[Tom Manning]

Good idea but why not splice the fuse into the original connector wires?

If you look at the circuit, heavy connections meet at 30 and 30a then connect to the BCM so the fuses must be between where the wires meet and the terminals on the BCM. This way the BCM is fused and the connections between the alternator, battery and K57 are retained. Why MB let this into production without fusing the BCM I will never know.

OP in the earlier thread used this type of fuse which is neater than mine. I just hung those inline fuses out the top so I can keep an eye on it for now.

I think I'll redo mine to this design now that I know it's all working. Note that he hadn't connected relay K57 which I have definitely retained.

No reports of water into the BCM that I have read - it's an internal (infernal?) issue caused by either shorted caps voltage regulators, of which there are several, which can short out and pass massive current.

So hopefully this mod is the way to go and more members can do a bit of PM insurance work themselves for a few dollars!

 

[Tom Manning]

Good, I'll make something up. Now it would be illuminating if the BCM decides to short out and blow the fuse, then at least I could trace it through and find out what's causing the issue. I guess if enough people make the fuse mod, one will blow in time and we'll know then...

I feel so much better having the BCM fused this way!

 

[Tom Manning]

If those caps should short to ground, it's going to draw a hell of a lot more current than 30A. That will blow the fuse for sure.

 

[Tom Manning]

I'll draw it up later, as a diagram will be easier to follow. In essence the two fuses are protecting only the BCM from sudden current surges, while all other functions such as the two emergency relays and the BCM itself are all still working exactly as originally intended.

 

[Tom Manning]

You'll get errors for sure - see the canbus wires out the side of the BCM monitoring it. Maybe you could do a work around.

Also you'd have to remove the 100A fuse in the prefuse block which would certainly blow if you're trying to start the car from the rear battery. That whole cable from front to rear might not be thick enough too.

Surely easier to do the fuse mod?

Here's how the two new fuses are wired, ref earlier discussion. You can see the normal battery connections are intact.

One other thing that occurred to me: say the rear battery goes low and kicks K57 in. During cranking, the ECU must signal the BCM to disengage K57, so as not to have full whack across F1.

OR does it follow that with a low rear battery, it's assumed there's not enough current to blow F1, so it keeps both batteries in during cranking ? Probably not though, as even if the rear goes low in voltage, it could still deliver massive current and blow F1.

 

[Tom Manning]

30 amps, shorted to ground, is of course plenty to cause a fire in any case...

I've been driving the car this week with no issues. One thing I haven't yet done is measure the current, with peak hold, through both of those fused circuits. It's going to be nowhere near 30A of course so I think I can safely drop the fuses down to 15 or 10A. We'll see.

 

[Tom Manning]

Right. That's for circuit 30A. I can't see it exceeding 10A though, even with a flat front battery.

 

[Tom Manning]

It would achieve the same result as fusing the two battery connections, as the ground is the same for both circuits.

What I haven't looked at are the connections on the side - those brown wires are just data I think so are only drawing milliamps or less.

I'll change the two 30A for 10A and see if they blow.

 

[Tom Manning]

I've packed mine away so can't look at it but guessed the BCM was grounded through the frame of the module, to the car body, not with a separate ground as Mike says. Perhaps both.
A 15A fuse might be the way to go as the minimum. I'll test it. The plan was to take it as low I can go until it blows then go up 5A.

I also check the 100A firewall fuse and it was intact. That makes it appear that the rear battery is the one feeding the fire. That's what I guessed, as the 100A fuse would probably melt if it was the starting battery causing the fires. So I might drop down the circuit 30 fuse to 10A and see how it goes.

Will you fit the fuse mod when you get the SL55 back together?

 

[Tom Manning]

FB user might have lost charging function to the front battery. All that battery has to do is crank the engine, so you'll get lots of starts out of it before it gives up the ghost.

 

[Tom Manning]

Yep. Perfect. I would expect if the BCM fails then that fuse will toast, so it's either working or it's shorted. At least if if blows I'll be able to find the shorted parts in the BCM pretty much straight away - probably the regulators in there, there are ten or more. Why this module was never fused I will never know.

 

[Tom Manning]

Could someone explain what the removal of the K57 relay accomplishes?

Don't remove it; it's there to connect both batteries in case the rear goes too low to power essential equipment such as the ECU and brakes.

****

Yes, this connector block looks good. It's a midi fuse holder. I was looking for a better mechanical fixing for the fuses, such as this one. I might attach them with small fly leads.

 

[Tom Manning]

If you want to fuse the connection to the front battery then, yes.

I don't think it's necessary as it's fed by a 100A fuse in the pre-fuse block which should blow if the BCM shorts through that circuit, but it's pretty clear that that is not what's causing the fires, but I did it anyway.

 

[Tom Manning]

Both, or if not doing the charging circuit, then just circuit 30 - the left side.

 

[Tom Manning]

Be interesting to know if anyone who's done the mod has had a blown fuse yet.

Here's how it's all wired:

 

[Tom Manning]

Send pics.

 

[Tom Manning]

Any reason not to install a 30a fuse directly at the battery terminal rather than 3 wires tied together at terminal 30?

Look at the F33 fuse block - that's getting full whack from the battery, and it contains a 60A and a 200A fuse.

Also the alternator is wired in series with a 200A fuse so that circuit will be drawing at least 60A.

 

[Tom Manning]

That's the 7.5A fuse in F33 IIRC.

 

[Tom Manning]

That fuse feeds the ignition module and the steering column. I've never had one go off, but presumably if an airbag is triggered, the engine will also shut down, along with the fuel pump - something to do with that control circuitry maybe?