Do I have a parasitic drain...
Thread Starter
Newbie
Joined: Sep 2025
Posts: 14
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2004 CLK320 A209
Do I have a parasitic drain...
New owner of a 2004 CLK320 Cabriolet. Previous owners are friends. They disclosed the car has a parasitic drain on the battery. I am trying to troubleshoot and have been having trouble confirming.
I have tried to measure the draw on the battery by hooking up the ammeter in series on the negative terminal of the battery and also by using a clamp style meter with the negative battery hooked up. The best I can tell, after about 6 mins, the draw was reading between 6 and 8 milliamps which seems really low.
I have also been monitoring the voltage in the battery to see if it goes down. After a short drive last night, the voltage registered 12.6 volts. This morning, it was 12.44 and 9 hours later, it is 12.33. The battery is relatively new (less than 12 months) and shouldn't be bad.
I thought a draw less than 50 milliamps was generally considered acceptable. My readings are under 10 milliamps. (Guage says .007)
Should I be concerned that the volts have dropped below 12.4 in less than 24 hours?
The local MB dealer determined the cause of the battery drain as "current draw on circuit 9" and further diagnosed it to "rain sensor control unit (in compartment above rear view mirror) was defective and not going to sleep" which is on that circuit.
I have a new (eBay) rain sensor ordered. While I am waiting for delivery of new sensor, I was trying to confirm the findings to make sure I understood how to test and troubleshoot so I can confirm it is fixed by the new part. Looking at my readings, I think I am doing something wrong, or I don't have a parasitic drain. What are your thoughts?
Thanks in advance for any response.
I have tried to measure the draw on the battery by hooking up the ammeter in series on the negative terminal of the battery and also by using a clamp style meter with the negative battery hooked up. The best I can tell, after about 6 mins, the draw was reading between 6 and 8 milliamps which seems really low.
I have also been monitoring the voltage in the battery to see if it goes down. After a short drive last night, the voltage registered 12.6 volts. This morning, it was 12.44 and 9 hours later, it is 12.33. The battery is relatively new (less than 12 months) and shouldn't be bad.
I thought a draw less than 50 milliamps was generally considered acceptable. My readings are under 10 milliamps. (Guage says .007)
Should I be concerned that the volts have dropped below 12.4 in less than 24 hours?
The local MB dealer determined the cause of the battery drain as "current draw on circuit 9" and further diagnosed it to "rain sensor control unit (in compartment above rear view mirror) was defective and not going to sleep" which is on that circuit.
I have a new (eBay) rain sensor ordered. While I am waiting for delivery of new sensor, I was trying to confirm the findings to make sure I understood how to test and troubleshoot so I can confirm it is fixed by the new part. Looking at my readings, I think I am doing something wrong, or I don't have a parasitic drain. What are your thoughts?
Thanks in advance for any response.
Thread Starter
Newbie
Joined: Sep 2025
Posts: 14
Likes: 0
2004 CLK320 A209
New owner of a 2004 CLK320 Cabriolet. Previous owners are friends. They disclosed the car has a parasitic drain on the battery. I am trying to troubleshoot and have been having trouble confirming.
I have tried to measure the draw on the battery by hooking up the ammeter in series on the negative terminal of the battery and also by using a clamp style meter with the negative battery hooked up. The best I can tell, after about 6 mins, the draw was reading between 6 and 8 milliamps which seems really low.
I have also been monitoring the voltage in the battery to see if it goes down. After a short drive last night, the voltage registered 12.6 volts. This morning, it was 12.44 and 9 hours later, it is 12.33. The battery is relatively new (less than 12 months) and shouldn't be bad.
I thought a draw less than 50 milliamps was generally considered acceptable. My readings are under 10 milliamps. (Guage says .007)
Should I be concerned that the volts have dropped below 12.4 in less than 24 hours?
The local MB dealer determined the cause of the battery drain as "current draw on circuit 9" and further diagnosed it to "rain sensor control unit (in compartment above rear view mirror) was defective and not going to sleep" which is on that circuit.
I have a new (eBay) rain sensor ordered. While I am waiting for delivery of new sensor, I was trying to confirm the findings to make sure I understood how to test and troubleshoot so I can confirm it is fixed by the new part. Looking at my readings, I think I am doing something wrong, or I don't have a parasitic drain. What are your thoughts?
Thanks in advance for any response.
I have tried to measure the draw on the battery by hooking up the ammeter in series on the negative terminal of the battery and also by using a clamp style meter with the negative battery hooked up. The best I can tell, after about 6 mins, the draw was reading between 6 and 8 milliamps which seems really low.
I have also been monitoring the voltage in the battery to see if it goes down. After a short drive last night, the voltage registered 12.6 volts. This morning, it was 12.44 and 9 hours later, it is 12.33. The battery is relatively new (less than 12 months) and shouldn't be bad.
I thought a draw less than 50 milliamps was generally considered acceptable. My readings are under 10 milliamps. (Guage says .007)
Should I be concerned that the volts have dropped below 12.4 in less than 24 hours?
The local MB dealer determined the cause of the battery drain as "current draw on circuit 9" and further diagnosed it to "rain sensor control unit (in compartment above rear view mirror) was defective and not going to sleep" which is on that circuit.
I have a new (eBay) rain sensor ordered. While I am waiting for delivery of new sensor, I was trying to confirm the findings to make sure I understood how to test and troubleshoot so I can confirm it is fixed by the new part. Looking at my readings, I think I am doing something wrong, or I don't have a parasitic drain. What are your thoughts?
Thanks in advance for any response.
MBWorld Fanatic!
Joined: Oct 2019
Posts: 5,152
Likes: 1,766
From: Colorado
G550
How old is your battery? Batteries can develop internal shorts that begin as leakages that slowly discharge the battery.and do so increasingly as time passes. Internal discharge can't be measured externally with an ammeter but can be monitored with a voltmeter as you have been doing. A good battery sitting disconnected shouldn't be losing tenths of volts within a day. 50mA of current draw when the car is parked isn't unusual. Have the battery load tested by the dealer or auto store.
Thread Starter
Newbie
Joined: Sep 2025
Posts: 14
Likes: 0
2004 CLK320 A209
At this point, I believe I am testing all things correctly and do not have a parasitic drain on the battery. Draw is about 7 milliamps within 5 minutes of turning off. Charge is holding on battery. Thanks to all who commented.
