Here's some info on CC diagnostices (at least for systems through 1992)

GDL Troubleshooting guide

You may want to pke around on this site a bit. It has a bunch off good info about cruise systems.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold