Intermittent no-crank: 0.9 V drop on the engine ground path

Question

The battery measures 12.61 V at rest and stays above 10.4 V during the failed crank attempt. The starter solenoid clicks. Battery positive post to the starter B+ stud drops 0.18 V, but battery negative post to the starter housing reaches 0.90 V. Adding a heavy jumper from battery negative to a clean engine point makes it crank normally.

The visible battery terminals are clean and tight. Before replacing the main ground strap, how would you split this reading to identify whether the loss is at the battery cable, the engine-to-body strap or a connection between them?

Scope &amp; Meter · Accepted Answer

Keep the circuit under the same failed-crank load and move only one meter lead at a time. Measure battery negative post to cable terminal, terminal to body ground, body to engine or transmission case, and case to starter housing. The segment that contains most of the 0.90 V is the fault area. Probe the actual post and metal casting, not bolt heads that may bypass the interface you need to test.

A resistance check with the circuit unloaded may miss a corroded joint. Once you find the segment, inspect both mating surfaces and cable crimps, repair it, then repeat the original battery-post-to-starter-housing test to prove the total path is now acceptable.