750 battery isolation switch test/bypass for my P1461 code (anybody got one?)

[RIboater]

I think this switch may be the culprit for my P1461 problem. If I throw the top battery on a charger for a few hours and start the car, I don't get the code. Yet just stopping for gas and starting the car again the code pops. Ironically, this is not the battery that starts or heats the cats.
I read a previous post from years ago where Edwin stated to test the switch, if you pull the negative from the top battery, it should switch over to the bottom battery for power, which mine never has. Car power goes out. But last night I undid the ground and the trunk light stayed on. So i'm perplexed. This is what Edwin stated SHOULD happen. Disconnecting the ground has always made the car go dead and last night, it did not, which is supposed to happen. bottom battery took over. Yet when I went back to the car hours later, it had no power until I put the neg cable back on. When the car is running, both batteries are showing 13+V. yet even driving of half hour, shut car off, wait 5 minutes and start again, the code pops.
Also, the E-Kat module under the passenger seat "appears" fine. No water damage or visible signs of failure.
Wondering if there is a way to bypass this switch? there is a small trigger wire I guess tells it when to be on/off/sleep.
Anybody have a spare willing to let me test it on mine? I did search Ebay and the only one that comes up is Murad who of course wants a million $$ for it.

Edit: after sitting overnight, both batteries read 12.6+. started car and code popped. Shut car off and disconnected neg from top battery and trunk light went out, as it always has, which apparently shouldn't (other than last night when bottom battery did take over and keep light on, a first) Edwin also stated the with the top battery disconnected, the car should still start. It does not. dead.

[psjr]

Have a read here to see what it should and shouldn't do:
http://files.psjr.org/BMW_Tech/battery26-30.pdf

[RIboater]

I did read that from some of the research i was doing. It's a bit confusing for me, (not hard lol). explains what it should be doing but not really how to test that it is.
TD signal on the Kbus? KL? Katon (low signal,which the P1461 mentions)
I would say Driving mode works. Both batteries receiving 13+v but not fully charging the circuit battery.
I suspect is using safety starting since I seem to often have a low Voltage circuit battery, and charging it before starting, the code does not pop. but this confuses me since stopping the car for just 5 min the code will pop again. maybe drive mode isn't working property to charge the circuit battery.
I always thought that if the starting battery was weak, the circuit battery would kick in to help. this says it will not.

[RIboater]

Since it's called an isolation "switch", I opened it up hoping it was mechanical and maybe a little WD40 would loosen it up. No such luck LOL
What I did find was what looks like a burned out or compromised little circuit thing. top one of the two in center. pic isn't super clear but it looks a little toasted/melty.
any electronics pros out there that could identify it? it's in a super easy spot to desolder and replace for about .10

Attachment 616788

[chrisn7]

Link not working for me..

[psjr]

Yep, pic missing for me too.
Regarding not charging the circuit battery, as shown in the diagrams in the document I linked above, the circuit battery is the one hooked directly to the alternator, and so should always be charging. The starter battery is only charged by the switch closing while the car is running.

To explain some more things -
KL = terminal
terminal (KL) 15 = ignition in position 2, KL R = ignition in position 1 (radio), KL 30 = constant power, KL 31 = ground
TD signal = engine RPM
KATON is a dedicated signal line between the battery switch and the E-Kat module, for the E-Kat module to tell the battery switch that the cats are on.

[chrisn7]

It does sound as though you have a problem with the switch, but I would get the circuit battery checked out before installing a new/repaired switch.

At start up, if the circuit battery voltage is too low, the switch closes to allow the starter battery to supply power to all systems for 30 seconds to permit starting, so perhaps the switch fault has been caused by a faulty circuit battery?

[Cubehacker]

Chasing the same problem both battery are new and fully loaded ithink it could be the battry switch or the ekat modul.

[RIboater2]

Houston we have a problem... the switch is discontinued. I've emailed multiple BMW dealers and parts suppliers who either state discontinued or back ordered with no ETA.
What's happening on mine is the switch is not letting the bottom battery do the job of starting the car and heat the Ecats. The smaller circuit battery is being asked to do it, which is why the code pops. it's not strong enough to start the car, heat the cats and also run all the other equipment. If I put the top battery on a 50amp charge for an hour and start the car, not code. confirmation that a stong surface charge gets the job done. but even though both batteries show 13.5 while the car is running, it's not enough for the top battery. even just stopping for gas and restarting will pop the code. Another indicator the switch has failed is if I pull the negative cable (car not running), the bottom battery should take over. It doesn't. Did once but a fluke. Car loses power (trunk light)

[psjr]

That's not an operating mode of the switch. The top battery is too small to use for starting, and the gauge of the cables that connect the two through the switch are too small for starting amperage, and the switch itself is not rated for that amperage.
Both batteries showing the same while running is just due to the switch closing to let the alternator charge both.
The switch is always open with the car off unless the starter battery is being charged (i.e. by a battery charger on the hood terminal).
There is definitely a possibility of a bad switch if you found some burned parts inside, but the behaviors you are observing are not indicative of a bad switch so far.
You can always isolate the two batteries by simply unplugging one or both battery leads from the switch. That would make sure there is none of the back feed you are suspecting. The only side effect would be the bottom battery would not be charged by the alternator. Also of course make sure the plugs are in the correct positions in the switch with the top battery plug on the right and the bottom battery plug on the left.

I bought a spare switch earlier this year, but not sure where it is or if it was even ever shipped to me (forgot about it until now)... I will have to take a look.

