I am getting code P1639 on my Chevy Venture van 2003. This is supposed to indicate a low 5 volt reference at the AC pressure sensor and the fuel tank pressure sensor. I have measured the voltage going to both sensors and get 5 volts. This is a gray wire in both cases. The AC has been working fine and I have no other codes. I measured the 5 volts. This is a gray wire in both cases. The AC has been working fine and I have no other codes.
I measured the 5 volt reference signal by disconnecting the AC pressure sensor and by sticking a needle through the reference wire while the sensor is connected. I am at a loss why the PCM thinks the voltage is low. The AC pressure sensor I checked is in the high pressure line and is located under the air intake box. I checked (loosened and re tightened) the ground strap located next to the PCM with no change in the immediate engine light.
When I check the 5 volt reference to body ground I get 5 volts. When I check the 5 volt reference to engine ground I get 5 volts. I am starting to think the PCM is bad but not sure if this would need re-programing. Can I safely measure the resistance of the ground wire to ground without damaging the PCM? What maximum resistance is allowable?
Alright, that’s a long one but let’s start with some information. The low voltage codes do not mean that the sensor or sensors are not getting 5 volts. It means there is problem with the voltage drop across the sensor. Checking the 5 volt reference to ground will usually always net you 5 volts so that is not the correct way to check the sensors. You actually cannot check the sensor itself for that type of code you would need to check the signal wire off the sensor and see what the reading is. One area of concern that you should check is the connector located about mid-way under the van, there is a bulkhead under the floor pan under the left rear seat. There are a couple connectors there, and the large white connector has the wires to the fuel tank pressure sensor and fuel pump. Quite often, on the Venture vans, this connector will get corroded and cause issues like the one you have. I would check that before anything else, as most times that is where the problem is.
I have looked at the connecter under the floor and did not see any corrosion. I did check the voltage after this connector in addition.
Ok, if the connector is fine (I would really look closely at it again), then you either have a faulty fuel tank pressure sensor, high side a/c sensor, or pcm. I doubt it’s a pcm, and out of either sensor I would bet on the fuel tank pressure sensor being the one that’s causing your code. It’s a guessing game, but you have to start somewhere.
I have not dropped the fuel tank yet. I can probably stick pin the wire coming from the fuel tank pressure sensor and check for operation by pushing on the seal tank. Do you know what color the wire is?
The 5 volt reference wire is gray and the signal wire is dark green.
Perfect!! I will check it out. Thanks
you’re welcome, have a great night.
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