horrible

I ask this because I read in the dual battery system brochure that if your accessory battery was dead you could still start your car if you opened the EIS door.

My rear battery was dead after my SL 600 sat for a couple months.
It was below 4 volts!

I checked the starter battery and it was well above 12 volts but it didn't do a thing when I tried to start the car.

I have replaced the rear battery and all is well.

bha

Starter (front) battery is only used for starting the car
Consumers (rear) battery is only used to run the electronics.

If your front battery is good, the car will start. If not, it won't start.

I have started the car many times with a rear/accessory battery completely dead, yet the front one is good. Not sure what you mean about "you could still start" if the rear battery is dead. You can start no matter what the condition of the rear battery.

Sounds to me like you need a new front battery. My front battery lasts longer than a few months, so your should still have enough juice to start the car.

Rear battery may be OK. Get the car started, drive around for 1/2 hour to charge up the rear, and then test it. Or you can trickle charge it for a few days.

Usually when you start the car and it says "consumers offline" every time, you know you need a new rear battery. If you start the car and everything works right away (heated seats, roof, etc.) your rear battery is in good shape. Note that all of this is assuming you drove the car at least 1/2 hr in the last few days to charge up the rear.

horrible

If you notice my front battery was well above 12 volts so I don't need a starter battery.

It must be a bad relay because it will not do anything with a dead rear battery.

Started right up when rear battery was replaced.

Still need an answer on the EIS door?

bha

That's definitely weird. Back battery should have nothing to do with it.

There is a controller that connects the front and back batteries. It's a module that decides when to connect the two, and when to disconnect. For example, when the engine is started and shortly after, it disconnects the two. Then it connects them once the front battery has charged, so the back one can charge up too.

Can't remember the name of it, but it's in the trunk and I had to replace it on my car. Symptom was the back battery would never charge. Even replaced the back battery but it still wouldn't take a charge. Did a diagnostics test of the car and it came up as the module being bad.

EIS is the ignition switch where you stick the key in. I believe the EIS door is that little flap that opens when you put the key in. I think it's a "smart" flap so it knows whether it's open or shut.

Whatever is instructing you to open the EIS door probably means you need to stick a pen in that hole to open the door. I suspect so that the car thinks a key has been inserted and "wakes up" something because of it. For example, I think the instrument cluster lights up in some cars when the key is inserted. There are probably a few other things that happen too.