Servotronic issue

[mckazok]

Hi everyone,

As title suggest my 2000 740i has developed servotronic issues.

About a week ago, shortly after driving on a very bumpy section on the highway, I lost power steering.
Initial thought was steering pump as I had a cheap re-manufactured pump and it was just past warranty period, so naturally I assumed pump has died at the end of warranty.

I found a used LUK from a 540i locally, installed but results are the same.

And until then I had no idea my car had servotronic, now I do and I'm sad.

Connected vehicle to INPA, activated servotronic valve while car is running, steering comes back.
This suggests valve is operational, pump is good, rack is good, GM is fine and of course all the cables and connector in between.

Scanned for codes, there are none, not in DSC nor in GM; It's not the 31st time some random failure happened, it just stopped working.

I want to diagnose it and fix it so please help.

[Dan750iL]

Not sure if it will be redundant after the INPA test but here is a troubleshooting guide for the servotronic that has worked for a lot of people.

http://www.bimmerboard.com/forums/posts/1236491/

[mckazok]

I read the posts thank you, however contrary to what I expect, it says if it works with a 9v battery then unit is bad. Please explain me this. Apparently valve actuates when powered with an external source yet this only proves that it is defective, how?

[Mayorchuck]

That is confusing. If you hook up the 9v and the steering becomes lighter, the solenoid is good. I don't think you activating it via INPA means the GM is sending the signal on it's own though. I've seen this fixed with a battery nap when the servo tested out good. Might give it a shot.

[mckazok]

True, INPA still doesn't rule out GM yet. I'm not sure if it's supposed to work this way, but when activated thru INPA, steering is light only for 2-3 seconds, then it goes stiff again.

Took out the GM, to test connections, I can read 10ohms from GM connector pins 9 and 11, this is when solenoid is connected.
PCB looks clean with no cracks on solder joints.
Physically in good shape.

I hope it's a software glitch.
Battery nap is next, so far I only unplugged battery and grounded the positive connection to simulate a "quick" battery nap, that didn't help but maybe sitting there unplugged for 8 hours may.
Other than battery nap, is there a way to reset those modules?

[setconstr]

Did you just lose low speed power steering assist or all power steering ? Low speed is the servotronic torque converter all power steering would be the power steering pump or belt. like mooll says you can rebuild the STC or as in my case I couldn't get one of the 2 screws out without damaging it so I purchased a new one ( $275.00 ) and its still a PTA to get to, for me it was easier to take the metal heat shield out of the way. I had to clear codes with INPA to get it to work ........

[mckazok]

I lost low speed assist. It is definitely servotronic related, I'm just trying to figure out which component gave up as I believe valve is operational; it passes battery test, it works when manually activated thru INPA. Problem is it doesn't stay on.
Update on the codes, I have one in error and one in shadow memory, both say short circuit valve or cable. I'm also having hard time believing this, I measure 10 ohms at GMIII connector, so there is no short, it's exactly what I measure at the connector by the wheel.
Also, on BTS412B FET which drives the valve, pin 4 is diagnostic feedback, which should be LOW on a failure, it was never low while activating from INPA, so in reality there was never a short circuit.
GM might have an issue though; thinks that there is a short on the circuit at all times.

[racer2086]

First thing to do is clear the codes and the shadow codes. When the GM has codes, sometimes it doesn't activate servotronic. It also will sometimes not activate if you have wheel speed sensor issues - be it complete failure or intermittent plausible values.

[Timm]

Yes, this is standard fare for the Servotronic - that error pops up for no reason at all - and after a few errors it inhibits the speed-related function (steering assist is still working). Delete the errors in the GM (or clear ALL errors) and off you will go again!

[mckazok]

After a battery nap it still didn't come back. I have been erasing codes yesterday but never got it to work that way.
As soon as ignition is switched on GM checks the valve and stores an error, and from there on it never operates by itself.

[purplecty]

In my experience i needed to rebuild my valve and had no GM codes. I also swapped in a GM with no changes before the rebuild of the servo valve. I think if you have codes in the GM just find a used GM3 and swap it in, code it and see what happens. That's the easiest 1st step to take IMO...

[mckazok]

Honestly, I'm leaning towards a GM failure.
Found two inputs on the microcontroller, one for open-circuit detection other for short-circuit. Just to make sure it won't register a short-circuit error, I cut the connection from FET pin4 to microcontroller and used a resistor to pull that input to HIGH. Result is the same, module still registers a short error as soon as ignition is ON.

Now, I need to get a used, hopefully working, GM to test.

GM3_mod.jpg

[Timm]

Well done for fault-finding to PCB level - don't see that often! As that area is connected to the outside world it is possible that an input on the processor has been frizzled....

[mckazok]

I just received a used gm3 with same hw and sw numbers as well as part number; it is an identical unit.
All I did was installing the part and syncing keys and every option I can think of so far is operational.
Do I have to code any numbers from the original GM? Or what happens if I dont?

[purplecty]

I just received a used gm3 with same hw and sw numbers as well as part number; it is an identical unit.
All I did was installing the part and syncing keys and every option I can think of so far is operational.
Do I have to code any numbers from the original GM? Or what happens if I dont?

Check PM does servo work now?

[Mayorchuck]

It varies with the GM. Some are luckier than others. The GM communicates with most every module inside the car. Just check things like a/c and fan at all speeds. Window auto down, passenger mirror tilts when backing, back door locks, trunk auto close, etc.just non-everyday stuff.

Seems like I did one years back and used NCS maybe. Whichever it was, it had an option to code all modules to the car. Been to long but some of the other guys can tell you better. But whatever works will continue to work.

[TNe38s]

Not sure if it will be redundant after the INPA test but here is a troubleshooting guide for the servotronic that has worked for a lot of people.

http://www.bimmerboard.com/forums/posts/1236491/

After reviewing the link, I noticed a brief mention of the abs unit...what about the abs unit?

my low speed assist has never worked in my 4 years of ownership. Right before I purchased the car a rebuilt abs module was installed. I have repeatedly cleared codes and the BMW dealer techs can find no faults. Is it possible this rebuilt abs unit could be responsible? If so, since these modules are a common repair item- I wonder if some of our problems with "no faults" lies with it?

[mckazok]

Check PM does servo work now?

Sorry, I typed all that but didn't mention servo.
Yes, servotronic worked right away.

[purplecty]

Sorry, I typed all that but didn't mention servo.
Yes, servotronic worked right away.

Nice! Success!

[dancerst]

A useful update for all who is trying to fix servotronic.
please read my DIY it’ll help to save you money ))

Written in Russian with lots of photos.

https://www.drive2.ru/l/519150359046783054/

[purplecty]

A useful update for all who is trying to fix servotronic.
please read my DIY it’ll help to save you money ))

Written in Russian with lots of photos.

https://www.drive2.ru/l/519150359046783054/

Where did you purchase the ZF module for $130? Have a link?

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[dancerst]

Where did you purchase the ZF module for $130? Have a link?

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Got it from FCP Euro - https://www.fcpeuro.com/products/bmw...caAl_7EALw_wcB

[purplecty]

Got it from FCP Euro - https://www.fcpeuro.com/products/bmw...caAl_7EALw_wcB

Wow never knew that was available thanks! So you just swapped the connector over to the wiring harness that connects to this universal servotronic module?

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