xjmarc

Really? That sounds pretty interesting. You going to do this experiment for us?




I have in the past. My grandfather taught me that one when we were stuck on our mountain property when I was a kid. It won't bring a really dead battery back but one that's low enough that it won't crank your rig it'll fix.

holycaveman

Simple test to see if your alternator is independent of the battery.

Start the jeep and remove the battery cables.

If it keeps running then yes spinning the alternator manually will charge the battery. If your jeep dies then it probably will not charge unless it can sense 12 volts. Some alternatotlrs run a rotor/coil set up that needs voltage to work.

salad

Or if it really dies you just fried the PCM when the VR goes ape**** from lack of reference voltage and lets the alternator go out of control...

Pro tip: don't do the above on any fuel injected vehcle.

holycaveman

lack of voltage fries PCM news at 11

LOL

you just make this stuff up as you go don't you?

salad

Won't be a lack of voltage if the alternator works...

And no, I get this from the FSM and electrical theory.

holycaveman

It was my turn to be a smart a&&.

Seriously, the electrical system does not operate like that. The PCM is designed to run on a specific voltage range. The alternator is regulated at this same range. otherwise when your battery died when you were driving the PCM would burn up. or when alternators go bad and start over charging your PCM would burn out. but the alternator has a protection circuit against these types of problems, and likewise the PCM.

Otherwise just driving the alternator would cause the PCM to fail.

5-90

Aspirin is a mild organic acid - so that makes some sense (although it won't give you much.)

If anyone's gotta take a good strong leak, that's better. It will get you started - just replace your battery when you get back to civilisation.

5-90

Don't do this, please!

Vehicles made after 1974 (electronic ignition became universal) have more risk of harm than useful results when you do this.

Why? When you disconnect the battery from the running system, you'll get a high-current transient through the wiring. Not for long - a few milliseconds - but it's enough that you've got about even odds of frying something useful.

It's not a lack of voltage that does it, but the sudden removal of a voltage sink - the regulator sees the change in loading, swings to forced full output, then recovers. This usually happens inside of 50ms - but it only takes about 25 microseconds for a transient to cook something, if it's going to!

You could pull the battery cable and blow the PCM as a result (yes, I have seen this happen. I've had to fix it after some other damned fool did it. Several times.)

Back when everything ran on points & physics, this wasn't a problem - and a perfectly valid test (just turn your radio off first.) But, with electronic bloody everything now, not so much.

I've posted a writeup on a safe way to qualitatively test your primary electrical system, using nothing more advanced than a DMM and fifteen minutes or so of your time. No, I haven't a link - but it should be easy enough to find.

salad

Thanks, you have way more patience than I do!

holycaveman

:brick wall:



Here is what you fellows fail to understand. IF they system has the old generator. NON fuel injected. You can run it without a battery in most cases. No damage to computer.

It has nothing to do with 1974. Its the type of charging and electrical system a vehicle has. We are talking about jeeps.

IF the jeep has FI its alternator is powered by 12volts dc from the battery. The alternator will NOT charge without battery voltage present.

So it does not matter at all if you yank the battery cable off while its running. The alternator instantly will shut down and not damage anything. And your jeep dies

These systems are pretty bullet proof. A runaway alternator that is charging too much still will not hurt the pcm. YOu can jump start the car with another one and it wont damage the pcm.

There are tons of rumors on the net about damage could be done without a battery because its a "buffer" These people do not know electrical systems.

Salad, no offense buddy but you should have not skipped class

salad

Sooo in other words, on vehicles that have absolutely nothing to do with this forum.

Which, in this thread, is a vehicle with a computer that drives fuel injection...

You might want to read your own post before throwing insults around. Or did you not know that XJ Cherokees are fuel injected with a computer? I can take pictures if you like. Don't just take my word for it. 5-90 has a lot more credentials than I do.

odgreen

You are truly special...... this thread is about xj's not a 1964 ford truck......

Anyone and everyone, never listen to holycaveturd..... he got his degrees from the dollar tree....yes salad I used your qoute

holycaveman

Hey, unlike most. I will completely admit when I was wrong.

I origionally stated that you could pull your battery cable to see if it would run or not.

I should not have said it like that. Newer FI jeeps wont run. Older ones may. So I should have said if you have FI, spinning the alternator will not work. If you have an older carb model it may.

It was late and I did not pay attention to what type of charging system the years/models had.

All I know is I don't surf to find the answer to my model. I do it and tell you from my experience. That way its not hear/say.

holycaveman

Few more ice cream seasons odgreen89on35s and you can sit with the adults

odgreen

The hell does that even mean? I'm pretty sure I'm older than you.