Monday, December 19, 2011

2002 Chevrolet Monte Carlo

A bright yellow 2002 Chevrolet Monte Carlo 3.8 liter with 117,263 miles on the odometer is at Mr. T's shop. The vehicle came in as a no start, no crank condition. The battery load tested bad so a new battery went in with the same result. I am informed that they can jump out the starter relay and the car will crank but will not stay running. Let's look at the starter circuit for starters.
Ok we have a starter relay which is called out as a crank relay in this diagram. The relay is PCM controlled. The PCM will engage the relay when it sees a crank voltage input and the gear input is correct. We don't want the vehicle cranking when in gear. But, there is one more piece of the puzzle. Lets look at some theory.

This is some Passlock (the vehicle theft system) theory from GM. I underlined the parts that pertain to our issue at hand. Passlock has been around since 1995/1996 on GM vehicles. On some years/models it will disable the starter relay operation and other years it won't. Check the description of operation if you are not sure. I know what I am doing next.

I hook up my Tech2 and perform a Class 2 DTC check. I use this to see what modules are reporting on the Class 2 data line. I see an issue here already. There is a module that is linked to our problem that is not here. The BCM is AWOL. Now, we have to find out why. I check the fuses that power up the BCM in the left side instrument panel fusebox which are good. My next step is to see if there is any add on items such as an aftermarket alarm that would be causing issues with the BCM.

The BCM is located under the left side of the dash buried up high. There is an aftermarket alarm involved that dropped on me as I dropped this panel. I disconnected the aftermarket alarm module and rechecked to see if the BCM came back online. No dice. We have to get right at the BCM.


There it is tucked up high. I wrestle it down and backprobed the powers and grounds right at the BCM they are fine. The next thing I do is to disconnect all the connectors at the BCM and walk away from the vehicle for 20 minutes. I return and reconnect the BCM. Let's recheck to see if now the BCM is reporting.
Well look at this. The BCM is back online. The car starts and runs now. This is what is called "logic lock" in the industry. The module was electrically locked and shutdown. Disconnecting it for 20 minutes and reconnecting unlocked it. I have seen this on Chrysler and Nissan vehicles as well as GM vehicles. There is much debate about what causes this and what is the proper repair. I believe logic lock is caused by voltage fluctuations, surges, spikes, or excessive ac voltage ripple. In this case I believe the weak battery caused the logic lock. I did check charging rate and for ac ripple as well. Now, the proper repair some believe would be to change the locked module for fear it will lock again. The theory there is that the module lock is a symptom of a bad module. I give the shop owner the options. He decides to let it go as is and to inform the customer of the possibilities. This vehicle has been out now for a month and doing just fine.

6 comments:

  1. your are a great troubleshooter.you have systomatic approach for hidden car problems.Thank very much for sharing your wisdom.keep up your good work!!!!!

    ReplyDelete
  2. I have a 2001 Chevy Monte Carlo SS after it starts it runs for a bit then it spits and spudders and then dies hard to start then when it does start you hold the throttle and feather it it clears up runs for a bit and dies checked codes came up with o2 sensor problem with the o2 sensors cause this or is it the PCM

    ReplyDelete
  3. thank you kindly for your input; the education seems good, and I will be looking at the issues involved tomorrow

    ReplyDelete
  4. I have an 02 Monte Carlo. someone hacked it up, installed a push button start. turn key to start and push button same time. It started no problem the first day I bought it. 5 months ago. have driven it 3 times since, because that push button locks in security mode cutting off injectors. push button is direct from battery. I had the battery out for a few days and it started when I put it back in, but shut it off and it doesnt start again.
    Im going to try the drain to see if it will reset the modules again. Im a mechanic but I have no equipment here with me its all back home 3000 miles away.

    ReplyDelete
  5. I have a 2003 MC, the turn lights quit working, 4 ways still work, when I use the turn lights, I hear a hum and other dash lights flash on, have repaced the flasher unit and turn signal arm, fuses are good, any suggestions?

    ReplyDelete
  6. Congratulation to you. I am happy after reading your post that you have posted in this blog. Thanks for this wonderful post and hoping to post more of this. I am looking for your next update.

    Thanks a lot!

    Auto Parts Suppliers in China

    ReplyDelete