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Red Battery Light Saga continues - why is the K57 Relay always warm?
Car is 2003 SL55 with about 140k miles.
Red Battery Warning message comes on the dashboard as soon as I start the engine.
Resetting the fault code (B1828) via STAR software doesn't help. It comes right back.
Within the last 2 weeks, I have replaced:
Both batteries are brand new
100Amp fuse in the foot well is brand new
K57 Relay - brand-new OEM relay
Both batteries are charging well when car is running (13.8-14.5V)
Alternator voltage regulator was replaced 3 months ago, using BOSCH regulator. Alternator is only 2 yrs old.
After all this, the red battery warning message still comes on, every single time I start the car !!
Here is the interesting finding I am trying to find an answer for - WHY is the K57 relay next to the BCM always warm, even when the car is off? It looks like the relay is constantly energized.
Per the documentation, this happens only when the charging system is in 'emergency' or 'limp' mode. If one battery is bad or not charged, then the relay energizes and stays closed (and will be warm).
On my car, both batteries are new and both are charging well ....so why is this relay constantly energized and warm?
Is this a sure sign of bad BCM module, since this module controls the K57 relay? Anything else I may be missing?
The BCM module is a $600+ part. I don't want to replace it for no reason if the issue could be hiding somewhere else, but honestly, I am really running out of ideas. I've replaced all the other possible weak areas.
Here is a new twist .... i just removed the K57 relay and now the warning light is completely gone! I started the car multiple times - no error! No warning! Miracle!
Both batteries are charging just fine when the car is running.
How is this possible? Why removing the K57 relay not only doesn't generate an error, but it actually cleared the red warning light? This makes no sense.
And before you say "you have a bad relay", i have tried with 3 different relays - 2 of them were brand NEW, OEM i just bought from dealer. Any of the relays were getting warm when plugged in, even with the car OFF.
However I do have some experience with auto electics on my old SLK350, and electronics in general, so maybe I can offer a little!
K57 is running warm, even when the car is turned off!
With relay removed terminal 30 is connected to rear battery +ve, terminal 87 to front battery +ve, terminal 86 to front battery +ve and terminal 85 to BCU connector 1 pin 5.
K57 is only energised when terminal 85 of K57 is grounded in the BCU.
So issue is either BCU giving ground, or ground in the wire to the BCU.
Diagnostics I can suggest:-
With key out of ignition...
remove K57, connect a SMALL bulb between terminal 85 and 86. If it lights then K57 IS being energised.
Remove connector 5 from BCU and repeat. If bulb is still lit, then there is a short in the wiring between BCU and K57. If not then BCU is suspect and you will need DAS to find out if internal switch has failed, or something is triggering it.
Remove connector 5 from BCU and repeat. If bulb is still lit, then there is a short in the wiring between BCU and K57. If not then BCU is suspect and you will need DAS to find out if internal switch has failed, or something is triggering it.
Do you have a wiring diagram for this relay?
Welcome to the SL forum and a huge thank you for replying.
My DAS tool unfortunately is not very helpful. It keeps pointing to "faulty relay K57" (replaced relay 3 times) or "no circuit 61".
As I mentioned, earlier when I removed the relay completely, the error/warning went away for some reason. I drove the car for an hour, no issues. At the end, I parked at a store, went to get something, came out, started the engine and the red warning light came back on.
Drove home, scanned and same error code was stored. I was able to reset it and the red warning light disappeared. Before, with the relay inserted, the code kept coming back as soon as I reset it and restarted the engine.
With the relay out, once I reset the error code - no red warning light, even after 3-4 engine restarts. It's bizarre that the system doesn't even detect the missing relay.
Just for fun, I inserted the relay, started the car...and no error message the first time. Stopped the engine, started again, and the red light came back on. Same error code stored. I could not reset it with the scanner. Relay was getting warm again and the red light was on with every engine start.
I removed the relay again, reset the code, and the red light is not coming back, for now. Clearly, when the relay is inserted something is acting up, keeps the relay energized and triggers the warning light.
I will try your testing method with the bulb tomorrow. One question... when you say "remove connector 5", which physical connector are you referring to?
On the BCM, there is one wide, multipin flat connector and 2 terminals with thick wires going to them. See picture. The K57 relay is next to the BCM (red cable goes to it)
Where is "connector 5" ?
I am referring to connector 1, pin 5. As only one connector it will be Pin 5 on it.
As to observations I guess (!!) BCU checks voltages on both batteries ONLY during startup.
If everything normal (no fault) both voltages will be high, but different by a small amount.
If one battery is very low voltage, gives an error and turns on K57.
But in your case, if it sees both batteries with identical voltage it flags an error as it assumes (sees) short circuit between both batteries.
Above is only a guess that explains what you observe, but seems plausible.
My guess is BCM is faulty, and turning on K57 permanently.
Test I suggested should help confirm this.
Reason I suggest bulb is to give small load to BCM, so small current flowing, to simulate relay coil.
Multimeter can give bad readings when used for electronic switching devices.
