W124 Headlamp Voltage

[Number_Cruncher]

I'm beginning to look at a problem with the headlamps on my 1995 E300D. They are a bit like me - too dim to be of real use!

Anyway, I think that I need to take off the headlamps and give everything a good clean - that's fairly simple, and I don't have a problem there.

However, before I began, I took a couple of readings with my multimeter.

Disconnecting absolutely nothing, and with the ignition on, and the headlamps on;

with the +ve lead of the multimeter on the +ve terminal of the battery, and the -ve lead on the back of the live terminal on the bulbs, I get 0.75volts drop.

with the +ve lead of the multimeter on the earth connection of the bulb, and the -ve lead of the multimeter on the -ve terminal of the battery, I get 0.25 volts drop.

So, in operation, the bulbs are running at battery voltage - 1, so, they probably aren't glowing as brightly as they ought to.

I'll find the earth point, and clean that up, but I'm not expecting a huge improvement. My main question is, on W124s, is 0.75v dropped on the positive side normal, or is it excessive? I know there's a big and expensive looking lighting relay behind the fuse box - surely that should ensure there's only a small voltage drop?

Has anyone any experience of this sort of problem with a W124?

Cheers

[JimFreeh]

Originally Posted by Number_Cruncher

I'm beginning to look at a problem with the headlamps on my 1995 E300D. They are a bit like me - too dim to be of real use!

Anyway, I think that I need to take off the headlamps and give everything a good clean - that's fairly simple, and I don't have a problem there.

However, before I began, I took a couple of readings with my multimeter.

Disconnecting absolutely nothing, and with the ignition on, and the headlamps on;

with the +ve lead of the multimeter on the +ve terminal of the battery, and the -ve lead on the back of the live terminal on the bulbs, I get 0.75volts drop.

with the +ve lead of the multimeter on the earth connection of the bulb, and the -ve lead of the multimeter on the -ve terminal of the battery, I get 0.25 volts drop.

So, in operation, the bulbs are running at battery voltage - 1, so, they probably aren't glowing as brightly as they ought to.

I'll find the earth point, and clean that up, but I'm not expecting a huge improvement. My main question is, on W124s, is 0.75v dropped on the positive side normal, or is it excessive? I know there's a big and expensive looking lighting relay behind the fuse box - surely that should ensure there's only a small voltage drop?

Has anyone any experience of this sort of problem with a W124?

Cheers

Since your in the UK, I'll assume you do not have the awful US spec headlights...

The relay you mention is for the lights out function, it's not a relay for the headlights power feed.

I would expect some voltage drop under load. Your measurements, if I read you correctly are not under load and appear to be more of a resistance check. Try an ohm (resistance) check of the wires.

Another possibility is that your reflectors have tarnished. Euro code headlight assemblies do not have sealed bulbs, and the reflectors can oxidise if any of the seals are comromised on the headlight assembly.

Good luck!

Jim

[Number_Cruncher]

Thanks Jim,

The readings were taken with the lights on - so it was under load. If the voltage drop reading on the positive side were below, say, 0.5 volts, then I would be happy. I'm just a bit concerned that 3/4 of a volt is being dropped somewhere.

I didn't realise that the relay didn't supply the power - I'll dig out the wiring diagram, and see where the multi-connectors are.

Since posting earlier, I have had the headlights out, and the lenses off. Yes, these are euro lights - but, being UK spec, they dip to the left, so the lenses are different again to most others in Europe.

The reflectors were filthy. I'm not sure why, because the units look to be well sealed. The only thing I noticed was that the plastic around the sidelamp housing had degraded - perhaps this has overheated, offgassed, and the condensed rubbish had landed on the reflectors. Anyhow, they're much cleaner now!

[hughet]

The lights on my 124 were flickering and it turned out to be a bad connection between the frame and the negative cable from the battery.
I cleaned off the corrosion and tightened the bolt and the problem went away.

[tankowner]

I have the same year and model, but it is the U.S. version. All I can say is that the one thing I noticed and really liked after buying the car was that the headlights are very bright. One of the lenses is even craked and gets a little condensation in it. I am in the process of trying to find a cheap replacement. Nevertheless, they are still bright. I'm afraid I can't say much about the electrical readings though. If worse comes to worse, I could always take a reading on mine and let you know what I get, but I would have to know what to measure with a little detail - electrical is not my strongest area.

