How to crank vehicle with coach batteries

[Flame]

1998 concourse Ford E-350 with Triton V-10 engine. Will leave on trip and my cranking battery under the hood is date coded january 2018 but...everything is fine and cranks good. Will someone explain to me for sure how our "override" cranking button works above my head in the cab? If my truck battery goes dead, then what exact procedure do I do to crank off of the carriage batteries? I am having difficulty determining if it is a simple "toggle type" switch or is it a push and "hold" to crank button? It does not say on or off on mine. It kinda acts like it is a 3 stage switch with "off" maybe in the middle. I have a 4000 mile trip to take and want no surprises. I will also carry a pair of jumper cables but would like to test the override system. How can I check it to know it will work if I need it?

[caconcourse]

I presume you are referring to the button labelled "Vehicle Boost Start" in the overhead switches. It is connected to the battery isolator relay (Sure-Power 1315) under the hood on the driver side panel. The relay is normally closed only when one side or the other is at a charging voltage, so that both sets of batteries are connected when one is charging. The boost button provides an override signal to the relay, to force it to close regardless of the voltage present on each side. This allows the coach batteries to combine with the truck battery to power the starter.

I believe the switch is momentary and the override is only activated when the button is held down, so you would hold it down while turning the key to start the engine when you need to add the coach batteries for starting. If you listen to the relay while someone presses the button, you should hear a click when the relay closes.

[Flame]

I will go out today and check that theory. Maybe I can get someone to help. What does the relay "look" like? It is in a fuse box?

[Roly]

Here is a previous thread on that.
viewtopic.php?p=18665#p18665


The Sure Power is on the front left fender under the hood.

Roly