4L65e Neutral Safety Switch and Reverse Lights

[C.I.Blazer]

I am working on wiring my PSI harness and had some questions on how to wire the NSS and my back-up lights. I did my best to research and wanted to confirm some of the information I found. The swap is an LQ4 and 4L65E from a 2004 Yukon Denali and I am wiring with a PSI Harness.

The car is a 1971 Blazer. It was originally a 4speed that was converted to a TH350 at some point. The wiring is a total mess and I am slowly working to clean it up. The old transmission had a floor shifter and I am using a Gennie Shifter on the new one. The shifter has a simple NSS contact switch on it. I believe I have found the original NSS switch (jumpered switch with purple wires) and the the backup lights (light and dark green wires) under the dash (see pic below).

I understand the NSS is not required for the PCM but helps adjust idle when in park and neutral and I would like to hook it up. So, my plan is as follows:

1) Connect the PSI harness wire to one side of the NSS on the shifter and a wire to ground to the other side. This would give a ground signal to the PCM when in park & neutral which I believe is correct.

2) Use the PRNDL switch on the transmission to drive two relays. One would serve as a NSS that would only allow the car to start in drive or neutral. This would essentially replace the jumpered mess in the picture. The other would drive the reverse lights when in reverse. I have the large style switch on the transmission and while I do not know (yet) how to wire this up, I believe this is doable. Can anyone confirm?

3) My other option would be to run the wire to the starter through the NSS on the shifter and get the PCM NSS switch from the PRNDL switch. Not sure if one option is better than the other. Any advise?

Feeling a bit overwhelmed by this wiring phase, but like this whole process it is fun to dive in an slowly figure it out one step at a time. Couldn't do it without this forum. Thanks for the help.

[ls1nova71]

Are you sure the truck was originally a 4 speed? I only ask because a 4 speed truck wouldn't have the purple wires, the ones that are jumped, under the dash. Also wouldn't have the reverse light plug there either, that would have been on the transmission. But, no telling what all has been changed in the last 50 years. Any way, to answer your questions, I have to first ask some myself. Does your PSI harness have any wires going to the NSS on the side of the transmission? Most aftermarket harnesses do not have that as they usually deem them unnecessary. Your option 1 sounds like to me that they just have one wire that needs to be grounded in park and neutral, correct? If that is so, that is how F bodies are wired, and the PCM needs to be programmed for that as trucks are wired differently.

If you have the plug for the NSS, you can easily use it to work both the neutral safety function and reverse lights. It would be wired as follows: For the reverse lights, the easiest way to do it would be to take the two green wires under the dash and hook them to pins 10 and 11. 11 is pink and 10 is gray, but since its just a switch, polarity wont matter. No relay is required if you're only using 2 1156 bulbs, unless you just want to add it, it wouldn't hurt anything, but would just be more wiring. GM has it going through the BCM, but the trailer lights go straight from the switch, so 2 lights wont overload the switch. Also, might want to jump that plug first just to make sure the rest of the wiring is good all the way back to the reverse lights first though.

For the neutral safety portion you will need a relay. To wire it you would use pins 1 and 12 on the NSS plug. Pin 12, a pink wire, will be wired to the wire that comes from the ignition switch, which is one of the heavy purple wires you have jumped under the dash. You will have to test it to see which one is hot when in the 'crank' position. Then pin 1, dark green, will be wired to the terminal 85 on the relay. Terminal 86 will be grounded. Terminal 87 will be a constant hot from the battery, usually through a 40a fuse, and terminal 30 will go to the starter solenoid.

This is how I wire all my swaps, never had any issues. Hope this helps.

[C.I.Blazer]

That does help!

So hard to know what was original on this truck. It has clearly been modified a number of times over the years. The SPID shows it as the four speed manual and the high hump has the hole for it. Also has the hole for the clutch in the firewall that was never been covered. I agree, I could not figure out why I had the NSS and backup switch under the dash.

The PSI harness has just the single wire that needs to go to ground when in park and neutral. There is nothing on the harness that goes to the NSS (is that the same as the PRNDL switch?) on the side of the transmission. I do have that switch there. I will go ahead and wire the harness to the switch on the shifter and wire the NSS and backup lights to the NSS switch on the transmission as you outlined.

Thank you so much for the help.

[C.I.Blazer]

Spoke with PSI yesterday and thought I would add the information I found out in case someone looks this thread up in the future. The PSI instructions say that there is an optional orange wire in the under dash harness that can be hooked up to "Park Neutral Signal". The harness itself has two orange wires. One is a long one in the under dash wire bundle as well as a short (~6") length of orange wire that comes out of the loom by the transmission connector. I mistakenly thought that this short section of wire is what I hooked to the park/neutral switch on my floor mount shiefter.

