'03 Sport V6 200K miles
New battery put in (after one died on me after 8 months) but battery light popped back on. Took to Oreillys and they said voltage regulator failed at about 12.8 after testing.
I've been reading around and some have said PCM? Should I buy a new alternator?
Anyone have any ideas on what my next step should be? Thanks in advance.
The KJs do not have a voltage regulator built into the alternator...they do have the diode pack inside the alternator which may cause problems and the brushes inside the alternator may be worn out.
There is a plug on the alternator with two small wires in it. This goes to the PCM/ECM and is the field coil for the alternator.
One side of this field coil effectively goes to +12 volts....the PCM monitors the positive voltage around about where the ignition switch is and accordingly switches the other side of the field coil to ground...using Pulse Width Modulation...in other words if the PCM decides that the voltage is too low...it increases the length of the pulse going down to ground to increase the current through the field coil and thus raise the voltage output to about 13.5 volts. If needs be the PCM will turn that wire completely to ground to get maximum output....you need an oscilloscope to see these pulses.
The thick wire that goes from the alternator to the positive side of the battery actually has a "fusable' link inside it which acts like a fuse...if you have a major short in this wire or someone puts in the battery the wrong way around then this fusable link blows. You can remove this wire and measure it with an Ohm-meter for continuity or you can put your voltmeter on this thick lead at the alternator side and you should read +12 volts there if the fusable link is OK.
So first make sure that your battery connections are clean and tight, the negative lead going to chassis is clean and tight and the grounding strap going from the engine to chassis is clean and tight and make sure the leads on the alternator are clean and tight.
With the engine off, you should measure about 12.5 volts across the battery. As you start the engine the voltage should fall no more than about 10 volts.
As the engine idles the voltage should run at about 13.5 volts. If not..you have to trace the wiring going to the PCM using a voltmeter.
Bear in mind that most early KJs have a battery temperature sensor sitting under the battery which adjusts the charge rate as the battery heats up. If you have this sensor sitting under the battery and it is faulty or has bad wiring then the same red battery light comes on....you need to pull codes out of the PCM to see what it is complaining about but I would not automatically replace the PCM.
You can download the 2003 Service Manual here..gives you a description of the charging system and has circuit diagrams:
Index of /manuals/Jeep/KJ
