground circuit repair numerous electrical problems over the years. This is especially true in older vehicles and vehicles subjected to extreme road conditions.
Pay Close Attention To Ground Voltage Readings
A lot can be learned as ground voltage is tested correctly. In Figure G10V below, the ground voltage is measured on the ground side of the load in Circuit #1 and Circuit #2. The DMM is grounded at -BATT. The ground side could be a logical place to start troubleshooting a circuit that appears to be not operating at all (Lamp is OUT) or not working properly (Lamp is DIM). But it’s always a good idea to always check known good circuits to learn what voltages you can expect in a good circuit. This makes it much more effective recognizing when a Vd is too high.
Notice the two ground side voltage readings above. These two readings tell us the ground circuit is good while electron current is flowing. The reason the voltage on the ground side is very small is due to the fact the ground circuit has very little resistance provided all ground connections are clean and tight and the wiring on the ground side is not damaged. These two readings confirm the ground side of the circuit for each circuit load is good.
By paying close attention to the ground side voltage readings we can reach some conclusions. DMM #1 indicates 0.04 V. From this we know the -BATT terminal is making good contact with the -BATT post. We know the accessory ground cable from -BATT terminal to G101 (sheet metal (1)) is good and ground connection G200 to sheet metal (1) is also good. We know switch, S2, is a good switch and the wire between G 200 and Pin 2 of Lamp Circuit #1 is a good ground wire.
DMM #2 indicates 0.05 V. The increase of 0.01 V from DMM #1 reading confirms the ground strap between Sheet Metal (1) and Sheet Metal (2) and ground connection G300 are both good and the ground strap adds 0.01 V to the ground circuit reading at Lamp Circuit #2, Pin 2. There is no need to do a voltage drop test between Sheet Metal (1) and Sheet Metal (2) to confirm the condition of the ground strap. The good reading at Lamp Circuit #2 confirms the ground strap is performing properly.
With this one circuit voltage measurement at the ground Pin 2 of the load for Circuit #1 or Circuit #2 while the DMM is grounded to -BATT confirms the ground circuit is in good condition for each circuit tested and saves time with other tests to confirm various portions of the ground circuit are good.
Detecting A Ground Side Problem
In Figure G11V below, we have a problem with Lamp Circuit #2. Lamp #2 is dim. The circuit voltage on the ground side of the lamp, Pin 2, is greater than 0.10 V indicating a ground side problem. Keep in mind what we said about the lamp circuit in the beginning of this series. We are using lamps to illustrate how circuit problems affect the lamp (load) because it is easy to visualize the lamp is working dim.
DMM #1 tells us what is good on the ground circuit. We can conclude the accessory ground cable, G101, G200 and Switch S2 are good based on the Vd at Pin 2 of Lamp Circuit #1.
DMM #2 tells us the ground circuit for Lamp Circuit #2 has a problem. The reading of 3.82 V clearly shows that Lamp Circuit #2 has a ground side problem. Possibilities to investigate would be the ground strap connections at either end, the condition of G300 and the wire between G300 and Pin 2 of the Lamp. Somewhere in the ground side of Lamp Circuit #2 expect to find a corroded connection or a damaged ground wire. From experience, I would expect to find a corroded connection on the ground side of the circuit.
The voltage of 0.04 V at Lamp Circuit #1, Pin 2, tells us a portion of the ground circuit is good at Sheet Metal (1). The bad ground reading at Lamp Circuit #2, Pin 2, narrows down the possibilities of what portions of the ground circuit are left to test. Further troubleshooting steps are required.
At no time did we need to measure the electron current in any part of the circuit to identify the circuit with a problem. If the electron current was measured, we would get a lower than normal current reading in Circuit #2 but that doesn’t tell us why the electron current is low or why the lamp is dim. That’s where voltage measurements must be done. Voltage measurement identifies the circuit fault as a ground side problem causing the dim lamp. The dim lamp is used to simulate how other circuit loads would be affected as we discussed earlier.
Suppose Lamp #2 was a DC motor instead of a lamp and placed in this circuit as the lamp is. How would the DC motor perform? Obviously, the DC motor would run slow (low rpm).
Suppose Lamp #2 was a solenoid. How would the solenoid perform? Obviously, the solenoid would be very sluggish and may or may not be able to perform its desired function.
Suppose Lamp #2 was a relay circuit which has two individual circuits; the relay coil circuit and the secondary circuit connected to the relay contacts. If the bad ground was on the relay coil circuit it may or may not prevent the relay from energizing. If the bad ground was on the relay contacts side of the relay circuit it would induce poor electrical performance in whatever circuit the relay is controlling.
Suppose Lamp #2 was an onboard control unit. How well would it perform if the main ground circuit was experiencing a significant voltage drop? It would be impossible to re-flash this control unit. Circuits that the control unit is supposed to turn ON or turn OFF may or may not happen.
You see the point? The lamp is used to simplify the teaching of circuit concepts. In future segments of this training program we will change the lamp into various electrical components and discuss some advanced principles of vehicle circuit troubleshooting.
In the next installment of this series we begin studying the circuit below. Look it over. We will focus on how circuits interact with each other in Figure G12V.
