I have a 98 Jeep grand cherokee 4.0, brand new battery, alternator is good, could it be the voltage regulator in the PCM?

Disconnect the battery at night and see if it starts in the morning. If it doesn't, then it's the battery. If it does, the it's an off draw. But please don't disconnect the battery with the engine running! This causes the regulator to full field the alternator and it will put out full voltage! And that could burn out bulbs, computers, and the radio depending on what type of alternator you have!

every one is right its sounds like a parasitic drain did you install a new stereo? that is where a lot of people mess up non factory parts not meant for that specific make and model. an it's a JEEP not a truck. Trucks fish tail in the rear end on corners role over and get stuck like a turtle on its back. Jeep role over and land on their wheels and if they don't it's easy with a couple of buds to rock it back over and keep having fun. and don't order a updated reprogrammed pcm from ebay. pull on at a junk yard pull a part got them for like 35 bucks cant go wrong with that. Disconnect your battery turn your headlights on press your brakes a few time and switch that pcm out reconnect your battery terminals black negative then red positive do the key dance or run the pcm with a odb2 and check for errors then start her up. if it fixes it your out 35 buck if not follow the instruction again like before and put the old one back in and get your money back or keep the pcm and sell it on ebay for 50 buck you made your money back and gas maybe a six pack and a sack never know.

If it starts fine and runs fine but dies overnight then you have a draw.

The voltage regulator is inside the alternator which no doubt you ruined by disconnecting the battery with the engine running!

Always test the alternator out-put with an inexpensive VOLT meter set @ 20 volts then hook the leads to the battery.

When you disconnected the battery from the charging system, alternator voltage had no-place to go.

Take it to a mechanic.

How are you testing parasitic draw?...some methods may not show draws as well or a timer is not shutting off for key off RAP power.....see links below. Check your charging voltage while the truck is running/driving...if it's low or zero...should read about 13.5 volts or higher up to 15.5 volts depending on load factors, engine speeds, etc... Check for a blown fusible link wire in the charging circuit wiring between the alt and battery...the short section of fusible link wire may be blown. Perhaps install a bypass kit which bypasses the voltage reg control of the PCM if needed.

https://alternatorparts.com/external-voltage-regul...

https://www.cargurus.com/Cars/Discussion-c1715_ds7...

Charging circuit wiring diagram...

http://arrc.ebscohost.com/searches?car_uuid=131596...

Draw tests...

https://www.youtube.com/watch?v=KF1gijj03_0

https://www.youtube.com/watch?v=P-wxG6U5TuY

NUMBER ONE! Removing the positive or negative battery cable while the motor is running is a very good way to fry an alternator. It puts the alternator into maximum output and over loads it.

Check the fusible links in the engine compartment fuse box. They are the larger fuses and one of them blows to protect the alternator and electrical system when idiots jump the battery backwards.

Use a volt meter to test voltage at the battery posts. With the motor running you need 13.7 volts or more or the alternator isn't working.

Get the alternator tested an auto parts store. As others said, I would not do the battery cable removal test you did.

If the battery was not charging then the car would die after driving for I some time, maybe 30 minutes.

I think you need to go back to the parasitic test.

Look up one on you tube by Eric the Car Guy.

If something kills the battery overnight then there is a parasitic draw. How many amps did you measure during the parasitic test? That vehicle working properly has a parasitic draw so zero is not the answer. Did you wiggle, bang on things to see if draw would appear? Did you lock the doors to see if the draw appeared?

Disconnecting the battery cable on a Chrysler with regulator inside pcm is a good way to destroy the pcm, Big no no. Doing that battery cable pull tells you what about the alternator? That it can produce it's full amperage? That it's diodes are good? That is almost pointless to do.

Never remove the battery terminals when the engine is running. Perform a true alternator test by measuring the voltage across the battery when the engine is running above idle speed. Readings under 13.8 volts means the alternator is bad.

Perform a parasitic drain test by disconnecting the negative terminal and measuring the current from the negative terminal to the negative cable with all all electrical circuits off. Any reading over 50 milliamps means a battery drain.