1971 TCS system general questions.

[Rick Church]

Greetings, I have a 71 small block (350/270), four speed with air-conditioning. I have tried looking for TCS information on the Internet, the Corvette Forum and other related web sites, but all I get are opinions and no real facts. I have the AIM and NCRS judging manual also but the questions still linger. Please allow me to ask the following: (1) Is the "temperature type switch" on the right (passenger side) cylinder head part of the TCS system? My switch isn't connected. (2) Does this switch (on the right cylinder head) have to be connected to get the TCS system to operate correctly? (3) The engine idle speed does not increase when the air-conditioning compressor is engaged (Comp cam 270 installed); is this related to the TCS solenoid? (4) The TCS solenoid is installed on the left forward area of the intake manifold. It has two small red wires and two vacuum lines connected to it. How do I test the TCS system for proper operation? (5) What is the correct part number for the TCS solenoid that mounts to the intake manifold? I appreciate your time and consideration!

Thanks you very much......Rick Church

[Joe L.]

Re: 1971 TCS system general questions.

Rick----

1) The temp switch on the right rear cylinder head is part of the TCS system. This switch will have "L" configuration blade terminals;

2) The switch is required for proper operation of the TCS system;

3) Idle speed is not related to the TCS system or the TCS solenoid. Idle speed with A/C is affected by the idle control solenoid mounted on the driver's side of the carburetor, if your car is so-equipped;

4) You should find quite a bit in the archives regarding the TCS system operation and testing procedures;

5) The correct TCS solenoid for your car is GM #1114432. It was discontinued a long time ago. However, NOS examples are "around". Except for restoration, most folks want to eliminate the system, not repair it, so there's not a huge demand for available parts.

[Jack H.]

Re: 1971 TCS system general questions.

(1) Yes, temp switch on RH head is part of the CEC system (TCS is sub-system of the '71-only Combined Emission Control system).

(2) Yes, to get full functionality of the CEC, the temperature switch on the RH cylinder head has to be connected and operational.

(3) The carb mounted CEC solenoid was NOT part of the CEC system with ONE minor exception; for A/C optioned Corvette, there were additional control relays mounted on the A/C box, passenger side interior behind the lower dash pad. On engine shut down, these relays acted to over-ride the center console setting of the A/C system and ENGAGE the A-6 compressor for 6-10 seconds to force additional drag on the engine and thwart potential engine run-on due to the wider throttle angle needed at the carb to insure smooth, no-stall, idle RPM.

(4.a) There is NO TCS solenoid installed on the RH intake manifold for 1971. This is how the more limited TCS system of 1970 worked and you can expect to find a NUMBER of errors (both omission and commision) in your '71 AIM as well as currently published electrical diagrams for the car regarding the one year only Corvette CEC system. Appears this feature of the car was 'rushed' to production at the last minute and the documentation we have never caught up with what the factory actually built.

(4.b) You test the '71 CEC system using the almost impossible to find Chevrolet Motor Division, 1971 Combined Emissions Control System, dealer mechanic training program booklet.

(5) There is no 'correct PN' for the TCS solenoid because it wasn't used. Your carb needs to have the correct set of mounting brackets (one version for Holley and another for QJet) and a Delco Remy 1114444 CEC solenoid.

Almost all of the parts necessary to correctly restore a '71 CEC emission system have been LONG discontinued by Chevy. Since I have the twin to your car ('71 SB A/C roadster), I labored long/hard to piece together the info (and the correct NOS parts) to get mine up and running. Talk to me off-board and we might find ways to get you from here to there!

[Rick Church]

Re: 1971 TCS system general questions.

Joe and Jack,

My home phone number is (931) 920-0031 (Tennessee). If you will call me, I will hang up and call you back to talk some more about the system.

Thanks a lot,
Rick Church

[Dave S.]

Re: 1971 TCS system general questions.

Rick,

I just went through all this TCS trouble shooting and its operation as I prepared my 71 for its PV. The NCRS Operations manual does not spell out the complete operation of the system and there are a couple of errors in it as well. With the help of a couple of members on this board and the 71 Chassis manual I was able to figure it out and ultimately prepare the 71 for the PV. It is a very confusing system but as with all Corvette repairs you can do fine with patience and help. I'd be glad to assist in any way especially with the operation and adjustment of the system.

[Rick Church]

Re: 1971 TCS system general questions.

First of all, thank you very much for calling Dave, it was a tremendous help. I went out to the car and sure enough, when power was qpplied to the ingnition circutry, the plunger on the TCS switch popped out. However, it doesn't come close to the carb linkage so I'm now seeking the proper adjustment to make on it. If you guys know the proper adjustmnent procedure for the plunger to carb linkage, I'd sure appreciate the info. You guys are really great. Thanks again for the call and the email!......Rick

[Patrick H.]

Re: 1971 TCS system general questions.

Rick,

Just unscrew it. The end of the plunger looks like a hex head bolt for a REASON!

Jack made a CD of all the info you need for the system. If he doesn't have it still, I do, and can send you a copy.

Patrick

P.S. Another 71 350/270 4 speed with AC.

[Patrick H.]

P.S.

Rick,

The parts really aren't that hard to find. With the advent of eBay, I've got duplicates of almost everything, and much more of others. If you know the part numbers, you can find the parts. In fact, due to eBay the prices of many of the parts for this system hve gone DOWN because people who previously had no way to sell old NOS parts now have a way to do so.

Patrick

[Joe L.]

