Drivetrain Neutral Safety Switch (nss)

fredfifty · Aug 21, 2017

Hey so taking a look at my trans under the car (Tremec 3550 5-Spd) I noticed the NSS isnt plugged into anything...its just hanging from the trans. Is that an issue? My car starts in neutral without depressing the clutch pedal FYI. Is this normal for 5-spds? Seems like it should be plugged into something, but there is nothing within reach to plug it into...i checked thoroughly.

Thanks

jrichker · Aug 21, 2017

I have a Tremec 3550 and the stock T5 NSS wiring harness plugs into it.

Without that NSS harness and a properly working NSS circuit you can't dump the engine running codes and have a potential for idle problems. A code 67 when the codes are dumped is the clue...

Code 67
Revised 18-Mar-2017 to include warning about the necessity of having a 5 speed O2 Sensor wiring harness when bypassing the wiring for test purposes

Cause of problem:
Clutch not depressed (5 speed) or car not in neutral (5 speed and auto) or not in park (auto) or A/C in On position when codes where dumped. Possible neutral safety switch or wiring problem. This code will prevent you from running the Key On Engine On tests.

External evidence from other sources claims that a code 67 can cause an idle surge condition. Do try to find and fix any issues with the switch and wiring if you get a code 67.

What the NSS (Neutral Safety Switch) does:
5 speed transmission: It has no connection with the starter, and the engine can be cranked without it being connected.
Auto transmission: It is the safety interlock that prevents the starter from cranking the engine with the transmission in gear.
What it does for both 5 speed and auto transmission cars:
The computer wants to make sure the A/C is off due to the added load on the engine for the engine running computer diagnostic tests. It also checks to see that the transmission is in Neutral (5 speed and auto transmission) and the clutch depressed (T5, T56, Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine Running test mode takes the throttle control away from the driver for several tests. This could prove hazardous if the computer was jumpered into test mode and then driven.

The following is for 5 speed cars only. Do not do this unless you are sure that you have a 5 speed O2 Sensor harness!!!! Smoke, sparks and expensive pain in the wallet may ensue if you don’t.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!

fredfifty · Aug 21, 2017

Thanks for the response!

Does it make a difference if my car was an original AOD? The PO swapped the tremec for the auto trans.

General karthief · Aug 21, 2017

Well that needs a stick shift harness I think, I'm wundren now if the o2's need repinning.

jrichker · Aug 22, 2017

fredfifty said:
Thanks for the response!

Does it make a difference if my car was an original AOD? The PO swapped the tremec for the auto trans.

If you have a computer marked A9P, that is the auto trans computer and it will work with the 5 speed transmission and 5 speed O2 harness.

fredfifty · Nov 10, 2017

Ran the codes and sure enough code 67. Damn it.

Mustang5L5 · Nov 10, 2017

Being a former AOD car, the trans harness lacks the plug for the NGS.

The harness runs from drivers kick panel to the trans tunnel under the drivers seat. You'll need one from a 5-Spd to swap in.

It's wired in parallel with the clutch pedal switch, so you can just keep the clutch pedal pressed in all the way when you dump codes, or jump the plug out with a 5amp blade fuse

fredfifty · Nov 11, 2017

Mustang5L5 said:
Being a former AOD car, the trans harness lacks the plug for the NGS.

The harness runs from drivers kick panel to the trans tunnel under the drivers seat. You'll need one from a 5-Spd to swap in.

It's wired in parallel with the clutch pedal switch, so you can just keep the clutch pedal pressed in all the way when you dump codes, or jump the plug out with a 5amp blade fuse

So i would need to remove the harness i have now completely, and swap in a 5 spd harness? Seems like a big job (to me).

Mustang5L5 · Nov 11, 2017

fredfifty said:
So i would need to remove the harness i have now completely, and swap in a 5 spd harness? Seems like a big job (to me).

Not really. The harness is maybe 5-6' long.

Pull the drivers seat out, pull the kick panel and door silk off. Lift up the carpet.

Go under car and unplug the VSS and nuetral harness, pull it through the trans tunnel and out from under drivers seat and follow it to the kick panel area and unplug. To install is the reverse.

