full beam has volts but nothing happens

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finisterre

Active Member
Joined
Oct 6, 2010
Messages
950
Location
Buxton
Car
Vito, Z4, 530i touring
hi

my old vito 638 is playing me for a fool - MOT fail on offside full beam
-if I put the headlights on and choose full beam the nearside light is fine and the offside is not working

first I tried four bulbs then tested them on the nearside. no progress.

I checked the earth - it made no difference when I linked the brown wire to the chassis.

Next, fuses, positions 2&3 under the steering wheel - sound, swapped them anyway and the problem persisted

okay- voltmeter across the terminals in the disconnected connector block, 11.98v switched to full beam on both sides between earth and the orange wire. They should be working then.

swapped the bulbs again, getting desperate. -no progress.

cleaned the terminals- no progress

checked the voltage between the terminals with a working bulb inserted - 0 volts. grrr


What is the problem? It is baffling me.

suggestions welcome...
 
Last edited:
Use a testlight not a voltmeter as this can give misleading results.
Then go back and check exactly the same points again.
Sounds like a faulty fuse connection.
A faulty earth will normally result in both the main and dip on that side glowing dimly so probably isn't that.
 
Points to a large voltage drop across a resistance in the full beam supply line. This may not show up by putting a voltmeter across the supply connector as a voltmeter has a very high resistance 10-20 megohms= very little current draw. Other possibility is a short somewhere in the headlight unit " bleeding" most of the supply voltage past the bulb directly to earth. one test to do is simply run a suitable wire from the battery directly to the bulb/light unit connector -- if the bulb lights its the supply-- if not its the headlight unit.
 
Could be a faulty relay in the Hi/Lo lamp circuit. Dirty contact not passing the current for Hi Beam. Have a look there.
 
cheers.

My next task is finding, identifying and replacing the lighting relay I guess.
The lack of a workshop manual is debilitating.
So I might try to sort one of those too.

Anyway, it looks like a cancelled MOT retest until it is sorted.
 
Turn on the ignition with engine not running, listen hard whilst operating the full beam stalk, you should hear the relay clicking and be able to locate it that way ;)
 
cheers.

My next task is finding, identifying and replacing the lighting relay I guess.
The lack of a workshop manual is debilitating.
So I might try to sort one of those too.

Anyway, it looks like a cancelled MOT retest until it is sorted.

try this

MBUSA Technical Training


and this

HELP!! TIPS/DIYS: General Info, Climate Control, Electrical, Videos - Benzworld.org - Mercedes-Benz Discussion Forum


There is documented evidence of problems with rear lamp clusters- don't know if this possibly points a finger at headlight units also?
 

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