Some Fuse Questions
 

Early this Memorial Day morning, while the family sleeps, I'm thinkin' about.....FUSES.

1. What are some effective ways to clean off the fuse holder contacts? I'm thinking a wire brush about the size and shape of a piper cleaner or some sort of sand paper pencil.

2. How many of you go to the trouble of replacing all old fuses and cleaning all contacts simply on principle? In my cars, they range from 20-26 years old.

3. Can the symptom of a bad fuse and/or contact be something OTHER THAN a system not working or working intermittently? Essentially I'm asking whether a bad fuse connection is ever known to produce "weird" symptoms.

Thanks.

Are these the pointy euro fuses? Take a look at a used fuse, there will be a thin ring where the fuse makes contact with the fuse block. This is the only area where current flows.

I've used the edge of a single edge razor blade to clean the hole in the terminal as that is the only area that matters.

Some fuses use a plastic body rather than the old ceramic, the plastic ones tend to melt at high current levels resulting in a loose fuse. I've even gone so far as to swap the element to a ceramic body.

My 1980 SAAB 900 has these and they are a pain to keep clean even though the fuse block is covered.

The irony here is that most of the OE ceramic-body fuses I've run across, have aluminum elements - not such a great thing when they start to corrode after prolonged contact with the copper strips in the fusebox. And while most aftermarket fuses have copper elements, too many use plastic bodies that melt under heavy load!
All things considered, I prefer the OE fuses to the plastic aftermarkets, and often grab a few spare OE fuses (if they're in decent shape and not too corroded) when I visit local salvage-yards.

I do think the best combination is having the copper elements on ceramic bodies, so I've done the element-swap trick a couple times. One was on the blower circuit of my first 240D, (a common problem on W123s) which kept melting the plastic fuses.

Happy Motoring, Mark

I would not want to "scrape" or hit the contacts with anything abrasive. They are plated brass and if you destroy the plating it's all downhill from there. It will only heat up worse than before which will increase the corrosion, which drives up the resistance and heat even more.

If the clip gets too overheated it will also lose its temper. This will mean that the contact pressure against the fuse will be insufficient and cause heating. Again this causes a downward spiral.

I just replaced all my fuses with the copper/ceramic. Only trouble spot I am having is the infamous #8 for the blower. Mine got hot enough to melt a plastic fuse. (The smell of burning plastic under the dash is the worst panic moment ever.):eek:

On Saturday VStech helped me service the blower motor and it is much smoother now.

Right now on high speed the load side clip for #8 is uncomfortably warm (can't keep my finger on it and it was getting increasingly warmer). On 1 or 2 it's fine. I am going to try Brasso on a q-tip to see if it helps. If not I am going to arrange an ATC style bypass fuse holder with a 16 A fuse.

The factory bypass fuse holder that is available is for 126's not 123's. It is a 30 A link fuse and that is inappropriate for a 123.

"Weird" symptoms are by definition hard to predict. They are more likely to occur in an electronic circuit that is voltage sensitive. I could imagine a circuit that carries a fairly heavy load through an old aluminum fuse that is covered with aluminum oxide and in a holder with a weak spring. The high current through that resistive fuse would drag down the system voltage beyond the fuse and that might cause problems in something electronic powered from that circuit.

In any case, new shiny fuses are always better than old corroded fuses, especially the "bullet" fuses used up through models 124, 126, and 201. I always go through a new-to-me car and replace most or all with new ceramic/brass fuses. It's one less thing to consider when diagnosing a subsequent electrical problem.

Jeremy

Every time I put an order in to one of the parts houses, I always pickup an assortment of fuses. I've always ended up with the copper ones. Now, I've got three drawers in my parts organizer full of blue/white/red fuses!

The first thing I'd always do when we bought any of the cars here was replace all the fuses with these newer ones. I didnt bother cleaning any of the contact points, as none of them (so far) have merited such action.

I've not had any "weird" electrical issues related to fuses either. The only thing that has really happened is blowing a fuse when I'm working on the radio or gauge cluster.

jay_bob, as for your blower motor fuse, I would move it to an external fuse holder. The 126 uses a strip style fuse at 30 amps. I'd be inclined to agree with you that 30 amp is inappropriate for the 123. The best bet would be to either use a similar amperage to whats there or measure the current draw on the circuit and pick a fuse a few amps slightly above that number.

Yes I am probably going to have to do this. I am hoping to find an ATC style fuse holder with pigtails that will fit in between the existing clips in spot #8. I would either find a gap or drill a couple holes in the casing for the wires and hook them up on the back side.

In fact I am getting ideas about rebuilding the entire fuse box with ATC holders in this way. You could strip out the existing clips and leave only the terminals on the back. Then using ring tonge crimps, attach the pigtails from the ATC fuse holders to the appropriate terminals in the back. Then use potting compound to hold the ATC fuse holders in position.

