Interlock Safety Switch

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4

4 on the floor

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I have a ’69 Dart 340, 727 automatic. When I bought it, it started and drove but had no backup lights and you could start it in any gear. There was no interlock safety. There was no wire harness off the transmission.

I found a junkyard 3 wire transmission harness and hooked it up per the factory service manual. Now the engine will not start. The starter is dead. With the key on (engine not running) I can put it in reverse and the backup lights come on but the starter is dead.

There is a wire (not factory) coming off the under-hood harness that was plugged into one of the wires that comes out of the bulkhead connector that would normally go to the transmission wire. I disconnected this non-factory wire so I could wire my new junkyard harness like in the service manual. Now it will not start even though it is set up like it is supposed to be. I can totally disconnect the new harness and plug in the non-factory wire to the bulkhead wire, and it will start and run but I have no interlock safety or backup lights.

The photo shows the non-factory wire shoved into the one of the two wires that would go to the transmission harness before I put the new harness in. It starts in this

bad wire.jpg configuration but I want the safety interlock and reverse lights.

Does anybody know what someone did to defeat the interlock and how I can put it back to factory?
 
The wiring for the neutral start switch is incredibly simple, and you should be able to trace it easily. The CENTER terminal of the switch on the transmission grounds when in neutral or park. That wire, from the center of that connector, goes up the firewall and plugs into one of the two "push on" connectors on the start relay. The remaining push on connector is the "start" power from the ignition switch when twisted to the start position.

Those two "push on" or flag terminals on the star relay are the magnetic coil terminals. So the power, in start, comes from the key, to and through the relay coil, then down to ground through the neutral switch.

If you do not have a service manual or wiring diagrams, run over to MyMopar.com and download them free
 
There is a difference in switches that make it nec to mess with the sender depth into trans.
This thread may help, but others will chime in.
One alternative suggests diss-assembly, thicker gaskets avoid that.
Good luck. will check back later.
www.forabodiesonly.com

neutral safety switch/ reverse light unit in trans

I need a reality check. I had some problems with my neutral safety switch engaging, so I got a new one several weeks ago. Now car starts fine, but I realized that I lost my back-up lights. I put the car up and unplugged the switch at the trans and jumped the outer two sockets in the wire plug...
www.forabodiesonly.com www.forabodiesonly.com
 
Thanks for the help. I have a factory service manual. I know where the wires go so I wired the transmission harness the factory way. The previous owner did something to defeat the safety interlock so it will start without a transmission harness.

Maybe I have a bad switch / sender depth. If I hook up the new junkyard transmission harness just like it shows in the service manual but pull the plug on the transmission what holes do I connect to make the car start? (This will test the switch)
 
Center pin is to ground the neutral/park start spade on the relay by the battery.
Outer pins are b/up switch (either way), that's the one, the # of turns will affect switch on/off, operation, spacer rather than other switches or trans combs was my solution, thick power valve gasket.
Good luck
 
4 on the floor said:
Thanks for the help. I have a factory service manual. I know where the wires go so I wired the transmission harness the factory way. The previous owner did something to defeat the safety interlock so it will start without a transmission harness.

Maybe I have a bad switch / sender depth. If I hook up the new junkyard transmission harness just like it shows in the service manual but pull the plug on the transmission what holes do I connect to make the car start? (This will test the switch)
Click to expand...
I already told you how it works. The center pin of the switch is grounded. That is one wire. It goes from the start relay, one of the flag terminals, down the firewall to the NSS. So start by grounding the relay terminal and see if it cranks using the key. BE CAREFUL as you have bypassed the switch function

Then check the wire, either check continuity, or reconnect it and ground the center pin.

Also hook it up as it should be and hold the key into the start position while wiggling the shift lever from park to neutral, which will check for poor switch operation, linkage mis alignment, etc etc.
 
What Del said above. I'd recommend removing the starter wire and just checking the yellow and brown at the starter relay. Show us a pic of the relay. You need a DC volt meter to do any intelligent diagnosis.
 
how about some photos of your starter relay. There were some relays that had one of the starter pins grounded so only the yellow wire from the ignition was needed (no NSS)
 
Thanks for everybody's help. I got it working right! I kept tracing the wires from the relay. The ground wire was put on the ignition lead at the relay and a jumper wire went from the ignition lead on the relay to the bulkhead connector where the transmission harness goes. The previous owner did this. I undid the crap and got the transmission harness in. I have backup lights and it only starts in park and neutral now.
 
it is so much easier when things are OEM stock!

Congrats!
 
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