Previous dates - up to about '86 1/2 don't use any form of a full duplex ECM - nothing that is actually controlling shift points or TCC, etc.
►There was HEI but I believe by 1988 1/2, they started using an infantile version of an ECM with a feedback carb system (TPS, ISC, CTS, 02S, etc) to run things until around early 1990 or so when FI CPFI was running.
My July 1989 K5 had a feedback carb, a THM700R4 (no real computer functions) a 4-wire distributor, and the first serpentine fanbelt.
- They used a governor, throttle cable and quadrant position for all automatic up and downshifts.
- The TCC application is modulated by the throttle cable and governor speed/pressure coupled with the quadrant position with some other considerations.
- There is also a small vacuum switch on the firewall that senses manifold vacuum to confirm TCC application/disconnect at various vacuum settings.
- That firewall switch also qualified the TCC disconnect if the vacuum dropped below a set threshold (eg: WOT)
- There is also a BOO switch to de-couple the TC when you apply the brakes.
- TCC was also not allowed in LOW, REVERSE, Park, Neutral nor 4WD.
- TCC is not supposed to apply below a certain road speed and/or when the engine coolant is not warmed up sufficiently - remember these were carbureted engines, not FI.
- THM700R4s never used that "little module" behind the speedo to regulate anything because that was ONLY a Hall effect signal generator for Cruise Control. I am positive of this because I took it out when I digitized my dash and deleted that sensor.
- I am not sure if that speedo signal generator was eliminate when an ECM was introduced - at least I think it wasn't in the early versions. If so, it would require a VSS to send a digital signal to the ECM to activate CC.
- There is a "burp switch" inside the transmission that momentarily released the TCC during an upshift to keep your teeth in your mouth.
- There was (probably still is) a thermo-switch inside the transmission that reads the ATF temp and forces TCC/LU in lower gears (except 1st/LOW) to drop operating temperatures inside the transmission, because gears don't generate much heat and clutches don't slip - just locking the convertor drops the big heat generator!
Sometime after 1990, they used a VSS, replacing the old speedo-drive with the cable.
Up to 1994 1/2, the transm was a Governor-throttle cable-quadrant controlled unit and had no electronics in it except the kick-down and TCC.
As to it not applying TCC at WOT - that's wrong. Just ask me how many sprags I've blown in a TCC Convertor at WOT!
