My car wouldn't start, lights were bright,tried to jump but nothing. I taped the starter a few times and it started right up when jumped. Today taped it again and nothing so I swapped it out with one that was in a parts car I have. turned for a split second then nothing. While jumped & after putting it in park then neutral a few times it fired up in neutral. I tried it a few dozen times again moving the shift around in neutral then still and it turned twice and started once more. Reverse lights work. My model does not have the jump relays in the inner passenger fender area. Is there a jump terminal in the 1980 300d 123's some where. I've looked and some symptoms suggest neutral safety switch and others are confusing. Please help with suggestions on NSS 'ie cleaning or repair or should I just replace. IF IT IS THE NNS.

is an interlock switch the same thing as a NNS.

Interlock and nss are the same. It could be either the NSS or a problem with the ignition switch. Both are known sources of this problem, as is the starter itself. I think you can use a jumper wire on the NSS to exclude it.

It's better to keep the diagnosis of one problem in the same thread so that everyone can follow the hypothesis and testing sequence.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I have 2 other ignition switches(by ignition switch you do mean the whole key tumbler assembly right). One is already removed because i removed the entire wiring harness from one of my 300d123's a month ago leaving most everything attached on the ends. You've been alot of help KERRY. I'll get back with results friday hopefully. Are they interchangeable on 123's

There are contacts in the ignition switch that get dirty. There's a thread on here somewhere that shows how to clean them. I think that thread has some pictures on it. If you can't find the junction box, you should be able to test the starter itself by jumping the terminals on the starter solenoid. Just make sure the car is in park or neutral. I've seen people get in trouble by jumping the starter while in gear.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13