Very Strange "Car won't start sometimes" problem
Thread Starter
Instructor
Joined: Apr 2007
Posts: 112
Likes: 0
Try this: When you have the NO START issue, turn and hold the key to START and while your holding it in start, pump the CLUTCH Peddle on and off the clutch safety switch. When you do that you will hear the Theft Deterrent Relay in the passengers foot well click. That proves all the components up to and including the relay coil is good.
Heres a picture of the relay:
Its the relay with FOUR wires. Yellow, Yellow/Black, Red and PURPLE
Heres a picture of the relay:
Its the relay with FOUR wires. Yellow, Yellow/Black, Red and PURPLE
Next time it happens I will test...
Tech Contributor
Joined: Jan 2007
Posts: 19,450
Likes: 1,155
From: Dyer, IN
Ok Bill this morning it happened again. I pumped the clutch while holding the key clockwise. The relay clicked. So thats good. I was not able to test with my voltmeter as I was late for work. But, atleast now I know all the components up to and including the relay coil is good.
Next time it happens I will test...
Next time it happens I will test...
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
lucky131969 - thanks for the pages out of the Service Manual. This is where the confusion comes in.
On page one, in paragraph 5, it says:
"If the correct resistance value is sensed when the ignition switch is in the RUN position, the BCM will ground the theif deterrent relay control circuit and enable the fuel delivery."
But on page 2, under "Body Control Module", it says:
"The BCM reads the key code from the ignition cylinder when the ignition is turned to the START position."
To me, the START position is when the key is turned to the position that cranks over the engine. So there seems to be some conflict in these statements.
My gut feeling is once the key is inserted and the ignition switch turned to the ON position without starting the engine, the Security light will go off if the key was read correctly, and the BCM/PCM will have then also been initialized and the starter should crank over if everything else is working properly.
If the Security light is on or flashing when the ignition switch is turned to just the ON position, then the key was not read correctly.
Thoughts?
On page one, in paragraph 5, it says:
"If the correct resistance value is sensed when the ignition switch is in the RUN position, the BCM will ground the theif deterrent relay control circuit and enable the fuel delivery."
But on page 2, under "Body Control Module", it says:
"The BCM reads the key code from the ignition cylinder when the ignition is turned to the START position."
To me, the START position is when the key is turned to the position that cranks over the engine. So there seems to be some conflict in these statements.
My gut feeling is once the key is inserted and the ignition switch turned to the ON position without starting the engine, the Security light will go off if the key was read correctly, and the BCM/PCM will have then also been initialized and the starter should crank over if everything else is working properly.
If the Security light is on or flashing when the ignition switch is turned to just the ON position, then the key was not read correctly.
Thoughts?
Tech Contributor
Joined: Jan 2007
Posts: 19,450
Likes: 1,155
From: Dyer, IN
lucky131969 - thanks for the pages out of the Service Manual. This is where the confusion comes in.
On page one, in paragraph 5, it says:
"If the correct resistance value is sensed when the ignition switch is in the RUN position, the BCM will ground the theif deterrent relay control circuit and enable the fuel delivery."
But on page 2, under "Body Control Module", it says:
"The BCM reads the key code from the ignition cylinder when the ignition is turned to the START position."
To me, the START position is when the key is turned to the position that cranks over the engine. So there seems to be some conflict in these statements.
My gut feeling is once the key is inserted and the ignition switch turned to the ON position without starting the engine, the Security light will go off if the key was read correctly, and the BCM/PCM will have then also been initialized and the starter should crank over if everything else is working properly.
If the Security light is on or flashing when the ignition switch is turned to just the ON position, then the key was not read correctly.
Thoughts?
On page one, in paragraph 5, it says:
"If the correct resistance value is sensed when the ignition switch is in the RUN position, the BCM will ground the theif deterrent relay control circuit and enable the fuel delivery."
But on page 2, under "Body Control Module", it says:
"The BCM reads the key code from the ignition cylinder when the ignition is turned to the START position."
To me, the START position is when the key is turned to the position that cranks over the engine. So there seems to be some conflict in these statements.
My gut feeling is once the key is inserted and the ignition switch turned to the ON position without starting the engine, the Security light will go off if the key was read correctly, and the BCM/PCM will have then also been initialized and the starter should crank over if everything else is working properly.
If the Security light is on or flashing when the ignition switch is turned to just the ON position, then the key was not read correctly.
Thoughts?
