I have a 96 LT4 which I recently posted about. The service engine light is on and I used an obd2 scanner which said it could not read. I went to autozone and their scanner said the same. I used all the advice that you guys gave me here and used a paper clip method. I got into the menu and for the CCM I got a C12 which indicates no stored DTC's for the CCM. I also navigated through the system and the RTD had no codes, ABS had no codes, DERM no codes. You can't read the PCM in manual mode so I am thinking that this is were the DTC must be. I still don't know why I can't get the scanner to work or how to proceed in fixing it. I really don't have any experience in diagnosing electrical problems, don't know how to use multi meters etc so I guess I need to learn. I did not get any error messages while in the diagnostic mode. I think I did everything correctly, I followed the FSM. Does everything that I did make sense? Am I correct in thinking that the DTC must be in the PCM and how can I access that. I am continuing to read and re-read my bible, The FSM, but any advice would be greatly appreciated. Thanks

 

The Service Engine Soon light is only illuminated as a result of the PCM having a DTC.

Verify the scanner works by plugging it into another OBD II vehicle. Assuming it does, then you have one of two problems...
- The wiring to the ALDL is bad
- The PCM is bad

For the first one, it's a matter of continuity checking the wires to the ALDL and/or rigging up separate wires to an alternate ALDL that you can plug into the scanner. The wiring schematic is in the Factory Service Manual (FSM).

For the second one, you need to replace the PCM.

If you don't know how to use a multi-meter, I HIGHLY suggest you take it to a reputable shop before you fry something and get to spend a few thousand dollars versus a couple hundred or less.

 

If the Check Engine Light is on, yes the code is in the PCM. If NO scanner can talk to the vehicle, I would make sure you have power to the Connector that hooks to the scan tool and ground as well. There should be a diagram in the FSM of the pins and which one is power and which is ground. Check those first, if it has these, the scan tool cannot talk to the PCM, this is a communication problem. Check fuses.

Long story short a wire runs from the Data Link Connector the OBD-II port to the PCM, the two communicate to one another on this, if there is a high resistance this two way comm. cannot happen. Only way to find the problem is this case is to disconnect scan tool from port and disconnect the appropriate connector at the PCM and do a resistance check on the Comm. wire.

There is a diagram of the communication network in the FSM. As well as connector pin locations and wire colors.

Given you do not know how to use a Digital Multi-meter this diagnosis will be VERY difficult for you. I hope you have wiring diagram reading/understanding skills as well, there is a section on this in the FSM as well. Read up.

FYI, if you decide to take it to a mechanic-(scratch that) to do this right you need a REAL technician not a mechanic, DO NOT let them tell you it needs a PCM without explaining why. For example if they can say "scan tool has power, ground at DLC, and the Comm. wire it good from DLC to PCM. The PCM has power and ground at all proper pins in connectors." Then a PCM might be needed.

There is no reason at this time to throw parts at it, sounds like a communication problem, of which 9 out of 10 times I have found to be a broken/high resistance wire somewhere in the Comm. circuit.

Hope this helps and best of Luck.

 

Thanks guys. I think I need a technician. although I would really like to learn how to do this, I don't want to fry anything or come to a wrong and expensive conclusion. One concern I have is that when I first bought the car I noticed a stain on the passenger footwell carpet. Ugly looking antifreeze stain, it was the heater I replaced the heater core but in the process found a massive mouse nest as well as several wires which had exposed copper. I went through all the wires and and fixed everything per FSM. This job really sucked because it was hard to get in there and see let alone reconnect tiny wires. I think that this is where the problem lies. I can't read wiring diagrams or do continuity tests etc so I guess I have to pay and I know this will take awhile. The engine service light just came on so I don't know why and it sounds like I can't get the DTC until I fix the PCM. Great! I am getting pretty good at taking things apart and putting them back together but it sounds like this involves more than that. Thanks for your time and advice. anyone have any recommendations for a technician in CT?

 

I was just reading over your posts again and I am wondering if I can figure out how and where to test the PCM and the DLC for power and ground and resistance is there any chance of me harming anything in the process? I just want to give it a shot. GS, I know you said I run the risk of frying something, is this possible just by checking for ground and power at the PCM? I guess I just want to gain a little knowledge so that when I do bring it in I can ask relevant questions and be somewhat informed. I know I am probably sounding ignorant as hell but I like to learn I just don't want to damage in the process. Thanks again guys

 

Post this in the Northeast Region section of the forum. It it is anything like it is out here in the Northwest for some beer and a pizza you will have at least 6 guys in your garage on a Saturday fixing your car.

 

I never realized that there were regional sections to post on. I hope I get the same results as you guys in the Northeast. I don't mind paying people either, I just hate paying $100 an hr. Thanks

 

How exactly do you post by region? I don't see any links on the home page.

 

just found it. Can't believe I have been on here for 4 years and never realized this. Probably not a good case for me to be trying to fix complex electrical problems