1972 Corvette:       Emissions Restoration Blog 




1972 Corvette (C3) 

Stingray Coupe
350/200 hp 

Engine Restoration and related technical information

Contact:  twoplays.25@comcast.net

 Transmission Spark Control (TCS) System

 Assorted Notes & Details related to the restoration of the 1972 Corvette (C3) small block 350 base engine with air-conditioning (AC) .

 The TCS system was often removed from vehicles because it was viewed as interfering with the transmission shifting.   The purpose of the 
 feature was to interrupt / control down-shift based on temperature and speed.    Certainly, a good working order TCS system was likely an
 asset.   A malfunctioning one could understandably need to be cut out if not repaired.      

 As part of the National Corvette Restoration Society (NCRS) flight judging process, having a fully operational and correct "system", based on 
 a "as delivered by dealer" baseline is the standard.     My particular 1972 was missing most of the hardware, but the wiring seems to be 90%

  The diagram below (not certain of source) is the best one I've found that clearly identifies the wiring diagram for the TCS harness.

 In determining what my car has & doesn't have, I've found some clues and information.   First off, let's 
 look at both circuits within the System.  For ease of following, there's the left hand (LH) side of the engine,
 which many call the driver's side.. and then the (RH) side.


 On the LH side, the first project is the idle stop solenoid, also known as the anti-diesel switch.    I wish they
 would just pick one name and get on with it.  Sure makes research easier.

 The Idle Stop solenoid is a fairly simple device.    If you understand solenoids, when power is applied to the 
  coil, it causes the plunger to move.   In this case, the plunger pushes out, increasing contact with the throttle
  on the carb, helping to raise the idle just a little when you put on the air conditioning for example.   With out
  the idle stop, you or your mechanic need to turn up the regular idle to compensate, which means when the 
  accessories are not on, the car is idling higher than needed.      Certainly puts a little more wear & tear on
  the moving parts.

 The checklist of parts for the Idle Stop Project include:

  1.   Idle Stop Bracket - attaches to the carb, holds the idle stop solenoid.

  2.   Idle Stop Bracket Screw - holds the bracket to the carb

  3.   Retainer - This piece goes between the idle stop solenoid, over the threaded end and under the nut to hold the 
        solenoid body from turning.

  4.   Solenoid Nut:    This nut threads onto the Idle Stop and presses the retainer against the Bracket, effectively
        enabling the idle stop solenoid to balance by it's neck in the bracket (like a bottle of wine in a holder).

  5.  Single Cavity Packard 56 Connector:  Fancy name for the little black plug connector that attaches to the Idle
       stop solenoid.    There's a piece inside that you crimp the idle stop solenoid wire to and then insert it into the 

  6.  Idle Stop Solenoid:    This is the actual solenoid that's required.


  7.  Idle Stop Solenoid Wire:    Tan (brownish) wire that runs from the rear of the Idle Stop Solenoid and curves
       180 degrees towards the firewall.    The wire "clips" into the two metal clips on the intake manifold side of 
       of the LH valve cover.  The wire runs straight down the valve cover and then towards the firewall, under the 
       LH ignition shield bracket.    

       Here's an example picture (not my engine) of the idle stop solenoid, bracket, retainer, nut, connector, wire and
       connectors on the valve cover.

       Based on the wiring diagrams, this wire connects to the tan wire that comes  through the firewall from the TCS 
       Relay (mounted under dashboard, near accelerator peddle).     Where the two tan wires come together on the Engine
       side of the firewall, the tan wire then travels inside a harness to the RH side of the engine and surfaces immediately
       adjacent to the evaporator box, right under the stop limit switch area (for the wipers).     
       The Tan wire is coupled with two blue wires and it appears that the trio run to the TCS Solenoid located on the RH
       side of the carb.


       12/11 - under construction

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