Voltage Output - No Load/Full Load - PeachParts Mercedes-Benz Forum

BrierS · #1 10-07-2004, 04:31 PM

My '87 300TD . . . though I will be replacing the battery and probably regulator brushes prior to Winter, I have done some testing. Comments appreciated.

1) Fully charged charging system tests normal

2) Put a 750 +/- amp load on and battery still showed marginally okay (green vs. red with my load tester).

3) At correct idle speed with nothing on charging system is at 13.4 volts

4) Same as above only all lights, heater fan, rear window defroster charging system is at 12.71 volts

5) Increase to fast idle and voltage increase to just over 13

6) Power seat relay has been remaining HOT and the battery drains

7) Pulled the above relay and waiting to see if battery still drains

TwitchKitty · #2 10-07-2004, 08:53 PM

You can check for drains by pulling a fuse and jumping across the contacts with an amp meter.

Same by disconnecting a battery terminal, jump across, read amps.

So on down the line. Pull a relay check for amps, etc.

Your voltage readings with the engine running sound a little on the low side.

BrierS · #3 10-08-2004, 07:58 AM

Phase one . . . '87 300TD started good this morning. I had pulled the power seat relay since it has been remaining hot. As those familiar with its location know, it is difficult to reach that deep into the holder to get a reading on amp draw (I will probably have to remove the relay holder eventually). At least I know I have isolated the drain on the battery.

Now I need to find the source of the power drain. The schematic helps but my guess is I'll need to pull the door panels or . . . at the seat under the mat???

TonyFromWestOz · #4 10-08-2004, 07:59 AM

Alternator output should rise to 15.0v after starting. This should only happen after the battery is recharged, so on a flat battery it may be some time.
After the alternator has hit the 15.0V mark, it should drop to 13.6 - 14.0v.

The battery voltage should be at 13.6 - 14.0 v at idle, with no accessories or lights on. Even with these on, the battery voltage should be at 13.6 - 14.0 with the engine doing >1K RPM.