Reverse lights on while in drive

[70Tom]

I'm guessing this is an easy diagnosis, but I'd like to prevent my reverse lights from being on when the transmission is in the drive position. It's a 1970 Sport Fury. Any thoughts on what might be causing this?

Thanks

 

[FinFury]

Incorrect neutral safety switch on transmission or the one you have has just failed if the lights have been working previously. I had same problem in my challenger and the problem was with wrong switch.

 

[70bigblockdodge]

Neutral safety switch or wiring to it.

 

[70Tom]

Thanks guys. Makes sense as this transmission was not from this car.

Is it hard to source a correct one?

 

[70bigblockdodge]

The 3 wires like you have are available the single pole earlier one is harder to find.

 

[Carmine]

I believe the trans switch provides a ground, which means that a pinched wire (to ground) could also be your issue. In fact, I've seen the harness pinched between the engine/trans after a trans R&R.

Unplug the harness, turn on the key and see if the lights go out. If not, switch is good.

 

[Mopars & Missiles]

It does provide a ground, but NOT to the backup lights. On the 3-wire NSS, the middle wire (brown) is the ground wire for the starter relay coil.

The two "outside" wires on the NSS provide power to the "backup lights" when the gear selector is in Reverse. +12 volts is ALWAYS present on one of the wires, and the NSS switch completes the circuit to the other wire when you shift to reverse. The "backup lights" have their own ground at the bulb socket in the rear of the car.

 

[Carmine]

It does provide a ground, but NOT to the backup lights. On the 3-wire NSS, the middle wire (brown) is the ground wire for the starter relay coil.

The two "outside" wires on the NSS provide power to the "backup lights" when the gear selector is in Reverse. +12 volts is ALWAYS present on one of the wires, and the NSS switch completes the circuit to the other wire when you shift to reverse. The "backup lights" have their own ground at the bulb socket in the rear of the car.

Was going from memory, so I'd probably defer to the above. However, do recall seeing this occur with a pinched wire in the NSS/reverse harness. Must have pinched just right I guess.

 

[Mopars & Missiles]

That's cool, I was just trying to clarify how the NSS and backup lights worked. Hope I've got it right as my memory isn't always crystal clear these days either. All sorts of ways this problem could occur, including pinched wires.

 

[Gerald Morris]

Looks like the switch is readily purchaseable on eBay. 1968 1974 MoPar Plymouth Neutral Safety Back Up Lamp Switch 904 727 Automatic

That one purports to be good for 1968-74, both 727 and 904 trannies. Happy to report that this particular dolor de mi fundio hasn't bit me yet.

 

[70Tom]

Thanks guys. Appreciate your help on this!

 

[MrMoparCHP]

Keep in mind that if you put single wire transmission in the car and simply putting a three wire switch in will not work.
The transmission needs to be built to activate the switch.


Alan

 

[70Tom]

Figured out the problem. It was originally a single switch transmission. The shop that rebuilt my transmission is going to fix the problem for me, thankfully. Hopefully I won't need to source a single prong switch.