.Howdy -
Some older cars are aided by having relays added into the headlight
circuits ? the 911 especially because the power for the headlight goes
through
the switch (as I understand it) and the relays reduces the load through the
switch. I?m told this makes it possible to use brighter bulbs safely. I
recall reading that the limiting factor for brighter bulbs on the W124 is the
plastic headlight housing and the Euro housings are metal and can take more
powerful bulbs. So, will relays make a difference on W123 or W124
headlight systems? Where would one find Euro headlight housings? Or are
there
other options/solutions?...
Before you start thinking about relays, you can do a voltage drop test on
your own car to see how much voltage drop the stock lights and wiring have.
And I'll bet you here and now that you won't find battery voltage at the
lights.
Set your voltmeter on DC volts and with the lights on and working (do not
unplug the bulb and leave the engine running to maintain a constant voltage),
measure from the yellow wire on the back of the plug to the positive
battery post. The voltage drop in that wire will register on the gauge. If
nothing
shows, switch beams. I can't recall whether high beam is yellow or white
but those colors are universal. After recording the drop from each 12v wire to
the battery, touch the brown wire (ground) and the other probe to a known
solid chassis ground or the ground post on the battery. This will measure the
voltage drop in the ground wire. Totalling the ground wire drop with each
of the others will give the total drop.
If you go back in time, the biggest voltage drop I've seen is in the 6 volt
Porsche 356 A B models which had less than 5 volts at the lights. And
people wondered why they couldn't see at night. Even my Porsche 944 with stock
sealed beams didn't have battery voltage at the lights. So, I added four
relays, one for each filament (two per side) and as close to the lamps as
possible. I used 12gauge wire to the relays with an MB twin-fuse block on the
fender well. The car now has full battery voltage feeding the 160/100 H4 bulbs.
If you are familiar with Herr Ohm and his law, you can calculate the
current flow increase from 55W to 160W which is substantial, thus the absolute
need for a relay.
If your car has a lamp out module, there will be a tiny consequence to
installing relays which is the lamp out light will be on permanently. The
module
expects to see a certain minumum current flow in the circuit and the relay
coil is below that so the module decides the lamp is out and illuminates the
warning.
I installed relays and 160/100 bulbs in my '95 C280 and the lamp out
warning wason from then on. But, in that car, the warning lamp was tiny and was
obscured by the steering wheel rim so I couldn't see it anyway.
RLE
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