Need to replace the battery in my 1996 SL600 (R129) as part of a maintenance action. No other problems; vehicle runs exceptionally well. But simply removing the battery will cause the ECU's to go into a 'Not Ready' state, causing OBDII to fail. While the infamous Check Engine Light will not come on, removing the battery will require a very, VERY lengthy series of drive cycles to reinstate the Ready condition within both ECU's. Obviously trying to avoid that scenario.

Looking for other member's suggestions on best practices to somehow keep passive (none ignition-related) power to the ECU's, radio, etc. while the battery is removed up to 30 minutes. Applying power through the cig lighter will not work, unless the ignition is on - a prospect I'm not thrilled with as all the electronics would then be energized. I would prefer applying power through the main circuit without having to energize all the electronics.

Cheers,
Jeff

 

Intersting question.I'm in a somewhat similar situation...I'm gonna be leaving my Bluetec unused for about 3 months,outdoors with no access to electricity,so I was thinking of bringing the battery inside and hooking up to a trickle charger.I was hoping to avoid the loss of any of the various settings...radio,transmission,etc,etc,etc.

IIRC there are small devices that can send a small (buy adequate) amount of power into the elctrical system so as to avoid said loss.But I'm an amateur and could well be wrong.

 

When I changed my battery, I took another battery and connected it carefully to my cables with some small light gage wires. If you are very careful it will work out fine.
As for long term sitting, you might want to add a (solar or plug in) battery tender to the car.

 

Thanks Dan for the suggestion. I am developing something along similar lines. However, I was looking for a more elegant solution, such as tapping onto a main 12v lug somewhere, preferably without tearing into coverings, etc. Obviously, the body/chassis is the ground. I'm planning the maintenance work next week and will generate a couple pics with a simple DIY through this fairly simple task.
Cheers,
Jeff

 

If you own a mid-90’s MB, especially an early OBDII version, then you are likely aware that removal of the battery is death to the OBDII ‘Ready’ condition. Removing and/or changing the battery does not necessarily disable the car, but taking power off the OBDII circuitry – even for a moment – resets the OBDII so that it requires a complete drive cycle to trigger the OBDII to a ‘Ready’ state. If you live in a community that requires an OBDII check during annual smog inspections, this can be a problem as during smog inspections the Inspector must not see the OBDII in a ‘Not Ready’ state. Simply resetting the CEL will not make the OBDII go to a ‘Ready’ state; only driving the vehicle extensively and under quite varied conditions.

So when I had to do maintenance on my ’96 SL600 that required the battery to be physically removed, I cringed. I searched for ways to keep power to the car, but didn’t want to turn the ignition on. With power to ALL the ignition circuits (and with a V12, there are twice as many of them) any glitch or issue had the potential of being disastrous. The only real suggestions were to provide power through the cigarette lighter. But that requires the ignition to be on; something I really didn’t want to do.

While scavenging around my local AutoZone, I came across the Schumacher SEC-12V-OBD. This slick little cable connects 12v directly to the OBDII circuits (through the OBDII connector) from an external 12v source. No ignition, no keys, no worries. Success!

Here’s how I used the cable to remove the battery from my SL600 for 3 days, while retaining the precious OBDII ‘Ready’ condition AND all radio codes, seat settings, window stops, etc. Arguably, the procedure is gross overkill for a seasoned DIY mechanic, but I wanted to write it for those more timid in trying such a task.

You’ll need the following items:
A) Schumacher SEC-12V-OBD Cable (~$15 at AutoZone)
B) 12volt Power Source (I borrowed an old battery from my local MB dealer)
C) Homemade cable – battery lugs to female 12v cigarette connector
For the cable, I used an old household extension cord. Remember, we are not trying to start the car; just maintain a keep-alive voltage to it. I soldered the lugs onto the cable to eliminate any chance of electrical noise.

Step 1 – Ignition off; key out.

Step 2 – Connect the Schumacher cable to the OBDII connector under the dash. Look for the red LED on the cable to light up. This confirms that the circuitry will accept an external 12v signal.

Step 3 – Connect the external battery to the male OBDII cable’s male cigarette connector. At this point, you will have 2 batteries feeding power to the car – the main in the trunk and the external battery.

Step 4 – Close all doors, open windows if need be, open trunk and disconnect the trunk light bulb. This ensures that there is no power draw from courtesy lights, interior lights, etc. You want as little power draw as possible.

Step 5 – Remove the ground lug from the main battery in the trunk. At this point, the main battery is electrically disconnected. If you have done everything correctly, the red LED on the OBDII cable should still be lit.

Warning and Caution – You are about to disconnect the positive lug off the main battery and there is still 12volts running throughout your car. Therefore, the positive lug being disconnected is still live with 12volts from the external battery. As a precaution, I wrapped my wrench with duct tape to ensure that if it slipped, it would not short out the 12volt power.

Step 6 – Remove the positive lug from the main battery in the trunk. Once the positive lug is off the battery immediately wrap the metallic end with a dry shop rag. This prevents the lug from coming in contact with ground.

Step 7 - Remove your battery and do whatever you need to.

The external battery will keep power to all critical electronics, preserving codes, etc. for as long as it stays connected. If you need to take an extended amount of time to do the work with the main battery disconnected, you can connect a small Battery Keeper – *NOT* a battery charger – to the external battery at the end of step 2. The Battery Keeper, being very low power output, cannot generate enough power to spike the 12volt line. Battery chargers are designed to drive a lot of power onto a battery; thus the horror stories about their use in MB’s. In my case, I used the Schumacher SEM-1562A Battery Maintainer. It’s designed for lawn tractor and motorcycle batteries, and only as a maintainer, not a charger.

When ready, reverse the steps to get back to normal.

Cheers,
Jeff

 

Interesting info,MB-Dude.I think I'll look for that cable and hope that it will be useful to me given that mine is a more recent model.