'87 350 TBI, custom application, looking to go bare-bones

**Mrgto68** · 03-24-2019

Ok, so I guess the P/N Switch is also the Neutral Safety switch.

"Regarding the P/N switch:
Some wiring harnesses ground the Park/Neutral wire so that the ECM always thinks the vehicle is in Neutral or Park. The reason is to prevent the SES (service engine soon) or Check engine light from being turned on due to lack of a VSS signal. The service manual states: The ECM uses the P/N signal as one of the inputs to control: Idle Air control; VSS diagnostics; EGR. If the P/N wire is grounded in drive, the EGR would be inoperative , resulting in possible detonation.
Without the P/N switch connected (grounded) the engine will run ok but engine speed may drop excessively when shifting into gear. "

I just looked at the Purple wire on the new harness, it has a tag on it which reads: Grounded when in Park and Neutral. Which would explain why when I data logged the motor, it showed that the vehicle was in "Drive" the entire time.

it's no trouble to wire it up., but I'm just curious..... if the ecu doesn't get a grounded signal when in Park / Neutral how does it effect the running of the motor?

**Mrgto68** · 03-25-2019

Hey Dave,

Following up on your findings on my last data log.

The bad VSS reading was a wiring issue, which is now corrected

I looked at the ESC module and the plug connections appeared to have some corrosion. I gently cleaned them up and plugged the unit back in.

I connected the P/N Wire and it now reads when the vehicle is in "Drive" or "Park"

I connected the AC Signal Wire to a toggle switch and when switched on the signal is received by the ECU and increases Idle about 200 rpm. this also sends the ecu into Closed Loop at Idle.

I put a voltmeter to the Knock Sensor and it tests good

Pulled the EGR valve, it's heavily packed with carbon. The valve moves and springs back when pressed, however it doesn't appear to hold vacuum. is it worth the time to try to clean these things or just order a new one?

I attached the data log while testing everything above at Idle.

Looks like I'm finally getting somewhere!!!!

Which programmer do you recommend?