Neutral Safety Switch R+R!

[Tomguy]

This is a step-by-step guide on how to remove, clean, Repair, and Replace your neutral safety switch. I don't know which models use this switch, but I would assume that most 108 and 109 chassis do, and probably others.

1: First, remove the neutral safety switch (found on the driver's side of the transmission) as well as the adjustment piece which moves the NSS.

2: Most likely, your switch is pretty crudded and dirty after over 30 years of service. Wipe as much of the dirt and grease off of it before proceeding, so that you do not get dirt inside of it.

3: Image 1: From the back side of the switch, using a 1/4" drill bit (you can use a larger one, just be careful to not drill through the plastic), drill the heads off of the aluminum rivets, If they spin freely, hold them on the other side so they do not move. There are 6 rivets in all.

4: Image 2: From the front side, with all the rivets out, push on the black circle that protrudes from the aluminum, as well as pulling gently on the pins on the opposite side. The plastic backing should slide out of the aluminum housing.

5: Image 3: Clean all of the pieces with alcohol. Make SURE to remove any dirt and grease from the inside of the housing and the plastic back. The gasket and O-Rings, as well as the non-exposed parts of the "lobe" piece, must be free of dirt. If you want, you may actually wash the lobe and the housing. Be careful not to tear the rubber seal gasket. Clean the contacts out with contact cleaner, and make sure there is no dirt where the plug attatches to the pins.

6: Image 4: Once cleaned, apply a light coating of clean, fresh grease to the O-rings, rubber gasket, and contact points on the lobe. Do not use too much grease! Put a small amount inside the backing and housing, where the O-Rings ride in, to help facilitate movement.

7: Image 5: Reassemble the back O-Ring, Lobe, and plastic back. Carefully move the lobe and make sure that in Park and Neutral positions, that the contacts are closed (test with a potentiometer, for 0 ohms of resistance). If they do not close, bend them slightly with needle-nose pliers. Do the same with the reverse switch. There should be no melted plastic prohibiting the contacts from moving - trim any with a hobby knife, if needed.

8: Place the front O-Ring on the lobe and the gasket on the plastic back. Carefully put the aluminum housing over it. Using (6) 1/4" x 1/2" flat-head machine screws, or similar, reattach them through the original rivet holes. Tighten them carefuly in a standard cross-torque pattern.

9: Reinstall the switch. Leave the adjustment screw slightly loose. With the transmission in reverse, and the ignition on (so the backup lights actually light), move the adjustment tab until the backup lights go on - a helper may come in handy if you don't have a garage door to see the light reflect off of. Doing this instead of testing park ensures that you get all three right the first time. To double check, turn the ignition off and move it through all the gears. Then, put it back into reverse and turn the ignition on. The backup lights should light. If not, readjust the tab until they do. Retest. Then, once that is successful, make sure the engine cranks in Park AND Neutral.

I hope this guide helps save some people ~$100 (or more, with labor) by doing this - it took me about an hour to do (at most), while taking pictures. Always remember standard safety procedures - chock the wheels and use the parking brake. Disconnect the coil wire to prevent accidental starting. If using a jack or jack stands, make sure your surface is hard and level, and your vehicle's weight is evenly distributed so it does not shift.

[WANT '71 280SEL]

Were you able to actually buy new gaskets and o-rings for it or did you just reuse them?
Thanks
David

[Tomguy]

I reused them. You could probably find O-Rings that fit and cut your own gasket, if needed, but mine were still in good condition (amazingly).

[Joker]

Hey Tomguy

Thanks so much for that. I have just bought a 300SEL with this problem and was thinking it was going to cost me BIG to get a specialist to sort it out for me.

Your step by step instructions with pictures are spot on!

Thanks for taking the time to produce the article

Joker AKA Rich

[Joker]

Sorry to sound like a dumb ass but is the safety switch inside the car on the drivers side of the transmission, or the outside of the car actually on the side of the transmission?

Thanks

[Tomguy]

It's on the transmission, where the linkage connects to the tranny. Very easy to see and access.

[Joker]

Thanks man

[tompaah7503]

What are the electrical connector pinouts?
I bought a used 722-tranny from a late 70's W123 and it does have a very similar device just where the shift linkage goes in to the tranny.
Mine has a cable attached to two of the pins, but the two others remains unused... what are they used for?
Thanks in advance for any light that could be shed on this..

Tomas

[abe g]

This worked for me on my 77 450sl. Spray wd 40 liberally at and around switch. This caused accumulated idrt to flush out and allowed proper operation of switch. (Same on parking brake switch and others with springs etc. good luck, Abe G

[drskipper]

Hey I was wondering what is involved in take the NSS off just take 2 bolts off the NSS and Unhook the arm? how's that done?
Thanks.

[aaan]

Hi
Am new to the forum,my w123 1982 Auto reverse lights never worked.
I read this thread got a replacement switch for the transmission, fitted ok, did some adjustments and Bingo. reverse lights worked.
Later at night , tonight no longer work.
I am looking thru Chiltons Manual and the circuit diagram has 2 switches,????
One called backup light switch , and one called
Start lockout/backup light switch which is the one I replaced. The cable seems to run up toward the gearbox lever?
Does anyone know if there is a switch under the consul? for backup light
Or Do I still need to adjust the switch on the gearbox.
I have 2, 4 pin switches now and could pull one apart as specified to see what makes it work n repair. The bulbs in the rear look like they have never been used so I am guessing the reverse light hasn't worked for ages.any help appreciated. aaan

[twinockchef]

Be careful on which model 108 you do this job to. My 71 280 sel's switch is different from Tomguy. The bolts that secure the switch to the transmission also go through the rivets. I the rivets are drilled out then there is no way to reseal and attach the switch to the transmission. I had to replace my switch because of this.
So everyone make sure that it's the same switch before taking it apart.