Hmmm… blew an unusual fuse. - Page 3 - PeachParts Mercedes-Benz Forum

vwnate1 · #31 11-19-2021, 12:49 AM

Yes, you can hook an ammeter into the blank space after you remove the fuse, this will show you how much current is being pulled .

As mentioned above this is where the fun stops ~ you need to isolate every item in this circuit and disconnect each one in turn and see if the fuse still blows until you find the thing that's drawing too much current .

If there's a chafed wire allowing a dead or partial short, it can be really hard to find, begin by looking for collision damage near any wires / harness .

I know this all isn't very helpful , electrical testing takes time, patience and knowledge, I learned the hard way....

jay_bob · #32 11-19-2021, 06:44 AM

The problem with that is most DMMs are only good for 10 A and you'll just be blowing the internal fuse in your meter.

A better trick is to put an incandescent bulb in place of the fuse.

Lots of ways to do this, get a socket with a tail light bulb off a wreck is one way.

Incandescent bulbs have a unique feature, their internal resistance starts low when cold but if current starts flowing, they start to glow. The wire heats up and the resistance goes up to the point where it limits the current flowing to that needed to make the bulb glow (usually on the order of a few hundred mA). So its double good, you have a current limiter and a visual indicator all in one.

Oldest trick in the book, the old school Edison screw base fuse boxes had the same thread pitch as a light bulb and this was how the old timers found shorts in a house with knob-n-tube.

Once you have the light bulb established, do whatever you do to make the fuse blow. The bulb should be glowing brightly. Then start lifting wires 1 by 1 off the back of the fuse block. When the light stops glowing you found the wire feeding the short. Put the other wires back on 1 by 1 and make sure nothing else has a short.

Now go into the schematics and figure out what that wire is feeding, either by looking at its color stripe(s) or by checking what device(s) are not working. This should narrow it down enough for you to trace through the circuit and see where the problem is.

If it is a multiple device circuit, find where the feed divides, and do the light bulb trick at that point to keep narrowing it down.

Shern · #33 11-19-2021, 11:20 AM

Thank you for this.

However, I’m not sure I understand you correctly: it sounds like under fuse blowing conditions, which could be a single amp over the fuse’s rating, the bulb will glow, but under normal operations, the bulb will not glow?

jay_bob · #34 11-19-2021, 02:16 PM

The bulb will glow brightly if you have something shorted. The bulb will not allow any more current to flow than what it takes to light the bulb to full brightness. This limits the current and prevents damage while you try to find the fault.

Dead short = fully bright
Normal load = glows dimly
Open circuit = bulb is dark
We do this trick at work in control panels all the time too.

Shern · #35 11-19-2021, 02:43 PM

Quote:

Originally Posted by jay_bob

The bulb will glow brightly if you have something shorted. The bulb will not allow any more current to flow than what it takes to light the bulb to full brightness. This limits the current and prevents damage while you try to find the fault.

Dead short = fully bright
Normal load = glows dimly
Open circuit = bulb is dark
We do this trick at work in control panels all the time too.

Brilliant... thank you for this.

Apologies if I'm getting too granular here: assuming it matters, what wattage bulb should I be using? Would a household 60w work or should I be using a tail light as per your explanation?

jay_bob · #36 11-19-2021, 10:32 PM

You want to use a small automotive bulb. Something with a 12 V rating. I would use either a marker light or a tail light. If you go to the parts store you should be able to find a generic socket with leads (you want something with 2 wires, going to both terminalsl of the bulb) and a suitable bulb to go with it, if you don’t have anything lying around at home.

Household bulb is set up to glow full bright on 120 V and so even if you put it straight across the 12 V battery you would get nothing or maybe just a little faint glow. So it won’t work.

vwnate1 · #37 11-19-2021, 10:48 PM

You can also use an audio device, I've used a weak (think MoPed) horn and a few times a circuit breaker, it will click until the short is gone, this can be handy when you're up under the dashboard and cannot watch the light bulb .

Shern · #38 11-20-2021, 12:32 AM

Thank you for this gents. I know it sounds a little remedial, but I have no electrical background : /

BillGrissom · #39 11-20-2021, 05:01 PM

I have only had problems with fuse 8 for the blower motor. It didn't fail in my 1985 CA 300D but rather melted the plastic fuse-holder. I soldered in an inline fuse-holder for that position which barely tucks under the cover. Seems a common problem in many 300D's and the blower circuit switches and fuses are a problem in many vintage cars due to the high current. I installed a relay for the "blower high" position in my 1960's Mopar cars. Similar in my two 300D's to power the "Blower Speed" relay box, so fuse #8 now just powers that relay's coil, with the main current on a wire I ran thru the firewall grommet from the battery to that new relay.

Someone showed the mono-valve is on the fuse of concern, so you might try changing that. In my 1984, I replaced it with a vacuum water valve driven by a vacuum switch actuated by the normal mono-valve wire. That was M-B later design and Klima kits those parts for retrofit (pricey). Being a cheap bastard, rolled my own from $15 of good ol 'merican parts (Ford & Chrysler).

vwnate1 · #40 11-21-2021, 11:45 AM

No one expects you to be a Mechanic, you're an Enthusiast who loves and cares about his vehicle and tries to keep it in good nick , this forum is dedicated to helping people like you Sherm .

Look at all the time they've given me good advice .

Just keep at it, don't rush, sooner or later you'll find it or make a work around like Bill did .