Parasitic draw down W/S124 Wagon - PeachParts Mercedes-Benz Forum

spock505 · #1 07-27-2017, 06:09 PM

Now down to the last electrical flaw which may affect rear hatch soft close.

There's a small intermittent parasitic draw associated with the courtesy light/door closure switches which I wanted to explore further.

Basically, a high draw with a door open which then drops slowly when closed, the speed of which is probably governed by the soft night dimmer rely.

All good, but is there any relationship between the door switches and soft close function as this operates also intermittently?

Wiring through the axis tail gate points has been replaced along with motor which worked fine for a while until above.

I have removed two of the door switches ready to order, is it just the four doors as no glove box (air bags) or illuminated cubby box as highlighted by Jeremy thread here:

Door Light/Buzzer Switch

That's one clear piccy

cornblatt · #2 07-30-2017, 10:25 AM

I just finished rebuilding the tailgate closing electronics for my 1989 wagon. I ended up designing a new circuit board for the closing motor since the original was so flaky / intermittent.

There are 2 microswitches mounted on the closer mechanism under a little black plastic shield. The one with 2 wires controls the interior lights. The one with 3 wires controls the closing motor.

They are independent; the door lights have nothing to do with the closer, aside from the door switch being mounted to the closing mechanism. I can send a drawing if that helps.

I had to carefully bend the arm on the door light microswitch just slightly in order to get it to shut off the lamps when the tailgate was closed. When I got the car the previous owner had removed all the interior light bulbs, and I believe this was why.

You mentioned soft dimming lights; I don't recall seeing this in my car. I'll have to check the next time it's dark.

jay_bob · #3 07-30-2017, 12:50 PM

The only soft light in the first gen S124 was the front dome light (between the visors). Not sure if the later models ran the rear dome lights through the convenience control module under the left (at least in US) rear passenger seat to effect a soft dim. The convenience control module did not make an appearance until the early 90s in the US, so I have never seen a car with one except in the junk yard.

I'm assuming you've isolated it to the dome circuit by placing an ammeter in series with the fuse?

I measured my standby drain on my S124 for another member a couple years back. There is so little standby drain on these cars that you literally can detect the clock ticking every 20 seconds. About 15 mA IIRC, and probably most of that drain is due to the aftermarket cheap Chinese remote unlock receiver that I installed. The clock movement draws about 50 mA for about a second every 20 seconds.

spock505 · #4 08-03-2017, 12:52 PM

Sorry for the delayed response, so many other things creep in with minor stuff getting pushed back.

cornblatt, that's interesting info on the micro switches; the previous soft close motor burnt out due to broken tailgate wiring, never really had the chance to see exactly what their functions were as installed replacement, tested working went onto something else.

I wondered if forward interior lights be part of the same circuit sharing the same fuse, when removing the red 'C' fuse you can hear a loud relay clicking on and off which happened today reinstalling the fuse. Bit of good news though as the tailgate motor started to wirr into action then stopped, sounds as though it could be a microswitch or possibly circuit board as you describe, will investigate further.

The pdf I have is for a 92 TE so could be a bit different, if you have something closer to mine that would be appreciated – thanks.

jay bob - these are the readings when first connecting the multie meter in series, .27 dropping to .16 is this okay or is there a draw somewhere?

cornblatt · #5 08-03-2017, 01:25 PM

Attached is a diagram of the closing motor assembly. You can see that all it does is ground the interior light circuit when the tailgate is open, which turns the lights on. When the tailgate is closed, there's no ground for the lights, so they are off. The lighting part of it is super simple.

The motor is more complicated. As shown in the diagram, there's a 2nd internal microswitch under the cover inside the motor housing. Its position is driven by a white plastic cam on the motor shaft. For the motor to work, both microswitches have to be in specific positions.

The internal switch was flaky on mine - it was getting into a state where it was in between the two contacts, so neither contact was closed. At that point the whole thing is dead and will never ever work until you open it up and reset it (or until it magically decides to flex into the right position again due to bouncing down a bumpy road or slamming the tailgate or something).

There's also a timing circuit to cut the motor if the mechanism jams. I am not clear on how this gets reset once it is activated; it seemed to lock things up for me too. That's why I just made my own board instead.

Your signature shows your car as a 1996 (?) W124 - I didn't even know that was a thing. So your part may vary from mine, since my car is older.

spock505 · #6 08-04-2017, 10:43 AM

Thanks again cornblatt, v helpful.