BCU will switch using transistor, which can fail.
If you have DAS/STAR in developer mode or vediamo you should be able to switch K57, but again I have no experience.
When I get my SL500 first thing I will do is scan it for a list of modules fitted and backup the settings. I can then find out what can be done.
Peter
Looking at the wiring diagram I don’t know how it would be charging both batteries, and if it is doing so, what are the downsides of constantly charging both batteries? Would a battery isolator do the same thing with less cost?.
the electrical engineering on these cars leaves much to be desired. And simplifying this system would be better.
Looking at the wiring diagram I don’t know how it would be charging both batteries, and if it is doing so, what are the downsides of constantly charging both batteries? Would a battery isolator do the same thing with less cost?.
the electrical engineering on these cars leaves much to be desired. And simplifying this system would be better.
When K57 is not energised, both batteries are isolated.
However there is a DC to DC converter that links them. A DC to DC converter takes one DC voltage, and either drops it, or increases it, to anothe DC Voltage.
In this case it takes the Auxilary battery and increases the voltage and applies it to the starter battery, and acts as a battery charger,
When k57 is energised, both batteries are joined in parrallel, in the case one battery is too low voltage (particularilly the auxilary battery), as no onboard systems could turn on.
Why seperate them? From WIS....Extends driving time for safety by disabling part of the electrics, Garantees vehicle will start if Aux battery is low voltage (e.g. electrical fault discharges battery, or lights left on overnight),
It is much more expensive to do it this way (and eats into profit) than to connect both batteries permantly together, so I am sure MB had a really good reason to do it!
If they are isolated,how are they both receiving the same charge from the alternator? If the relay is constantly energized when the car is off, wouldn’t that drain both batteries eventually?
I wonder what goes south in the bcm to make k57 constantly energized? Sounds like switching k57 to an engine run 12v would be the backyard fix to the constant energized relay, probably not the battery light though.
If they are isolated,how are they both receiving the same charge from the alternator?
They are not!
Auxilary battery is charged from the alternator.
Starter battery is charged from the DC-DC converter, which is connected to the auxilary battery, and acting as a battery charger.
Originally Posted by cdk4219
If the relay is constantly energized when the car is off, wouldn’t that drain both batteries eventually?
No and Yes!
If K57 is energised when car is off, all that happens is the two batteries are connected in parrallel, making a single battery with higher A/h rating. This in its self will not drain the batteries, unless one of the batteries is faulty.
However if there is a drain on the electrical system (parrasitic drain) then eventually it will drain both batteries. Hence why they are normally isolated.
Originally Posted by cdk4219
I wonder what goes south in the bcm to make k57 constantly energized? Sounds like switching k57 to an engine run 12v would be the backyard fix to the constant energized relay, probably not the battery light though.
The BCM energises K57 by GROUNDING terminal 85 (i.e. switching connector 1 terminal 5 to ground) Terminal 86 is connected to battery +ve terminal.
The switching inside the BCM is likely solid state, i.e. a transistor, FET or SSR. When these fail they can fail "open circuit" (fail to turn ON) OR they can fail "short circuit" (fail to turn OFF). This second failure mode is likely what has happened, but needs diagnostics to test.
It is not desireable, or intentional, to ever have the two batteries cnnected by K57. K57 will only energise in an "emergency" mode if one of the batteries is seen to be at too low voltage to start the car.
Hopefully below may offer an explanation to your questions..
Originally Posted by cdk4219
If they are isolated,how are they both receiving the same charge from the alternator?
They are not!
The alternator charges the auxilary battery directly.
The starter battery is charged from the DC-DC converter. The DC-DC converter is powered from the auxilary battery, presumably only when the engine is running. Imagine it as a smart charger powered from the auxilary battery instead of mains electricity.
If the relay is constantly energized when the car is off, wouldn’t that drain both batteries eventually?
No and Yes!
When K57 is energised, both batteries are connected in parrallel making one big 12V battery. This in its self will not drain either battery, unless one of the batteries is faulty.
However! If there is a drain on either battery (parrasitic drain) then this will eventually drain both batteries (hence why they are normally isolated).
I wonder what goes south in the bcm to make k57 constantly energized? Sounds like switching k57 to an engine run 12v would be the backyard fix to the constant energized relay, probably not the battery light though.
K57 has terminal 86 connected permantly to the starter battery +ve. The BCU switches on K57 by GROUNDING terminal 85!
The switches inside the BCU will likely be solid state (transistor, FET or SSR) and have two possible failure modes. Open circuit or short circuit.
Open circuit means switch is always open (or OFF), so K57 never energises.
Short circuit means switch is always closed (or ON) and K57 will always be ON. I suspect the latter to be the case here. Only diagnostics as described or possibly DAS developer or vediamo, can indicate if this is the case or not.
Under normal conditions K57 should never opperate. Only during starting, if it sees one battery too discharged to enable starting, or if it detects one battery discharging during driving, will it energise and indicate a warning message.
It looks like when starting it checks the opperation of the relay, and flags an error if it is not operating correctly.
