W203 side lights not working

[SiPreston]

I have a problem with the side lights on my 2005 C Class. The on board computer says the bulbs have failed, however they have had new bulbs fitted and still do not work. Any help?

 

[brucemillar]

I have a problem with the side lights on my 2005 C Class. The on board computer says the bulbs have failed, however they have had new bulbs fitted and still do not work. Any help?

You need to check the voltage at the bulb holders. Possible front SAM or Switch failure.


Sent from my iPhone using Tapatalk

 

[SiPreston]

You need to check the voltage at the bulb holders. Possible front SAM or Switch failure.


Sent from my iPhone using Tapatalk

Thanks, I’ll check that tomorrow. The problem is on
both sides. Can the SAM be repaired if it’s faulty?


Sent from my iPhone using Tapatalk

 

[brucemillar]

My sidelights failed on my W203 C55. It was the front SAM. Both bulb holders showed voltage of around +3vdc. I had mine replaced. I believe they need coding, but may be wrong.


Sent from my iPhone using Tapatalk

 

[SiPreston]

Just got round to checking the voltage and I have 13 volts with the engine running, 11-12 with the engine off

 

[Ted]

Test it with a testlight, not a voltmeter

 

[SiPreston]

Test it with a testlight, not a voltmeter

Testlight doesn’t work. It works on the main headlight so the testlight is ok, just not lighting up on the side lights, even though voltage is there

 

[brucemillar]

Hmmm. Ted is your man here for this electrickery stuff. But to help Ted out have you tried your test amp across different earth points or the same one that you tested your main lights from? 12vdc should drive the side lights assuming that it has load & earth, which your test lamp should indicate. Bad earths are common on these cars.

 

[SiPreston]

Hmmm. Ted is your man here for this electrickery stuff. But to help Ted out have you tried your test amp across different earth points or the same one that you tested your main lights from? 12vdc should drive the side lights assuming that it has load & earth, which your test lamp should indicate. Bad earths are common on these cars.

I tested the lamp on the headlight (which works anyway) from the same cluster and it worked. The earth point was a bolt into the bodywork. As the earth for lights seems to branch from one light to the next (giving one common earth?) I assume the fault may lay elsewhere?

 

[brucemillar]

Are you sure that you have bulbs and NOT leds? leds are polarity driven so if they are led? try rotating each one in turn or, just get some bulbs and try them.

 

[SiPreston]

Definitely bulbs, 501 W5W just as the manual says

 

[Ted]

Mmmm...Voltage at connection but doesn't light up a testlamp with a good earth does suggest, as Bruce says, a faulty SAM.

 

[190]

Hopefully not to confuse the issue but is the test light preferred because it's a relatively low resistance device compared to the very high input impedance of a digital voltmeter which wouldn't be able to distinguish the "CANBUS" voltage from the lighting voltage.

 

[Ted]

Exactly that.
The high input impedence of a dvm will show a voltage even though the source impedence is in the megOhms range, so is unreliable for checking in this case.
A testlight - say 5watt will draw enough current to indicate that the current source is ok.
It has the added value of giving a rough indication if the source is good (bright) or not (various levels of dimness).
It is also a good indicator of varying voltages which a dvm doesn’t indicate well. Try across a set of speaker terminals with voice/music to see what I mean (or some SAMs which pulse to check the current draw (or not) of a bulb.

A voltmeter is great for measuring voltages - charge voltage, discharge voltage etc. A testlight is used for checking a circuit is ok.
You can also use a testlight - a 10 or 21 watt for diagnosing a short to earth. Remove the fuse, put it across the fuse terminals. When it is full brightness the short to earth is there. Any other brightness the short is gone.
A bit harder on high current consumers, but a bit of experience helps.

 

[Ted]

Oh, and Halfords have decent ones - not the crap ones you get for a couple of quid. They come with a 24 volt incandescent but I swapped it out for a 12 volt as it gives a better indication and draws more current.

 

[brucemillar]

Ted

Great tip there. I just popped into my local Halfords and bought their Halfords Advanced Heavy Duty Tester (as per your post). A solid bit of kit and comes with their lifetime replacement guarantee. I have a trade card which makes it a really great price.

Cheers

Bruce


Sent from my iPhone using Tapatalk