New to MB but not to German engineering. Could use a little help diagnosing!

Rkanuch88

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Hello! I'm terribly sorry if this is in the wrong section. I'm new to the forum and to Mercedes Benz as well. I've owned audis and bmws my whole life and recently decided to give MB a try and I am very pleased so far. I got a 2009 c300 sport with the AMG wheels.

Anyhow I'm having a few issues. I bought the car and had to drive it home 2 hours. About an hour into the trip the temp gauge on the dash started fluctuating from 90 to 100 to 115 then back down to 90. It was just all over, it never reached the red however. About a minute from my house the upper radiator hose blew out, I was able to get it home safely and replaced that hose the following day.

My issue now is, the temp gauge still fluctuates like crazy, but only if I go on a longer drive, like 30-45 minutes. Also my AC blower slows down significantly when I come to a stop, and then speeds back up when I start driving. I'm fairly mechanically inclined, I've always done all of my own work on my previous cars successfully but am not too sure where to start diagnosing these issues. If anyone could shed some light that would be awesome! Thank you!
 

PobodY

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Any chance your alternator is on the way out? - I'm just wondering if the fluctuating gauge and the varying blower are because the power drops when the engine slows down.
Even a loose connection somewhere (so the voltage gets through, but not at the right current)?

Any problems starting or other gremlins that might help diagnose it?

Hopefully someone with a C-Class has got a better idea than me.
 
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Rkanuch88

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Thanks for your reply! The only other real issue is that the engine cooling fan comes on high once up to temp (95 C) and it stays on regardless of whether or not ac is on.
 
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Rkanuch88

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And the newer ones might be, but mine is a 2009 it's not a hybrid.
 

bob6600

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They didn't make a hybrid in 2009 did they?

Not sure about a C300 either? :dk:
 

bob6600

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Alternator is fairly easy to check with a multimeter. Battery should show circa 12v when off and 14+ volts when started
 

markjay

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Is this a UK car? Or are you based in the States? I may be wrong, but I don't recall C300 being sold here in 2009?
 
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Rkanuch88

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Okay I'll make sure to check that as soon as I'm home, also I'm in the states
 
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Rkanuch88

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So I just checked with a multimeter, the battery reads 12.38 while the car is not running, and 13.57 while it is running.
 

PobodY

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Does that voltage go up when you increase the revs? - As above; it seems low when the engine is running.

I don't think it's a dead battery because it seems to have charge with the engine off, but it's not impossible that it's creating a capacitive load on the charging circuit.
 

Major774

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Have you blead the cooling system? I assume these cars require this (I have DIY'ed a lot of Japanese cars haha).
 

brucemillar

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It does sound rather like an air lock. You could try getting the nose of the car as high as you can (on ramps) when cold, leave the radiator/header tank cap off. Run the engine at idle for 45 minutes. That should allow the car to burp out, any air locks in the system.

Be careful when doing this not to risk having a peep inside the radiator/header when the car is running. If it burps when you are looking in, it may well scold you.
 

grober

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THE C300 has a 3 litre version of the M272 v6 petrol engine. Not sure that model was ever marketed in the UK.
Seems you might have either an electrical problem or a cooling system problem. They can overlap. Electrical problems might effect any electrical cooling fans or an electrically coupled water pump if one is fitted. The car is exhibiting classic overheating symptoms which vary according to airflow thro the radiator from the car moving forward [ram effect] and inadequate cooling when moving slowly or extra loading such as aircon is being used when fans are required to provide extra airflow through the radiators. It may be your aircon is downrating itself at idle for that reason. Aircon compressor, alternator and water pump are all driven off the poly V BELT [ FANBELT] so can effect each other. If the alternator or main battery is faulty this can give rise to a mixture of weird faults in modern cars that are heavily dependent on electronic controlled ancilliaries so your problem may be purely electrical. The only sure way to get the bottom of this is to get the car to a garage with access to Mercedes Star diagnostic system and get the fault codes read. In the absence of any obvious mechanical failure or leaking radiator etc this is your best option.
 
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So I just checked with a multimeter, the battery reads 12.38 while the car is not running, and 13.57 while it is running.

Both voltages are low. 12.38 indicates the battery is not fully charged. If (and that's a big if) the car is not drawing excessive current with the engine off then you should see 12.7 volts on a fully charged and rested battery.

The engine running voltage of 13.57 is also low but it would be if the battery isn't fully charged. To be honest diagnosing battery state of charge and charging system health on modern cars is not as simple as it once was. Fully charging the battery with a bench charger and then leaving the car to rest overnight is a good idea as a first step before doing any measurements.
 
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Rkanuch88

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Okay so, the battery reading while the car is running remains 13.57. I had my wife sit in the car and rev it up to 3,000 rpms, the reading dips to about 13.23 then goes up to 13.59 and doesn't move really. I saw the voltage drop to 11.47 a fee times while the car was running as well but that could have just been my multimeter, not sure. The temp gauge was fluctuating again from 95 degrees, up to 105 degrees irratically, as well as the ac cutting just about off at a stop light (with max cool on) and taking a second to blow again once I started moving. I bled the system of air for nearly an hour too. I'm stuck
 

markjay

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While a low voltage reading (with engine switched-on) indicates an under-charged battery, a healthy voltage reading does not necessarily indicate a healthy battery.

The way to test the battery's health is by using a load tester, e.g:

https://www.amazon.com/s/ref=nb_sb_noss_1/131-6602114-9062448_sa_automotive?url=search-alias%3Dautomotive&field-keywords=car+battery+load+tester&rh=n%3A15684181%2Ck%3Acar+battery+load+tester&ajr=0

Best tested after the car has been left overnight, to see if the battery keeps it's charge.
 
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Rkanuch88

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So I pulled up the engineering menu today while driving to monitor battery voltage, and when my temp gauge went up and my ac cut off while stopped, the voltage dropped from 13.5v to 12.2v and stayed there until I started moving again. Seems pretty low to me, I still need to check the battery with an actual load tester instead of just a multimeter.
 

190

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I mentioned earlier that diagnosing battery state of charge and charging system health on modern cars is not as simple as it once was. What they have done is given over control of the voltage regulator to the ECU so that the charge voltage can be adjusted dynamically. For example the charge voltage will be reduced when electrical load is low and will be increased on the overrun to utilise free energy. I don't know if all this applies to your car but it might explain the strange behaviour. This technology was available in 2009 because my car has it.

Even given this as a possible explanation for the voltage changes you are seeing there is still something odd about a maximum of only 13.59 volts. While I saw changes while driving the car I saw a higher maximum of 14.5 volts.
 

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