Battery light on but alternator fine? [Archive] - Jeep Patriot Forums

: Battery light on but alternator fine?


jack4012
10-26-2012, 02:15 PM
Hi All,
Hoping for some help here. I have a 2008 Patriot 2.0 CRD and the battery light came on and stayed on whilst driving the other day. After checking all connections to alternator and battery and found them to be fine I concluded it must be a faulty alternator. I have now changed the alternator to a brand new one but the problem remains. Battery light on whilst engine running, and the battery voltage does not increase from engine off to engine on which I believe it should show an increase of about 2 volts. A friend suggested the voltage regulator might be faulty but having looked this up, it is an internal part of the PCM (power control mudule) and the whole PCM would need to be replaced and reprogrammed to my vehicle but I don't want to go down that route yet as it sounds very expensive. Has any one else out there had any experience of this?
Any ideas would be most welcome! Thanks.

jack.allwardt
10-26-2012, 07:10 PM
Not knowing for sure, but...most alternators today have an internal regulator built into the brush ass'y. So, if you get 12V (+/-) with engine shut off, then with a properly operating alternator, you should see around 14V--maybe as much as 14.8. Check the output right at the alternator plug. I don't believe you have TWO bad alternators...let us know!

jack4012
10-27-2012, 08:53 AM
Thanks for the reply Jack. The output on the alternator is showing just 4 volts which is obviously 10 or so volts short. The multiplug that plugs into the alternator is showing 12v so is providing power to the alternator. With the belt working perfectly am I right in thinking it simply has to be another faulty alternator?

ti-claude31
10-27-2012, 01:06 PM
have you tested your battery? the voltage when fully charge should be about 14.2 v and the load test should read no less that 10 v .

todde702
10-28-2012, 10:39 AM
Actually a fully charged 12v battery will show 12.6 volts (there are six 2 volt cells in a car battery and they can charge to 2.1 volts each when fully charged). Most mechanics will want to see a minimum of 13.6-13.8 volts of output from the alternator. This would be the minimum to run everything and keep the battery charged.

I'd double check any and all engine ground cables and the battery terminals for tightness and good connection, then I'd say you got another bad alternator. It happens from time to time.

jack4012
10-29-2012, 12:38 PM
Hi guys, this is now SOLVED. For anyone else with a similar experience, I have found that even though there was correct power to the alternator, fully functioning alternator/ belts and inadequate voltage coming back from the alternator output doesn't neccessarily mean the alternator is at fault. The charging system is wider than this and also includes the starter motor. Having found this out and removing the protective undertray to the engine I found a cable broken away from the starter motor which obviously plays a part in obtaining the correct output from the alternator. A simple new connector and refit to the starter motor cured the problem immediately. Hope this helps someone else out there.

jack.allwardt
10-29-2012, 05:05 PM
Could this broken wire been a ground to the alternator? That would make sense...

todde702
10-31-2012, 01:50 PM
Could this broken wire been a ground to the alternator? That would make sense...

More then likely the hot 12v feed through to the battery.

FluffysPatriot
10-02-2014, 02:18 AM
As Jack found, I had exact same issue and had already replaced alternator. It's a poor rated eyelet to starter motor. Mine broke too and I have upgraded to a higher csa cable. For advice, check with multimeter set to 12 or 24vdc and test to B+ stud on back of alt to battery then classic ground. With no electricity loads you should get 14.5 v. If it's at battery voltage, alternator failed. If different then broken cable. If varies to ground then maybe lost ground to engine. Also check pcm and control Twin wires not open circuit. On the Jeep, I would say order of failure probability is 1). Broken cable at starter, 2). Failure alternator (regulator) 3). Failed gnd or control wires 4). Failed pcm 5). Failed battery cell dragging the system down (if you see 10.4 v). Hope this helps

FluffysPatriot
10-02-2014, 02:21 AM
Ah! That's chassis gnd not classic. Spellchecker. Grrrrr