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Weird problem with electrical.

Old 02-27-2019, 08:23 PM
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Default Weird problem with electrical.

Okay so we've got this 1997 jeep cherokee with 4.0 auto and it dies when you disconnect the negative cable. Now there is a new battery, new alternator. Alt is putting out 13.95v and I've checked power at all the grounds and cleaned all the ground points and ground strap from engine to firewall. I tried another ground wire and wiring harness for the starting system,coil and negative cable from another 97. The charging sytem is good working order. The negative cable supplies the ground for the coil and I though that maybe when disconnecting the neg cable it was somehow opening the circuit on the coil and killing the jeep that way but when you disconnect the neg cable it's still connected to the fender apron. So I'm at a loss. I've tried another ECU as well and problem still persists. None of my other jeeps die when disconnecting the neg cable. Typically when the sytem isnt charging and you disconnect the neg cable it will die. But as stated charging sytem is doing its job.
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Old 02-27-2019, 08:26 PM
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I canít tell if youíre trolling or if youíre serious ?
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Old 02-28-2019, 01:07 AM
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No I'm serious. I know these cherokees well and I cant figure this out. Dont make any sense to me..
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Old 02-28-2019, 03:16 AM
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Why would you expect the jeep to run with the battery disconnected?
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Old 02-28-2019, 05:31 AM
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I know for a fact some vehicles will run on powered by the alternator if the battery is disconnected, but this is ill-advised, as it can damage the voltage regulator and diodes in the alt

maybe thats why yours doesnt run like that anymore
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Old 02-28-2019, 09:01 AM
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Voltage regulator is controlled by the PCM. It wont damage the PCM as the voltage stays where the computer tells it. That jeep dont care if there is a battery there as once its charged the PCM tells the alternator stay at constant 13-14volts. I've already tried another PCM so I know it's not damaged and I know that running it without cable attached wont damage it. I've jumped one before with no battery and drove it home when battery got destroyed. The jeep drove fine for years after without any PCM issues
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Old 02-28-2019, 09:35 AM
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The jeep dies instantly and it shouldn't. If you had a dead battery that failed while driving it would be the same circumstance. I've never had a car with a dead battery kill a voltage regualtor or alt because battery died or got disconnected. I'm not running the jeep around without it. You disconnect it dies. The jeep will drive fine then you go to start and it's dead. Charging sytem is good and battery is new and everything is making proper voltage. Problem has nothing to do with me disconnecting the negative cable. It's just telling me I have a problem in that area I've cleaned all grounds and positive cables and even tried new cables so I'm baffled.
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Old 02-28-2019, 09:39 AM
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Originally Posted by Morat View Post
Why would you expect the jeep to run with the battery disconnected?
because every car I've ever owned including multiple XJs will run if alternator and regulator are working but if you disconnect and it dies means alt or regulator is bad but I know both are working properly.
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Old 02-28-2019, 09:42 AM
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Originally Posted by awg View Post
I know for a fact some vehicles will run on powered by the alternator if the battery is disconnected, but this is ill-advised, as it can damage the voltage regulator and diodes in the alt

maybe thats why yours doesnt run like that anymore
I've changed alt and battery and PCM and wiring since problem. So most defintaly not because I disconnected the neg cable and it died instantly. There is no way that would cause damage. Other wise every time your alt went out it would kill the PCM. That's just not realistic. Yes some vehicles are sensitive to this but these jeeps are not.
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Old 02-28-2019, 10:36 AM
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Originally Posted by MoparNJeep View Post
I've changed alt and battery and PCM and wiring since problem. So most defintaly not because I disconnected the neg cable and it died instantly. There is no way that would cause damage. Other wise every time your alt went out it would kill the PCM. That's just not realistic. Yes some vehicles are sensitive to this but these jeeps are not.
So you're criteria is that it doesn't behave the same as other Jeeps when doing something stupid? Pulling the negative cable while it's running is a great way to fry the electronics. Without the battery in there, the alternator is putting out some serious voltage spikes up to 30-volts. I personally watched a self-professed mechanic try this "test" and fry two Ford ECMs in a row, claiming they must have been bad. The customer wasn't happy that he also fried the radio. When I open up the ECMs, the zener diode that regulates the voltage was fried and allowing the full voltage to the cpu which also had a big hole in it.
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Old 02-28-2019, 11:28 AM
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I dont remember where i got this, but...

