Dead Battery

Alternators and voltage regulators often go hand-in-hand.

Your local auto parts store can do an charging system review in a few minutes, often for free.

What are you driving and what engine?

You are describing a bad connection.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

curbrider wrote:

91 jeep cherokee 4.0 replaced alternator and it over charge even with the battery disconneted

Now you are talking a dead computer because it got fried from the alternator surging with no battery buffer.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

curbrider wrote:

How can the alternator even work with the battery gone? Generators work fine with no battery, alternators do not since they need +12 volts from the battery for the exciter voltage before they will work.

Jerry

Mike Roma> Now you are talking a dead computer because it got fried from the

i only pulled positive cable off for 30 seconds

There is enough residual flux to self excite most alternators - but I've only seen one that would avalanche (or bootstrap) enough current to actually be functional. Once excited, it's another story - the alternator needs on the order of a couple of amps in the rotor to produce full output so if you keep the battery on long enough to get past a certain point (and it doesn't take much) then disconnect it, the alternator will continue to operate at up to near rated output until it is shut down in some way (stopped, etc.). The ones I've played with have had miserable regulation and horendously noisy outputs, but they do work. A few even got so far out that they exceeded the voltage ratings for their diodes and fried them.

It is a newer Jeep with a 'generator' or 'half an alternator' isn't it? The regulator is in the computer.

I don't know how they work with no battery, maybe they got excited before the cable was pulled off, but the only times I have seen that happen on a Cherokee, the computer toasted right away, the stereo fried up in smoke and the 'half alternator' died.

Does that 'half alternator' even need an excite?

Mike

Jerry Bransford wrote:

Well, if you got extremely lucky and didn't fry any electronics, then I would be chasing cable connections.

The main battery cables need cleaning at 'both' ends, pay close attention where the negative hooks to the block. Then there is a wire mesh cable that runs from the rear of the head to the firewall, if ratty, it can cause the alternator to read all over the place while not putting juice to the battery.

The case to bracket ground on older Jeeps can go bad too. I test that by taking a booster cable and run it from the battery negative to the alternator case to see if the volt readings level out. If they do, then you have a ground path issue.

Good luck!

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

curbrider wrote:

I know my wife's Grand Cherokee will not even idle after jump starting it if the battery is totally dead, it dies immediately. Her battery suddenly went totally dead last week and while I could jump start it, it would die once I let it idle since there wasn't enough power in the battery to continue exciting the alternator. I had a hell of a time even driving it one mile to a battery store to get it replaced, it even died and literally left me sitting on the railroad tracks at one point, lucky there were some workmen there that pushed me off the tracks so I could jump start it again.

Yep, both electronic theory and my recent personal experience says it won't run without the battery.

Jerry

Mike Roma> It is a newer Jeep with a 'generator' or 'half an alternator' isn't it?

Something has changed then from the Cherokee system to the GC's. I have heard the Grands won't run with a bad battery several times and places.

The Cherokee will run if the battery cable is taken off after starting like the OP said he did. The one I saw doing it was a 92 and the OP's is a 91 I think. Not for a long time, but it will run.

I saw the 92 do it twice and burn things up twice after the same shop left the battery cable loose a second time and it fell off while off roading once again.....

Well, he saved the stereo because I realized what I was seeing as it happened so hit the off button fast and the mean green 'alternator' was just a smokin'.

Mike

Jerry Bransford wrote:

I really would not suggest disconnecting the battery on an alternator based system. The alternator will then put out enough voltage to make things interesting, if somewhat short lived, for the computers that don't like somewhere between 40 to 80 volts open circuit. Plus the battery acts as sort of the filter capacitor for the simple diodes in the alternator output, so you get a rather rippling waveform.

Mike Romain proclaimed:

If you have a cheap 0'scope you can find a popped diode if it is failed to open--the waveform is pretty obvious and the deader the battery the easier it is to see. If it is failed to short and there is anything left in the battery, the diode will tend to signal by all the magic smoke it lets go of. An awe inspiring amount of smoke even on a presumably dead battery.

L.W.(ßill) Hughes III proclaimed: