two days ago i went to start my car and surprisingly it wouldnt. i then jumped it and charged it and began checking the system. so far this is what i have checked: battery charges fine(on a charger), the brushes on the voltage regulator are good, i tested the alternator: it passed, the drivebelt tension is good, the wires on the alternator are good, the battery wires are good....so i did a volt meter test and at idle it stays at 11.98 volts and at 3500 rpm it goes to 12.35 volts, i know that is no where near where it is supposed to be but i cant seem to find what the problem is. any help would apperciated .

 

how could i test the voltage regulator? ....to be sure that it is what is causing the problem

 

FYI: Longer answer

A 12 volt battery needs to charge at more than 12 volts. A good range would be from 13.8 (minimum) to 15 (maximum). A 12 volt battery will not charge at all unless it is exposed to a higher voltage than 12 volts. Anything below 13.8 I would say is going bad, but not quite dead yet. This is true for any battery. A 3.6v battery for your cellphone will usually charge at 7.2volts or somewhere near that for example.

If your charging system is not exceeding 12.9 volts, especially right after starting, the regulator is bad.

Good news is, you only have to replace the regulator. 2 screws on the back of the alternator. Don't replace the whole alternator unless the bearings or something are bad.

Hope this helps.

ttfn

 

You did not say if your No Charging Warning Light was glowing red when your Engine was running.

If the Bulb in your No Charging Warning light is burned out or there is an open circuit in the Wiring your Alternator will not charge; even with a good Voltage Regulator.

To check it turn your Ignition Key to the first position and all of your Warning Lights should light up including the No Charging Light. If they all do the Bulbs are good and a bad bulb is not causing the no charging of your Alternator.

To test the Wiring remove tha Contector Plug at the Alternator. You should see 2 large Female slots. If you probe them with your Voltmeter and the other probe connected to ground you should get the same Voltage as the Battery even with the Key off.

There is 1 small slot on the Connector. That is the one that goes to the No Charging Lignt. With the key off you should get no Voltage there. When you turn the key on (do not run the Engine) you should get just slightly less than the Battery Voltage.
Re-install the Alternator connector.

To see if your Battery or Chassie to Engine Ground Strap/Cables are the problem; while the Engine is running get one Jumper Cable and connect one end to the negative Terminal of the Battery and take the other end of the Jumper Cable and rub it or clip it on the meatal of the Alternator Housing. If it starts to charge agin you have a bad Ground connection.

 

Of courese it could be a bad Regulator.

But, since has a Voltmeter the checks that I mentioned would not cost anything.

He has a choice to trouble shoot by replacing parts or you can try to find out what is wrong and fix that.

No one has mentioned that he could also pull off his Alternator and have it Bench Tested for free. Since the wiring setup on the Bench Tester is likely to be in good order that would remove the wiring as a source of the problem if the Alternator tested no good and somewhat target the Regulator as a prime suspect.

 

hi, i have now done more test to find out what is wrong....i cut open part of my wiring harness and found the blue tester wire from the back of the alternator i checked it with the volt meter and reads 13.8 volts, meaning the regulator is good.by the way my battery light works fine. i then found the same blue wire on the back of the instrument cluster there it reads 11.67 volts....i am now suspecting a break in the wire or something of that nature, also just for kicks i checked the back of my fuse box and everything was fine no melting or burning and i replaced all the fuses, just to eliminate that possiblity and it was pretty cheap. i dont know if anyone else knows if there is anything else that could be causing this mess, but if you do PLEASE lemme know...

 

I have never checked the Voltage of the Blue Wire while the Engine was runing.

When you were getting the 11.67 Volts was the Engine Running and was the No Charging Light lit?

 

the car was running and no lights were on

 

I have a 96 e300d, and my alternator is set up a little different than yours, but I had similar results. At 2500 rpm, my voltage reading was between 12 and 12.5. Leave the car off for 3-5 hours, and the battery goes dead. Disconnect the battery, recharge using a charger, and the battery holds the charge, and starts the car with no problem. Decided to check the alternator first, and I disconnected the large wire going to the battery from the alternator post. Put my multi tester in continuity mode, and with the engine and ignition in the off position, I thouched the alternator battery post and the alternator case. I had continuity, which meant that one or more of the diodes had failed. It is an easy test to do, and will eliminate that end of the equation. Good luck!!

 

The light will usually still go off if it is a diode failure in the voltage regulator. Make sure all your grounds are good and it really sounds like you need a new voltage regulator.