[brabus3.6]

After some advise from George Fraser I purchased & installed a second battery, the largest yellow top Optima YT S 5.5 (75Ah, 900/1125CCA) in the boot. I have the standard Bosch 66Ah 510CCA battery under the bonnet. After wiring up the second battery by running some 4ga cable from the terminal the alternator (150A) is wired to to the Optima I noticed the ABS Light turning on. It comes on then goes out when ignition is switched on as it should. When turning over it briefly comes on then goes out. It then proceeds to turn on once I've reached 10MPH or so and doesn't go off until I switch off the car. Any suggestions? It seems like too much of a convenience for it NOT to be related to the installation of the second battery.

 

[Yal]

I am willing to bet that would be a blown fuse in the OVP.
Thats way I never jump start someone elses car with my W124. It could be that the swapping of the batteries caused the same kind of condition. Go ahead and check it out. If its not that then i'll put my money on the brake switch near the brake pedal.

 

[brabus3.6]

Yal thanks for your quick reply. I checked the OVP fuses already & they seem fine. I also changed the OVP over with my old one (which was letting the ABS work previously) but still no change. How could the brake switch you speak of cause this problem? Is it the one which causes the brake lights to come on?

 

[Kestas]

Yal could be right, and it could be happenstance that the brake switch decided to give out just as you replaced the battery.

It's the switch just above the pedal. It costs around $10 from the dealer. A very common problem.

 

[brabus3.6]

If it was the switch wouldn't my brake lights stop working too?

 

[brabus3.6]

Battery or alternator?

Today I decided to get my multimeter out on the problem so I checked both the front and rear batteries with everything off. Front battery ~12.3V, rear battery 12.46V. I then satrted the car which places no strain on the rear battery as it's purely wired for the soundsystem. Rear battery still 12.46V front now down to 11.9V, with lights etc on down to 11.6V. I decided to give it a quick rev up to 2500RPM then let it drop to idle. Sure enough the lights brightened up and the multimeter now read ~14.3V accross the front battery at idle. I'm unsure as to whether this is the battery or the alternator, I suspect the battery as it's been left to fully discharge once which could well have damaged it.

 

[Kestas]

brabus3.6 - 3/31/2005 3:54 PM
If it was the switch wouldn't my brake lights stop working too?

The brake lights would still work. But I believe cruise control would stop working at the same time.

 

[brabus3.6]

So is their a way of testing it or do I just have to replace it? The light comes on once I travel over about 10mph then remains on. It doesn't come on under braking. Thanks Kestas

 

[Kestas]

I don't know of any way to check it other than swapping parts.

 

[brabus3.6]

Tried a new brake switch - no difference

Swapped out the brake switch for a new one but the ABS light is still coming on. Any more ideas? Thanks

 

[Yal]

I would jack up the front and clean out the two front sensors. I would do one side, drive, then do the other side, that way you can isolate the bad one.

 

[NordStern]

RE: Battery or alternator?

Hi Brabus.

It would be interesting to know exactly how you connected the second battery. Where did you get ground? When you say separate circuit, are you sure? Does the lights on the stereo turn on with the rest of the dashboard lights?

What you experienced with your mulimeter is normal. Even if your battery has been drained, it might still be OK. Always top up a flat battery with a good battery charger.

One thing to consider, is that current travels the easiest route, just like water. That's why it's important to place ground correctly. If you screw up here, you might bypass the OVP and other weird consequenses. One circuit might suck up current unexpectedly through earth, and confuse sensors and controllers.

Imagine you have an old diesel truck with a main switch disconnecting the negative battery pole from the chassis. (frame) Keep imagining you can't find it, but you know you can roll it and let up the clutch. It will start, and you will see that lights and other electrical things work, even if the minus pole is not connected to to ground. Why? Because current from the now running generator will spread out on all the other ground connections so that they will seemingly work fine. But the battery will not be charged or used. So if a main switch gets disconnected somehow during driving, you might not notice it. Until the smoke burns your nose or something.

So lets hear how you connected this rig of yours. So we can chip in.

