OBD ll code P1639

Tiny
BILLMILL5050
  • MEMBER
  • 2002 PONTIAC FIREBIRD
  • 3.8L
  • V6
  • RWD
  • AUTOMATIC
  • 87,000 MILES
Where would I check "5 volt ref circuit 2 "? I do not know what that means.
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Wednesday, August 31st, 2016 AT 2:44 PM

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Tiny
CARADIODOC
  • EXPERT
5.0 volts is developed inside the Engine Computer and is the supply voltage for most of the engine sensors. In your case it sounds like there might be two separate 5.0 volt supplies. The easiest sensor to find this voltage is probably the throttle position sensor on the side of the throttle body assembly. It will have three wires attached to it. Use a digital voltmeter. Back-probe through the rubber seals alongside the wires while the ignition switch is on. One wire should have 5.0 volts. One will have very close to 0.2 volts, and the middle one will have around 0.5 volts at idle

If there is no 5.0 volts, that is the circuit that must be diagnosed. Usually it is missing because something on that circuit is grounded, then the computer turns that supply off to protect it. Once the short is identified and corrected, you must turn the ignition switch off, then back on to reset the supply voltage. Check for a wiring harness that got cut from laying over a sharp metal bracket, or some of the wires rubbed through against the body sheet metal. You may need to unplug sensors, then cycle the ignition switch off and back on to see if the 5.0 volts comes back. If it does, plug in one sensor at a time until you see the 5.0 volts go away again. That will be the one that is shorted.

If you do find 5.0 volts at the throttle position sensor, check at the other sensors the same way. Some sensors might run on 8.0 or 10.0 volts so do not be surprised if you find that in a few places. You are looking for a sensor that doesn't have at least 5.0 volts on one terminal. This does not apply to the coolant temperature sensor or intake air temperature sensor. Those will have just two wires, and those circuits operate differently, so ignore those.
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Wednesday, August 31st, 2016 AT 5:57 PM

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