Park Neutral switch 1990 with 700R4 Winters Sidewinder Shifter install.

[RShrimp]

I am installing a Winters Sidewinder Floor shifter. It comes with a Park Neutral switch and a reverse light switch.
I was looking under my dash for the P/N switch and it is missing and the wires/plugs are hanging. I looked at the wire diagram and it shows that the switch closes with in Park or Neutral. Obviously mine never does. The truck starts and runs fine (All stock) I would have thought that the P/N switch would prevent you from starting it in gear but that is not the case. I dug further into the wire diagram and found that on Manual truck they break the purple wire between the ign switch and the starter with a clutch switch which closes when the clutch is depressed. I plan to break the purple wire and run it through the P/N switch on the new shifter so that issue is solved.
Question: What does the P/N switch do? It is connected to the ECM only and does not prevent starts. That function is done mechanically in the steering column. So, What does it do? I know it tells the ECM that the truck is in P or N but why?

 

[GoToGuy]

If your starting with gear selector in any position, it's already been bypassed. I would trace the wire and find the bypass. Use the wires off the original P/N switch, there available, circuits are in place for use.

 

[RShrimp]

It's not starting in gear and the park neutral switch has nothing to do with when it will start (I thought it did too but in fact it does not) Start in gear prevention is done mechanically in the steering column. (The key wont move to the start position if the column shifter is in gear.) With the PN switch disconnected as mine is, the circuit is open and in the same condition as when the vehicle is in gear. The circuit closes when in Park or Neutral. If the PN switch prevented start in gear then my truck would not start. It starts fine.
I post this stuff so the next guy can find it. That is why I love forums but hate FaceBook.
So, back to my original question. What does the PN switch do (It simply tells the ECM if the vehicle is in gear or not.) Buy why does the ECM need to know this and what changes does the ECM make because of it?

 

[RShrimp]

Here is the wiring on a 1990. You can see that the PN switch feeds directly back to the ECM. The purple start wire in uninterrupted on an Automatic. My plan is to break the purple wire as does the clutch switch on the manual and run that through the PN switch on the new shifter. As you can see in the diagram the current GM PN switch does nothing to prevent starts.

 

[RShrimp]

Install went well. When you install the cams to hut the switches the Park Cam sticks out the bottom of the housing about a half inch and causes it to hang up in Low and second gear. I made some wedges to raise the front of the shifter off the floor about a half inch. Problem solved. I was able to hook up reverse lights and break the purple wire through the new PN switch in the shifter to prevent in gear starts. Very happy with outcome. Next is a Transgo Reprogramming kit.

 

[PlayingWithTBI]

Buy why does the ECM need to know this and what changes does the ECM make because of it?

With an automatic trans, the ECM will adjust the idle when in neutral, based on temperature. IDK if you may want it so your idle will raise when warming up?
In this case it's 850 RPM when engine is warm.

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[RShrimp]

Nice and Thank You! I love learning stuff, which brings up a follow up question. What Software/Scanner is that? I am considering an OTC 3211 or equivalent. Should I get it / will I learn things? found one for $150

 

[PlayingWithTBI]

What Software/Scanner is that?

I'm using TunerPro RT for reading and modifying the .bin (AKA chip, EPROM, etc) on my laptop. That was a screenshot of a random .bin on TP. For ECMs like the 1227747, I had it modified to a ZIF (Zero Insertion Force) socket to accept a flash chip like the 27SF512. Then I can flash it with TunerPro RT and a Moates Burn2. I use TP to monitor/data log also.

With the EBL, I use his WUD (What's Up Display) to flash .bins and monitor/data log.

No, I don't think you can do all this stuff with a Scanner but, one may be what you need if you're not gonna do any "Tuning"