I did this exact same task on my car a few weeks ago. Took me 20 mins

Hardest part is finding a 5spd harness. eBay is saturated with AOD harness but the 5-Spd ones get snatched up quick

fredfifty · Nov 13, 2017

Mustang5L5 said:
Not really. The harness is maybe 5-6' long.

Pull the drivers seat out, pull the kick panel and door silk off. Lift up the carpet.

Go under car and unplug the VSS and nuetral harness, pull it through the trans tunnel and out from under drivers seat and follow it to the kick panel area and unplug. To install is the reverse.

I did this exact same task on my car a few weeks ago. Took me 20 mins

Hardest part is finding a 5spd harness. eBay is saturated with AOD harness but the 5-Spd ones get snatched up quick

Okay, so it sounds like i dont have to pull the wntire harness out? Just up to where it connects to a harness at the kick panel? Thanks for your info

fredfifty · Nov 13, 2017

https://lmr.com/item/LRS-8793MTH/Mustang-Manual-Transmission-Harness-87-93-50L

So this is what i need? And it will plug right up?

fredfifty · Nov 13, 2017

Also, what would the clutch switch (at clutch pedal) need to be looking like to operate properly? I see it up there but not sure if its connected

Mustang5L5 · Nov 13, 2017

yes, that's the harness. Direct swap and the NGS on top of the trans should plug in. (assuming 3550 NGS is the same as T5)

As for the clutch pedal. there is a switch located on the clutch pedal that accepts two plugs. They are taped up hanging in the dash harness so you'll need o fish for them. One has a jumper in it already. Remove the jumper and plug this into the clutch switch. It's the starter circuit interrupt. The other plug doesn't have a jumper and is the other, parallel, circuit for the NGS. So when clutch is pressed in, it also tells the ECU the vehicle is in the neutral state.

fredfifty · Nov 13, 2017

good info, thanks

fredfifty · Nov 20, 2017

Mustang5L5 said:
yes, that's the harness. Direct swap and the NGS on top of the trans should plug in. (assuming 3550 NGS is the same as T5)

As for the clutch pedal. there is a switch located on the clutch pedal that accepts two plugs. They are taped up hanging in the dash harness so you'll need o fish for them. One has a jumper in it already. Remove the jumper and plug this into the clutch switch. It's the starter circuit interrupt. The other plug doesn't have a jumper and is the other, parallel, circuit for the NGS. So when clutch is pressed in, it also tells the ECU the vehicle is in the neutral state.

ordered that harness...hoping after install, i can check codes and have a better idle. also, does anyone know if i were to replace the ecm from my current a9p to an a9l, is there any special install process?

Mustang5L5 · Nov 20, 2017

You'll want to verify the jumper on the o2 sensor harness under the MAF. They are pinned differently depending on year and trans and can burn the ECU out if incorrect.

There is a lot of info on this if you search "Mustang o2 sensor jumper"

fredfifty · Nov 20, 2017

Mustang5L5 said:
You'll want to verify the jumper on the o2 sensor harness under the MAF. They are pinned differently depending on year and trans and can burn the ECU out if incorrect.

There is a lot of info on this if you search "Mustang o2 sensor jumper"

that was my next question. i am sure i have the auto o2 harness...ill search that.

fredfifty · Nov 20, 2017

jrichker said:
If you have a computer marked A9P, that is the auto trans computer and it will work with the 5 speed transmission and 5 speed O2 harness.

i believe i have the aod O2 harness, should i get the 5 spd one? or is the auto one okay?

fredfifty · Nov 20, 2017

https://lmr.com/item/LRS-8793XSHM/Mustang-O2-Sensor-Harness-Manual-Transmission-87-93

should i replace my current harness with this one?

fredfifty · Nov 29, 2017

anyone?

can I use the 5 Spd trans harness with the AOD 0xygen sensor harness? I currently have an a9p ecm if that should be mentioned. any insight on this before I put in the trans harness would be appreciated. I don't want to (possibly) fry my computer.

Drivetrain Neutral Safety Switch (nss)

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Drivetrain Neutral Safety Switch (nss)

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