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I checked the ignition switch out of the car and it seems to be functioning correctly. I have voltage at the fuses at the kick panel and if I jump the purple and red on the theft deterrent relay, the starter jumps. After clearing all the codes I still get a B2721 H C and U 1096. The switch reads the key correctly. And now I would like to check the neutral safety switch . Is it on the shifter or the transmission? If its on the shifter can someone tell me what pins to measure. That and any other help would be appreciated . The other thing is that my security light has been flashing while driving for a few weeks I'm new to these "modern Corvettes" Thanks Dennis
Last edited by dstrayes; Jul 27, 2021 at 11:34 AM.
I replaced my ignition switch about 5 - 6 years ago. I did not replace the lock cylinder. All was well until this year with a bunch of random no crank/no start issues. New battery. Car only has 40k miles so I was initially convinced the lock cyl couldn't be "worn out." But issue happened with both ignition keys, so I assumed it was the contacts in the lock cyl. Eventually decided to replace the lock cyl and keys. Problem resolved.
You state that the key is read correctly, how do you know this? The B2721 code is the pass key code, when you turn the key to the On position does the Security light go off before you turn the key to the start position? I do not belive that there is a problem with your cars neutral safety switch, but I could be wrong. My suggestion is to spray electronic contact cleaner on to the keys resistor pellet and inside the switch and insert and remove the key multiple times to clean the switch contacts that press against the key resistor pellet.
My key reads 7.5 k and is the same reading at the end of the pigtail when inserted. its is solid as shaking the key does not change the reading. I was thinking about checking the neutral switch as soon as I find the pin out. thanks for the help. Dennis
Ok, sometimes people have to much extra stuff on the key ring this can cause the flashing Security light while driving. So if everting is working fine including the neutral safety switch you should hear the TDR click on, have you considered that maybe the TDR is the cause of your cars no crank problem.
Simple explanation of starting circuit, 12v is sent from ignition switch thru the neutral safety switch and then to the positive TDR relay coil. The BCM completes the negative/ground connection to the other side of the TDR relay coil if everything is ok with VATS. When you jumper across the red and violet wires of the TDR socket you are bypassing everything, and I caution everybody to make absolutely sure that the car is in Park or neutral because the starter will crank over.
You should check if the Body control module ground the TDR relay . If not the relay will not energize.
If you measure with a DVOM on the TDR A1 or the C3:B7 lug on the BCM you shal measure 12 volt when the ignition key is in ON (not start) .
When you set the key in START the BCM will(shall ) ground the TDR relay and ground the voltage to close to 0.1-0.2 volt .
If this does not happen you should check(measure) the grounding in engine bay , below battery and the splicepoint in passenger footwell
shown on photo below for good continuty .
You should check if the Body control module ground the TDR relay . If not the relay will not energize.
If you measure with a DVOM on the TDR A1 or the C3:B7 lug on the BCM you shal measure 12 volt when the ignition key is in ON (not start) .
When you set the key in START the BCM will(shall ) ground the TDR relay and ground the voltage to close to 0.1-0.2 volt .
If this does not happen you should check(measure) the grounding in engine bay , below battery and the splicepoint in passenger footwell
shown on photo below for good continuty .
Little Confused. If key on why does it not pull the switch closed when its energizing the coil at A1???
The TDR relay does not energize before the A1 side of the relay is grounded through the BCM .
Then the current will flow through TDR relay coil and the contact set C1 -A2 will be closed and provide voltage/current from Fuse 52 to the starter solonoid
I just pulled the ground apart . It was clean and no sign of corrosion. Checking the TDR relay I have 12 Volts on both sides of the coil when trying to crank. Same voltage back at pin on BCM. Tested the vats key again. Now I have had the security light blinking while driving the past couple of weeks. It still blinks now and a code of B2721 is consistent. I disconnected the pigtail from the ignition switch and get a solid security light when turning on the ignition. Seems to me it recognizes the key as being correct. I get codes of B2721 and B2723 when I do that. B2723 goes away when I reconnect. I do all this testing after clearing the codes. 12+ volt on the battery all the time and I had it tested. Its only 6 months old. I just measured my vats key at the connection to the BCM and it reads correctly. Is this looking like a bad BCM?
Last edited by dstrayes; Jul 28, 2021 at 02:18 PM.
Have someone try to start the car while you verify that the TDR is being energized, you should here it click. If the TDR is being energized but the car won't crank over it's the contacts of the TDR that are bad. You can find a ground point and run a wire to the yellow/black stripped wire of the TDR socket if it cranks then yes the BCM is not providing the ground connection. Have you tried disconnecting the battery for several minutes to reset the BCM.
Since the A1 side of the TDR is not grounded when start , it could be caused by a bad BCM or the ground path of the BCM could be open somewhere .
Do you have the schematic of your car ? If you locate the ground pin in your BCM connector ( I think it is C1:A9 on my car )and measure the resistance between this pin and a groundpoint is should be close to zero .
I did a check on my own car last year on this BCM ground loop shown on the photoes below
I removed the TDR relay in my car and put one lead in the relay socket (first photo) and put the second lead on the nearest ground point(second photo) .
My DVOM showed 3 Mega ohm (open circuit ) between BCM input and the ground point with ignition ON , engine OFF .
When I put my key in START position the DVOM showed zero ohm and indicating my BCM switching the TDR A1 side to ground as it should
If my BCM was bad this 3 Mega ohm to zero ohm will not have taken place .
Hope this info will help you out to verify if your BCM is bad or not
I just grounded the lead from the BCM pick coil and the starter will turn. I think I pretty much narrowed it down to the BCM. Can a Chevy dealer test it out of the car? What I see about the BCM, they are a pain when it goes bad. Any suggestions what I do about that.
Last edited by dstrayes; Jul 28, 2021 at 03:02 PM.
Did you check if the ground path is ok ? If not , a new BCM may not will not help you out
If the ground wires between the BCM and G104 in the engineroom does not have good conductivity the BCM will not be able to ground and energize the TDR relay.
Another forum member had the same problem as you and his problem was caused by a loose wire in the splicepack 208 shown in my photo above
I checked the splice pack you mention and it was clean and seemed to be ok. I cleaned the ground under the battery. where is the other one that is in the diagram. I will try to trace the wire from the ground pack to the BCM on Friday. Can't continue working on it till Friday .Thanks to all
Well I just got my BCM back from a repair depot. Said they fixed a hardware issue.. St have no crank and still getting Vats errors P1631 and B2721. Can these BCM's have a problem where they don't recognize the original key anymore and it's looking for a different resistance? I ordered a resistor assortment to give it a try. Any more info you great people can give me? Thanks again.
c5 Auto, no start or crank
