Wolf_n_Kat wrote:
Hey Griff, got a really stupid question for you!
We've started working on The Beastie, but over the winter it decided to 'not start'. Charge up the battery, get it jump-started, and it runs fine, but once you shut it off, it's 'dead battery time'. Fair warning - rats got into it, and tried to make a nest on the engine!
Apparently they decided to dine on some wiring while they were at it. I've got a wire coming off one side of the solenoid that I have NO idea where it goes, what size fuse goes in it (it looks like they went for the fuse holder), is it actually supposed to be fusible link, etc etc etc.
Included for your viewing pleasure (or if you just like cringing!)...
No need to cringe ... your wiring, etc. is quite clean compared to some I've run across ... and unf**ked.
Interesting question ... and something I'm going to need to think about.
I'll start with WAG. (For those who don't know, WAG is an acronym for Wild A$$ Guess.)
Check your battery and possibly replace it with a known-good or new one, at least temporarily. The battery you have may be dying and very close to death.
The main purpose of engine batteries (more properly called SLI batteries) is the tremendous burst of energy need to crank the engine. Once the engine starts, the alternator provides all the electricity needed to run the engine, plus a little extra for lights, heater/AC blower motor(s), wipers, gauges, radio(s), and so forth.
When the engine is running, and the alternator is putting out current, the SLI battery is essentially just along for the ride.
If the SLI battery is close to dying, it may still produce some current but not enough for the starter to get the crankshaft rotating. In this case, jump-starting would get the engine running and the engine would continue running using electricity from the alternator.
Likewise, an SLI battery that close to death would not be able to accept and store enough of charge to restart the engine once you turn it off. Using a battery charger also won't work because, even though the battery appears to be fully charged, the charge "bleeds" off very quickly. (I've seen almost dead batteries appear to accept a full charge but drop below 9.5 volts in as little as 15 minutes, without any load connected.)
It's actually possible to completely disconnect the SLI battery from a running engine but is definitely NOT recommended. (I'm done that at least twice by inadvertently turning the battery switch to OFF.) Although the SLI battery is essentially just along for the ride, it does have a relatively important/minor role when the engine is running. (Minor role for the battery and engine but important for the electrical system as a whole and especially for sensitive electronics.) It acts a a buffer, helping the alternator deal with sudden significant demand spikes. (Without the SLI battery, turning on the headlights or the heater/AC can create a brief drop in system voltage that is sufficient to cause some electronics to have a hissy fit and drop dead.)
In terms of your mystery wire, I assume you're referring to the one in from of the white windshield washer fluid/radiator overflow bottle in the picture. If so, it could be a "red herring." Apparently unused wires/connections like that are very common in vehicle wiring harnesses and are connections for options not included on your vehicle. They may also be for end-user convenience, placed there for conversions for emergency and commercial vehicle applications.
Finally, bear in mind what I've rattled on about previously on this thread. Starters initially require a brief but immense surge of energy, in the range of 500 to 1,000 amps, to get the engine rotating. On the other hand, most vehicles need less than 50 amps (and as little as 15 amps) to operate once they've started running. (The exception is rolling boomboxes and other vehicles with power-hungry, aftermarket accessories.)
