P0502 Help!!

[jstatham]

I have a 2012 Camaro SS Convertible with 36k miles. Its tuned headers,cam, no cats etc. Its make 485 at the rear wheels. Long story short the battery died and my emissions is due. My rear O2's are deleted out so all the other monitors have to be in the ready state. You are allowed 1 no ready and every year mine is the O2. The problem this year is I cannot get the evap to run. I have the tech2 vxdiag setup and ran the test got it to pass and got a code for the O2. I replaced the 02 reset the codes and then when I tried to run another Evap test it runs for about 2 minutes then says engine running test aborted. Lost enabled conditions. I then check codes and I get a P0502. I then replaced the vehicle speed sensor reset the codes and drove around again and have everything ready but the evap and 02. As soon as I hook up the tech2 and try and run the evap test I get the P0502. I check the wiring for the VSS and it all good and I used a new Delphi sensor. Clear codes drive around again everything is ready but evap and O2. I try the tech2 evap test and runs for about 2 minutes and fails engine running test aborted. Lost enabled conditions. I am not sure what to try next. The evap has to be over 39 degrees to pass and after today that's not going to happens for a while. The other thing I noticed is I cannot get it to go into reverse when I 1st crank it up. Is it possible the reverse lockout solenoid is bad and causing the VSS issue. They are all controlled by the same 7.5 amp fuse. Doe anyone know how many volts the VSS should have with key on. I am showing right at 3 volts. Anyone have any experience with this or have an idea on what to try next. Any help is greatly appreciated.

[gtstorey]

Find the service manual diagnostic steps for your particular model for that code and follow them. Diagnostic is available here.
https://www.acdelcotds.com/subscriptions#

[jstatham]

Thanks but I have access to Mitchell and All Data and followed the steps the circuit is good. There is something else happening.

[gtstorey]

They usually will point you to a problem even if it's the "if nothing else found, replace the ECM".

I don't think anything on GM will be 3v. It's either 5v or 12v. I don't know off hand which without a wiring diagram. If the feed comes from the 7.5amp fuse, it's 12v.

[jstatham]

Thanks it does come from a 7.5 fuse I will get under it and check again tomorrow. I was trying to hold the 2 test leads and the light and see the volt meter at the same time. I will also test the reverse lockout as well for voltage. They are all on the same circuit. I can also back probe the connectors at the ecu and see what coming out at the top VS at the plug.

[Mouse330]

Do you have a 2 wire VSS(Tremec) or the 3 wire VSS(Aisin)? The SS comes with the M10 Tremec. The 2 wire M10 Tremec is not fused has signal low and high. The MV5 Aisin is fused and has Ignition V, Ground and VSS signal. Make sure, Good Luck

[jstatham]

I have the 2 wire VSS(Tremec). What voltage should I see from both wires with the switch on?

[Mouse330]

There's no voltage testing just Ohms testing, Look at the wiring harness from the VSS to the ECM carefully. Did it contact the exhaust etc....P0502 is a short to ground or voltage, open/high resistance in the low or high signal. Ill copy/paste these tests....

1. Disconnect the ECM harness connector X1. (X1 connector is black)

Gry/blk is the signal high circuit, PUR/WH is signal low circuit

2. Measure for 1,300-1,500 ohms at 25°C (77°F) between the VSS high signal circuit terminal 71 and the VSS low signal circuit terminal 72.

♦ If the value is greater than 1,500 ohms test the VSS high signal circuit and the VSS low signal circuit for an open/high resistance. If the circuits/connections test normal, replace the vehicle speed sensor.

♦ If the value is less than 1,300 ohms test the VSS high signal and the VSS low signal circuits for a shorted together condition. If the circuits/connections test normal, replace the vehicle speed sensor.

3. Measure for more than 10 ohms between the VSS high signal circuit terminal 71 and ground.

♦ If the value is less than the specified range, repair the short to ground.

4. Measure for more than 10 ohms between the VSS low signal circuit terminal 72 and ground.

♦ If the value is less than the specified range, repair the short to ground.

5. Reconnect the ECM connector. Disconnect the VSS connector.

6. Ignition ON, test the VSS high signal circuit terminal B for a short to voltage.

♦ If all circuits/connections test normal, test or replace the VSS.

7. If all circuits test normal, replace the ECM.

[jstatham]

My resistance between Terminal 71 and 72 was 1,826 at 45 degrees outside I wasn't sure if that made a difference since the temp on the spec was at 77 degrees. I then disconnected the VSS and hooked a 9 volt battery to the connector and then my volt meter to Terminal 71 and 72 and it reads 9 volts. That should prove the wire is good. One thing I did notice is the 1,826 was the same resistance I got on the new VSS the old was 1,700 ish. Do you know what the resistance should be on the VSS? I can reset the codes and drive around fine but as soon as I hook up the tech2 and start the Evap Test It kicks a P0502 in about 2 minutes. I am pretty sure the wiring is fine I looked and its nowhere near the exhaust and all the loom looks clean. I am going to look more tomorrow if I can get the garage warm enough to work in it. Another note I just ran this test a week ago and it passed but had to replace an O2 and reset and now I am fighting this.

[gtstorey]

Using the tech2 may be the problem. 5th Gen Camaro is listed as GDS2 supported not tech2. I don’t know what the differences are there but it’s the place I would look if the p0502 never shows up when just driving it.

Beyond that a two wire VSS should generate a ac voltage system if you can move a piece of iron in front of it fast enough or test with the wheels off the ground. But if you get a vehicle speed when driving and this is only a problem with tech2 connected, that is likely your problem.

[jstatham]

The Tech2 goes up to a 2013 but I wonder if the gds2 has the test because I have it and could at least try. I have ran the test successful 5 days ago with no issues. I am going to retest the resistance of the 2 wires and pull out the new sensor and see if anything catches my eye. The sensor works fine because I can see speed on my scanner with driving and looking at live data. I wonder if the tech2 can somehow tell the resistance is to high on the low voltage wire and kicking the code. Hopefully it will get above freezing today or tomorrow and I can investigate. It has to get above 39 to run the evap test. I now have a battery tender so once I get it set I shouldn't have to fight it again.

[jstatham]

Well I did some more checking today and I found the old sensor is 1861 Ohm and the new sensor is 1852 ohms. I then tested Terminal 71 and 72 with the sensors hooked up and reads the same ohms as the sensor so itself so that tells me my wiring is good. So now I am leaning more towards a bad sensor. I went to Oriellys and checked their sensor and they read 1920 ohms. I am going to the dealer tomorrow and check the OHMS on a new OE AC delco one. I don't see how the resistance will ever be 1200-1500 OHMS when the sensor is over 1800 out of the box. Anyone have any ideas?

[gtstorey]

Tech2 is only through 2013 on certain vehicles. Camaro is not one of those.

This is not a problem of a sensor reading a few ohms different.

[jstatham]

I agree but the spec is 1300-1500. The tech2 does allows me to pick the vehicle and perform 2 way test. I have done the Evap test successfully several times.