Is it possible to disconnect the neutral safety switch permanently and still have the car start and run normal ? It's for a 1990 500SL

It's a bad idea.

With a manual transmission, you know immediately if it's in gear when you hit the starter. On an automatic, there's no power to the transmission input until the engine gets spinning, whereupon it can launch the vehicle.

Considering that it's your neck and liability, note that it is indeed possible and fairly easy.

Do you want to disconnect it simply because the original one has failed? You'll probably want a wiring diagram if you're trying to permanently remove the NSS from the starter/ignition circuit.

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE

In addition to what Matt L said depending on what model you have the neutral safety switch is also used by the cruise control\Idle Speed control module in the 104 engine with HFM-SFI to control idle speed and alter throttle response in reverse gear. It’s also pretty cheap around $30.

I had an old '68 Cadillac Deville that I did that on when I was young and broke. The car idled high, and tended to run on when shut off, so i used to shut it off while still in drive. I sometimes forgot to put it in Park afterwards. This is not good when the neutral safety switch is bypassed. There were a few times I almost nailed the car in front of me when I started the Beast up.

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1995 S600, 1 of 618 (sold)

"Speed is just a question of money...how fast you wanna go?"

LONG LIVE THE W140!
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My 1974 F250 WILL leap forward if I forget the neutral safety switch isn't working and that I've left it in drive.

Not intending to pat my own back but you don't want this to happen to you.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120