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
There is no "RUN" position, but I guess they probably mean the "ON" position. There is a difference between the "ON" and "START" positions.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Tech Contributor
Joined: Jan 2007
Posts: 19,450
Likes: 1,155
From: Dyer, IN
I don't think both statements necessarily mean the same thing, as the terminology means something specific. There are 4 positions on the ignition switch ... namely: OFF, ACC, ON and START.
There is no "RUN" position, but I guess they probably mean the "ON" position. There is a difference between the "ON" and "START" positions.
There is no "RUN" position, but I guess they probably mean the "ON" position. There is a difference between the "ON" and "START" positions.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Security Light
This light comes on to
remind you to arm your
theft-deterrent system. If it
comes on and stays on when
your ignition is on, there
may be a problem with your
theft-deterrent system.
So, if I had to surmise, I'd say that when the ignition is turned to the ON position without cranking the engine, if the PASS Key system is working correctly, the "Security" light will go out. If the PASS Key system is faulty, then the yellow Security light will be on before you even try to crank the engine ... and cranking will result in a no-start.
... 
Last edited by ZeeOSix; Jan 27, 2011 at 05:15 PM.
Instructor
Joined: Jan 2006
Posts: 114
Likes: 0
From: Sachse Texas
@lucky
2 codes
tcs - c1288 H
bcm - B2721 H
Does not happen all the time. Car starts just fine when it does happen and after 2-3 mins of driving the security light comes on and stays on. Here lately it has happened every day.
2 codes
tcs - c1288 H
bcm - B2721 H
Does not happen all the time. Car starts just fine when it does happen and after 2-3 mins of driving the security light comes on and stays on. Here lately it has happened every day.
Tech Contributor
Joined: Jan 2007
Posts: 19,450
Likes: 1,155
From: Dyer, IN
The B2721 is the problem. It's for the pass key detection circuit. You can try using your spare key. If that does not resolve the issue, a new lock cylinder may be required.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Tech Contributor
Joined: Jan 2007
Posts: 19,450
Likes: 1,155
From: Dyer, IN
I can buy that ON = RUN. But the discrepancy is that the manual says the BCM reads the key code when the key is in RUN and in START. I think it really reads the PASS-Key and grounds the thief deterrent relay control circuit and enables the fuel delivery before the key ever goes to the START position. Just a conflicting detail in the manual.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
This is the "C5 Tech" forum, right? The manual is contradicting ... you obviously don't care.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
Did you write that section of the shop manual or something? 
Advanced
Joined: Oct 2010
Posts: 65
Likes: 0
Well, NOT to steal anyone's thunder but I've been having some start issues and as of late, the codes are P1631 (H) (Theft Deterrent Enable Signal Not Correct), and B2721 (H) (Passkey Detection Circuit). I have tried the spare key but to no avail but havn't tried the clutch pumping technique to see if the relay was clicking. I've noticed that when the car DOESN'T start, the "Security" light flashes continuously but if its going to start after I put the key in the ignition, the light comes on flashing and then goes out. So now I'm trying to decide if I should replace the ignition switch or the TDR. Any suggestions? Thanks in advance.
Le Mans Master
Joined: Mar 2002
Posts: 5,955
Likes: 161
From: PNW
I've been having some start issues and as of late, the codes are P1631 (H) (Theft Deterrent Enable Signal Not Correct), and B2721 (H) (Passkey Detection Circuit).
I have tried the spare key but to no avail but havn't tried the clutch pumping technique to see if the relay was clicking. I've noticed that when the car DOESN'T start, the "Security" light flashes continuously but if its going to start after I put the key in the ignition, the light comes on flashing and then goes out. So now I'm trying to decide if I should replace the ignition switch or the TDR. Any suggestions? Thanks in advance.
I have tried the spare key but to no avail but havn't tried the clutch pumping technique to see if the relay was clicking. I've noticed that when the car DOESN'T start, the "Security" light flashes continuously but if its going to start after I put the key in the ignition, the light comes on flashing and then goes out. So now I'm trying to decide if I should replace the ignition switch or the TDR. Any suggestions? Thanks in advance.
Did you try cleaning both key pellet contacts? With age, the metal gets a coating on it ... could be both keys are coated. Take a soft pencil eraser and burnish the pellet's metal contacts then wipe with some rubbing alcohol. If that doesn't work, then I'd say the ignition switch is defective and can't read the key's resistor.
Also, your codes are History (H), so clear them and see what happens after cleaning the key pellet.
Last edited by ZeeOSix; Jan 28, 2011 at 08:09 PM.