Geabox contactor unit service and repair

[s class]

The 116 cars have a contactor unit bolted onto the outside of the gearbox, where the selector shaft goes into the geabox.  Its function is to provide override for the starter lockout circuit and actuate the reverse lights based on the position of the gearbox selector. 

Its mounted on the LHS of the gearbox, external to the gearbox proper.  These items commonly give problems.  Here's a description of what goes wrong with them, and how to fix it. 

The picture below is looking from under the car, and shows the location of the contactor unit.  The first step is to remove the selector lever's crank lever, indicated in blue, by loosening the indicated bolt. 



This next photo shows the crank arm removed, and how it performs two functions - it operates the gearbox selector shaft via the keyed hole in it.  The indicated flat metal tabs engage the contactor unit and rotate its central cam (which is visible in subsequent photos). 



The next picture shows the rear view of the contactor unit after it has been removed.  It is held together by 6 rivets along its perimieter, which I have already drilled out in this photo. 

[s class]

PART 2 : how the contactor works.

Inside the unit is a central rotating cam unit, which actuates two sets of contacts when appropriate

This first photo shows the unit as it would be with the selector lever in the "P" position.  As indicated in red, the cam has actuated the starter lockout override, enabling the starter circuit. 

Indicated in green is the reverse light contacts, which are open as appropriate for the "P" position.



The next photo shows the cam rotated as it would be for the "R" position.  On the right you see the starter lockout has not been overriden, and on the left the reverse light contact has been activated (via an intermediate rocker). 



The next photo shows the unit in the "N" position.  The SECOND lobe of the starter lockout cam has actuated the starter lockout override contact at the right of the photo.  The reverse light contact is open (hidden beneath the cam). 



In this last photo below, the unit is positioned as it would be for L, S, or D.  Both the reverse light switch and the starter lockout override are deactivated. 

[s class]

PART 3 : what was wrong with my contactor and how I fixed it

I was fortunate to have a spare unit.  I selected the best bits from each and made one. 

The picture below shows the units with the cams removed.  The left hand one shows burning on the reverse liight contacts, and loss of tension in the phosphor-bronze leaves.  I chose to use the better item on the right. 



The photo below shows wear in the cam units.  The reverse light actuating portion is particularly flimsy and prone to wear. 

I chose the right hand one, which in real life looks considerably better than the other one. 



HEre's a closeup of the wear in the reverse light actuating cam on the poorer unit. 



The reverse light cam activates the reverse light contact via an intermediate rocker.  In the photo below you see the one out of my car (right) has significant wear.  I chose to now use the much better one on the left. 




When I had selected which bits to use, I put a light coating of red rubber grease on the rubber 'o'-rings that seal the two sides of the cam and I also greased very lightly the various friction surfaces of the cams. 

I riveted the unit together and installed.  It works beautifully now. 

Ryan

[WGB]

Really Good Write up.

This also needs to be in the Library.

Bill

[gregdeklerk]

It must be great to have working reverse lights.

[oscar]

Hi s class, great post.

In the event of another spare not being available, is there any chance of building up the cam wear spots?  What are they made of?

[koan]

In the event of another spare not being available, is there any chance of building up the cam wear spots?

I've fixed a few plastic parts with a soldering iron and scraps of plastic that look to be of the same type. Ski shops sell sticks some sort of plastic or nylon material intended to for repairing the bottom of snow skis.

koan

[s class]

That's a good suggestion koan.  My feeling is it shouldn't be too difficult to find a better unit - remember mine has nearly 500 000km on it to get that kind of wear.  The same units are found on 114's, 115's, 116's, and probably 107's.  So you should be able to find a lower mileage item at the breakers. 

You can test them on the bench with a multimeter for correct operation, but being the thorough sort I had to open up and see what's going on and service the contacts. 

I must say when you see how flimsy the cam points are and how much force they need to exert to operate the contacts, its a surprise they last as long as they do. 

PB - part numbers will be welcome if you have them.  It may be easier than tracking them at the dealer. 

Thanks