RepairPal is your advocate for car care advice and guidance

Q: Why is my security light still flashing after recommended repairs? on 1998 GMC 2500 Pickup

I have changed both the ign cyl&key, and the passlock sensor ass. After learn procedure, the light will go out after starting, but after a few ignition cycles the light will begin flashing again, with no start condition again. I have performed relearn procedure again, and again. What else is involved?
Get a Repair Estimate
Guaranteed by certified locations nationwide. Learn more
RepairPal estimates are guaranteed at over 1,700 quality certified locations nationwide. Learn more
| |
Sounds like you may have a poor electrical conection on wires at passlock sensor. There are some pinpoint test to check system. May have to get a Pro. to do those test.
Potential Causes are listed:
Body Control Module (BCM)
Ignition Switch
Passlock Sensor
| |
Assuming you have P1631 fault is a short cut test I have listed below.

Trouble code P1631. Intermittent no start.
1. Test voltage values of the Passlock sensor and compare the values to the voltages found when the vehicle is a no-start.

2. There are three wires at the Passlock sensor to test.

3. Test voltage from the Red/White wire to the Orange/Black wire, back probing with a sensor connected.

4. Should have 9.0v or higher at: IGN 0, IGN 1 and CRANK.

5. Test voltage on the Yellow signal wire.

6. Should have 4.7-5.0v at: IGN 0, and then a drop to a specific value should occur as the key is rotated to IGN 1 and/or CRANK.

7. Compare this voltage value to what is detected when the engine is a no-start.
Initial test at ign0 reveals no voltage from red/white to orange/black, nor yellow to orange/black. At ign1 10.6V from red/white to orange/black/ .86V from yellow to orange/black. Is the abscence of the 5Vref signal significant?
| |
Sounds like pass lock module to me. Here is another test that you can do to determine sensor or module. What fault code is it setting?

1. At the passlock module, verify 12 volts power feed to the Pink wire pin B3 and to the Orange wire pin B8 when the problem is present.

2. Check ground on the pin B1 Black/White wire.

3. Check for 9-12 volts coming out of the passlock module on the Red/White wire pin A5.

4. See if there is a 5 volt reference coming out of the module on the Yellow wire pin B7.

5. If power and grounds are good but no voltages coming out on the other two terminals, then replace the passlock module.

6. If all the voltages are correct, then check for the Yellow wire to have a voltage drop when the vehicle is cranked. If no change in voltage, then replace the passlock sensor.
| |

EVO/Passlock Module

Manual shows picture of it , cannot copy and paste here. Says trim panel and ash tray need to be removed to get to it. Looks to be in center of dash below radio. Called EVO/Passlock Module.
Have located passlock module, voltage at yellow wire still .87V, other values check out. Engine will still start with this voltage at yellow wire, and light goes out. Also, with the module unplugged, engine cranks and runs,with no security light on. Is it possible to run the truck with the module unplugged?
| |
I don't what you say it will. That module also supposly works the variable rate steering if equipped. Have you tried to drive truck with it unplugged.
Well, I have to retract part of that statement, the truck started and ran with the module unplugged twice but not again, but does go through the learn procedure and recognizes the cly&key, but after a few key cycles returns to flashing security light. Is there a sensor in the column that could set this light, because a scan has indicated a problem with a steering sensor?
| |
Was that code there all along or did it appear when you unplugged module. If the truck has Variable rate steeeing it has a steering wheel speed sensor and that sends signal to the module you unplugged. Suggest you clear codes and see what comes back. Let me know what codes you get. Its starting to appear that you may have a problem with BCM. BCM can cause this problem and I have had to replace a few of them. If it does need one it has to be programmed. You may need to take truck to a shop if it goes that way.
| |
When you installed a new lock cylinder was it OE or after market? If it was not a OE remove the new lock cylinder and see if there is a magnet on the outside of the lock cylinder. If no magnet install an Original Equipment Manufacturer (OEM) quality part that has a magnet.
I have replaced the cyl&key twice, still have the first set, it has magnet. Both sets have the same problem.
| |
I think you should take this to someone that is equipped with compatible scanner preferably a Tech2 that can do a good diagnosis. I do not want to lead you wrong direction. BCM can do what going on but its too costly to do on a guess.
What is a Tech2 ? Where do I find one?
I have a starting problem/security light too, on my 2000 Monte Carlo. It is a real pain.
| |
Tech2 is the scan tool that gm dealers used to diagnose and repair gm cars. Best computer scan tool to use on a GM of the year model you have. All GM dealers and many independant shops have made the investment to properly diagnose problem like you are having. You probaly cannot rent such a tool. You have been dealing with this issue for a long time now. I suggest you find a good shop in your area that has the knowlege and tools to diagnose and fix your problem.
Didn't find what you were looking for?