Having problems with your 2002 Pontiac Grand Am ?

I have a Pontiac hard to start and will not rev up the code that comes up is PO502

Posted / edited by AnonymousUser on : 15-01-2018

Answers :

DTC P0502: VSS Circuit Low Input

Circuit Description

The OSS provides vehicle speed information to the PCM. The OSS is a permanent magnet generator mounted to the transaxle case. The OSS produces an AC voltage as the speed sensor rotor teeth pass through the sensor's magnetic field. The AC voltage level increases as the speed of the vehicle increases.

The PCM converts the AC voltage into a digital signal. The PCM uses the vehicle speed signal to determine shift timing, TCC apply, TCC release, and gear ratio calculations. When the PCM detects a low output speed and the vehicle has a large engine speed in a drive gear, then DTC P0502 sets.

Conditions For Running DTC

DTC will run under the following conditions:

  • No MAP sensor DTCs P0107 or P0108.
  • \015\012
  • No TP sensor DTCs P0121, P0122, or P0123.
  • \015\012
  • No ISS DTCs P0716 or P0717.
  • \015\012
  • No TFP manual valve position switch DTC P1810.
  • \015\012
  • The transaxle is in a drive range.
  • \015\012
  • The throttle angle is greater than 12 percent.
  • \015\012
  • The input speed is greater than 1500 RPM.
  • \015\012
  • The engine torque is greater than 50 ft. lbs. (68 N.m).

Conditions For Setting DTC

DTC will set under the following condition:

  • Output speed is less than 150 RPM for 3 seconds.

Action Taken By PCM

PCM performs the following if DTC is set:

  • Illuminates MIL after 2 consecutive trips with a failure signal.
  • \015\012
  • Freezes shift adapts from being updated.
  • \015\012
  • Commands maximum line pressure.
  • \015\012
  • Calculates output speed from input speed, engine speed and commanded gear.
DTC P0502 will be stored in PCM history.

The Vehicle Speed Sensor (VSS) may also be referred to as the Output Speed Sensor (OSS).

Your hard start and this code maybe unrelated.

P0502, Vehicle Speed Sensor circuit has a issues and needs to be addressed.

There are three parts of the Vehicle Speed Sensor (VSS) circuit; the\015\012VSS, the Power-train Control Module (PCM) and the wiring between the\015\012two. The problem could be in any one, or more, of these parts.The most likely problem would be with the VSS itself. You can check the\015\012VSS signal with a DVOM set to read AC volts. Raise the front wheels off\015\012the ground and support the vehicle with jack stands. Never, ever work\015\012on a vehicle that is supported by a jack alone. Having a vehicle fall\015\012on your chest could ruin your whole day.

Connect the leads of the DVOM to the yellow, C1-64 and purple, C1-65\015\012wires of connector C1 (blue) of the PCM. Be very, very careful you do\015\012not damage the connections of the wires. I use a single strand of a\015\012multi-strand wire to insert into the connector then connect my leads to\015\012that. There is an electrical test kit you can buy but the single strand\015\012wire will do quite nicely in a pinch.Put the key in the ON engine OFF position, place the transmission in\015\012DRIVE or NEUTRAL and slowly rotate the right front wheel. You should\015\012get a reading of 0.4 volts AC. If the reading is good set your DVOM to\015\012ohms and measure the resistance of the whole circuit. It should read\015\012between 1650 to 3180 ohms. If not you have a problem in the wiring. If\015\012both check out good, then the problem is in the PCM.

The PCM will run about $321.45 and about an hour labor to replace and\015\012reprogram. The VSS will run about $35.17 and about a half hour to\015\012replace. Figure about one hour labor to diagnose the problem. If the\015\012problem is in the wiring; that is charged by the actual time it takes\015\012to locate and repair the problem.

The VSS is located on the end of the transmission. Remove the right\015\012front tire and you will see it. To replace it simply unplug the\015\012connector, remove the attaching bolt and remove the VSS from the\015\012transmission. Then install the new one, secure the attaching bolt and\015\012plug in the connector. Simple as that.

With thing information, and if your not to checking everything as it's time consuming. You may just want to inspect the wire leads toe VSS and or replace it. 90% of the time it's the VSS that has gone bad and the other 8% is wire leads damaged due to corrosion.

Good luck and keep me posted.
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