How to find out if your alternator konked out?
 

Having this problem where if my car don't start for 2 days, i will need to jumpstart it... happen for 3 weekends in a row...

Changed the battery to AC Delco MF but still same problem... switch to another AC Delco batt but problem still persists....

However, when car starts, charging show 13.0 Volt .... sometimes when idle at traffic light.. drops to like 12.2 Volt .. and even once to 11.5 Volt ....

And when engine is off, its about 12.4V only ... is this a sign that my alternator is a goner? pls help ... problem still unsolved...

Thought it was battery drain, checked alarm, audio, etc .... can't trace ...

Only things that i have changed before this problem :

1. Single fuel pump to Double fuel pump.
2. Original 80A alternator changed to a used 120A alternator.

Put a digital voltmeter across the battery and you should get at least 13.4 volts when you rev it up. If you do, then use the digital meter in the ammeter setting in series with one of the battery cables and the battery. With EVERYTHING turned off, you should see less than a hundred milliamps of current draw.

Good luck,

It could also be the voltage regulator.

And check the ground wire between the engine block and the body.

To Jim Anderson's point.

The volt. reg. is underneath the alt. 2 phillips screws are removed and out it comes. Slip in a new one and reinstall screws. I say this assuming it's the problem. New VRs have brushes that are 1/2 inch long. Yours may be much shorter. If they are, try one of the $15 German VRs that you can get from this site. They used to be about $15...maybe they gone up a few bucks.

It takes longer to setup for this than to actually do it. A really easy DIY job.

larry,
being a noob about mercs w124, whats the difference between voltage and amperage, i understand that a high voltage does not mean it is having a proper amperage, why is this?

when i rev heavily, normally the charging do not surpass 13.0V actually.... :(

Jim & 1991300SEL,
did not know that a w124 has a voltage regulator beneath the alternator, will do a quick search on this ..... :bulb:

Grounding checked, btw.. is it true that the fuel pump electrical wiring is linked to a relay? could this be the problem because i was previously using a single pump, now that its a double pump, need i change the relay accordingly? cheers for the leads... :)

To begin with the voltage regulator is not underneath the alternator, it is on the REAR of the alternator. If you are only getting 13 volts across the battery, I would click on the Fastlane above and just get a voltage regulator. The last time I bought one from them it was $14 and change and has lasted a long time.

This regulator has an electronic voltage regulator and the brush pack. When the brushes wear out, it usually stops charging altogether, but low voltage could very easily be caused by a bad regulator. Before replacing it, make sure all connections including ground straps are tight.

As far as the difference between Voltage and Amperage, that is a question about electricity, not MB's. Voltage is the amount of Electromotive Force that is available to push current through the circuit. Amperage is the measure of current flow. The amount of current flow (Amperage) will increase as the Voltage increases IF the load (resistance) remains the same. In the case of an automotive electrical system, the load is always changing. That's why a voltage regulator is needed to adjust the voltage applied from the alternator to the battery to match the load.

The alternator load is changed by the different accessories that are operational and the state of battery charge.

Since Voltage is the measurement of electromotive force, you measure across the battery with the voltmeter, while Amperage, being current FLOW is measured with the ammeter in SERIES with the circuit.

An oversimplified way to think about Voltage and Current is to think about a water hose. The pressure at the faucet is like Voltage. The gallons per minute through the hose is like Current and the size of the hose is like Resistance. If the hose stays the same diameter, the more you turn on the faucet, the more pressure you apply to the hose thus you get more water flow. If you put on a smaller hose with the same faucet setting you will get less flow.

Even though Ohms law is quite simple to understand, troubleshooting the electrical systems of a car is not that easy. But it certainly is not easy if you don't understand Ohms law.

Hope this helps,

larry,

excellent metaphore used to explain amperage and voltage.. couldn't be better than this... :)

my mech also says that the voltage regulator brush is actually located on the alternator itself.... definitely takin' it out and changing it is gonna be a pain ....

however, the is still a current leak... my mech disconnected the batt terminals when the car engine is dead and stationary... and put through a simple wire with a Bulb to it between the connectors and the batt terminals, '-ve' side of it i think, and the Bulb lights up very brightly.. so there's something which is draining the batt heavily.... errrmmm... any ideas?

ohh .. btw the way, i failed to locate the Overvoltage relay, checked the ECU compartment and did not find any relay, checked other forums that it is silver in colour, but couldn't find any.... :(