I replaced the neutral safety switch on a 1985 300SD today. I couldn’t find any reference to the job in my Mercedes manual (I have only the Chassis manual—Volumes 1 and 2) and thought it might be helpful to post an after action report for those who may be considering this repair on a 126 chassis

Problem/symptoms: After an hour or two of high speed highway driving, if you shut off the engine and try to start it, on many occasions the starter does not engage—no click, no response. After several minutes of jiggling with the shifter and the key, it normally will come to life and start. It will then function normally as long as the driving is in town rather than high speed. Several threads have covered this problem and the most likely causes (not in order) are: (1) bad starter/solenoid; (2) bad neutral safety switch; (3) worn bushings in shift linkage that fool the starting system into thinking the car is not in Park; and (4) bad ignition switch.

I found it difficult to replicate the problem for debugging, so I decided to play it safe and replace the starter (it has 203K miles on it) as well as the neutral safety switch. The safety switch is a concern because on several occasions, the car did start when placed in neutral. Despite this, my gut feel is the real problem is an intermittent short in the starter solenoid that acts up when the engine gets hot during highway driving. Will get to the replacement starter next week.

Repair procedure for Neutral safety switch: The part number for the switch on my 300SD is: A000 545 49 06. It runs about $55. at the dealer—I’m sure it can be had as an after market item for less. The switch is located on the left side (driver’s side) of the transmission and is accessible from beneath the car. My "school of hard knocks" procedure to replace it is as follows:
1. Place the shift lever in Park.
2. Jack up the left side of the car and support with a jack stand.
3. Locate the switch on the left side of the transmission. It is approximately 2 inches by 3 inches by ¾ inches thick and is made of black plastic. It is held in place by 2 each 10 mm bolts, one of which is hidden by a large round electrical connector (about 1 inch circumference). A shift rod from the transmission case protrudes through the switch and is connected to the shift linkage by a clamp (similar to the clamp used on a car battery) with a single 10mm bolt and nut. Thus, as the shifter in the car is moved, the internals of the neutral safety switch are engaged.
4. Remove the electrical connector from the switch. DO NOT PULL ON THE CONNECTOR—it is locked into the switch body by a white ring. To unlock it, you must rotate the white lock ring slightly—counterclockwise as I recall. This will disengage the plug and allow you to pull it straight out. The plug fits fairly tightly, as it has 4 pins and a square lug in it. If you have the replacement switch look at it closely and you will see how the white ring rotates.
5. Remove the single bolt and nut holding the linkage clamp on the shift rod. To get the clamp off of the rod, you will need to “spread” the clamp by wedging a screwdriver in the opening as you would on a battery clamp. A little penetrating oil will help as well. Note the shift rod is not round—it has 2 flat sides, so the linkage clamp can only go back on one way—no slipping—good design. Also note above the linkage clamp, there is a small hole that engages a plastic arm from the neutral safety switch—a “tit” on the plastic arm fits into the hole. When the new switch is mounted, be sure this plastic arm is engaged properly. Now, push the linkage clamp aside—you’ll need the room to remove the 2 bolts that hold the switch to the car.
6. Remove both 10mm bolts and the switch will lift off easily. Pivot the switch slightly to clear the shift rod.
7. Now’s a great time to check the bushings in the linkage rods—they are right there in front of you. One is right above the linkage clamp. Follow the linkage rod back about two feet and there is another bushing where the rod turns and moves up into the cavity where the Shifter is mounted. Both of these bushings were fine on my car. In fact, I would expect these not to be problematic on the 126 chassis, as they appear to be pretty substantial bushings. But now’s the time to check it….
8. Reverse the above to install the new switch.
9. When installing the linkage clamp on the shift rod, if you find alignment difficult, you may find it necessary to check the position of the shift lever in the car. I found that in trying to remove the 10mm bolts from the switch body, I had bumped the linkage out of Park into Reverse. Thus, the linkage clamp would not align with the shift rod. When I moved the shift lever back to Park, everything aligned fine. There is no way you can hook this up wrong—if it doesn’t line up, recheck the shift lever position.

This is certainly not a difficult job. A good DIY’er can do this in about 30 minutes easily. Don’t let my longwinded description dissuade you from making this repair, if you are sure your switch needs replacing.

Now, for that starter replacement, I’ll try and post the results next week…

From now on, you can take digital pics and see if the Webmaster@Mercedesshop.com will do a DIY feature.

__________________
Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

Be aware that for the W140 chassis / 722.xx transmissions, the neutral safety switch should only be installed w/ all 4 wheels on the ground & there is a calibration procedure w/ shifter in neutral & bolts loosened to put a pin through the arm & the switch body. Not sure on your earlier chassis though. FYI.