NIGEL CALDER REPLIES

I am reasonably certain you are experiencing one of two scenarios: 1. When you try to crank you energize an electromagnet inside the solenoid that drives a heavy metal washer into the main battery terminal and the main starter motor terminal, making a circuit between the two. Every time this happens, there is a spark that slowly burns the surface of the washer and terminals. Given that you are seeing a small voltage drop when cranking, the solenoid is almost certainly being powered up. I’ll bet you are also hearing a “click” as the washer gets driven into contact with the two terminals. The problem then arises if the washer and/or terminals are sufficiently burned that the connection is not good enough to power up the starter motor. After that, because the washer is free to rotate when not engaged, when you try to crank again it turns a bit and a new surface comes into contact with the terminals. Sometimes this is good enough to pass cranking current and sometimes it is not, hence the intermittent problem. 2. Of course, in your case you have a newly serviced starter motor, so here’s another possibility. When the solenoid is activated it draws several amps. Therefore, if the wiring from the ignition switch to the solenoid is at all undersized, and especially if your batteries are showing their age, the voltage drop in the circuit may be such that the solenoid starts to drive the washer into engagement with the main terminals, but then the voltage falls, the solenoid trips out and the washer disengages. The voltage then climbs and the solenoid tries again. If this is the case, you may notice a rapid clicking without cranking. The answer here is heavier-gauge wiring to the solenoid and/ or a healthier or better-charged battery. Another approach sometimes used is to install a second solenoid in the cranking circuit. However, that just adds extra wiring and another potential failure point, so we won’t go into that! s