You are right about the blower fuse, I will do no more than a 15 A ATC. I looked at the FSM wiring and the blower is fed from a 2.5 mm2 wire which is approximate to a 12 AWG. In my world that is a max 20 A fuse. However they also feed the a/c thermostat control and clutch relay and recirc flap actuator off a 0.75 mm2 wire which is approximiately #18 AWG which we cannot fuse at more than 7 A. (UL508a)

I also wonder what the contact point continuous and short circuit ratings for the ignition switch are. It's not just the continuous rating (thermal) but what happens in the case of a downstream short, you can weld the contacts if the fuse is overrated and can't clear in time.

I'm sure the Mercedes design standards were different and automotive is a totally different standard, both on this side of the pond and over there, than what I have to live with for control panels. But I still do :eek: on some of the fuse ratings vs wire size, or the number of unprotected circuits straight from the battery.

I am seriously considering installing a fuse in the 6 mm2 line from the starter to the light switch. Way too scary for my taste to have unprotected power from a Group 85 battery running through the dash. Just have to calculate a size that would never trip under normal circumstances but blow in the case of a catastrophic short under the dash. I'm thinking in the 80 to 100 A range.

Unfortunately the regulator circuit for the alternator depends on this path, if the fuse were to open up you would lose regulation on the alternator. Although in that case you have isolated everything but the battery so you would not be operating in an overvoltage condition for very long as you would have no electrical system operating at all.:D

Anyone have access to a high capacity dc shunt to get some measurements of current on the system? I'm curious what some of the systems actually draw during operation. I have a cheap meter at home with a 10 A dc rating, I blew the fuse the other day checking out the blower before VStech fixed it.

JamesDean is sure right.
One of the first things to do on any new MB is to replace all the fuses REGARDLESS of appearance.

I also use Tarnex on the fuse holder block making sure each side of each fuse connection is clean. If you have any doubt about what value goes where, refer to the printed sheet inside the fuse block cover. The fuses indicated there match up with the small numbers on the fuse block.

I have a clamp dc ammeter somewhere around here. I upgraded my 190E's alternator to 143A a few years ago and wanted to install a re-settable breaker. I bought one and installed it, found out it was tripping way early. I think it was 200A or something. Its sitting in my garage in a cabinet right now.

I doubt the electrical system is drawing more than 30-40 amps under normal driving conditions, especially in a early diesel with minimal electronics.

I wouldn't go too overboard with fusing everything. There are, for sure, somethings that ARE under-fused on these cars, like the auxiliary water pump.

I know that on the 126 560SEL, the wires leading to the ignition switch are 4mm and feed quite a bit of systems. The connector is pretty robust too.

You can get ATC fuse holders in a block, some are stackable for length. Look at a truck body building supply web site or a company like Painless wiring.

For proper fusing, the max load plus some % would be more proper than wire size. A clamp on amp meter is very helpful here,make sure it will read DC , the less expensive ones only read AC. ( Tenma www.mcmelectronics.com/manufacturer/TENMA/01001018 is a decent house brand , Extech is great www.extech.com , Fluke is high end.

Fuse curve matters too. Take a look at Digikey , MCM , ( Farnell for the Brits on this list ) or similar for fuse data sheets. You will see a fuse can carry more that it's rating for X amount of time. As a side note, 60's - 70"s Brit cars used fuses of a different rating and installing a USA rated fuse will cause a melt down if the circuit is overloaded.

On my 70's International backhoe ( For German content, it has a D239 Nuess engine and Bosch VA injection pump. ) I added fusable links to the main feed from the battery to dash, seperated the alternator output from the main wiring / added a fusable link and added 4 push circuit breakers for lighting.

You could run a fresh wire from the alternator output to battery and add a fusable link/ fuse along the way. A fuse should be as close to the powersource as possible. Consider the battery the power source.

Here's another affordable AC/DC clamp style meter:
Triplett 9310-A AC/DC Digital Clamp-On Meter, 1000A AC/DC Voltage to 600V, 40MO Resistance, -4/1472F Temperature Range, 10Hz/10MHz Frequency Range: Amazon.com: Industrial & Scientific

Here's some Bussman ATC fuse blocks:
Bussmann Automotive Fuse Blocks and Others | WiringProducts

They had some nice looking 6-8 circuit units that might prove helpful to you. You could relegate all high current applications to that if you liked. Blower motor, rear window defroster...

What is it about an old but not blown fuse that makes it a candidate for replacement?

Well, I'd say mostly is just owner preference unless there is something physically/visually wrong with the fuse.

Well, I have heard people say that a fuse, though not blown, can not be performing correctly. Just wondering why that would be the case (if in fact it is the case)

I believe, technically speaking, its this:
Galvanic corrosion - Wikipedia, the free encyclopedia

Often you have aluminum strapped fuses installed in the holders. The holders themselves are copper.