Compare DC evidence under load
Locate a loss in a supplied DC circuit without mistaking one reading for a component diagnosis.
The short version
- Compare source and load during the same operating condition.
- Name both lead points before interpreting a voltage.
- Separate the measured interval from the cause inside it.
A supplied circuit, not a probing procedure
A lamp is dim in an original training circuit. Your job is to explain the supplied readings, not connect a real meter.
Battery circuits can deliver damaging fault current. This class does not authorize access, isolation, live probing or component replacement.
The drawing uses a dedicated return wire. A different coach may use another return arrangement, which requires its own verified diagram.
All voltages here are invented problem data. They are not manufacturer pass limits.
Compare the same operating condition
The first card shows 12.6 volts at the source with the lamp off. The second shows a dim lamp during Run A.
Those records describe different conditions. The resting reading cannot tell you what the source supplied while the lamp was on.
Open the Run A source card before deciding whether the source collapsed or the path lost voltage.
Run A records 12.2 volts across the source and 9.7 volts across the lamp. Both readings belong to that same loaded condition.
Predict what the next record separates
The missing voltage could be distributed along the feed and return paths. A low load reading alone does not identify either side.
Before the reveal, choose which record would distinguish them: source positive to lamp positive, or another resting source reading.
The first comparison measures the feed interval during Run A. The second repeats a different condition and leaves the interval unresolved.
The supplied feed record is 2.3 volts. It identifies a loss across that measured interval, not yet a failed connector.
Keep the return in the explanation
The Run A return record compares lamp negative with source negative. It shows another 0.2 volts across that interval.
The supplied values reconcile: 2.3 volts in the feed, 9.7 across the lamp, and 0.2 in the return total 12.2.
This arithmetic checks the fictional records for consistency. It does not establish that 0.2 volts is acceptable on an unidentified real circuit.
If lead references or operating conditions differ, do not force the readings into one voltage budget.
An interval is not a part verdict
The next fictional record divides the feed interval. It shows 2.2 volts across the named joint and 0.1 across the remaining feed.
That result narrows the investigation to the joint interval. The photographs and lead labels must still establish what the measurement includes.
A lead placed on a cable lug and one on a stud can include their connection. Different lead placement can include additional wire.
Record the observed loss and request the applicable inspection or service decision. Do not translate it automatically into a replacement order.
A second run changes the conclusion
In Run B, the source is 9.8 volts, the lamp is 9.6, the feed loss is 0.1 and the return loss is 0.1.
Would replacing the Run A joint explain this new record? No. Run B places the change at the supplied source terminals.
The records do not identify why the source changed. Its supply, state and connected loads still need a model-appropriate investigation.
If the complaint is intermittent, keep Run A and Run B separate. A good-looking later run does not erase the earlier evidence.
Verify the claimed change
A proposed correction needs a comparable record: the same named points, requested function, relevant load and reported symptom.
A bright lamp proves only the observed operation at that moment. The work record must also say whether the original conditions were reproduced.
When the required conditions are missing, record a limited observation and an unresolved verification need.
Use the linked diagram activities to collect virtual evidence. Then try the separate intermittent case before claiming the reasoning transfers.
Make the conclusion no larger than the evidence
First match identity, point names and the operating condition. Then compare source, load, feed and return records.
Describe the interval supported by the readings and the unanswered cause question inside that interval.
A correction and its verification are separate decisions. Neither a quiz score nor a virtual meter reading is hands-on sign-off.
On the job
- Match the circuit identity and operating condition.
- Read source, load, feed and return records from the same run.
- Localize the interval and state what remains unknown.
- Choose a bounded follow-up and a comparable verification record.
Shop tip
A voltage is between two points. Preserve both point names and the load condition in the work record.
Where this comes from
- https://www.fluke.com/en/learn/blog/automotive/electrical-automotive-troubleshooting
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.