Alternator replacement issues

[wysubey]

'94 EA82 Loyale. Removed alternator due to bearings failing. I drove it for a couple days while charging it at home. Installed the new Carquest reman alt, I have these lights on while running: charge, brake fluid & parking brake. Voltage isn't getting above 12.5 and alternator is putting out less than 10amps even at 3k rpm. When I pull fuse #9 (charging), all of these lights go out, obviously still not charging. Parking brake light behaves properly with the ebrake handle. Replaced all the warning bulbs. Replaced the diode in the LH warning lights cluster. 

Been reading this thread about alternator issues. Battery 12.6v. Voltages on the "L" connector are as follows. Engine off, key on: Blk/wht 12.5v, wht/red 1.05v. At the charge light: 12.2v, after charge light 1.17v, after diode, 1.07. If I disconnect the L connector from Alt and jump the wht/red pin directly to +12v, it charges just fine. Less than 1a running through that jumper so I'm not too concerned about that for a few minutes. Seems the end result of that thread was that the replacement alternators need >3v to charge. Ideas?

[wysubey]

Ground to the engine & alternator is good 0.1ohm

[Rampage]

2 hours ago, wysubey said:

Replaced all the warning bulbs. Replaced the diode in the LH warning lights cluster. 

You are  right about the 3 volts.

If the bulb you put in has a higher resistance than the old one then the voltage at the L terminal will be lower. The same goes for the diode (forward voltage drop). Different part numbers of diodes have different forward current and voltage drop characteristics.

Chances are the replacement alternator requires a lower resistance (higher wattage) bulb to make it work.

[naru2]

Chances are the replacement alternator has a shorted field winding.

If you have the 3 volts w/the old alternator hooked up electricly,then the new alt.is the problem.

[wysubey]

Ok, so I'll check into putting a bigger wattage bulb, or put a resistor in parallel with the bulb to reduce the impedance. The voltage drop of .1 for the replacement diode seems low enough, but its only a 1a rated rectifier diode. Should be enough, given that 12v to the L terminal is pulling 1a exactly. Is there a way to test the field winding without dismantling the alt? Unfortunately I traded in the original already. 

[Rampage]

4 hours ago, wysubey said:

Ok, so I'll check into putting a bigger wattage bulb, or put a resistor in parallel with the bulb to reduce the impedance

The only 1994 wiring diagram I could find is for an Impreza. It looks the same as our 95 Legacy. 

The plug on the alternator has 2 wires. The large one hooks to the wire on the alternator stud then goes to the fusible link then to the battery (hot all the time). The smaller wire goes to the batt light bulb. The other side of the bulb goes to fuse 15 to get battery voltage when the key is on.

When the alternator is charging there is an output on the small wire of the plug to keep the light bulb from lighting. If that voltage is not there the light will turn on. Battery voltage on one side low voltage on the other side.

I just checked our 95 Legacy. Engine at idle, the large wire is 14.2v and the small wire is 13v.

I don't know if they use the same bulb, but I just measured the resistance of the batt bulb on our 95 Legacy. It is 11.5 ohms and draws .104 A (104 ma) at 13.8 v.

I don't think it will make a difference but I guess you could try hooking up another bulb from the battery + to the small wire on the alt plug and see if the voltage changes.

This diagram does not show the fuse between the ignition switch and the light.

[wysubey]

Thanks Rampage! Very helpful diagram, I've been a bit fuzzy on what the pins on alternator actually do. So, I unplugged the 2pin connector. Alligator leads with a larger bulb to +12v. Still weak charging. At this point I'm taking the alt back for testing Monday or Tues. I'm getting 8v on the lamp wire at idle and less than 8amps from the alt, 12.5v at the battery with only headlights and HVAC fan on 2. Tired of futzing with this especially with the weather turned cold. I want my seat heater ASAP!

[Rampage]

Hopefully the replacement alt will work.

I am lucky in that I live about ten miles from a shop that repairs alternators and starters and they know me (I do computers). I took an alt in one time needing brushes. I had it apart and he soldered the new brushes in for me (which I could have done) and gave me the new bearings and I put them in. It cost me about 12 bucks.

Good luck.

[wysubey]

Success! The alternator was quite bad, tested only 10v at the store. They replaced it for free, very nice. New one charges right up to 14.2 and spins freely. Unfortunately the threads stripped on the lower mounting ear. The bolt was long enough so I just put a bolt on the backside. Pain to get to, but it works. Thanks Naru and Rampage.

[Rampage]

Cool. You got one that works. Thanks for letting us know.

To play it safe, I would keep an eye on the voltage. 14.2v is ok, but also the limit for most vehicle electrical components. I have seen a few new ones at 14.6v.

Think about this, a light bulb for an auto is rated at 12v and a certain wattage or Candle Power. Voltage times current = Wattage. But it is used at 13.8v or 14.2v and that shortens their life. That's one of the reasons you see so many newer vehicles with burned out headlights and parking lights.

A 12 volt lead acid battery is supposed to be charged at a constant 14 volts and the current drop off as it charges. If the voltage is higher it will slowly eat away the lead plates and cook out the acid. I learned that from GE many years ago.