Hi
I'm having a serious issue with an 02 Grand Am, 3400. The car has gotten progressively worse, can't reach highway speeds anymore. When you accelerate, the engine seems fine at first. When it reaches around 1500-2000 rpm it suddenly seems like it'* under a massive load, eg: brakes applied foot on accelerator. It will hardly accelerate and may even slow down. I pulled codes for a random misfire and PO502, Vehicle Speed Sensor Low Input. Replaced speed sensor, no change.
Here'* where I get very confused. I put the vehicle on stands. No problems while in park or neutral. Had the problem in drive and reverse. I also have the problem when the vehicle is in neutral with the wheels still spinning. When I shift from drive into neutral with the wheels spinning, the speedometer immediately goes to 0mph. Looking at my code reader, I can see my MPH. I pulled the plugs, they seem to be running lean, but the fuel trims seem fine. I can't find anything. I'm checking the wires from the PCM to the VSS for a short. What else could cause this?

you can see the mph on the scanner when the gauge gauge drops or the problem happens?

I see my MPH all the time on the scanner, but when I shift from drive into neutral I immediately lose the dashboard gauge.

if you got the right vss, then i cant think of what else it would be other than the plug or wires to the pcm

I just got done checking the wires, I disconnected everything from the PCM and the Speed Sensor. The wires are not shorted together. The purple wire (circuit401?) was "shorted" to ground. The book I have tells me specifically to check the yellow wire (circuit400?) though, which is not shorted to ground. I don't see why that purple wire would be grounded before it reaches the PCM.

I also can't find where that yellow wire enters the PCM. I tested every wire for continuity multiple times. So I figure maybe that wire is broken. I hotwire the yellow wire and patch it into the back of the wiring harness where I am fairly confident it should be (don't have pictures). No changes.

I'll do a thorough check on the wiring harness when I get a chance, maybe tomorrow morning

if you have the connector off on both sides it should read open on both wires. the diagram i have says the la1 is C1 64 IS yellow and 65 is purple. if you dont have continuity like you said its open

Today I hotwired the VSS directly to the PCM. I cut the wires so a short could not interfere with the signal. Yellow to yellow, purple to purple. The engine will no longer turn over, and I do not hear the fuel pump with the key on. Those wires must have some other junction that they pass through. Without a wiring diagram, I'm not sure where they go to. I will reconnect those wires. Back to square one.
I still can't figure how I can have a speed reading on the scan tool but no reading on the dashboard when I'm in neutral with the tires still spinning.

i am not sure about that either because the signal goes to the pcm and the dash is a serial connection with a bunch of different data on it.

you make sure it was c1(blue) 64 and 65? if the thing wont crank, either the connector didnt get put back on right or you got the wrong wires.

the diagram i am looking at is on autozone, if you sign up they give you free repair guides that have some diagrams for popular stuff, just if something doesnt agree i wouldnt trust it

To be on the safe side, I suggest going to Alldatadiy.com and get the information you need. It'* like $12-15 for a year and it covers the entire vehicle. You can even download and save schematics etc.