1991.
1. FORD does NOT install any Coach charge relay's at all. The Coach Maker installs the Coach Battery charge system
2. Even as old as 1991, there can be 2 types of Charge systems for the coach battery. The Older 80/90's style on C body was a standard silver continuous duty solenoid. It was activated by the Ignition side of the Key switch. When you turn the Key to START the engine, the Ignition side of the Key switch is opened until the engine starts and you release the spring load of the key. Yes, If you have an Ignition side charge solenoid activated it WILL close for both battery banks when you turn the key to the Ignition indent, but NOT to the 2nd spring loaded Engine start indent.
3. Other systems back then used a Dual Battery alternator Isolator. NOT to be confused with current Intellitec type BIRD system. Here is a link to that older type system.
https://www.sterndrive.cc/product/battery-isolator-130-amp-bi-1203-3a.html?gclid=EAIaIQobChMIuOnWhaiS5wIVBdvACh07UAuBEAQYAyABEgJe6vD_BwEThis link shows a 3 battery but they make the 2 battery system.
