New Alt not Charging Battery
 

Hello!
Alt tested low voltage, was warrantied and replaced and battery still not charging. Looked at the voltage regulator and it is new. What else can this be?
Thx!
quirks of the car: have to jiggle the key to get the heater blower to fire up and I pulled the trunk light as a precaution...

Where did you get the alternator from?

what year vehicle?

all MB alts have a trickle voltage (field) feed, and current feed.
key on, measure voltage to small wire, to case of alternator, and large wire (wires) to case on alternator.
report results.
did you have them test the replacement alternator in the store? I've had several bad in the box...

Check the ground strap as well.

Is this an SD made from 85 or older?

"Alternator tested at low Voltage"; was that an on the Car Test or was the Alternator removed and taken for a Free Test at an Auto Parts Place?

If the Test was done on the Car the Charged State of the Battery and the Wiring could influance the test.

When you first turn the Key all of your warning Lights including the Charging Light should go on. That tests to see if the Bulbs in the Warning Lights are OK.
If the Charging Lignt does not light up during this test the Bulb may be no good. Without that Bulb in the Circuit to excite charging your Alternator is not going to charge properly.

And, if your Iginition Switch is having issues that could als effect the operation of the Alternator.
Measure the Battery Voltage after the Car has sat awhile and not when it is running for the next stuff.
If your Alternator has the Connector/Plug with 3 Slots. The 2 Large Slots will have Battery Voltage all of the Time.
The single Smaller Slot connects to the Charging Light and the Ignition Switch. When you turn your Inition Switch on without cranking the Starter you should get slightly less than the Battery Voltage.

84' Tested on the car, wire to ignition tested good. Autoparts store initial test showed Low Voltage or 12.9 w load. Will check the batt bulb now.
Thx!

Ok, the bulbs appear to be fine...4 light up, batt, glow plug, alt,etc and a solid red light on the far right that I have not paid attention to before?
There is a gremlin that I'm not sure is related, the instrument cluster lights-including the heater/radio lights cut out intermittently.
Right, Alt is from a shop and had a lifetime warranty. Did both free test at parts store and tested the small and large wires before replacing the alt w the warranty. Too cold to test now...no shop space...ugh!
Thx!

I'd check the condition of your wiring - especially at the alternator.

Here is a simple check to see if alternator itself is good:

Check AC voltage across battery with engine running. I checked mine the other day and it was 24.9 VAC. Battery wasn't charging so it seems that my rectifiers were bad. Replaced alternator. This test does not tell you if Voltage Regulator is bad.

Ideal DC voltage should be between 13VDC and 14.5VDC across the battery. This is a pretty good Voltage Regulator test.

NEVER disconnect or connect battery cables from battery with the engine running.

I've bought rebuilds but alway put a Bosch regulator in.

In addition, make sure you have at least the negative battery cable disconnected when you hook the alternator up.

Regarding the VAC test across the battery terminals with the engine running, pulsed DC may read as VAC depending on your meter as another board member pointed out.

I'll have to verify my original assertion before I can again stand behind it 100%. Wish I had a good oscilloscope.

On my car, I was getting 12.75VDC across the battery engine running, so I switched to AC and got the 24.9VAC. For me, that was enough to diagnose the bad rectifiers.

The store clerk condemned my alternator. I ignored him. Glow plugs were pulling voltage down but battery still would not charge from the alternator. I couldn't find signs of a parasitic drain so kept looking at charging system. Battery tested good.

I found a thread where I could solder a resistor into the regulator to increase charging voltage. That worked. Now voltage was too high but it confirmed that alternator was capable of exceeding proper output. (I posted the link but don't remember details.)

After at least 2 bad out of the box Bosch and other regulators, I ended up with another Bosch that is still working. Battery and wiring need to be in good condition also.

Thanks for the help trouble shooting!
I cleaned the alt connections w toothpick, it had some corrosion, then used some sandpaper on the slots and plugged in and removed the connection a few times.
No change. I have a new battery.
Have to get ramps to trace down the ground to the frame, to wire brush it. Is it always in the same spot?
I'm trying to get the alt warrentied, the shop didn't bench test it before installation. If I can't get them to stand by their work, I'm considering an upgrade to 65amp alt. any exp w that?
Thx!