I agree with Yal that a constant glowing ABS usually means a defective sensor or controller. So you could ohm the front sensor coaxial cables, and should find between 1100 and 2300 ohms. Isolation resistance in the sensor itself should be over 20 kilo ohms.

Regards

Geir

 

[brabus3.6]

RE: Battery or alternator?

Hi Geir,

Okay, what I did was ran a wire from the trunk of the car with the 2nd battery to the cars OEM distribution terminal from the alternator which is near the fuse box. Both my primary & secondary batteries draw their current from this distribution point. My Alpine HU dims when I turn the lights on as it should, although now my instrument lights (intrument cluster, heater dials etc.) are all inoperative but I don't think that it's linked to this.

 

[NordStern]

RE: Battery or alternator?

Brabus.

Did you take ground at the same place as the other battery? Or somewhere else?

Regards

Geir

 

[brabus3.6]

RE: Battery or alternator?

Somewhere else! My second battery is in the trunk where I believe all the new model Mercs have there batteries, it's ground is to the vehicle's body at the rear. The primary battery's ground is the stock ground, also to the vehicles body but next to the suspension mounts. Thank You Geir

P.S. my instrument cluster lights now work. It was a bad solder joint on the dimmer (cluster out job!).

 

[spanners]

RE: Battery or alternator?

Hi brabus

I know it seems like a coincidence, but the ABS light coming on after 10mph is a definate sign that one of the front ABS sensors has gone bad.

The ABS system carries out a self diagnosis test when the ignition is turned on. If it goes out after a second or to then no fault is detected.

Below 5mph, the ABS system does not work so the light will stay off.

After 5mph, the system becomes active and if the light comes on after this point, it is probably a fault with one of the sensors ( nearly always a front ).

 

[brabus3.6]

RE: Battery or alternator?

Hi spanners,

I'm pretty sure that your right. I have cleaned all three ABS sensors but to no avail. I guess one of them has given up the ghost so to speak. I'm having a look at www.alldatadiy.com for the info on testing ABS sensors, apparently a special ABS diagnostic tool is needed []. There is surely another way, else I'll have to get a pro on it because thee sensors are too expensive to just use trail & error! Thanks

 

[spanners]

RE: Battery or alternator?

There is a way of testing it without the expensive tester but it's that long ago I can't remember the readings.

The way it is done is by disconnecting the sensor to test at its connection with the loom (on the fronts it is on the inner wing, the rear is under the back seat). Then connect a multi meter between the inner and outer cores and with the wheel jacked up, spin it by hand. When it reaches 5mph, you need to measure the Ohms.

Although I don't know what the reading should be, if you test all three, one should give a completely different reading to the others.

 

[brabus3.6]

RE: Battery or alternator?

Cheers spanners. I had a hunch today that it may have been caused by the magnetic field of the wire that passes very close to the ECU going to the second battery. I completely disconnected it for a test drive to no avail, so it must be one the sensors. Unfortunately I won't be able to resolve this for a while as I'm going back to Uni tomorrow. I will update when it's resolved.

 

[NordStern]

RE: Battery or alternator?

Brabus.

If I were you, I would take ground at the same place as your original battery, or at the very least, at the same place as you grounded your main amps. Did you fuse the secondary circuit? What stops the second battery to suck current through their common plus point?

A parallel connection is preferable, unless listening in a off-engine mode is desireable. A dual battery isolator is needed if you want a separate circuit. A smart one can steer the charging to where it's most needed. And where to take power according to specific or variable parameters. Be careful when operating with a drained secondary battery, it can strain your generator. A capacitator would help, but costs.

I agree with the others as far as the symtoms concurs with a faulty sensor, but it might also be the controller itself, or you might have tampered with ABS wiring somehow when installing the second battery. Maybe you shorted the plus at a vunerable place or circuit?

Using Spanners method will give you an indication of error in the aft sensor, because it has a different resistance value than the front ones. Measuring the resistance is easier. Besides, the one in the back very seldom fails. By reading what you write, I get a feeling of ECU malfunction. Did you get any response out of the DTC reading?

But the very first thing I would do, is take a proper gauge black copper wire and ground the negative pole of the second battery same place as the other.

Regards

Geir