P1639-erratic shifting

[clemsonblozz]

I have a 2001 Silverado Z71 with the 5.3...

 

I have experienced twice in the last 5 days the following issues.. The truck will upshift and downshift erratically,, idles about200 RPM higher.. There will be no check engine light,,, UNTIL the vehicle is restarted.. That is when the 1639 will trigger..

 

Now... I have a scangauge2 on my truck,, and since this problem surfaced this afternoon,, I monitored the TPS percentage.. it would be 10-12 % at idle,,, and would jump up and down even with the truck on cruise..

 

If the truck sits for 10-20 minutes,, it will run ok again...

 

I have read that this might be a fuel tank pressure sensor,, TPS, wiring issue, or even a PCM..

 

Haven't seen where anyone had the 1639 code with the erratic shifting,, so I am hoping it's a TPS..

 

Help!

[Chris]

The code says PCM voltage is low

[gmtech4]

have battery and alternator checked and have truck code cleared,,if it comes back ,have code diag'd,,,dont throw parts at it ,,,,,so uncool lol

[clemsonblozz]

 

Battery and alt seem to be fine.. I load tested the battery and the alt is putting out 13 volts..

 

What bothers me, and it might be a huge coincidence, this happened after trying to jump start a guys kubota tractor.. There just seems to be a lack of info on this code,, and the truck does not do this all the time.. But when it does it,,, it's fairly Irritating and slightly dangerous

[clemsonblozz]

I have a 9.2 volt 5 volt referance when the truck acts dumb,,, when the truck is hot.. Goes back to 5 volts when then truck cools.. Ideas?

[gmtech4]

code has to be diagnosed ,,,thats it ,thats all

 

DTC P1639

Circuit Description

The powertrain control module (PCM) uses a common 5–volt reference circuit as
a sensor feed to the fuel tank pressure sensor and the throttle position (TP)
sensor.

The PCM monitors the voltage on the 5–volt reference circuit. This diagnostic
trouble code DTC sets if the voltage is out of range.

Conditions for Running the
DTC

The engine operating.

Conditions for Setting the
DTC

• The 5–volt reference circuit is out of range.
• 
  
• The above condition is present for more than 2 seconds.

Action Taken When the DTC
Sets

• The control module illuminates the malfunction indicator lamp (MIL) on the
  second consecutive ignition cycle that the diagnostic runs and fails.
• 
  
• The control module records the operating conditions at the time the
  diagnostic fails. The first time the diagnostic fails, the control module stores
  this information in the Failure Records. If the diagnostic reports a failure on
  the second consecutive ignition cycle, the control module records the operating
  conditions at the time of the failure. The control module writes the operating
  conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the
MIL/DTC

• The control module turns OFF the malfunction indicator lamp (MIL) after
  3 consecutive ignition cycles that the diagnostic runs and does not fail.
• 
  
• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
• 
  
• A history DTC clears after 40 consecutive warm-up cycles, if no failures are
  reported by this or any other emission related diagnostic.
• 
  
• Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

Important: Remove any debris from the PCM connector surfaces before
servicing the PCM. Inspect the PCM connector gaskets when diagnosing or
replacing the PCM. Ensure that the gaskets are installed correctly. The gaskets
prevent water intrusion into the PCM.

Using the Freeze Frame/Failure Records data may aid in locating an
intermittent condition. If you cannot duplicate the DTC, the information
included in the Freeze Frame/Failure Records data can help determine how many
miles since the DTC set. The Fail Counter and Pass Counter can also help
determine how many ignition cycles the diagnostic reported a pass and a fail.
Operate the vehicle within the same Freeze Frame conditions such as RPM, load,
vehicle speed, temperature etc. that you observed. This will isolate when the
DTC failed. For an intermittent condition, refer to Symptoms -
Engine Controls.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. • The 5–volt reference circuits for the sensors are connected together
     inside the PCM. Both DTCs P1635 and P1639 set at the same time, indicates a
     5–volt reference circuit is shorted to a voltage.
   
   
   • The 5–volt reference circuits for the sensors are connected together
     inside the PCM. When only P1639 is set, this indicates a short to ground on one
     of the 5–volt reference circuits.
   
   
   • This step determines if a 5–volt reference circuit is shorted to
     ground.
   
   
   • The 5–volt reference circuits may be shorted to another PCM circuit.
     The shorted circuit may not be apparent when the PCM harness connector is
     disconnected. Testing continuity from each 5–volt reference circuit isolates the
     shorted circuit.

