On my 2003 R53 I have a battery drain issue. I get a small amount of current draw from the battery when the car is off and typically the battery will drain completely in a week or two of no use. I traced the drain to F21 in the footwell fuse box. I understand this fuse powers the speedo. I figured out that disconnecting the tach solves the drain issue, but otherwise the tach and speedo work fine. Actually everything in the car works fine. I noticed that the V_BATT signal from the speedo to the tach (pin 7) always has 12 volts on it, even when the ignition is off. Its not clear if this is correct. Should pin 7 at the tach always have constant 12v or is the speedo supposed to turn this off when the ignition switch is off?


Can anyone check their car and see if pin 7 on the tach always has 12volts or does it turn off with the key?

 

You need to check the idea at that door when the key is off there should be no current draw. All modern cars have a finite trickle of current draw 24/7, including our Minis. By some modules. I am quite sure without looking up that the Kombi module which includes the instrument cluster is responsible of ignition switch's antnthieft functions.

You didn't say how much current you are seeing. Without it is a waste of time. Also what is the condition of your battery? Is it a rarely driven car?

Without a key, press the Tripmeter reset and you will see the instrument is "woke" up.

 

Ok... sorry for not explaining everything done and investigated already. Battery is not an issue as its been replaced several times which led me to investigate steady state current draw. Everything i read indicates normal current draw with the car off and doors closed and sitting for 15 minutes, to allow systems to shutdown, to be on the order of 10's of milliamps. In my case it is more like 100's of milliamps. Which is enough to drain the battery to the point where it won't start in a weeks time. Simply unplugging F21 or the tach connector reduces the current draw to a more normal 20ma or so. So the question remains, should v_batt (pin 7) have constant voltage or not?

 

Yes. 10's of milliamp is normal. Over 100 mA is excessive. If the Kombi is drawing 100s of mA something is wrong. I don't feel like looking up F21.

One possible fault of instrument cluster drawing excessive current - assuming no one connect something to it that shouldn't is ESD damage. ESD damage can happen if someone open them to customize LED color.

 

Here you are! Per 2006 wiring diagram, F21 is hot all the time for the instrument cluster. I would begin by checking someone didn't hookup some carp to it somewhere. Check too the ODB connector.

History of your car?

 

I have been the only owner of the car and the instruments are stock. I agree that F21 should have constant voltage so that wasnt the question. According to WDS, F21 goes to the speedo directly. From there the v_batt signal goes to the tach on pin 7. Should v_batt have constant voltage or not?

 

Good luck with asking people to tear apart the instrument just to measure the voltage so you can compare. To successfully troubleshoot you need to have some inkling the schematic of this subsystem, and obviously you don't. Checking voltage with a DMM on these pins will rarely get you where you want to go. I am speaking from decades of experience troubleshooting electronics.

Even if someone took the trouble to, and tell you his pin-7 is not "hot" that does not mean anything at all as this is an active circuit, and can be in difference stage of being. pin-7 looks to be a communication bus if that help.

 

Thanks for the input but i dont believe you have a good handle on what it takes to do what i asked. The tach is 2 screws to take off. 5 minutes tops to measure pin7 on the tach connector which is v_batt on 2003 car with no Nav. I have all the wiring schematics and am an electrical engineer with plenty of experience troubleshooting electronics. If i knew if v_batt was constant or not it would narrow down where the problem lies. In the tach or the speedo.

 

Let's just say I have been considered as the fixer to go to with the toughest system issues when 100k of $ is at stake and a solution is is needed fast.

 

So then where did "tearing the car appart" come from then? Is that what you call 2 screws?

 

Let me give you a few tips since you are an EE.

• Think conservation of energy if you know there are 100's mA going into the tech. The key is to find out which component is consuming excessive power.
• Determine if the current that went in, if there are any the leaves and where.
• Use an IR thermometer to see what component is dissipate too much heat.

You should have access to more than just a DMM. An IR thermometer may be useful. A scope may be useful. To measure the current going into and leaving the module can give insight. Reverse engineer base on the circuit components and PCB layout can give a lot of clues as to the functionalities.

 

Something else that might help to know is there are stages at which the car shuts things off after the ignition is off and the door is shut and or locked and the dome light shuts off.

You need to be able to check draw after roughly 30 minutes with the key out of the car and all doors having been closed for the 30 minutes and not reopened to do the check.
I would recommend a inductive amp clamp and set the meter somewhere you can see it without sticking your head in the car (IR sensor is activated).

All BMW products (Mini, Rolls Royce, BMW) are known to be hard on batteries if the car is not started on a regular basis. All the brands sell battery tenders I keep one on mine to combat it as it only sees limited use currently.

If you are not already using a top quality battery I would recommend getting an Odyssey or equivalent but stay away from Optima.

 

did you ever solve the problem with the tach drawing power when car is off? I am having the same issue

 

Problem was fixed. I found a solder short between two pins of an IC on the tachometer circuit board. It was enough to cause an extra 100-200ma of current draw. I removed the short with a solder iron and the problem is gone.

I suggest removing the tach and applying 12v DC source and measure the current draw. It may draw tens of ma but no more than that.

 

did u by chance take a picture?

 

Here is a picture showing the IC with the solder short (arrow).


This is an obvious manufacturing defect that was never caught in QC. I never noticed the problem until I stopped driving it regularly.

 

I've been troubleshooting this problem on my R53 for 3 days now. I found the current draw at the battery when I pulled fuse 4 (General Module BC1), fuse 21 (Instruments), and fuse 24 (EWS; alarm siren; ultrasonic sensor). Just like you, these all made sense except for the instrument cluster. I just stuck my ammeter directly to the fuse receptacles and fuse 4 and fuse 24 were pulling like 0.03 A and 0.08A, but fuse 21 was pulling way more, 0.22 A.

Thank you so much for investigating this and finding the cause (though not the root cause... I wonder if this solder flows? It seems like a common issue).

 

I consider the short on the IC to be the root cause. I can confirm that another tach I purchased did not have the same short. Do you also have the same short?