FLU 419 Blown Head lights aka Kermit vision problem

I have a blown high beam one side and a blown low beam other. It came with a blown headlight. I replace one already and it blew shortly after. As parts are hard to come by I want to be pretty sure what part needs replacing. I'm thinking it is the alternator regulator. Haven't checked yet if it internal or external. You manually shut off the fuel to stop the engine. There is a built in battery cut off and the standard key switch (almost called it ignition switch) . I've been cutting off the engine, Shutting off the key switch and lastly disconnection the battery. It seems to me the battery would buffer any spikes doing its capacitor thing.

Tips?, Tricks, Testing before blowing more headlights? Things I can do before buying parts?

Well thanks to a tremendous gift, A truly righteous FLUKE 179 true RMS DVM, bless you Ken, I'm able to properly trouble shoot the problem. the meter Has a high/low capture function. Going to hook up the meter to the Head light plug and trying different switch on/off start stop scenarios checking for voltage peak and sags.

Don't forget to check the rubber isolator mounts. There should be 3 of them per headlight. After a while, they get hard and stop dampening the vibrations, so your literally vibrating the filaments to death.

One other thing is, whats your idle rpm? I keep mine up near 800 and it helps keep bulbs from going out.

Hi Butch,

How is the SEE set up for fuses? Circuit breakers? Mercedes typically puts high beams and low beams on separate circuits and fuses. You would then have to check four fuses/breakers to ensure the headlight circuits are good.

The first thing I'd check with the voltmeter is the battery voltage with engine running. That will tell you if the regulator is ok. You should see somewhere around 27, 28 v but could run higher and still be ok.

For checking loads on the truck, a digital voltmeter is a poor test tool for continuity like checking voltage to lights, etc. A digital meter presents almost zero current load to a circuit. A fuse can have a poor connection and not power up a light bulb at several amps, yet it will still show good continuity/voltage on a DVM.

Did you verify the filaments are actually bad? Obviously you can check continuity of a headlight filament with a DVM/Ohmmeter.

A better test tool is a 24v light bulb because it will draw some current and reveal a high resistance connection. A quick check is to work across a terminal block, or fuse block, or rack of breakers, and check input voltage with the test light. Then check output from the fuse/breaker/terminal block. What I'm getting at is that your bad headlights might be poor connections on a vehicle that has been setting for a long time.

Bob

PS you guys have me wanting a SEE really bad.

My first gut feel is voltage spikes. Gimpyrobb mentioned vibration. Definitely going to check that. I'm either burning out or breaking the filament, loosing continuity. Could be I just got a pair of cheap poor quality headlights. But Id hate to install a new set and have them blow again.

Great point on using the DVM for continuity not putting any load .Never realized that. Doesn't have breakers , has those ceramic pellets fuses with a strip of metal all out in the open. The kind my 1959 Opel had! The fuse block is very prone to corrosion as you well know. Bet it is the same set up on your 404's and 406's.

The running battery check is just what I am looking for. Would anything over 28V be considered excessive? Should I be shutting off the lights before killing the engine? or does it matter.

Thanks everybody for the input. Very much appreciated.

Kermit is going to the rally in off road mode, no bucket or back hoe.

Pretty sure your charging system should be at 28.8v I would think your safe voltage wise to 30v.