PSI told me yesterday that this short length of wire is only for a 4l80 transmission (not sure why it is on my harness when I specified a 4l60) and that it does not get hooked up. They also told me that the long orange wire should not be hooked up to anything and that it was simply a test wire. Not sure why it specifies how to install it in the instructions. I asked about the park/neutral switch being used to control the idle RPMs when in park and neutral and was told that this was already done via the transmission connection.

Anyway, seems a bit odd to me but will follow there directions. Does that make sense? Hope this helps someone down the road.

[bigmoe]

My neutral safety and reverse light wiring are independent of the pcm and engine wiring harness other than interrupting the start connection. 08 5.3 and 4l60e.

[dayj1]

Originally Posted by C.I.Blazer

I asked about the park/neutral switch being used to control the idle RPMs when in park and neutral and was told that this was already done via the transmission connection.

Anyway, seems a bit odd to me but will follow there directions.

The range switch on the side of the transmission is used (in the OEM application) for reverse lights and to provide a signal to the gear indicator in the instrument cluster. The powertrain controller "knows" whether the transmission is in gear or not based on the internal transmission sensors and wiring.

[C.I.Blazer]

Thanks! Received some further clarification from PSI. The long orange wire can be hooked up to ground in park and neutral and will indeed be used to adjust the idle. Also needed for a "crank relearn". Need to understand what that is down the road. Most people do not hook it up and do not have any problems. They also confirmed that the short 6" section is for the 4L80 and not needed in my application.

[derotoreut]

I'm with Bigmoe on this. I'm doing a 5.3 LM7 swap in my 70 K10 and currently wiring it. The NSS and BU signal is independent of the PCM. My NSS is on the column and has the two parallel tabs the right. They control the actual starting of the engine. The other two leads are the back up. One wire off the NSS goes to the ignition switch and the other goes to the starter solenoid. No relay involved. The other two tabs are for back-up lights. One tab goes to a fuse that is hot all the time. The other tab goes to the back-up lights.

[ls1nova71]

Originally Posted by derotoreut

I'm with Bigmoe on this. I'm doing a 5.3 LM7 swap in my 70 K10 and currently wiring it. The NSS and BU signal is independent of the PCM. My NSS is on the column and has the two parallel tabs the right. They control the actual starting of the engine. The other two leads are the back up. One wire off the NSS goes to the ignition switch and the other goes to the starter solenoid. No relay involved. The other two tabs are for back-up lights. One tab goes to a fuse that is hot all the time. The other tab goes to the back-up lights.

That's fine if you're using the factory column shifter, but the OP said he was using an aftermarket floor mounted shifter. Also, the transmission mounted switch is more accurate since it is mounted directly on the transmission. Column mounted NSS switches can sometimes be kind of vague due to slop in the linkage, which can be exacerbated by the fact that the column shifter was designed for only 3 gears and is now being used for 4. Just kind of the nature of the beast.

[C.I.Blazer]

ls1nova hit it right on the head. Floor mounted shifter so I will not be using the column for NSS and reverse lights. For now I am just working to get it running and will have to be careful to ensure I am in park or neutral when I start it. Maybe a manual switch for reverse lights. I will tackle trying to craft a pigtail for the switch on the transmission down the road.

[Ocjason]

ls1nova, I am doing a LS swap with 2005 Silverado LM7 with 4l60e into a 55 GMC truck. I am using the NSS switch with a Painless universal starter switch. Based on your instructions above, does this diagram look correct? Thanks.

[lutronjim]

Dayj1

Does the switch provide prn1234 - when you said dash indicator ?

Thanks

[ls1nova71]

Originally Posted by Ocjason

ls1nova, I am doing a LS swap with 2005 Silverado LM7 with 4l60e into a 55 GMC truck. I am using the NSS switch with a Painless universal starter switch. Based on your instructions above, does this diagram look correct? Thanks.

I'm not familiar with that ignition switch, but that looks right. Generally the back up lights would be fused in the fuse box though.

[dayj1]

Originally Posted by lutronjim

Dayj1

Does the switch provide prn1234 - when you said dash indicator ?

Thanks

Yes. But there are 7 gear positions (for a 4Lxx trans) and only 4 gear position signal wires. So, it’s a switch matrix instead of discrete outputs. In other words, it takes some electronics to decode the signals into what gear you’re in. I don’t have it handy, but the information is in the GM schematics.