Read all the pages now it makes better sense. Looks like the bcm is in one of the emergency states at the bottom of the diagram. Not seeing the correct voltage from one or more of the inputs. Or star to reset.
If you have a few bucks to invest, this tool would help solve the problem, Power Probe IV.
Just picked one up used, but have not had a chance (reason) to use it (yet).
New BCM arrived today. I disconnected both batteries (rear first, front after) and I swapped the modules. Then i connected the batteries - front first, rear last.
The new BCM one is identical, except the HW date is 35/11 and the HW date on the original module is 44/00. Looks like the new module is about 11 years newer.[img]data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7[/img]
Inside, the components are visually the same, but the chips have different part numbers. I can post some pictures for comparison.
The good:
No Red Battery warning on the dash, even after i started the car 5-6 times, in short sequence, after a 10-min drive, after sitting idling, etc. No more red light. No errors. No fault codes.
The K57 relay does not get warm anymore.
Rear battery is charging great - getting 14.2-14.3 Volts at idle.
The bad:
- Front battery is getting no charge or very little charge. It stays at around 12.7 volts when the car is at idle. I measured it 5 times, and it's always around 12.6-12.7V.....didn't go down, even after i restarted the engine 5 times. This is also a brand-new battery, so it's strong.
The 100amp fuse in the foot well was replaced 2 weeks ago, so why is the front battery not getting more volts now?
Or is this expected behavior since the BCM now works properly? Does the front battery start charging ONLY if it goes under a certain voltage? I need to review the documentation again. I am getting confused after weeks of troubleshooting. [img]data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7[/img]
So, overall, I see some progress after I spent $670 on a new BCM. I just need to figure out the front battery charging situation now. Likely I will check the 100amp fuse again tomorrow. If it's blown, the BIG question is, what caused it to fail again? This is a big fuse. It's not easy to melt a 100amp rated fuse. And I was extra careful to disconnect the batteries before i touched anything.
Well done on what looks like a successful diagnostics/repair.
Originally Posted by danmm7
Update:
The bad:
- Front battery is getting no charge or very little charge. It stays at around 12.7 volts when the car is at idle. I measured it 5 times, and it's always around 12.6-12.7V.....didn't go down, even after i restarted the engine 5 times. This is also a brand-new battery, so it's strong. [img]data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7[/img]
I just need to figure out the front battery charging situation now.
Is the front (Starter) battery actually discharging?
I think you will see it maintain 12.6 to 12.7v, except during engine cranking.
Reason......The starter battery is NOT charged from the alternator, so will not see the alternator voltage, nor the Auxilary battery voltage!
The Starter battery (as mentioned) is charged by the DC-DC converter, which takes the auxilary battery voltage, and uses it to charge the starter battery, just like a battery charger connected to the mains.
It will maintain the front battery at optimal voltage (trickle charge it).
To prove it, start and stop your car several times in quick succession. Does the voltage drop below 12.6V. If it wasnt getting charged then you still have an issue, but I suspect it will maintain the 12.6V regardless how often you start the engine.
Your old BCM can probably be repaired by electronic engineer, as it seems the Solid state switch on pin 5 is permantly short, and should be easily replaced by SMD re-work. If you were in the UK I would love to take it off you and investigate the circuits.
Yes it sounds like the bcm is doing it’s job, before with k57 on it was seeing alternator charging, now it’s being charged from the converter as stated above. Both of my cars show alternator voltage at the starter battery, and both have battery drains I would guess both have bad bcm’s
Disclaimer: not an electronics expert, just know enough to be dangerous.
The bottom half of the board is clearly the DC-DC converter. The lower 6 MOSFETs are all part of that and not used for relay actuation. I'm not sure about the top large MOSFET, possibly used for board power supply. There are also appear to be two other power transistors, one in the lower left corner of the 'upper' section of the board and one above the long row of caps, right of the processor. My guess is that these are used to power the K57 and K75 relay coils.
I would be interested in your old board to evaluate and attempt to repair.
My friend, in electronics, all these components may look great from the outside, but the inside is what it counts. Check the voltages of the front and rear batteries with the car running. Must be around 14.7 volts. I four rear is 14.7 indicated alternator is good, if the front is below 14.7 (12 vdc) it means your BCM is not doing the job. Your could check the starter battery fuses by checking continuity between the positive cable (disconnected) and the terminal 30a disconnected from the BCM.
If there is continuity and starter battery is below 12.8 VDC, I would send the BCM for repairs.
Yes, I scanned for codes. I initially got B1828 K57 Relay. I replaced the relay cleared codes, but the red warning light returned right after start up.
Now I have lost communication with the BCM and all items on that CAN BUS.
Sent BCM out to have caps replaced and board checked.
I received my BCM back from the electronic repair facility. Plugged BCM in with fuse mode, red battery light not showing up on the dashboard. I can communicate with the BCM, but not any other module on CAN BUS B.
11-10-2021, 01:45 AM
Red Battery Light Saga continues - why is the K57 Relay always warm?
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