Burnt 100A fuse to the rad fan

Ray Diagnostician Ontario Posted   Latest   Edited  
2005 Mercedes-Benz CL500 5.0L 7-spd

I was called to a shop to diagnose a burnt 100A fuse to the rad fan. I checked the power wire possibly shorting to the right fender, between the 100A fuse on the positive battery terminal, to the rad fan module and I found that the power/ground connection terminals, on the module, were touching. When I pushed on the terminals, the new fuse blew, so I separated the connections.

What do you think of the 86A motor draw at 90% duty cycle.

Albin Diagnostician
Albin Default

That current looks pretty high to me. Any time I see a load over half the fuse rating, I start looking for a problem. Your load is getting pretty close to the current rating of the fuse. I would imagine the problem you found with the terminals touching was only a symptom of the problem. It is pretty hard to argue with the test you have used to determine the circuit load.

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Ray Diagnostician
Ray Default

Thanks Albin, the fan "feels" good when I spin it by hand and the shop owner is considering getting a used fan/ module assembly.

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Dmitriy Analyst
Dmitriy Default

The 86A value is the peak. The fuse (being a resistive heater) sees the RMS-averaged current (somewhere in the middle of the hash). The DC average current (at 90% duty cycle control) on this waveform is 67A, while TrueRMS=68A. diag​.​net/file/f2j1m4z01… ​ The standard rule for DC motors is to use a fuse rated at 150% of the full load current or the next size up. NOTE: edited to…

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