Electrical Issue

[mendelo]

A few weeks ago I posted a problem I had where the motor died while driving, and took with it all the accessories that work through the ignition (windows etc.). Turned out to be a fusible link which I replaced with a 50 amp maxi fuse. It looked like the insulation on the fusible link had split and the wiring corroded. So I thought nothing more of it. It's been driving, oh about 3 - 4 weeks with no problem.

However today it did exactly the same thing. I pull over (good reason I spend most of my time in the curb lane) and check the new wire. The 50 amp fuse had blown. Being resourceful I had bought a spare fuse so I plugged that in. Everything worked fine, windows, dash lights, gauges, until I started the motor. I noticed that immediately the ammeter dropped right down to the bottom of the guage (less than 8) and the motor ran fine for 10 secs before the new fuse blew. (At least this time I had the foresight to roll up the window so that I did not have to abandon her to her fate). Did I mention resourceful? I had yet another fuse. Plugged this in, turned her on, ammeter sank, shut her off before the fuse could blow. Since I can't drive home in 6 second spurts, she was rendered hors d'combat right there. And there she remains until I can sort this out.

So does anyone know why the fuse will remain fine until the engine actually starts. Gotta be a short I imagine, but I do not know what extra current draws occur when the motor is running as opposed to simply having the ignition fully on but not running. Coil I guess but any other ideas?

THanks
Peter

[RWalker]

Have you checked to see what voltage your alternator is putting out while the Jeep is running? It could be overcharging.

[mendelo]

I have not, as the jeep was running just fine up to that point, showing as usual 13.5 on the meter. I am assuming that; if the engine is running, the ammeter gets dragged down to 8 and the maxi fuse blows after about 10 secs, I have a short somewhere. Is it possible that the alternator itself could contain the short? I know it is newer (about 1 year) but that doesn't mean its not faulty.

Thanks
Peter

[GWChris]

Wait - I'm confused here. "the ammeter dropped right down to the bottom of the gauge (less than 8)" - I'm assuming you meant voltmeter?

[RWalker]

The alternator could be the problem but I would first start with disconnecting accessories and see if the problem persists. I woud disconnect the power to the seats, window motors, heater blower motor, rear defroster, etc. I haven't had an alternator blow fuses but I have had one drain the battery. The alternator was staying charged with the ignition off and draining the battery in about 3 hours. You can connect an amp meter between the negative battery cable and the negative battery post to see if there is any drain with the ignition off.

[mendelo]

Yes, sorry, voltmeter.

[mendelo]

Thanks for all your suggestions. I do not profess to be an expert here, but the wiring diagram on Tom Collins site is helpful. As mentioned the problem only occurs when the engine is running. All the accesories that are attached through this one ignition switch wire (windows, etc.) do not blow their respective fuse, it is the maxi fuse that fails. Only the ignition module, coil and charging system are not protected by additional fuses but rely on the fusible link (and now maxi fuse) for their protection. The ignition module seems to be driven by it's own fusible link , it just uses the ignition switch to provide an on/off signal. I do not know if it's possible for a coil to have a short and still run the engine just fine until the fuse blows. Or back to the alternator, which has been suggested and is the most likely (at the moment) source of the problem. Does this make sense? This is a tough one to diagnose because without the engine running I cannot find a short anywhere and the engine will not run long enough for me to get any readings from a multimeter that might indicate a short.