I have a Mercedes 124 estate, 230 TE. Built in 1989.
The alternator has recently been replaced and seems to be functioning okay. However the battery loses charge when running with the lights on; and eventually the car fails to run. (Not very convenient during these short winter days!) With the lights off the battery seems to get enough charge. Using a clamp meter I have measured the alternator output at 25 amps when the engine idles. The battery is not 'seeing this' and only shows a flow of about 2 or 3 amps. This changes to 'minus' 10 amps when the lights are switched on. So it seems there is a 25 amp drain on the alternator; but only when the engine is running. Removing various fuses one by one doesn't make any difference. The traction control is in fault and I have removed the controller; this makes no difference to the current draw. The cruise control hasn't worked for the last 10 years. The battery is 3 years old, Bosch Silver, but seems okay. The ignition coil is as 'old as the hills', but doesn't seem to be drawing large currents.
Does anyone have some advice?
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Have you checked the voltage at the battery with the engine running? Could it be a faulty alternator?
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The electrical systems on these are fairly straightforward and reliable.There are a few simple tests that may pin-point the fault.Check the poly-vee alternator drive belt for worn ribs and cracking.Check the voltage at the alternator output terminal and compare this with the voltage at the battery.A difference of more than 0.5 volt could indicate a high resitance in the power supply wire.If the alternator has a 3-pin connector,then you will need to unclip the back part of the plastic connector to access the terminals.hth
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Dear Simon and elekie&a/c doctor,
Thank you for your prompt replies.
The poly vee belt is brand new, as is the alternator (Bosch).
The voltage across the battery with the engine off is 12.75V (measured using a regular wired multimeter).
With the engine running the voltage is 13.95V across the battery. The voltage between the alternator power supply cable and earth is 13.92V; note this is measured at the terminal block just in front of the battery. I flipped the back off the alternator 3 pin connector to try and measure the voltage. I got non sense readings of about 100mV (DC) between either of the two thicker wires and the thinner one. I tried to measure the resistance of the cables and the meter showed about 0 to 0.1 Ohm for each of the heavier cables.
Should I be reading 13.95V DC between either of the thick wires and the thin one on the alternator terminal block?
Edited by Senecio on 05/01/2009 at 15:16
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The 2 large wires (power supply) are connected together internally within the alternator so are both the same voltage point.The thin wire is the alternator charge light circuit.From the readings you have given ,the charge rate seems to be within spec.Readings are taken from supply wire (large) to earth.hth
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Although the battery is 3 years old, has it been on this car for 3 years?
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Dear Elekie&a/c doctor and Number_Cruncher,
Thanks for your further comments.
Okay; I have measured the voltage between the alternator heavy wires and earth and I have a reading of 13.70V (for each wire). The voltage at that time across the battery was 13.60V; so the losses are not much. (I am actualy disappointed by this; the notion of there being a chronic power loss between the alternator and terminal block would fit the symptons of the problem perfectly.) I am still measuring 25, or so, amps from the alternator.
The battery has been on this vehicle for the full 3 years. It has been charged using a 1 to 3 amp battery charger from time to time in recent weeks. The reason I haven't replaced the battery is the understanding that once the engine is running the alternator should be taking care of the electrical needs of the vehicle and current should not flow from the battery. Current does flow from the battery when I switch on the lights when the engine is running.
Any further thoughts?
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I'm a bit concerned that you aren't ever going to charge a silver battery with under 14v from the alternator - does the charging voltage rise when you rev it? Typically, you should check charging voltage at 3000 rpm rather than at tick-over.
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At 3000 RPM the voltage across the battery is 14.07V
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Looks like your battery could be in trouble.Get it tested by someone who knows what they are doing.
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Dear Elekie&a/c doctor & Number Cruncher,
If a new battery will solve the problem I am willing to gamble the cost. At the worst this will rule out the battery being a problem. I am away for the next few days so it will be a while before I can report the outcome; but I will let you know how I get on.
Thanks again, you've been most helpful.
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Dear Elekie&a/c doctor,
In your post from Jan 5th at 16:07 you refer to the 'light circuit'. Do you mean 'light weight duty' or 'circuit for the lighting'? I have not traced the thin wire to the terminal block so have no measurement of the voltage at this point. If this circuit is powering the lighting then I have a strong suspicion it is in fault as the battery does hold charge when the lights are not on, but loses charge when the are lit. WIll make further measurements on Thursday.
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Just to clarify the situation;the thin wire to the alternator (in the 3-pin plug )is the signal wire for the battery warning light in the instrument pod(red).It is needed to initiate alternator output and warn of a no.charge condition.hth
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Okay I have fitted a new battery, a Banner 74Ah 'with calcium'. (I couldn't get another Bosch Silver)
Now I get the following readings;
ENGINE ON IDLE;
Battery +3amps (meaning being charged)
Alternator 47 amps
Potential across battery 13.92V
ENGINE ON IDLE + HEADLIGHTS;
Battery -5amps (meaning being drained)
Alternator 44 amps
ENGINE AT 3000 RPM;
Battery +2.4amps (meaning being charged)
Alternator 46 amps
Potential across battery 14.03V
ENGINE AT 3000RPM + HEADLIGHTS;
Battery +1.5amps (meaning being charged)
Alternator 50 amps
So the situtaion is improved.
However the underlaying problem still remains, why the alternator is required to work so hard. (The headlights should draw about 10amps) The power take off from the alternator goes to a terminal block just in front of the battery. The power is then split; one way directly to the battery and the other way towards the bulkhead of the engine bay. This second power line is taking the bulk of the ampage. This current is not drawn when the ignition is on and the engine not rotating; it only occurs when the engine is running.
So what could be drawing such a high current only when the engine is running?
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The only consumer drawing the most current with the engine running is the fuel pump/s.Some of these estate models are fitted 2 pumps in line.Another possible is that the car is fitted with an inlet manifold heating system,but I have only seen this on cars destined for cold climate conditions.hth
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Interesting comments; thank you. The car was first registered in Vienna (I bought it when I lived in Holland). Maybe it is a cold climate edition; what should I look for?
Don't the fuel pump(s) run briefly when the ignition is switched on to presurise the system? I just looked at the current flow on the power take off from the terminal block towards the bulkhead (as mentioned in my earlier post) when the ignition is turned on (but the engine not started). This peaked at 20 amps and then fell to about 8. So it is not out of the question that the fuel pumps are 'eating' all this power. Maybe I could place my trusty clamp meter somewhere convenient to measure the current flow to the fuel pumps; is there an obvious location?
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