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My hobby is electronic repairs of high end test equipment.
I regularly reverse engineer such boards, in order to produce schematics so I can fault find on them.
I take several high resolution photographs of both sides (if it is not a multilayer board) and then painstakingly draw them out.
I have been looking for a BCM for about 1 year, so I can reverse engineer it.
However here is the problem, no one wants to give away their old BCU. I saw a burnt out BCU on eBay for £300!
This is wonderful news. To have an expert be able to get this to the next level of finding a solution is fantastic. Now we need to find a used BCM Maybe from a Pull Yard. I am going to ask my used car guys who go frequently if it is possible and reasonable. Gene I am excited!!!!
If you read the other thread on this subject a fuse is discussed but there would be a fire anyway in the wiring harness which contains 12-16 gauge small wires. They are the fuse and they burn up. In other words the fire is in the harness and bcm not before it. So a fuse is useless. Think of it this way the fire starts before a 250 amp battery fuse would blow.
Fuses are literally made to prevent overloads and fires. The BCM does not pull anything close to 250A. A much smaller fuse can be used. Honestly there are suprising very few wires to the BCM, front battery (fused), rear battery (unfused!), control lines for K57 & K75 (gound side), CAN B hi & low, and a couple of chassis grounds. The front SAM feed line taps in there, but only for convenience. Same with the K57 relay. Only the front and rear battery can supply energy and the front one is fused.
Collectively we can probably nail this issue - at least design a preventative fix, if not determine the root cause. I would think it more likely that the shorts are caused by failed regulators in the BCM rather than shorted smoothing electrolytics. That answer may come to light.
I don't understand why the rear battery has such thick cables and is unfused, because it supplies electrical power for the entire vehicle (including the starter solenoid) but with the exception of the starter motor armature. That is from the battery systems PDF. So there is a practical limit to how much current it will provide. So why not fuse all the connections to the BCM and see what happens? Surely that 100 A fuse in the prefuse block could be safely reduced (with an additional inline fuse). I would rather have the inline battery fuse blow than it burn up. The car will start and run with the rear battery disconnected, as that's when relay 57 kicks in - when the battery is missing or very low.
There is a reason why the rear battery is not fused to the BCM and I would like to understand why.
I think that both the legal and regulatory paths are pretty limited at this point. If it occurred with the same frequency on 15 year old F150's, something might get done, but not on a low production car like this. I'm more concerned about preventing damage beyond the BCM and I think that is very doable.
K57 bridges the two batteries. It should only be energized when the car is woken up and rear battery voltage is low. If there was a short between the BCM and the firewall fuse, that front battery wire could be overloaded by the rear battery. K75 turns off the cigarette lighter and trunk socket. I believe that they are both fused. The coil power to both relays is the front battery feed with the 100A fuse.
Mike, here is a solution that kills two birds in one shot.:
Using a golf cart continuous duty battery switch, it stays normally open, when energized using a low current, it closes. So, connecting this relay between the any of the poles of auxiliary battery allows to disconnect the battery when the vehicle is not in use and will connect the battery when power is applied to the coils. Since the coil current is low, it will stay engaged as long as needed with no significant drain.
So, the only way that BCM will burn, if unlucky, will be when driving the vehicle! This also could be prevented by using an oven capillary switch to monitor the BCM area temperature...with that the connecting relay could be disconnected and problem solved!
If nobody knows how the problem starts, how can you go about fixing it? At this point the only way to fix this is to disconnect both batteries. I am inclined to believe the relays and rear battery condition and charge are to blame.
The relays that are used with the large gage of wire going to them is questionable at the least. If the bcm switches on the relay when the service battery is low, why wouldn’t it do the same if it were disconnected? If the r car can start and run with a bad rear battery, it makes little to no sense that disconnecting the rear battery is going to solve anything.
If nobody knows how the problem starts, how can you go about fixing it? At this point the only way to fix this is to disconnect both batteries. I am inclined to believe the relays and rear battery condition and charge are to blame.
The relays that are used with the large gage of wire going to them is questionable at the least. If the bcm switches on the relay when the service battery is low, why wouldn’t it do the same if it were disconnected? If the r car can start and run with a bad rear battery, it makes little to no sense that disconnecting the rear battery is going to solve anything.
There is a wakeup wire direct from the BCM to the EIS. When the rear battery is dead, all modules except for the BCM are dead. The only way to wake up the car is to insert the key. This causes the BCM to activate K57 and K75 and send can messages disabling the convenience features. K57 is only activated for brief periods on wakeup, otherwise the front battery would discharge along with the rear.
Gents, so far I have been lucky and my SL500 2004 BCM has been responding as intended. But well aware of the potential issues I have been collecting all the info I find just in case I need guidance in the future. In the meanwhile, I keep my fingers crossed while finding ways on how to prevent a catastrophe.! I have the attached piece of information that may be useful for some to comprehend how the system works.! So, let keep on digging to see if one day before our cars get on fire we find the perfect solution..!
