As long as we're on the subject of spare parts, you should consider carrying a spare starter relay.
Like ignition switches, starter relays have internal mechanical components that can wear out through repeated use. (I've never had one fail but the potential is there.) If the one currently in your motorhome looks like it came from the factory, you may want to replace it with a new one and save the old one as a spare.
Like the ignition switch, the relay could signal pending failure through intermittent or erratic operation. However, it also could simply fail without any warning.
For those who don't know, a relay is basically a remote switch. It allows a low current switch and wires to control (switch on and off) a high current circuit. (Relays can also be used to perform certain logic functions but that's a topic for another discussion.)
In terms of 12VDC circuits, 'low current' typically means 40 or less amps and usually means less than 10-15 amps. 'High current' typically means 30 or more amps. (Yes, there's an overlap due to variations in definition and use of the terms.)
Before continuing, note starters have two circuits leading to them. One is a 'control' circuit, which is what we're discussing here and is a usually a 'high current' circuit with relatively large (10-12 gauge) wires. The other is a 'power' circuit, with very large (4 to 4/0 gauge) wires capable of handling hundreds to a thousand or more amps. (These wires are sometimes called cables, as in 'battery cables.')
Note: Do not confuse high tension/voltage wires (i.e., spark plug wires) with what we're discussing here. Spark plug wires carry tens of thousands of volts while we're talking exclusively about 12VDC circuits. (There's a distinct difference between voltage and current.)
In very old vehicles (prior to the mid-50s), there were separate ignition and starter switches instead of the combined ignition/starter switches in modern vehicles. (Yes, I just called our roughly 40-year-old motorhomes "modern.")
The ignition switches in those old vehicles were effectively identical to our 'modern' switches, minus the START position. (In most cases, the similarity extended to using a key, instead of a knob or lever, to control the switch.)
The starter switches were massive pushbutton switches, capable of handling the high current needed for the starter control circuit. (The switch for the starter's power circuit has always been, to the best of my knowledge, inside the starter solenoid.) If the starter switch was on the dash, it also meant very large (6-10 gauge) wires leading from the battery into the dash and back out to the starter.
(The starter switch on my '47 Willys Overland wagon was on the starter, with the foot-operated pushbutton poking up through a hole in the floorboards. When it rained, my feet got wet.)
You might think a single switch and simplified starting procedure is the main advantage provided by starter relays. (Old style: Turn on ignition switch and press starter switch. New style: Turn key to start position and release key.)
However, safety is the primary benefit. Relays permitted moving the high current wires out of the dash and allowed shorter, more direct routing. The possibility of a short-to-ground or similar problem increases with wire length. In smaller low current wires, the wire usually acts like a fuse, melting through and stopping the flow of electricity. The same process occurs in the larger high current wires but takes much longer. The result is always spectacular, often with enough sparks and heat to start a fire. (It's never good in a vehicle but especially dangerous in the dash, close to the driver and passengers.)
There's also economics involved. Longer and larger wires means more copper, which means more expense. The 'scenic route' for low current circuits is acceptable, as long as the wires are slightly larger to compensate for the added length. ('Not good' but at least acceptable, given the wire size caveat.) The wires in high current circuits need to be as short and direct as possible.
In my next post, I'll provide pictures, part numbers, and other info on your starter relay. In closing, I will tell you finding your starter relay will require a flashlight or trouble light (aka droplight). It may also involve a significant bit of physical contortion. (They're rarely located in one place ... instead placed wherever is most convenient for the designer/manufacturer.)
