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Juan Crespo
Juan Crespo, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1513
Experience:  A.S.E. Master Technician, Advanced Level, Emissions - Asian, Domestic, & European
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Just changed the ECM on my 92 Corvette LT1, now it wont

Customer Question

just changed the ECM on my 92 Corvette LT1, now it wont start. I even put the old one back on, wont start Cranks but no start?
I believe the theft deterrent has it locked out. do you know how to reset the ECM?
Submitted: 10 months ago.
Category: Chevy
Customer: replied 10 months ago.
I also put the new ECM back in but it still wont start, cranks but no start. I seems to me the Theft deterrent system has me locked out but I dont know how to reset it?
Expert:  Tool Hut replied 10 months ago.

Hi, My Name is ***** ***** for posting your question to just answer. I am a independent contractor for just answer, and ASE Certified Master Auto Technician with 20 years experience. I am confident we can resolve your problem together.Please remember I don't know your skill level. I can't see, smell, hear, or touch the vehicle. It may take a few back and forth replies to get to a satisfactory resolution.There will probably be some equipment needed for a proper diagnosis. If you are not comfortable with this or do not have any test equipment please let me know ASAP. I also try to give all of the information you may need expecting a positive rating. I am not compensated for my time until you leave a positive rating. Your rating should be based on the service you received.

It is more likely you have a blown fuse, The security system controls the crank not the start on this system.

Did you transfer the PROM from old ECM? If you cross the top 2 pins on the ALDL it should flash a code. 12 and then something else. can you try this?

Customer: replied 10 months ago.
I did transfer the PROM, The code is telling me the ECM is not communicating..I use the top right and lower left pins on the ALDL. I still believe the theft deterrent system has the fuel shut off so it cant start?
Expert:  Tool Hut replied 10 months ago.

Do you have a security light on? Attached is the aldl picture. cross a and b

Expert:  Tool Hut replied 10 months ago.

You should have a code 46 if it is security locked

Expert:  Juan Crespo replied 10 months ago.

Hi there. Please allow me to assist.

First of all I'd like to make something clear; as experts on this site, our only goal is to help you solve your car issue, we are not here to belittle or criticize any one. We're just trying to help you.

Now, to the issue at hand; as expert Brooks indicated, the anti-theft system prevents cranking of starter and fuel delivery to engine. The cranking/fuel interrupt function operates independently of the alarm system and provides protection even if the alarm system is defeated and vehicle is left unlocked.

In other words, if you turn the key to START and the starter engages, the anti-theft is disarmed.

Also, as expert Brooks indicated, you need to jumper across terminal A and B of the ALDL in order to get codes. Terminals A and B are next to each other horizontally.

Lastly, it would greatly help us help you solve your problem if you tell us what condition prompted replacement of the ECM.

Best Regards.

Customer: replied 10 months ago.
when i jump a and b on the ALDL i get no codes, the system light flashes 3 times and the security light flashes 3 times.
when i jump a and g on the ALDL i get code C41,H41,ERR,H64 and H72.
I did swap the eprom. the reason i changed the ECM was i have been having intermitten problems with it running poorly and backfiring through the intake
Customer: replied 10 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Juan Crespo replied 10 months ago.

I understand. However, I want you to understand that by jumpering A and G, you're accessing the Central Control Module. At this time, we need to access the Engine Control Module, which should be done by jumpering A and B in the ALDL, which should result in the Service Engine Soon light flashing code 12 three consecutive times. After code 12, any other codes in memory will appear three times as well.

I should also mention the fact that the anti-theft system should disarm automatically when the correct key is inserted and the ignition switch is turned on.

So, I would suggest arming and disarming the anti-theft as you normally would before driving the vehicle and then try to retrieve codes from the ECM.

Please let me know how you make out.

Best Regards.

Customer: replied 10 months ago.
but im not getting any codes when i jump a / b, just the lights flashing,SYS and SECURITY, at the same time every 20 seconds.
Expert:  Juan Crespo replied 10 months ago.

If the SYS and Security lights are flashing, then the anti-theft is not disarmed. This could be caused by several different things, such as a bad key, an open circuit, faulty module, etc. If you have two keys, you could try reprogramming the CCM:

  1. Insert first ignition key into ignition switch. Turn key to RUN position. Turn key OFF and remove from ignition switch. Program mode has now been entered. CCM will prompt technician to insert appropriate key into ignition switch by flashing the SECURITY lamp.
  2. Lamp will flash a number of times corresponding to the key number that is being programmed. At CCM prompt, insert key No. one into ignition, but DO NOT turn key. During this step, the CCM determines the resistance value of the key and stores it in memory.
  3. If a valid key is sampled, the CCM will prompt the technician to insert key No. 2 into ignition switch by flashing the SECURITY lamp twice. If an invalid key is detected, the CCM will exit the program mode and a trouble code will be stored.
  4. Process will be repeated for key No. 3, with the SECURITY lamp flashing 3 times as a prompt. After the third key is inserted into ignition switch, and all keys are detected to be valid, system will finish programming itself and automatically exit programming mode.

Other than that, the other two options are a certified locksmith or a qualified Checvrolet repair facility.

I have done the best I can to help you solve your issue. I hope you find my efforts worthy of a good rating.

Best Regards.