W124 Fuse Question - PeachParts Mercedes-Benz Forum

Jmana · #1 09-19-2006, 07:44 PM

When I purchased my car, the aux fan fuse was blown. I had replaced it and it immediately blew, so I put a 25 amp fuse in. Thought the problem was solved, but apparently it got too hot and completely melted the plastic around the bottom terminal. So today I went and bought a 30 amp fuse holder that I intended to use to replace the faulty fuse circuit. But when I pulled out the fuse block, I realized that this fuse isn't like all the others. The one that is bad is the D fuse, and the bottom terminal is tied into the bottom terminal of the C fuse by was of a copper strip on the side of the block. There is also a small wiring harness that is plugged into the area below the C and D fuse. Are these 2 fuses tied together? I checked the C fuse, and the top part seemed like it had some damage to the metal, if they are tied together, would a bad C fuse cause the D fuse to melt? Also, how should I go about wiring up this new holder to replace the D circuit?

Jmana · #2 09-20-2006, 08:02 AM

Would it be possible for me to just attach one wire from the new fuse holder to the spring clip on the fuse block, and the other wire to the metal strip that connects the two bottom terminals for the D and C fuses? I really cannot figure out which wires go to the D fuse circuit, so wouldn't doing it this way essentially be the same thing?

TimFreeh · #3 09-20-2006, 08:31 AM

If you continue to attempt to "fix" this problem by substituting larger capacity fuses in a circuit that was designed for a 16 amp red fuse I'd advise you to keep a fire extinguisher handy.

You did notice your fuse box melted right?

You need to find out why your aux fan circuit is drawing excessive power - and then fix it.

Jmana · #4 09-20-2006, 08:56 AM

I don't know why it would be drawing extra power, the fan spins freely without any noise. But I know something is wrong somewhere with the wiring, because the only way to get the fan to work is to jump the two wires at the temp sensor. I replaced the temp sensor and it still wouldn't work, so I just left the jumper wires in place, which is kind of nice because the fan works all the time, and helps to keep the temps down. I don't know what else could be wrong with it to cause the fuse to overheat, or to blow a smaller fuse, unless the C and D fuses are supposed to be utilized as one circuit, and maybe if the C fuse is bad it puts all the power through the D fuse? Maybe the C fuse controls the temp sensor, and that's why it wouldn't work? Is that plausible? If that's the case, I should be able to replace the C fuse, put in this extra fuse holder to replace the D circuit, and I should be able to put a smaller blade fuse in that without blowing it. But I can't test my theory until I figure out how to replace that circuit with a seperate fuse holder. I do realize the danger of having a 30 amp fuse in there, being that is about twice of what's supposed to be there, so if I put in a smaller fuse and it blows I'll just not put a fuse in at all until I figure out what's wrong. I figure I have all winter to find out the problem, as I won't be needing the aux fan now that it's cooling down.

gsxr · #5 09-20-2006, 03:28 PM

1) You have either a wiring problem or a bad fan - doesn't matter how freely it spins, what matters is the current draw at the fan. It should work fine with the stock 16A fuse.

2) Why, oh why, do you want to jumper the fan to run all the time? It is designed to run on high speed when the temp sensor sees coolant over 105°C, and run on low speed when A/C refrigerant pressure exceeds either 16 or 20 bar (depending on the color of the switch). If the car runs cooler with this fan on all the time, you probably have a failing mechanical fan clutch.

Jmana · #6 09-20-2006, 03:48 PM

I am sure you are correct, that the fan clutch is bad, but the reason it is jumpered, is because it wouldn't come on at all without it. I did change the temp sensor for it, but it still wouldn't come on. The previous owner is the one who put the jumper wire in, so it must have been a reccuring issue. I can live with it being on all the time, to me it's better than it not coming on at all.

gsxr · #7 09-21-2006, 12:36 PM

It's not *supposed* to come on unless engine temps are above 105°C, which on a OM603 with a properly working cooling system, is almost impossible to reach. Mine never runs except when the A/C is on (and then it's the low speed setting), and then it's only when refrigerant pressure is high enough to trigger the fan. If you have a new switch (see below), then just fix the fuse blowing problem, and remove the jumper - let the switch do its job.

Jmana · #8 09-21-2006, 07:11 PM

Yeah, I tried that, but with the AC on, the fan doesn't come on and the temps rise to 110 degrees.

gsxr · #9 09-22-2006, 11:05 AM

Quote:

Originally Posted by Jmana

Yeah, I tried that, but with the AC on, the fan doesn't come on and the temps rise to 110 degrees.

Exactly my point - the electric fan simply can't move enough air to cool an engine that's getting to 110C. And it was never designed to do that - it's an aid to the main clutch, not a replacement. You need to fix the root cause, which is probably the fan clutch, the radiator, or both. It could be something simple like crud in the radiator (or condenser) fins, a bad T-stat, bad rad cap, but it's not likely. BT, DT, on more than one 1987 300D.

If your fan clutch is original (metal blade) I can almost guarantee it's shot. If the bearing is good, you can try refilling with silicone (search the forum for that thread) if you don't want to shell out for the expensive replacement. Aaron found that the 606 turbo clutch & fan fits and is much cheaper - he posted a thread on that, too.

With a good clutch & radiator, the electric fan could be removed and you'd never get over 105C.