A sensor wire chafes through on a chassis bracket, touches the body, and the ECU has a few milliseconds to decide whether it is looking at a real signal, a short to ground or a broken sensor. If that decision was never tested on the bench, it will be tested in the field. Fault injection is how you move that test back to the lab: you deliberately create the electrical faults a harness will one day suffer, in a controlled and repeatable way, and you check that the ECU detects them, reports the right diagnostic code and falls back to the right degraded mode, without being damaged.<\/p>\n
This guide explains the fault types, how a switching matrix is built, how to size it, how to keep the setup safe, and how to make every injected fault traceable.<\/p>\n
Most electrical fault campaigns start from five basic faults, applied to each relevant pin:<\/p>\n
For each pin, the test specification should state which faults apply. A short to battery on an output that is already driven high may be pointless; an open circuit on a redundant ground may be the most important test of the whole campaign.<\/p>\n
The fault-injection unit sits between the harness and the ECU under test<\/strong>, in series with every line you want to disturb. Picture it from left to right:<\/p>\n A test sequencer drives the relays, so each fault is a defined state of the matrix. In practice, the fault-injection unit is often combined with a breakout box<\/a> so that every line also remains accessible for measurement.<\/p>\n A shared bus keeps the relay count manageable: with N lines, you need roughly one series relay plus one or two bus relays per line, rather than a relay for every possible pair. The trade-off is that only one fault (or one pair) can be active on a bus at a time. If your specification requires simultaneous independent faults, plan several buses.<\/p>\n The relay must survive the worst case it can see, not the nominal current.<\/p>\n Example:<\/em> a low-side ECU output, switched on (line held close to 0 V), and you inject a short to battery through a 10 \u03a9 current-limiting resistor. The current is roughly I = V \/ R = 12 V \/ 10 \u03a9 = 1.2 A. The resistor dissipates P = V\u00b2 \/ R = 144 \/ 10 = 14.4 W while the fault is active. A 0.25 W resistor would fail immediately; a power resistor, a short fault duration, or a higher resistance is needed. The ECU driver must also be rated, or protected, for that current.<\/p>\n Example:<\/em> on a sensor ground carrying 50 mA, a relay contact of 100 m\u03a9 adds a drop of 0.05 A \u00d7 0.1 \u03a9 = 5 mV. On a ratiometric sensor with a few millivolts of resolution, that is not negligible. Count every contact, connector and wire segment in the path.<\/p>\n Fault injection is deliberately destructive in intent, so the setup must limit the damage to what you actually want to test.<\/p>\n A fault campaign is only useful if you can prove later what was injected and what happened. Each injected fault should be a test step with:<\/p>\n\n
2.2 Why a shared fault bus is a design decision<\/h3>\n
3. Sizing the relays and the paths<\/h2>\n
3.1 Selection criteria<\/h3>\n
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3.2 Worked example (illustrative values, not RAYNOV specifications)<\/h3>\n
4. Safety: protect the ECU, the bench and the operator<\/h2>\n
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5. Traceability: one fault, one test line<\/h2>\n