So I changed the water pump and thermostat on my 8 after 2 months of sitting and fired it up for the first time, started no problem first bump of the key, but I had the battery light come on.

I have an Optima yellow top freshly charged on the trickle charger all winter. Tested good 12V at about 600CCA. The battery was good so I figured it could be the alternator.

I changed tension on the belt hoping that could be the problem, no dice. Still battery light, checked the connections and still battery light. So I picked up a low mileage alternator from someone with a wrecked 8 locally that was parting out.

Swapped in the alternator and still have the battery light followed by check engine light and I scanned it and came back with P0131.

Now Im wondering what else it could be, hoping its not the alternator I just bought. I tested voltage at idle at the battery posts and it reads about 11.4V. So I'm not 100% sure if thats the correct way to test alternator output since Im a noob at electrical systems.

But that looks like its not giving a charge to the battery because I do know the alternator is supposed to give anywhere from 13-14V to the battery.

Also the check engine light indicates low voltage at the front o2 sensor so I'm not sure if thats whats causing the battery light or is the alternator I just bought bad. Anyone have this problem?

EDIT: Not sure if this is relevant but when doing the thermostat I did unplug the wires/harnesses to the PCM and moved them out of the way so I could reach the thermostat easier. Not sure if its a PCM related problem causing my battery light too.

 

The alternator is controlled by the ECU/PCM. The battery light generally indicates a charging system fault. Why did you replace it without testing it? Any alternator shop can test it.

 

Engine ------> Charging system ------> Generator inspection.

M A Z D A

I assume you disconnected the battery when you changed the alternator right?

 

Ok so I followed that and tested and its output at the battery is 11.9V. So the battery isn't getting charged. And yes I disconnected the battery and removed it when I installed the alternator, also tested it the simple and stupid way, disconnecting the negative while the car is idling. It cut right off.

My problem is, would the P0131 cel be in any way related to this problem, or could it be any problem from disconnecting the PCM thats causing this issue of no charge getting to thet battery, maybe a wiring related issue or is it just unlucky that I have 2 bad alternators.

 

You need to take the alternators to get bench tested. Once you know they are good then you need to look at other possibilities.

 

Yeah this weekend Ill take both of them to a local auto electric place to get bench tested. Testing them on the car only told me the battery isn't taking a charge from them.

I'm not entirely sure its not a wiring issue I'm having. And I'm pretty much a blind man when you put a multi meter in my hands. Can't do electrical for the life of me.

 

Well throwing parts at it is not the right way. Try and go to an alternator rebuilding shop if you can. they will be able to tell you for sure if the alternators are bad or not. The odds of you having two bad ones are rare but trust me it happens.

 

Do you have an aftermarket pulley kit installed?

 

No I have stock pulleys. This problem only came about after I changed the water pump and thermostat.

The car has been sitting for about 2 months with the battery in my basement on and off the trickle charger. After the change on the first start I noticed the battery light.

 

I went thru two Optimas in two years, my father went thru a similar fiasco with a Yellow Top on his Jeep. I went with an Odyssey and never looked back. Optima was apparently sold at some point and quality went to ****. That said, if you load tested it then it should be fine.

 

It was implied earlier, but not stated clearly.

Was the battery checked with a proper load tester?

 

It was tested at the local advance auto parts after I charged it up. I think they just use a basic digital tester, not too sure if it tests load.

 

Hmm, they should load test it, the tester they use should do it automatically, they just have to enter the battery specs. But I would not assume, night want to call and ask.

 

I've seen this failure with a bad connection to the battery. If the ECU detects a fault (probably from voltage spikes/droops) it will cut the alternator voltage down to around 12 volts and give the battery light. Check the terminals/connectors to make sure you have good connections. When it happened to me the car would start just fine, but have issues after that.

 

Yes it could definitely be a connection issue, always check the obvious stuff first. I also know from personal experience that the car will die and you will get a battery light if the ground from the lower intake manifold to the chassis is not hooked up,

 

Well I took both alternators to a reputable local auto electric place, where all they do is strictly rebuild alternators and starters, no services, no oil changes, etc. Just alternator and starter rebuilds. They tested both alternators and they both came back good, BOTH.

Installed the alternator back in my car along with the optima freshly recharged again overnight. Connections all are snug, grounds are tight all around. I even cleaned my MAF sensor. And belt tension checked over and same thing.

Battery light and P0131. Checked voltage while the car is off and at idle and battery is reading ~12V both off and at idle so no charge from the alternator again.

So I'm baffled at this point unless its a PCM related issue. But I took the car on a short test drive this morning after I confirmed the alternator is good and re-installed it.

And everything seems fine. Cars shifts thru all gears and revs thru the rpm range fine. No limp mode nothing out of the ordinary while driving, except for it running directly off of the battery.

I didn't turn on any accessories besides wipers because it was raining here this morning and they ran unusually slow. So any suggestions?

I would suspect from sitting for maybe 2 months maybe animals might have chewed the harness somewhere along, but the harness seems fine. Nothing out of the unusual and no rips or tears or frayed wires. No sign of animals.

 

Yeah I would remove the ECU cover, make sure everything is good there and thru to the alternator.

 

It should be kicking out a P2502 or P2503 code, so it seems like a wiring, connector, or pcm issue

 

Pulled all the PCM connectors and looked at them and they all seem to look ok, no burns or corrosion. I plugged them back in until they clicked and all the connectors are snug.

I also pulled the UIM to follow the alternator harness and the wiring seems to look ok as well no shorts or breaks in the harness.

But same problem battery light and no charge going to the battery from my alternator. So I'm baffled. I think tomorrow I'll swap batteries with my dads 24F series battery in his Pathfinder just to see if I get the same thing with a different battery.

But any one know any ways to test the PCM for problems?

 

Did you check all the wires for continuity?

 

Actually I didn't do that. I'll check that tomorrow.

 

So I ran a continuity test with the multimeter and found something interesting.

The alternator positive ring terminal has perfect continuity. Beeps for continuity at the battery terminal so there's power from that connection going to the battery uninterrupted.

The I went to the connector in harness and tested continuity of the pins. Stuck the prong in the negative pin and then touched the negative battery terminal and beeps for continuity along with all the other chassis grounding points, also beep for continuity.

So it's not a grounding issue. But when I stick the prong in the positive pin and touch the positive battery terminal I get nothing, no beep for continuity.

This pin right here for this connector. Assuming that is the positive because the other pin beeps for negative continuity at all the grounding points along with the negative battery terminal.


Those are the two wires right there, negative and positive and they don't look to have any problems with the connection to the connector. Would it be the connector that is bad?

 

Could be, you need to "depin" it and see if you see anything.

 

Actually just found this and ordered it, if its the connector thats my problem I might be better off replacing it and for $14, can't hurt replacing.

http://www.ebay.com/itm/MITSUBISHI-A...e25cb8&vxp=mtr

I'm doubting its a break in the wiring because the harness is pretty tightly wrapped and I can barely access it.

If it was to corrode or short out I can't see just the single alternator positive wire going bad without shorting out the rest of the wires in the harness.

I'll give replacing the connector a try first.

 

I think it's working like it should from that end. The positive lead is what the ECU controls for field strength to control output voltage. At least that's from my limited understanding. I still think it's a connection issue, either the battery or grounds somewhere.