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help: 300sd alternator

Hi Folks,

Battery in a 1982 300SD died the other day, so I took a multi-meter to check whether there was any short in the circut. It turned out that the alternator was partially shortted: the resistance between the two battery cable poles was in the teens with the battery disconnected and alternator connected, the resistance with the alternator disconnected was infinity.

So I took the alternator out and studied how it should work. The diode pack seems to be OK(resistance around 7 ohms one way, infinity the other way). The voltage regulator seemed to be working: when I applied

12 volts to the regulator, there was currect between the two brushes; when I appied 14 volts to the regulator, there was no currect between the two brushes.

The voltage regulator only has a over all resistance of 7 or 10 ohms, since there is a resistor on the regulator that said 68 ohms on it, I assume it should be this way.

The problem is, the positive pole of the battery is always connected to the volgage regulator positive pole(resistance is zero), so there is always a drain through the voltage regulator(resistance at most 68 ohms), if my assumptions are correct so far, there must be some switch to shut off the voltage regulator, where is this switch then? Ignition switch?

Help is greatly appreciated, thanks in advance!

Reply to
jingyuanw
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You need to look at a schematic which will probably show that you are correct.

Reply to
calcerise

There isn't any switch to disconnect the voltage regulator and alternator. These are built with diodes, so that the current only goes one way, ie to charge the battery, not discharge it.

Reply to
trader4

Thank you all for the reply.

The puzzle is partly solved. There is a short in the radio/hazardLight circuit that somehow "shorted" the regulator. The regulator actually is controled by the ignition switch. Any ideas what is the cause of the short in the radio/hazardLight circuit? I used the hzard lights for more than 1 hour before this problem occured, can it be the cause?

Should not have taken the alternator out at the first place(stupid me)! Now I have broken the 68 ohm resistor on the voltage regulator, where can I get a replacement?

Wasted too much time on this car, but it is running too well to be thrown away!

Reply to
jingyuanw

Check any legit electronics store.

Have you considered a Delco upgrade? It demands a little mechanical fabrication ability but onec done you get twice the juice and replacement alternators are much cheaper.

Reply to
calcerise

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