[chrisn7]

Good point about the plugs to the switch - not only in the right positions, but seated properly

[RIboater2]

thought about the plugs because i've had the switc\h off before but they are dummy proof. they won't fit in the wrong hole.
So does this sound right: disconnect top battery, trunk light goes out (has stayed on before but rare). go to start car, get just one clunk from starter, car is dead. Go back to trunk and light is on so the switch must have changed battery being used. Considering the bottom battery starts and heats the cats, or should, why is it the top battery that i have to overcharge (50amp at least an hour) to not get the P1461 code? And the car won't start without the top battery connected.
does this point to a bad switch??

[psjr]

When you disconnected the top battery then turned the ignition to position 2, the switch detected the top battery voltage was low, and initiated "safety starting" (using the bottom battery to power the electronics for 30 seconds or until engine start). That is why the trunk light was on for a bit.
The fact that the car didn't have enough power to start in this configuration IMO points to a weak bottom battery.

[RIboater2]

It is 5 years old so at first I suspected that as well. took it to be bench tested twice and came up full charge,no dead cells and above rated cca.
no problems starting the car. so far.

[chrisn7]

In post #4 you stated that the battery switch 'looks a little toasted/melty' (we weren't able to see the pic). If that's the case don't you need to resolve this first?

[RIboater2]

Not the entire switch. I opened it up to peak inside and one of the diodes, resistors, whatever it is, did appear compromised. And yes i need to resolve that but none are available. Part is discontinued by BMW

[tjpike]

Not the entire switch. I opened it up to peak inside and one of the diodes, resistors, whatever it is, did appear compromised. And yes i need to resolve that but none are available. Part is discontinued by BMW

Search for part# 61356906383

RealOEM says they're still available.

http://www.realoem.com/bmw/enUS/showparts?id=GJ03-USA-07-1999-E38-BMW-750iL&diagId=61_0121

[RIboater2]

Here's a strange update. We had a 10 day stretch of 5-10 degree weather and I was not getting the code on start up. Unfortunately I had a faulty thermostat and was getting a code for low coolant temp so I still couldn't take the car for inspection. grrrrrrr
I've now replaced the thermostat so other than hoping for another stretch of 5 degree weather (can't believe i'd hope for that) still getting the p1461 code.
What or how could the outside temp factor into this?

[RIboater2]

was able to find a used battery disconnect switch and did not stop the code from coming up. But I suppose this switch could also be bad.
both new batteries.
nothing works other than that stretch of arctic cold, which is weird, or putting the top battery on a 50 amp hyper charge before starting the car. Which doesn't stop it from coming up next time you start the car.

[740ilDuke]

That stinks that BMW no longer makes those. Discontinuing stuff like that will be a PITA for 99-01 owners. I've had a couple 01's over the years and now have the 1998 750 - which doesnt have the dual battery setup or electronic catalyst, thankfully.

I had similar issues with my last 01 and it was the switch. Another option that might serve you better would be wiring the batteries differently and having the code removed that creates your fault light. That was my approach for the Ekat module faults. In Europe, the EKAT system does not trip SES lights. Both the isolation switch and eKat module look exactly the same, btw - although of course they are not.

If I see one Ill link back.

[RIboater2]

I've tried various battery configurations including adding a second battery in parallel and running a H9/95R as the "top" battery thinking if the car insist on starting using the top battery, give it one. I can return this disconnect switch, and will need to try another to rule out this one is also not working. Then theres the ekat module like you said. I pulled the passenger seat out to visually inspect it, just because of all the stories of water getting down there, but it "looked" fine. How did you remove the code?

[740ilDuke]

I've tried various battery configurations including adding a second battery in parallel and running a H9/95R as the "top" battery thinking if the car insist on starting using the top battery, give it one. I can return this disconnect switch, and will need to try another to rule out this one is also not working. Then theres the ekat module like you said. I pulled the passenger seat out to visually inspect it, just because of all the stories of water getting down there, but it "looked" fine. How did you remove the code?

DUDMD removed the code for me on my previous 01 750il. It was cheaper than a new module. https://www.dudmd.net/

In reflecting on your post about charging the battery, it makes me think you could just have a weak/bad battery. That's typically what activates the
switch IIRR. Top runs accessories, bottom cranks and heats cats. I replaced both with AGM batteries upon purchase of my former 01, which narrowed
the issue to being the switch.

[RIboater2]

I've messaged him. to $$$ for my liking because he will only do the full performance mod and not just turn off the ekat. I'm also not sure I'll pass RI emissions inspection.
it's not a weak battery. I bought a brand new top battery and within 3 months exchanged it for another brand new one thinking i got a bad one. running a 50amp charge for an hour or so directly on the top battery will prevent the code on first start but not after just running to the store an come out 10 min later and start it.
I just bought a brand new bottom battery and that didn't work either and was able to return it, but before doing so, connected it as a top battery. that didn't work either. thought it might being a bigger stronger battery trying to mimic the hypercharge i've put on the smaller battery that worked.
Both batteries are receiving a charge with the car running. The heated catalyst monitor never comes online either. I can have two monitors offline and pass inspection, as long as the CEL is not on.
I'm still very perplexed why a stretch of 5 degree weather also kept the code from popping.

[740ilDuke]

Out of curiosity, whats the cluster say the alt is kicking out?

Double check the + / - cables for tightness too.. hmmm.

[RIboater2]

13.4-13.6 confirmed at both batteries. The cables have been on/off many times at this point and are tight. I believe it's just that disconnect switch not telling the bottom battery to start and heat cats. I believe that's why putting a heavy overcharge on the top battery is a temp fix. But again, don't understand using a brand new H9 with all that cca as a top battery didn't.
I need to find another disconnect switch to rule out the one I just got isn't also bad.