So have you checked the voltage at idle in park? Could be what Yamae says, but you would think the auto parts place would be smart enough to figure that out. Could be Wiring issues, corrosion etc etc.
What is the battery voltage with the ground wire disconnected (shorted bad battery)? So you are saying when a rebuilt alternator is put in your car the car kills it?
Maybe the power wire has a bare spot and is shorting intermittently some place and causing a problem. Need to see what the alternator is putting out on a scope. You need to take it to someone that knows how to really check it out, or spend some money on the right tools and learn how to do it yourself.

 

My 96 behave the exact same way, at red light voltage will go down to 12.6v but will go back up when the car move. Replaced the old dead battery it had and its still behave this way...

My guess the "problem" is the way the car adjust the voltage regulator and not the alt itself.

 

Im trying to remember where I read it, but dont these have a clutch style alternator that will always test bad at idle but once under load it wont?? Something odd like that with these...

 

Is your alternators a genuine Denso's?
The left photo shows a re-manufactured Denso alternator with a non Denso regulator. On the other hand, the 2nd photo shows Denso regulator.

I sometimes find non-Denso alternators not charging while idling shifting the AT to D.

 

I took mine to a alternator/ starter repair place I use. Mine was not completely worn out with the brushes - so I just had the brushes replaced and they also checked the bearings. I did this when I fixed my leaking power steering problem and since the alternator was covered in oil and grease, I just decided to have it checked out. Cost me just $ 15.00 for new brushes and a check out. The brushes were about gone - so it was time to get them replaced while I had it out. BTW: I use Autozone. Bought a replacement starter there a couple months ago and it is working great! I have always had good results with them.

 

Ok Ill have to check that

One thing I discovered yesterday is that the problem was much worse after going into the car wash. Voltage was dropping to 11.8v (from the ODB port) while in park at normal idle RPM, I've rev the engine up to 2k but the max I was getting is 12.6v, no belt slipping I could hear...

Next step is to make sure the OBD reading is right compared to a DMM reading at the battrery. If it is, my alt is probably dead.

 

So I headed to the garage today since the problem is getting worse and asked if they could have it rebuilt. They sure did and charged me 300$ CAD for everything.

Regultator was shot (not denso), when they tested it, under load "heavy" load (headlight,wiper, heater fan...) it would die. Problem now is that they didn't had Denso regulator so the new one was also without the fins like the previous one.

Now, I've tried all accessories at the same time and voltage is now staying in the 13.9-14.3 range while driving or after startup, but at idle when hot (650 rpm) voltage is dropping again...

Ill go see them tomorrow and ask if they can get the denso regulator instead.

 

With a problem like this/these i would go back to the basics and start at the beginning of the diagnosis and start checking off everything methodically. I have seen owners and "lube-shop mechanics" make some pretty silly mistakes that have caused problems like this.

1. is the belt routing right and how is the tension on the belt and in the tensioner?
2. is the pigtail connector properly plugged in? Are the terminals on the connector corroded at all? What is the wiring like? Any wires loose on the back of the connector?
3. what the condition of the battery terminals, main power connection on the back of the ALT and the ground points on the chassis and engine?
4. what is the voltage at the battery with the car off, idle in park, 1800-2000rpm in park and idle in D,R,L or any other gears
5. what is the voltage at the back of the ALT compared to the above? but be careful testing the voltages at 1800-2000 in gear, you might get some road-rash or have a car fall on you it you do not support the car properly! haha




No, please stop talking.

 

Thanks but everything point the the regulator going into protect and taking some time to come back after I asked a bit more that just power the basic electric need. Amp meter on the alt wire was going up to 40amp just before dying.

Apparently the rebuild shop didn't replaced the regulator since they didn't had any in stock Monday and tested "good" after doing the rebuild... They received some yesterday afternoon and my mechanic installed the new one this morning. He just left me a message that telling me that the car is charging fine now at idle with all accessories on.

 

ok problem persist at low RPM, my mech will make calls tomorrow...

 

That's the typical symptom of a cheap regulator which doesn't provide enough current to the rotating coil. Denso's regulator has a well designed circuit that send the coil enough current at low rpm.

 

yup I believe it is, I've played a little bit with the car this morning and as soon the RPM drop under 800 the voltage drop. Me and my mech are trying to source a used factory alt locally, but no luck so far...

 

My mechanic found a rebuilted Denso with Denso regulator, will try that tomorrow morning.

 

No luck with that one too... We need to find what's the original pulley size and see if there's a mismatch.

 

Ok i've run the car without alternator to measure the load

66A@12.1V with headlight, fog, rear defrost and both seat heater. This is simple the stock 80A alt can't make 66A at idle... Most 2LS owner probably have the same issue if they use some power hungry accessories.

Now hunting for an alt that will give 100A at idle