Re: 1971 TCS system general questions.

Patrick-----

I don't understand how this 71 system works without a solenoid. I did some further checking and found that the part number that I provided may have been a 70-only number. 1972 used a different part number for the solenoid and I can find no specific reference to a 71 solenoid.

However, the solenoid is, basically, the "heart" of the TCS system. How is the vacuum advance signal "interrupted" on your 71?

[Dave S.]

Re: 1971 TCS system general questions.

Joe,
The vacuum advance is effected when the plunger is extended and that occurs during the first 15 seconds of start up, cold start(under 82 degrees), and 3rd and 4th gear engagement. Adjust the plunger so you get about 1200-1400 RPM when engaged. You can adjust it by hand and that setting will always be the same for the 3 items listed above.

[Joe L.]

Re: 1971 TCS system general questions.

Dave-----

I don't understand which "plunger" that you're referring to.

In any event, the operation of the TCS system that I'm familiar with involves several components, depending upon year. The transmission switch, engine temp switch and relay(s) all serve to control the action of a solenoid. The solenoid is what "executes" the interuption of the vacuum signal to the distributor vacuum control. Without a solenoid in the vacuum line between the vacuum source and the distributor vacuum control, I don't understand how any of the other system components can have any effect on the vacuum signal applied to the distributor vacuum control.

Certainly, 1971 TCS systems included the trans switch, temp switch, and relays. If there was no solenoid, then what function did these components serve?

[Dave S.]

Re: 1971 TCS system general questions.

Joe,
I would call the 444 TCS switch, the solonoid. Correct???? It is powered and/or controlled electrically via the sensor, relays 12V, transmission switch etc., and by vacuun from the distributor and carb. The plunger inside the solonoid interrupts the vacuum when it is extended and returns it to normal when retracted. The extension/retraction of the plunger is an electrical function, but by its movement the vacuun is either open or closed. After preparing this system for my PV I was ready to comitt "hary carey". Ricks 71 also has A/C which further complicates the system. I didn't have to deal w/ that.

[Jack H.]

Re: 1971 TCS system general questions.

Adjustment proceedures for setting CEC solenoid travel/throw are documented in your copy of the '71 Chassis Service Manual....

[Joe L.]

Re: 1971 TCS system general questions.

Dave------

OK, now I've got it. The on-carb solenoid combines both idle control and vacuum interupt functions. From Jack H.'s post above, it seemed like he was saying that there was no solenoid used for the 71 system. However, without a solenoid, there could be no TCS functionality. It's just that there was no seperate solenoid used for the TCS function.

But, this raises another question: I wonder what purpose that the idle control solenoid function plays on manual trans, non-A/C cars?

[Jack H.]

Re: 1971 TCS system general questions.

The '71 Combined Emission Control system is a 1-year only Corvette innovation. Was met by the buying public with GREAT DISDANE and I can't start to tell you how many 'clever' ways I've seen to 'castrate' and circumvent it out there!

Basically, the buying public (aided by mechanics) saw the CEC as the culprit that robbed them of horsepower/performance, when, in fact, this was the first year of tougher Federal Emissions standards and the basic engines underwent a drop in compression ratio....

The 'heart' of the CEC system is a pair of control relays mounted back-to-back on the firewall adjacent to the wiper motor in front of the driver. A CEC solenoid is bolted to the carb(s) (one bracket version for Holley another for QJet). The carb solenoid serves two purposes:

(1) Dynamically 'bump' curb idle (adjustable by the hex on the solenoid plunger)
(2) Simultaneously open/close vac line to the distributor's vac advance.

The pair of control relays on the firewall (one is a early exercise in DTL, diode transistor logic, do NOT try to 'test' it as a conventional mechanical relay with powersupply/ground; you'll smoke da sucka) receive several inputs and based on a truth table built into the DTL logic of the main control relay, turn the carb solenoid ON or OFF.

Inputs are:

(1) Elapsed time since first cold start engine fire.
(2) Which gear is the transmission in?
(3) What is tri-stated analysis of engine coolant temp (cold, normal or overheat)?

The 'smart' relay processes these inputs and picks the most intelligent output state for the carb mounted CEC solenoid. The solenoid has two states:

(A) Engine curb idle boosted from mechanical norm by approx 75-150 RPM AND full vac advance fed to the distributor.

(B) Engine curb idle allowed to relax to carb's normal mechanical stop AND all vac to the distributor's advance interrupted.
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There is one other CEC 'state' that was omitted by prior thread discussion. What happens when the engine is about to enter overheat/danger/thermal runaway? This situation is NOT tested/verified at NCRS under Operations Check or during PV....

There, the CEC logic fires the carb relay to boost engine RPM, spin the fan faster, draw more air through the radiator and effect an automatic cool down. Works under serve stress conditions like A/C on, engine idling in stop/go traffic on a HOT summer day!

Happened to me at a July 4th parade where several Corvette clubs donated cars/drivers.... FIVE straight axle and MY Corvettes 'bit the dust' puking their rads from idling for TWO HOURS in 100F ambient behind parade bands, marchers, floats, and horseback riders.

My car ('Lil Red) saw the temp gauge steadily rise to and INTO the red overheat warning zone! The CEC system automatically engaged, bumped idle RPM and the temp gauge SLOWLY drifted down below 'danger'. The CEC sensed 'normal' engine temp and cut out. The process repeated itself 3-4 times during the parade and 'Lil Red survived to drive home passing the other Corvette 'casualties' sidelined along the parade route!