Serial Data Line CCM - what does it do?
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2nd Gear
Joined: Jul 2025
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Serial Data Line CCM - what does it do?
1995 Corvette, was attempting to communicate with ECM using OB2 cable and open source software. Didn't work. Using jumper method found a H72 error message. After taking apart my entire electrical system I believe I narrowed it down to a bad CCM. Found a high impedance 2.5kOhm short between serial data and ground. Disconnected CCM and now I can communicate with ECM. Question... Car seemed to be running fine, no errors other than diagnostic serial communication with ECM. Is the serial data line only used for diags and programming?
Advanced
Joined: Dec 2020
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From: New York City
1995 Corvette, was attempting to communicate with ECM using OB2 cable and open source software. Didn't work. Using jumper method found a H72 error message. After taking apart my entire electrical system I believe I narrowed it down to a bad CCM. Found a high impedance 2.5kOhm short between serial data and ground. Disconnected CCM and now I can communicate with ECM. Question... Car seemed to be running fine, no errors other than diagnostic serial communication with ECM. Is the serial data line only used for diags and programming?
Melting Slicks
Joined: Nov 2016
Posts: 2,235
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From: So Cal
2023 C4 of the Year Finalist - Unmodified
There is a lot of data on the "800" circuit that interconnects the ECM/PCM, CCM, TCS/ABS, C68 Programmer and the ALDL. The CCM acts as the master, and the other modules are slaves. The CCM queries the PCM, then 'listens' for a response. If three sequential requests are sent with no reply, Code C/H 41 is set in Module 1. If the 41 is Current, the SYS message is displayed in the speedo LCD. H72 is a frequently-observed lighting code. It means nothing, has no influence on operation, and can be ignored and cleared.
I am not personally familiar with this, but I have observed several posts over the years that say that the CCM will interfere with the data from the ECM/PCM and must be disabled when using 'opensource' software. I believe you pull one of the CCM fuses to disable it. I do not know which fuse of the 3 are the 'best' to remove in a 95. If the VATS system is still functional, you will have to remove the CCM fuse after the engine is running.
I don't think there's anything "wrong" with the OP's CCM.
Last edited by IHBD; Jul 21, 2025 at 07:12 PM. Reason: Added last sentence of paragraph one.
