Roof closing problem wont latch to windscreen cowl panel - FIXED.

[Bill H]

Just had a problem on our 120i where the roof wont latch to the top of the windscreen when it closes.
The whole mechanism seems to work up until the final locking but then the hooks just dont move - so was then serenaded by the BMW bongs as you drive along.
After 3 days of intense problem determination have finally solved it - and I reckon it could be a common problem so hope this helps someone else.

The solution was one of the Hall Effect sensors on the mechanism that had become displaced by a few millimeters, managed to push back in place and secured with a cable tie. The sensor that caused this is called "Tensioning bar extended" but what it really means is the hydraulic ram is extended that pushes the rear window up and down.

There are a total of 8 Hall sensors on the mechanism - these are on a RH drive car I presume they are the same for LH drive as well.
1 in the cowl panel above the windscreen
2 under the rear storage compartment on the LH side of the vehicle (looking forward)
5 behind the rear RH quarter panel (when looking forward).

These can be really difficult to find as some are only exposed at certain points as the roof goes up/down, its easiest to find the cable and follow it to the sensor, - there are 2 towards the back of the mechanism that are obvious when the storage compartment cover is up, a third can be seen from the same point but looking forward under the rams, a fourth is buried deep down near the seat belt , and the 5th was the devil - it was on the hydraulic strut itself and very well camoflaged, the cable to it was tied to the hydraulic lines and had pulled tight - it had pulled the sensor out by just a mm or two. I cut off the cable ties and tied the sensor to the strut itself.

There was an awful lot more to the diagnosis than that, I ended up taking the Control Unit out and used a oscilloscope to monitor the sensor signals as the roof moved, once you've located the sensor you can actually test it by monitoring the pins on the control unit with a 'scope or digital voltmeter as you pass a steel bar in front of the sensor.

[olds350]

great fix! Glad you were able to chase the problem down...