Your description sounds a bit like mine, will hone in on this when removing tailgate door card.

"The internal switch was flaky on mine - it was getting into a state where it was in between the two contacts, so neither contact was closed. At that point the whole thing is dead and will never ever work until you open it up and reset it (or until it magically decides to flex into the right position again due to bouncing down a bumpy road or slamming the tailgate or something)."

Checked several times for any draw, started as above but then without doing anything apart from re-testing it eventually went down to 0.01 - hopefully not doing something daft with multi meter, assume the read out is in milliamps?

jay_bob · #7 08-06-2017, 03:20 PM

Sorry I have been traveling all week as usual and just got back to this.

The switch on your meter is pointing to 10 A range. Your display shows 0.01 A.
This is also expressed as 10 mA.

1 mA = 0.001 A, or 1000 mA = 1 A

This is "down in the dirt" so to speak so the 0.01 A reading is pretty useless.

You can safely reduce the dial setting to the 200 mA position. You will probably also have to shift your red lead into the other jack (most likely labeled 200 mA).

At that point your meter will give you a reading in mA. So if it says 13.2 that is 13.2 mA.

Again anything in the low double digits of mA (say less than 50) is acceptable for a standby drain.

Near the top of the thread you had 0.27, that is 270 mA, that is too much.

spock505 · #8 08-08-2017, 01:46 PM

Cheers Jay, have saved your post - thanks, will report back

spock505 · #9 08-08-2017, 02:24 PM

Jay, followed your instructions, results as below:

"You can safely reduce the dial setting to the 200 mA position. You will probably also have to shift your red lead into the other jack (most likely labeled 200 mA)."

Changing the dial setting to 200ma works but only with the jack plug remaining in 10a, when moved it reads 0.

200ma dial setting, jack plug in 10am - high reading which settles down

As above, dropping down

Lowest reading 00.1, please note LCD missing bar or two

200ma dial but jack into lower socket - is that black bar on left of display indicating incorrectly tested?

jay_bob · #10 08-09-2017, 01:38 AM

I think you have a blown fuse in your 200 mA circuit in the meter, which is giving you a zero reading there.

Chances are the reading is not meaningful with the lead in the 10 A socket and the knob on 200 mA range. Most of these inexpensive meters are not set up to read the 10 A sensing path correctly with the knob on the low ranges.

Remember the way this works is there are two precision resistors inside the meter. One is in the 10 A path and the other the 200 mA path. Each has a fuse in series with it so that if you overload the resistor (for example by placing it across a voltage source) it will not cause the meter to self destruct. The meter actually uses Ohms law to determine the current based on the voltage difference across the resistor. When you are plugged into the 10 A jack and the knob on the 200 mA position you are not getting the proper connection across the 10 A sensing resistor to be able to determine the voltage drop across the resistor.

You are going to need to replace the fuse (with an identical type and rating, this is a safety related item) and try agin on the 200 mA scale in the 200 mA socket.

spock505 · #11 08-09-2017, 03:00 PM

Cheers Jay, understood - the meter is very old with a damaged read out, proper nuisance for inexperienced user.

Going to order another one but in the meantime popped the old,one apart, fuse looks okay but battery flat as proverbial pan cake, went to change it pulled the terminal off lol

Soldering iron out this evening, will have another go tomorrow

Picked this one out due to Amazon reviews, it looks like a dummies version for..don't say it..

eBay item number:172690594894

spock505 · #12 08-10-2017, 07:48 AM

Re-soldered, tested again still same readings as above, will wait for new meter to arrive - checked standing voltage, 12,31v is that a bit low, should be around 12.6v?

Tested alternator once glow plug relay kicked out, 14.17v, looks okay.

jay_bob · #13 08-10-2017, 09:17 AM

12.3 not unreasonable with glow plugs running, those plugs draw a pretty heavy current. 14.17 at idle just right after glow shutoff is good, the alternator is working trying to recharge the battery.

spock505 · #14 08-10-2017, 04:10 PM

12.3v was as standing, no doors open, engine off and key out - i do quite a few short journeys of under 10 miles, probably not enough to get battery back to where it was despite high capacity 210 alternator.

spock505 · #15 09-09-2017, 03:09 PM

Thanks for everyone's input especially Jay, bit of a confession to make - in the end I winged it as the new multi meter instructions were super complicated, lol still in it's box..but the good news is nothing has happened, battery remains charged despite lots of small journeys.

If it's okay, i might dig it out and stick a pic up here for basics.