Wiggle test
The purpose of this test is to check circuit integrity during a wiggle test.
How to perform the test
- Disconnect any devices connecting the PC to the vehicle earth (e.g., a scan tool or data logger).
- Power down and disconnect the suspect circuit(s) at both ends.
- Connect the PicoBNC+ resistance probe to Channel A.
- Connect the PicoBNC+ resistance probe across the suspect circuit(s) [see Further guidance].
- Start the scope.
- Take hold of the shielded loom or wires containing the suspect circuit and gently wiggle them about their normal resting position.
- Stop the scope as required to view and save your waveforms.
Continuity check of circuit resistance during wiggle test.
Further guidance
Wiring faults occur because:
- the insulation breaks down, causing the conducting core of an unshielded wire to touch another or some other part of a vehicle.
- the conductor corrodes due to moisture ingress or breaks down due to the stress and strain caused by movement or flexing.
The typical circuit faults are:
- Open circuit
- Short to positive
- Short to negative
- High resistance
Often the faults are intermittent in nature and occur only when the faulty part of the wiring moves to a specific position. In these cases, a wiggle of the wire, or the loom in which it is enclosed, can cause the fault to occur. A wiggle test therefore saves diagnostic time that might be spent waiting for the fault to occur by chance.
Several circuit checks can be conducted during a wiggle test when appropriate connections of a PicoBNC+ resistance probe are made. The table below describes the nature of the checks. Note, in all cases, the suspect circuit is disconnected at both ends. The accompanying example circuit diagram for a typical sensor circuit illustrates the locations of the measurement points (A, B, C and D) for each check.
| Test type | Measurement points | Possible failure mode | Expected resistance |
|---|
| Continuity | A - B | Open circuit or high resistance | < 0.5 ohms |
| Insulation | A - C | Short to ground | > 10 kilohms |
| Insulation | A - D | Short to voltage supply | > 10 kilohms |
Where it is impractical to peform the wiggle test and operate the scope, you can use Masks and Alarms to alert you if the resistance deviates away from the expected tolerance.
The absence of a changing resistance during a wiggle test does not guarantee the absence of a fault. Instead the resistance-based checks should be considered a first look", which can quickly identify faults if they are clearly present. Volt drop testing with a circuit under load should be used to fully confirm that there is no fault present.
Please also see:
Graphing resistance with the PicoScope.
Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
