Strange charging/electrical problem - Page 2 - PeachParts Mercedes-Benz Forum

boneheaddoctor · #16 10-20-2004, 12:05 PM

Quote:

Originally Posted by GregS

I tested my car last night according to "Old deis'" suggested method, and here is what I got:

1) first test with engine idling and no accessories on - 14.01v;

2) turn on radio, headlights, and heater fan - drops to 13.95v;

3) turn on high beams, defroster, and a/c - drops to 13.65 (engine still at idle, and volts increase a little when the idle is increased (to about 13.68v or 13.7v));

4) turn off all accessories and test again at idle - back up to 13.9v and 13.95v, goes to 14v when I rev up the engine.

O.K. This does not sound like a bad alternator does it? There is serious amp draw when all these accessores are on, and with such good voltage readings I just don't feel comfortable accusing the alternator. Could I still have an amp output problem with such good voltage readings? On the other hand, I still don't know why my battery would be weak at times (reading as low as 12.24v when the engine was off), or whey my starter won't always engage. Is the starter not engaging because the battery is low (from a charging problem), or is the battery being drawn low because of a worn starter?

GregS

You could still have a bad alternator.......I had an intermittant regulator, that did everything from working fine, to WAY , WAY overcharging (17.9Volts running under load with accesories on) to discharging the battery overnight (4 times in one year) Don't assume because it works now its fine. Requlators are cheap. Get one to put in glove box if you don't put it in right away.

Voltmeter is all you need to determine charging system health.

If the Charging current drops, your voltage will drop due to load. If you are driving along with lights on , stereo blasting and its around 13.8 volts you can bet the bank its not discharging.

psfred · #17 10-20-2004, 06:57 PM

you must check the actual output -- bad diodes will give proper voltage up to when the load exceeds the output when voltage will drop.

You also need to check cranking voltage (must be higher than 9 V) and starter current draw.

Finally, you must be aware that you CAN run the battery down making large number of start and stop trips if the engine does not start instantly warm. If you have to crank 10 or 15 sec each time, it won't recharge in a mile or so, and eventually the battery will go flat.

Failure to engage the starter motor is either a bad solenoid or a tired neutral safetly switch -- this can also be missing viton bushings on the shift linkage so that the switch isn't always engaged in Park. That is an easy fix, anyway.

If it cranks slow and the headlights dim quite a bit while cranking, it's too much current draw and you need a new starter.

Peter