[babymog]

Are they standard 55/65w H4s or has someone put 100W bulbs in them. If 100W bulbs (illegal for street use), I'd say that the voltage drop is due to the wiring not being of adequate size. I changed the wiring gauge on mine (inside the light housing).

[Number_Cruncher]

With new 60/55W H4s in (the old ones were 60/55 too), and the clean reflectors, the lamps are much better - on the verge of being tolerable when dipped, and quite adequate when on full beam.

Until I took the headlights off, I didn't realise that there was a connector on the headlamp unit itself - it's much more usual on European cars for the wiring just to pass through a grommet into the lamp unit.

I'm still not happy about the 0.75 volts drop, and I'll make another measurement to the headlamp connector, to see if the bad connection is in the headlamp itself, or upstream in the engine bay or light switch.

Thanks all for the help so far - it's really making a difference.

Although the aim is currently OK, my headlamp positioner doesn't work. Where does the unit draw vacuum from? I have traced the vacuum pipe (pink!) back as far as I can, and it disappears under the heater unit to the passenger side of the car (RHD). What level of vacuum should I measure at the dashboard switch?

[babymog]

Do you have any vacuum to the switch/valve at all?

On the Euros I've had, both lines to the adjusting switch/valve were the same color, but I don't know the source.

[JimFreeh]

Originally Posted by babymog

Do you have any vacuum to the switch/valve at all?

On the Euros I've had, both lines to the adjusting switch/valve were the same color, but I don't know the source.

Taps off the line to the brake booster.

Jim

[Number_Cruncher]

Jeff, Jim,

Thanks for the info. There's no vacuum at all at the switch/valve. The brake servo is working well though, so there is vacuum in the large line between vac pump and servo.

This looks like the perfect "problem" to keep me out of the way, in the garage, during the worst of the grisly entertainments of the season - excellent!

[scottmcphee]

..picking up an old thread...

I've just done major fiddling with my 87 w124 headlight system, much brighter now. These are the USA DOT 9004 POS design. I know I could have got Euro assemblies to replace these.. $$$ if I was lucky on ebay etc.

But the mods I've done apply to either kind of assembly, so there is cumulative benefit to be had if I do change over to Euros. For now, I've got probably the best light output possible for 9004's.

Here's how:
1. clean the inside of the lens assemblies. Take them out of the car, take bulb out, get a little dish soap and hot water in there, half full and shake and swish and risk with hot water. Air dry in hot sun.

2. new bulbs, you pick a brand lots of other threads on these. I chose some over-wattage 80/100W bulbs. (DANGER! these get hot.) (Warning! these are illegal on the street, so now I just use my w124 for you know, off-roading.) Pro-tip: In order to safely use 80/100W, without risk of burning up your wiring harness, sockets, headamp switch, etc... the next step is mandatory, not just optional. Not only is it mandatory, you'd be wise to substitute high temperature sockets for the 9004's to replace the stock ones. Don't get the made in china ones off ebay buy real ones. I found USA made ones and compared with the china ones that fell apart in my hands, can you say cr*p? Hands down winner: USA brand. That's not patriotism because I'm Canadian, that's just a fact. Now then....

3. upgrade voltage to the headlamp. This is what this original poster was asking about 3 years ago. There is too much voltage drop through the MB + side of the supply to bulb, more on that later. Here's how to minimize it: bring both plus and ground from the battery posts to the lamp area on minimum 14 AWG wires, 12 AWG is better, these beefy new wires must be fused at the battery side. I used 20AMP per wire for a good reason. Pro-tip: avoid having to run 4 wires by running a common rail of 2 wires that passes behind by each headlight. Rotate the second side of the car's connections to the rails so it's high beam shares the low beam rail of the other side of the car. Both wires are active for low, and and high - just the roles reverse. When you do this, one blown fuse means one side of the car remains working on low beam and the other side of the car on high beam - but you'll still have some light at all times. If you notice this failure pattern it's probably your fuse and not two filaments in different bulbs going at the same time. Beware, it is possible to hold the light stalk just so (as if doing flash to pass in slow motion) to get ALL FOUR beams ON at the same time. This is why I chose 20AMP fuses for the 2 wire idea. If you're antsy run 4 wires. Back to the program... cut the three wires going to each headlight lamp (3 wires for 9004 bulb), leaving enough length on the socket wires to remain useful. Repeat this for each side of the car: wire the car-side of the bulb supply with 2 relays, one relay coil for low beam, other relay coil for high beam, using car-side bulb ground for the relays only. Now when you operate headlights you'll only get relays clicks and no light until you connect your beefy + supply from the battery through the relay contacts to the bulb socket. One rail and a relay for low beam, the other rail and relay for high. Attach the bulb socket ground to your beefy ground wire. You can't get lower voltage drop than this for any headlight style assembly or bulb type!