"Your battery does more than just provide electricity. It also shorts AC spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in your car. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and even the car's stereo.

Even if your computers and stereo remain intact, in a great many cases removing the battery burns out the diodes in the alternator, necessitating a new alternator. If disconnecting the battery interferes with the voltage regulator's control voltage input, it's possible for the alternator voltage to go way over the top (I've heard some say hundreds of volts), frying everything.

Even the initial premise was wrong. If you disconnect the battery and the car conks out, you don't know if it conked out due to insufficient alternator current, or whether the resulting transients caused your ECU (the car's computer, which controls fuel mixture, timing, and much more) to spit out bad data, shutting down the car.

Nobody should EVER run your engine without a battery.

And yet when you tell them not to, they'll roll their eyes. "I'm a professional. I do this every day. It's fine!" They'll sound so authoritative. So commanding. So in charge. So intimidating. But they're wrong.

The problem, of course, is that disconnecting the battery doesn't always damage something. It does it only sometimes. Less experienced jump start professionals and automotive technicians figure if they got away with it a few times, it must be OK."

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Old 02-28-2019, 11:39 AM
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Provided nothing else is damaged:

Primary ignition (at coil) gets positive from ASD relay and a pulsing ground from the PCM via its 2 wire connector. This is the 12v side of the coil.
Secondary ignition (at coil) gets positive from the primary side and its ground comes from the engine block. This is the 15-20k volt side of the coil that drives the spark plugs.

So if ignition (spark) is dropping out, one explanation could be that the block finds ground when the battery is connected, but does not find ground when it is not. Set a test lamp to show the ground available at the block, then try your test again and see if you lose ground to the block.
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Old 02-28-2019, 11:41 AM
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IMO the ASD is shutting everything down because of a greater than expected voltage variation. You can check this by pulling the ASD and bypassing it with a paper clip. Disconnect the battery ground while running. If the ECU catches fire you'll know ASD was the culprit (or Savior, depending on how you look at it).
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Old 02-28-2019, 04:02 PM
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Originally Posted by MoparNJeep View Post
Voltage regulator is controlled by the PCM. It wont damage the PCM as the voltage stays where the computer tells it. That jeep dont care if there is a battery there as once its charged the PCM tells the alternator stay at constant 13-14volts. I've already tried another PCM so I know it's not damaged and I know that running it without cable attached wont damage it. I've jumped one before with no battery and drove it home when battery got destroyed. The jeep drove fine for years after without any PCM issues
As the regulation is controlled by the PCM, its possible the PCM may also shut down if runaway over voltage is detected ?

over voltage is generated by the alternator itself in that no load condition, its like an unbalanced power grid, the regulation will "try" to stop this

the sudden shock of dropping the main load will generate a transient spike every time, in that situation, electrical engineering 101

You could add a few additional earth wires, and check the + side

if I was you however, I would consider a Lithium jump starter, (just on a simple merit) and ease up on disconnecting the battery, as it is against good practice

I have noticed one of my spare XJ will not run properly with the very old battery thats in it...upon being jump started with Lithium power pack, it will not run unless the throttle is held at @2000rpm for a few minutes, presumably, until the battery has taken enough charge...starts & runs perfect with a good bat

Last edited by awg; 02-28-2019 at 04:06 PM.
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Old 02-28-2019, 05:25 PM
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Originally Posted by GreaseMonkey17 View Post
I canít tell if youíre trolling or if youíre serious ?
+1

Originally Posted by MoparNJeep View Post
Voltage regulator is controlled by the PCM. It wont damage the PCM as the voltage stays where the computer tells it. That jeep dont care if there is a battery there as once its charged the PCM tells the alternator stay at constant 13-14volts. I've already tried another PCM so I know it's not damaged and I know that running it without cable attached wont damage it.
Originally Posted by MoparNJeep View Post
Problem has nothing to do with me disconnecting the negative cable. It's just telling me I have a problem in that area I've cleaned all grounds and positive cables and even tried new cables so I'm baffled.
Originally Posted by MoparNJeep View Post
So most defintaly not because I disconnected the neg cable and it died instantly. There is no way that would cause damage. Other wise every time your alt went out it would kill the PCM. That's just not realistic. Yes some vehicles are sensitive to this but these jeeps are not.
Google " Munchausen by proxy "
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