carlson03

I have a 2003 ML500 that has had a battery drain problem. So last night I decided to charge the battery and pull it in the garage. I have been testing the drain using my multimeter and running it in circuit of the negative. When I touch the two connectors (Negative side of battery and ground on the body), I hear a relay click and the power spikes to over 1amp. After a few seconds it drops to .85 amp or so, then down to .45 amp or so, then after a few minutes, it goes down to .16 amp and stays. I figured its normal for that high spike, then drops. I pulled fuses and seen that it doesn't do much to stop the draw.

The relay that clicks is K6 in the hood breaker box, Brake Lights. I pull the relay out when the meter is at .16amps and it drops to .03amps, plug it back in, it goes up. I have swapped the relays with similar ones on there and it does the same thing. Also, the relay is very hot, is this normal for it?

The battery dies after about 5 days of not driving, so its not a massive drain, but still enough to be a pain when I go out to start it and there is a lot of snow on the ground. Its not my primary driver, but it makes it pretty unreliable.

Thanks for any suggestions!

Critter

Relay should not be hot, shows large current draw to somewhere. Silly but is the battery fairly new and has it been tested for load.
In the mean time buy a battery maintainer .75 amp or 1.5 amp to hold the charge of the battery when not in use. Sitting for days at a time is not good for long term use of a battery and failure of the battery occurs sooner rather than later.
Do you have a trailer hitch on the vehicle and does it have a trailer plug for extra brake lights on the trailer, some have 12V power for electric brakes, possible salt corrosion at the external plug is draining the battery and causing the hot relay. Good Luck

carlson03

Thanks for the reply! Battery was replaced this summer, thinking it was bad, plus it was older anyways. I have had the battery and alternator tested by auto zone and advance auto and both said are working fine. I do have a trailer hitch on the vehicle, but does not have a brake control. Fuse is out of the power to the trailer wiring. The relay is also hot even when the battery is plugged in, so probably something is drawing power. With the battery connected and I pull the relay and put it back in, I hear the click.

Critter

Next would be leave the relay out and see what does not work. When I mentioned the trailer hookup, are you sure it is independent to the main brake light and fused separately. Check trailer connector with a meter to verify no voltage.

carlson03

I will have to check that out, I know that the unit gets power from fuse 18, but the brake connection is hooked up to the brake line that is at the factory trailer connection...I know its not seperate fuse to the brake line

Critter

My concern is that the external plug has moisture or salt water inside which is causing a voltage bleed, seen this before with the plugs that have the spring loaded cover, traps moisture.
When you leave the relay out, what does not work? Just the brake lights

carlson03

The lights and the towing hitch harness stopped working when pulling the relay...I now have the tow hitch harness completely removed from the car...battery is hooked back up, still clicks when I put the relay in, switch with another to see if it gets hot

Relay is still getting hot too

Critter

Has the amperage draw changed at all?

Critter

Do both rear brake lights shine the same, parking lights on, same brightness on both?

carlson03

Still same thing as before I removed the wiring for the towing hitch

Critter

Apologize for getting you to remove trailer wiring, but it was a shot, Fresh out of ideas,
Can you see a switch under the hood that is used to verify hood closure. Close the car totally after getting this switch to stay closed somehow IE a heavy weight, use the key fob to lock the car then recheck draw after 1/2 hour. With this scenario the car will think it is locked but the hood will be open. Giving you access to the offending fuse box, check for heat on the relay after sitting for the 1/2 hour. Amperage draw also

carlson03

No problem at all, I appreciate all suggestions that might fix this...I have removed the battery too many times on this car, I am a pro at it! I used the car tonight, so engine is warm, I will see about trying this in the morning!

Relay is still hot after sitting with the battery plugged in...

carlson03

I know this isn't the proper way to test, well the night before I took a reading of the voltage of battery (obviously its not the best test) with everything plugged in, all fuses/relays in...12.74V the night before, get up in the AM 8 hrs later, 12.64V.
I tried the same thing last night, 12.38V, but last night I pulled the relay K6, left all the other fuses in...let the car sit in the garage for 22.5 hours, went back and read it, 12.38V...
I know its not the most accurate test, but could it be safe to say it is that relay? Sorry to repeat myself, but just wondering?

Critter

With the K6 relay removed does the alarm system still operate, try locking the car with the key fob then open the door with the key removed from the fob, this should set off the alarm if it is still active. Is the K6 relay still hot or warm to the touch. .1 volts is a small drain none the less

carlson03

I will give this a try when my wife isn't home

carlson03

Sorry I didn't have a chance to try this...but after almost 5 days with the relay out, the car fired right up, where as before, it wouldn't have started. When I turned on the car without the relay, there were no warning lights on the dash, just the brake lights didn't work.

carlson03

Does anyone have a diagram of what goes to the relay? Also, what part number should the relay be on the 03 ML500? Mine is black, but not sure if its the wrong type? Its the one that has been on the car the whole time as far as I know

mertkara

@carlson03, as you’ve mentioned in the first post on this thread that the current drawn by battery reduces with time, that’s normal. It’s because some of the car parts are actually charging their capacitors. They draw more current in the beginning but then it drops down to zero. If it’s not dropping completely to zero then there has to be some issue in the relays, they can leak current if faulty. If the current drawn is too small, that could also be because the wireless receivers need power to stay powered up to lock and unlock the car.

Bezalel

I've had a drain on my battery for a while - I thought it was just an old battery, and not coping with going in and out of the car a lot (dealing with a leak in the footwell - three cheers for heat shrink, dbrace). I deferred dealing with the problem by using a 12V 20Watt solar panel propped on the dashboard, plugged in the 12V outlet under the dash. It holds the charge fine, which if the battery was completely dead it might not, and has made me think it is a drain somewhere.

Even when I've solved the problem, I'll still use the solar panel out of habit!

Did you try just changing the relay K6, and have you solved the problem yet?