4. oh-oh... bulb out indicator on dash is now rightly confused for headlights for both low beams and high beams and comes on when you use headlights. This is that pesky "N7 Exterior Lamp Failure Monitoring Unit" (a.k.a. that huge "relay" in the back of the fuse box, you have to unscrew the lid to see). It's not a relay, it's a pair of circuit boards that you modify - giving it just enough of a frontal lobotomy to forget about low beams and high beams, but continue to serve its useful purpose for every other exterior bulb on the car. Here's the deal, one of the reasons MB has voltage drop in the headlights is N7. Voltage must flow THROUGH it for lamps to work. There are copper foil traces handling full headlamp power, a short resistor wire, and the contacts of the N7 all fighting against and adding to voltage drop, not to mention the thinnish gauge wires going to and from N7 to the headlamps. Yuck. But none of that matters now for headlights, because it's only powering and monitoring your relays - which are happy to get even minimal power. To make the world more blissful it's time for that lobotomy, open up the shell. Once N7 is out of the car, it can easily be pried apart it with a flat tool chasing around its joining edge - it's snap together fit. A picture tells a thousand words, so I'll post a couple... in short, you cut two traces (one on each circuit board) then put it back together and into the car. Easy, and just like a real lobotomy.

5. Now, looking at your dash alone, you will never know if your headlights are out... but if you happen to look out the windshield at night it will all become pretty obvious, duh! On the bright side, happy motoring! Aside from using good electrical practices throughout, making good connections, motherhood, apple pie, at-your-own-risk warnings, etc, that I can never understand why people put in DIY forum threads (!?) this is the best you can do with USA DOT 9004 design lights, for pretty cheap.

[scottmcphee]

I said I'd post a few pix of how to do quick surgery on your N7 bulb out indicator module. Turns out it takes a double frontal lobotomy to make N7 not pay attention to the headlights on the car. One nick for the low beams, the second nick for the high beams. I'll leave it to you to figure out which is which.

You can do this mod if for any reason you are getting dash lamp out indicator for headlights or highbeams, and they seem to be working fine for you. Maybe you can't figure out why, or simply don't want to figure out why, and it's just annoying because things are working just fine. Maybe your bulb wattage is off, or you upgraded to HIDs, or LED lights, or whatever... I did it because my "lamps" had been converted to relays, as noted in above post.

Here's how to do it.

Find and remove N7 module from the car, it's in the fuse box. Take it out and open it up.

Note the WHITE arrows I painted on the photo pointing to the plastic peg(s) that prevent wrong insertion. This will orient yourself as to which side of the unit you should be working on.

The PURPLE circles show what to cut on one circuit board.
You can cut that thin trace leaving both the encircled pads. Or you can do what I did, heat and lift those two diode leads from the board. Just left them hanging in air a bit off the board. (This way I can re-insert the leads in the future if I ever want to revert.)

Flip the unit over. Note again the WHITE arrows to re-orient yourself to correct side of the unit.

The BLUE circles show what is cut on the other board. Cut either that trace going between the two encircled pads, or lift the jumper wire out at one end.

Now pop it back together, and re-insert into the car.

Doing this mod leaves all other bulb indicator warnings intact.

[scottmcphee]

Oh, here's how I open these things.

Stuff coffee stir sticks all around the sides, then pry one end with the screw driver (you can see it is already popped in the photo), then pry the other end.

POP! it's apart.

[Renntag]

Scott,

Great right up. Thanks for the extra bit of info on the N7. I have lamp warning lights on from time to time and I have no lights out. I have dim headlights and have cleaned the grounds and chased wires. While my lights are still dim, I drive on High beam at night, clearly i have not resolved this issue. As a last resort, I did think of doing just what you have described.
Now that I am trying to install some Euro lamps I think I may perform this relay mod.

I would still like to get wiring diagrams for the lighting system on an '87 300TD.

Thanks in advance to anyone that can help.

[oldiesel]

Renntag
Here is a scan of the headlight /fog light circuit , hope it comes out clear enough to be of some use. If its not clear enough send me an email and i will email it to you it might be better that way. Don