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Engine Running Off Battery???!!

C

Cherokeekid88

NAXJA Forum User
Location
North Carolina
So i just put a new higher output altenator in my XJ about 2 months ago and a new better battery like2 weeks ago for the winter. Well christmas eve night i drove up to my grandparents and everything was fine (From what i could tell) and i drove back to my GF's house and still everything seemed fine. On the way back to my house i noticed that my Battery gauge was EXTREMELY low. Hovering right above the 9 on the guage. I immediately tured my stereo off and watched it as i drove home. I parked it and got out and noticed that my headlights were much dimmer and there was a kind of knocking noise comeing from under the hood. and its been sitting for the past 2 days and i went to crank it up today and it had a slow crank but it fired right up but still VERY low readings on the batter guage. Seems to me its the altenator>?????
 
If you have a voltmeter or know someone with one you can check it yourself. Turn it to DC read across the positive and negative and you should get over 13v if the alternator is charging. Check your connections on the alternator and battery. It should still be under warranty if it is bad.
 
the "knocking" noise is your engine misfiring because the spark plugs are not getting enough juice. you are going to ruin your battery. your alternator is bad or has bad connections. take it off and fix it.
 
Turn on the jeep, and while running unhook the battery. If it is still running, it will be running off the alt. If it dies, then the alt. isn't working.

At least thats what we used to do to our old plow truck. Assuming it's the same on xjs.
 
98XJSport said:
Turn on the jeep, and while running unhook the battery. If it is still running, it will be running off the alt. If it dies, then the alt. isn't working.

At least thats what we used to do to our old plow truck. Assuming it's the same on xjs.
Click to expand...

NEVER DO THAT, IF IT IS WORKING YOU CAN BLOW YOUR ELECTRONICS.
Do the voltage tests. I would look for the knocking noise.
 
How would that blow the electronics?
Just curious, the old truck we had to do it on barely had any, and the ones it did have never worked to begin with...
 
98XJSport said:
How would that blow the electronics?
Just curious, the old truck we had to do it on barely had any, and the ones it did have never worked to begin with...
Click to expand...

Listen to Langer on this one. It doesn't always happen, of course, but it can blow the diodes on the alternator, and if there is a voltage regulator problem or shorted diodes, removing the buffering effect of the battery can result in serious overvoltage, or spikes of unwanted AC that can do a number on other electronic components. Sure, it won't usually happen, but why take chances? You can buy a cheap and accurate digital meter for under 10 bucks these days, and test it right.
 
Like I said, just curious. I can see the problems if theres a voltage reg. problem, but this would also have already done a number on your battery, too. But again most times when Ive done this its not a matter of running to the store and getting a meter, its a case of I need help, am I going to make it home from this jump start or am I gonna crap out a mile down the road...

Theres always a "right" way to do things, and it should always be persued. And theres usually a "wrong" way to do things, that works but has risks involved. I just like to learn about the risks to see if they are justified...
 
The battery is a part of the charging circuit. It helps smooth out the ripple in the voltage generated by the alternator. If the alternator voltage regulator is flakey and trying to put out too much voltage, the battery tends to regulate that. Disconnecting the battery can allow the output voltage to go too high and to use a technical term, fry the computer.:read:
 
The voltage regulator don't even need to be flaky. Alternators regulators monitor battery resistance to determine a full charge if you disconnect the battery the voltage can soar out of control and damage anything running on it.

While on the Old school subject don't ever pull a plug wire off todays ignition systems (Use a spark tester) that spark is so hot if will blow a coil if it can't find a path to ground.
 
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So a voltage regulator can output significantly more than +-14V during proper operation? Does the ECU not have any filtering or capacitor protection? Based on my electrical background, it just seems kinda like one of thse "you can do it, but you shouldn't", but the response on here is along the lines of "thats like checking to see if you have a full oil tank with a match". Just trying to find out why. And always glad for the info, hopefully its helping others as well.

Edit: Definately agree on the spark plug part, checked my plugs and the next morning Jeep was running horrible. It was still dark, opened the hood, and had a cracked plud. Boy didn't that thing light up when it fired...
 
98XJSport said:
So a voltage regulator can output significantly more than +-14V during proper operation?
Click to expand...

Yes. As an example, an onboard welder using a stock alternator puts out 40-60 volts of rectified ac. This won't bother a points setup but it can sure destroy electronics like computers and nice radios.

I inadvertently fried a Ford EEC-IV engine computer by doing this. I won't take all the blame though. The person who wired in the battery disconnect didn't realize that the charging wire from the alternator goes to the battery side of the switch. We pulled the battery disconnect and the engine kept running for about another 10 seconds until the ecu smoked. The engine computer really didn't like the 40-volt spikes and it burned up its internal voltage regulation (which was nothing more than a few caps and a big 12-volt zener diode).
 
Those are big spikes, Im used to working with electronics that are usually around +-10%... a regulator that far off seems very poorly built, but I guess for the application it wouldn't matter... but a 20 volt spike range for example going across a battery circuit...unless the battery is dead, I don't think it would have that much dampening. But what do I know...
 
The ECU actually has an internal regulator. It even has a 3 terminal unit for the 5.0v to the sensors. That does not mean that spikes can't hurt it.

Tom Houston EE/CS/ME
 
98XJSport said:
Those are big spikes, Im used to working with electronics that are usually around +-10%... a regulator that far off seems very poorly built, but I guess for the application it wouldn't matter... but a 20 volt spike range for example going across a battery circuit...unless the battery is dead, I don't think it would have that much dampening. But what do I know...
Click to expand...

It's not really a true Voltage regulator even though thats the common term, a better name would be a charge controll.
 
langer1 said:
It's not really a true Voltage regulator even though thats the common term, a better name would be a charge controll.
Click to expand...

Although the "voltage regulator" associated with the alternator is only a charge controller which does not regulate the battery output, I'd be pretty surprised if the PCM didn't also have a regulator for its own internal use. The sensors run at 5 VDC and the internal computer might also.
 
All good info to know. It works if you have to check it that way, but definately not the best way to do it. But in a pinch...
 
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