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Old 04-07-2003, 04:20 PM
Registered User
Join Date: Mar 2003
Location: New England
Posts: 213
Testing the neutral safety switch

It appears the nss on my 84 300SD has toasted. Key on, dash lites and glowplugs go, key to start and solonoid doesn't engage. Lights don't dim, no click, nothing to indicate an additional drain on the battery. Putting it in neutral makes no difference.

Where can I plug in my ohm meter or volt meter to test it? Can I jumper around it to start the car? Could there be something else that could cause the same symptom?

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Old 04-09-2003, 02:43 PM
Registered User
Join Date: Feb 2003
Location: roslyn, LI,NY
Posts: 445
possible defective neutral safety switch

Heres a possible solution: On my 450 sl (1977) I had difficulty occassionally with starting car een shifting all stops. Get under car and spray wd 40 at and around switch location on transmission. Did the trick for me, (corroded contacts), and no problem since. Let me know if this helps, good luck, Abe G
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Old 04-09-2003, 04:21 PM
bhatt's Avatar
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Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421

My car wouldn't start earlier this winter and I tried to test the NSS. Everything I read (in MB manuals and online) said that if the car started in Neutral but not Park, the problem was the NSS. Mine wouldn't start in either so I was convinced it was something else.

Mechanic replaced the NSS and it has been fine since. He said the switch can fail and the car will start in Neutral -OR- not start at all.

I haven't seen the actual switch, but if it is a two-wire switch you should be OK jumping the two terminals to see if the engine turns over. I believe in my '86 300SDL (W126) the switch was on the left side of the transmission, under the car.

Depending on your climate, this could very well be the problem. Mine was original and subjected to salty water and cold winters for 15+ years. My guess is that one day the corrosion finally got to it and killed it.

'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi)
'07 GL320 CDI 4MATIC 289,000km (162k mi)
'07 S550 4MATIC 170,000km (105k mi)
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Old 04-10-2003, 07:40 AM
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Join Date: Mar 2003
Location: New England
Posts: 213
Downloaded the starting system electrical diagram from Alldata. Connector location for C105 they gave me was wrong, but color code was ok. Disconnected battery and measured resistance from positive cable to the solonoid connection. When key turned to start and shifter in park, resstance goes to 0 ohms. NSS is good, starter is toast. One of my teenagers will wrestle with starter this afternoon, may have the car on the road again tonight.

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