Open circuit in relay control wiring

A break in the low-current side of a relay — the coil, its feed, or the module output that switches it — rather than in the heavy circuit the relay carries.

Every relay is two circuits that share a housing, and separating them is most of the diagnosis. The control side is a small coil drawing a fraction of an amp; the load side carries the current that actually does the work. This cause is entirely about the small one, which is what makes it cheap to fix and easy to misattribute to the expensive component the relay feeds.

The break can be in three places and they are worth checking in order. The module's own low-side driver, which switches the coil to earth and is the part that fails when a coil has been driven without its suppression diode. The coil itself, which goes open circuit with age and heat — measurable in seconds across two pins. And the feed to the coil, usually a small fuse in the same box.

The symptom is complete absence rather than weakness, and that is the useful distinction. A relay whose control circuit is open does not click, so the thing it powers simply never operates: no crank, no fuel pump prime, no fan, no heater. A relay that clicks and still does nothing is the opposite fault — burnt contacts on the load side — and it points at a different repair.

Two ordinary causes account for most of them. A socket whose terminal tension has relaxed, so the relay sits in place and touches nothing, common in boxes that get hot. And corrosion in an under-bonnet or underfloor box whose seal has aged, which reaches the thinnest pins first.