Control Module

Help!!
I purchased a 1998 Chevy Venture from a local auction and I am having a problem with the check engine light. I have purchased a scan tool and get the error code P1639 Control Module Analog to Digital Performance.
Anyone have any sugestions for me on this?

Help!!
I purchased a 1998 Chevy Venture from a local auction and I am having a problem with the check engine light. I have purchased a scan tool and get the error code P1639 Control Module Analog to Digital Performance.
Anyone have any sugestions for me on this?Where did you get that description? I show DCT P1639 for GM vehicles to be "5 Volt Reference 2 Circuit".

Where did you get that description? I show DCT P1639 for GM vehicles to be "5 Volt Reference 2 Circuit".

Hmmm, That came right off the computer. I Am using Auto Enginuity's ScanTool for PC w/GM Extended codes. I'm loosing hair over this one.

Either your computer's wrong (possible), we are describing the same error (no likely...sound too different), or every GM ODB-II reference list I can find is wrong (even less likely). I'll check my shop manual tonight to get the official word though.

I got the 98 service manual in front of me and the DTC P1639 is indeed for 5 Volt Reference (B) Circuit. The 5 volt circuit is used for sensor feed for the A/C refridgerant pressure sensor as well as the fuel tank pressure sensor according to the wiring diagram. The DTC is set when the PCM detects a voltage out of tolerance on the circuit for more then 10 seconds. The tolerance is 5 +/-.5 volts. Check connections, as well as engine ground points.

I got the 98 service manual in front of me and the DTC P1639 is indeed for 5 Volt Reference (B) Circuit. The 5 volt circuit is used for sensor feed for the A/C refridgerant pressure sensor as well as the fuel tank pressure sensor according to the wiring diagram. The DTC is set when the PCM detects a voltage out of tolerance on the circuit for more then 10 seconds. The tolerance is 5 +/-.5 volts. Check connections, as well as engine ground points.

Ok that gives me a clue as where to startlooking for the problem. The rear A/C unit is not working so now Im wondering if that could be where the problem is. Do you know if I could short the wires going to the damper control swicth out to bypass the unit? That swicth is not working and I have not been able to locate a replacement as of yet.

Ok that gives me a clue as where to startlooking for the problem. The rear A/C unit is not working so now Im wondering if that could be where the problem is. Do you know if I could short the wires going to the damper control swicth out to bypass the unit? That swicth is not working and I have not been able to locate a replacement as of yet.No idea. AC system is beyond my experience. The troubleshooting chart in the shop manual mainly talks about poor connections and wiring faults. Presuming that the wiring/connection is good, it looked like a bad PCM.

I just realized something. You might want to double check that error code and see if there were any other codes present. P1639 won't set the MIL (Malfunction indicator light) according to the manual.

My manual says the following for Code P1639...

The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.

So, it looks like the MIL will come on for this code.

You said that the rear air conditioner doesn't work, how about the front air?

Ok just spent most of the afternoon trying differnt things and tearing down the rear A/C unit. What I have found was the rear swicth module that controls the damper door from hot or cold air is dead. I took apart the module and found that the motor works on the bench but the IC circuit is bad. My Haynes manuel does not even show this side of the A/c Unit or the control module. Well I put it all back together again ran the van for about an hour or so and no Service Engine Light. Sounds great right? Not, after driving around and patting myself on the back for a job well done the darn thing came back on. Well back to the drawing board, Maybe I'll try the brakes next :)