DTC P1639

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls
Schematics

1

Did you perform the Diagnostic System Check–Engine Controls?

—

Go to Step 2

Go to Diagnostic
System Check - Engine Controls

2

2. Install a scan tool.
3. 
   
4. Start the engine.
5. 
   
6. Idle the engine for 2 minutes.
7. 
   
8. Monitor the DTCs using a scan tool.

Did both DTCs P1635 and P1639 fail this ignition?

—

Go to DTC
P1635

Go to Step 3

3

Did DTC P1639 fail this ignition?

—

Go to Step 5

Go to Step 4

4

2. Turn ON the ignition, with the engine OFF.
3. 
   
4. Review the Freeze Frame/Failure Records data for this DTC
   and observe the parameters.
5. 
   
6. Turn OFF the ignition for 15 seconds.
7. 
   
8. Start the engine.
9. 
   
10. Operate the vehicle within the conditions required for this
    diagnostic to run, and as close to the conditions recorded in the Freeze
    Frame/Failure Records as possible. Special operating conditions that you need to
    meet before the PCM will run this diagnostic, where applicable, are listed in
    the Conditions for Running the DTC.
11. 
    
12. Select the Diagnostic Trouble Code (DTC) option, the
    Specific DTC option, then enter the DTC number using the scan tool.

Does the scan tool indicate the DTC set this ignition?

—

Go to Step 5

Go to Diagnostic Aids

5

2. Turn OFF the ignition.
3. 
   
4. Disconnect the PCM connector C1. Refer to Powertrain
   Control Module Replacement.
5. 
   
6. Connect the test lamp to B+.
7. 
   
8. Probe the PCM connector 5-volt reference circuits for the
   following components:
   • The TP sensor
   • 
     
   • The fuel tank pressure sensor

Does the test lamp illuminate for any of the circuits?

—

Go to Step 7

Go to Step 6

6

Test continuity from each 5-volt reference circuit to all other PCM circuits
at the PCM harness connector using the DMM. Example: probe one lead to the TP
sensor 5-volt reference at the PCM harness connector circuit and probe the other
lead to each terminal at the PCM harness connector. This must be performed for
each 5-volt reference circuit. Refer to Testing for
Continuity in Wiring Systems .

Do any of the circuits indicate a resistance within the specified range?

0–2 Ω

Go to Step 8

Go to Step 9

7

Repair the short to ground on the appropriate 5-volt reference circuit. Refer
to Wiring
Repairs in Wiring Systems .

Is the action complete?

—

Go to Step 10

—

8

Repair the short between the appropriate 5-volt reference circuit and the PCM
circuit that had continuity. Refer to Wiring
Repairs in Wiring Systems.

Is the action complete?

—

Go to Step 10

—

9

Replace the PCM. Refer to Powertrain
Control Module Replacement.

Is the action complete?

—

Go to to Step 10

—

10

2. Use the scan tool in order to clear the DTCs.
3. 
   
4. Turn OFF the ignition for 30 seconds.
5. 
   
6. Start the engine.
7. 
   
8. Operate the vehicle within the Conditions for Running the
   DTC as specified in the supporting text.

Does the DTC run and pass?

—

Go to Step 11

Go to Step 2

11

With a scan tool, observe the stored information, Capture Info.

Are any DTCs displayed that you have not diagnosed?

—

Go to Diagnostic
Trouble Code (DTC) List

System

[clemsonblozz]

I was generally trying to avoid paying someone to diagnose my issue, thus the post here ... I work for a diesel engine supplier,, and our engine systems use the same basic sensor setup(with the 5 volts),, and I've seen ECM's get hot and do some funky stuff..

I ordered a pcm,, and I am pretty sure that will take care of my temp sensitive super voltage issue.. If not, I will get 5 volts from another sensor circuit and rock on.

[Chris]

The PCM will have to programmed at the dealer

[gmtech4]

ya dont fix something by guessing bud ,but good luck to ya lmao !!!!!

[clemsonblozz]

You might not fix things by guessing,, but not being stupid has it's benefits...

 

Very few things add voltage.. Even less of them are temp sensitive... Now,, usually voltage drops with temp rise,, but anything is possible..

 

A PCM from a sister vehicle took care of my issue..

 

Now I'll wait for my PCM to arrive and hope I am competent enough to install it..

[gmtech4]

glad ya got it ,,,its got to be programed at the dealer or any shop that has GM subscription and a tech2