Gentlemen, I have on my hands the low drain electric golf cart relay. All I need now is to find a positive key on/key off connection somewhere in the trunk area where to connect the relay coil. I am sure some of you have done such a search that have the answer right in your memory. Please let me know where to find that green "+".
I'm pretty sure with this configuration you will always be starting in emergency mode and convenience functions will be disabled for a time. If the car is locked, you will have to use the hard key to unlock it. You will need to reset the steering angle sensor and probably normalize the windows.
If you put the positive battery cable and cable to the front prefuse together on one terminal and then run a new switched wire to the BCM, you can keep all of the other modules powered so that your fob will unlock the doors and you will not have to reset the steering angle and windows. I think it will still start in emergency mode, so convenience functions will still be disabled.
One step further would be to do the above, but use a remote control switch using battery or circuit 30 power instead of ignition switch power. These are available as a theft prevention device. You would 'unlock' the BCM, then unlock or open the door, waking up the car. The car should wake with good voltage seen by the BCM and all functions should be normal. You do have to remember to 'lock' the BCM when you park the car. It is not as failsafe as using ignition power, but avoids the issue with unavailable convenience features.
I still think that fusing the rear battery connection to the BCM should provide adequate protection. The BCM is not some magical source of energy. If you minimize the energy available, the risk of a fire is greatly reduced or eliminated.
I still think that fusing the rear battery connection to the BCM should provide adequate protection. The BCM is not some magical source of energy. If you minimize the energy available, the risk of a fire is greatly reduced or eliminated.[/QUOTE] How would you do this wire?? and what fuse would you use? thanks Gene
I think I would mount a megafuse fuse holder to the left of the rear prefuse. That allows access without removing any trunk lining. Run the branch of the positive battery cable that goes to the BCM and the cable that goes to the front prefuse to one end of the fuse, make a new cable going from the other side of the fuse to the BCM.
As far as fuse sizing, you need to account for the K57 relay function. If you leave the K57 wire attached at the BCM, emergency power will run on this fused wire. Since the K57 supply from the front battery is fused at 100A, I would use a 100A megafuse to match. I am confident that will be more than adequate for the charging function as well where current will flow in the opposite direction.
Another option would be to reroute the K57 output to the battery side of the fuse. This would probably allow a smaller fuse (30A?) to be used as it would only see current for front battery charging and not emergency BCM operation.
Mike, 100 amps still could do some damage..! I have an alternate idea......
Use the switched connect/disconnect on the hot side, then in parallel, place a 30 amp fused connection. So, when I disconnect the battery on shut off, the 30 amp fused connection will maintain the steering and windows functions ok while the vehicle is at rest, then when ignition is back on, the 250 amp connection will bring the system to normal.
Any thoughts on this?
The question would be if there is ever going to be over 30A on that wire without the ignition on. My fear is that when a door is opened and the car wakes up, you might get several modules drawing a few amps and then maybe the SBC or PSE pump kicks on and takes it over 30. Also, if the rear battery is low, the BCM needs to activate emergency power before you will have circuit 15 voltage. When that happens, the front battery connects to the rear one at the BCM. I would not be surprised to see >30A load in that case. If we route the K57 relay to the battery side of the fuse as I mentioned above, we allow for 100A emergency power, 30A DC-DC converter power for the BCM, and everything is protected. The dual battery information says that the max. charging rate for the front battery is 15A. Even with the loses in the DC-DC converter, 30A should be more than adequate. You could also remove the front battery supply from the BCM and add a 30A fused lead. That would limit the BCM to 30A from the rear battery and 30A from the front.
Another thought, preventing a fire is simple by covering the BCM and surroundings with a rockwool or fiberglass mat so the fire is contained within. The problem is to reconstruct all that mess and find the "almost extinct" replacement BCM.
The idea is to that in case of failure, to preserve as much as possible of the BCM and connectors since a "electronic board repair service" could salvage the part.
I tried to get a spare BCM and the holders know the scarcity and discontinuity issues. This makes them the holders of a precious commodity at $$$.
Another thought, preventing a fire is simple by covering the BCM and surroundings with a rockwool or fiberglass mat so the fire is contained within. The problem is to reconstruct all that mess and find the "almost extinct" replacement BCM.
The idea is to that in case of failure, to preserve as much as possible of the BCM and connectors since a "electronic board repair service" could salvage the part.
I tried to get a spare BCM and the holders know the scarcity and discontinuity issues. This makes them the holders of a precious commodity at $$$.
I think the addition of some kind of fire suppression device would also be a good idea such as:
08-10-2023, 07:08 PM
