Having just finished up my 3800SC/4T65EHD swap into my 88GT, I spent some time to research and re-use a now unused wire from my existing cruise control wiring loom to get the "V8" style electronic cruise control to work without installation of the interface relay. This may have been done/posted before, but I thought I would share my experience if it can help others. First off, I used the tried and true wiring diagram from Sinister performance for my schematic with everything wired as shown except for the following wire which I used for my Brake input N.O. signal. This is the white wire ( pin G) of the electronic cruise control module that needs to see zero volts with no brake , and then +12VDC with the brake applied. After installing all the other wires to convert the stock vacuum cruise control over to the electronic version, I realized the tan wire which used to be connected to pin B of the vacuum servo harness is not used. I traced it back in my manuals and found the other end comes from pin F of the original cruise control module connector which is also not used in the electronic cruise swap. Now all I had to do was find the correct wire from my brake pedal switch. It is the white one on Pin A, ( note pin B is orange and Hot from the fuse block, pin C is the blue wire that feeds the stop lights). I simply stripped back a little bit of my white wire at the brake switch and soldered another wire to it and then fed this new wire down and spliced it to the tan wire from my original cruise control connector ( formerly pin F). Next at the new connector in my engine bay for the electronic cruise control, Pin G (white) I soldered it to my tan wire from the original cruise harness and voila! the correct input was there without using an interface relay. It works perfectly.
To recap, just tie into the white wire of the brake switch on top of the brake pedal with a new wire, hook that new wire to pin F (tan wire) of the original cruise control module connector, then on the other end in the engine bay, tie the same Tan wire to the white wire (pin G) of your new electronic cruise control connector and you have just installed the brake lamp N.O. input which all the electronic cruise control modules need to work properly. I used a "V8" electronic cruise control, but this should be exactly the same for the AR version as the wiring is the same. I hope this helps someone. I just didn't see the need to install another relay when the wires for mine were already there. It should also be noted that I did not hook up any of the cruise control wires from my PCM as Ryan at Sinister noted they were not necessary. Also, both pins E (ground) and H (cruise inhibit) of the new electronic cruise control connector were grounded, I tied mine together with the existing original ground wire (black) from the original cruise control servo. The cruise engaged signal (pin J) was not used as shown in the wiring diagram.
I like the redundancy of having two opposite signals coming from the brake switch.
I don't like splicing existing wires in the engine compartment however so I route them back to the 3800 harness, through the firewall and then make the spices at the cruise control control box under the carpet. I add a disconnect near C203 so the harness can be pulled in the future.
