TCS = Transmission Control Spark ('74, 4spd, LS4)

[Raymond W.]

I'm in the process of hooking up the Transmission Controlled Vacuum Advance System. I've installed the TCS switch in the manual transmission and the appropriate wiring plug is attached to the TCS solenoid along with all of the various vacuum hoses. I'm stuck in my understanding on how to hook up the TCS wiring harness and whether the temperature sending switch is connected into this system with a manual transmission. I've got one end of the wiring harness connected to the TSC temperature switch jumper...what's confusing me is the V shaped connector on the opposite end has two terminals. I was thinking that this would be connected to the switch in the transmission. Do I have the wrong TCS wiring harness? Is anyone familar with this system for a '74, LS4, 4-speed)? Thanks.

[Jack H.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Try reading your copy of the Chevrolet Chassis Service Manual. You should find a description of the system in the book's emission section. In your mental modeling of the system, you're leaving out parts and it's a small wonder you're confused!

There's an intelligent relay in the system that acts as the 'traffic cop' making the decision on when to tell the vac solenoid to open/close and give the distributor active vac advance. One input to the controller (relay) is what gear the transmission is in but there are other input(s) such as the engine's temperature (cold engine, normal engine temp, engine starting to overheat).

I think things will become clearer once you read Chevy's thumbnail description of how things work....

[Terry M.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

I once prepared a 1974 for PV, and as I recall that system is Temperature Controlled Spark -- but I believe that car was Auto, and the systems may be different depending on the transmission. I'm certain there was no connection to the transmission on that car, though. I spent a lot of time looking for that connection. One will need the Service Manual for a 1974, and I don't have one of those.

The temperature sensor (in the passenger side head, I believe) has two terminals set at a right angle. Wires should be green to both temperature sensor connections. One wire loops back onto the other one, so that electrically both terminals are connected together -- just like the earlier C3s.

[Jim T.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

I don't have a 1974 Service Manual, but I do have the 1970 Chevrolet (69 Corvette) and 1971 Chevrolet Service Manuals. Also the 1970 Corvette supplement. The 1970 Chevrolet Service manual Section 6T covers the Transmission Controlled Spark TCS. This distrubutor vacuum control solenoid is mounted on the manifold of my 70 with turbo 400. The turbo 400 has a two terminal electrical switch installed on the left side. One terminal is connected to the kick down switch on the gas pedal. The other terminal is connected to the TCS system. It's function is to distinguish when the transmission shifts to third or is put into reverse. This identifies when the TCS can give engine vacuum to the distributor vacuum advance (third gear and reverse). 4-speeds will have the full time vacuum when shifted to third or forth. The 71 cars have the different design of about the same thing except it is mounted on the carburator and has a new name, Combination Emissions Control System CEC covered in Section 6T. The automatic transmission still tells the system to permit engine vacuum when the turgo 400 shifts to third and when reverse is selected. 4-speeds get full vacuum in third and forth.
Both systems allow full engine vacuum when the engines are cold. The 71 manual says below 82 degrees. The 70 manual just says when cold. Another function of the extra temp indicator in the 70 and on some 71's is when the engine temp goes above operating temp the system will decide that full engine vacuum is needed to help in cooling. This may occur when idling or in heavy stop and go traffic usually. The 71 manual mentions that only the 71 Camaro LS3 with automatic and air,71 Corvette base engine with air and automatic, and 71 Corvette LS5 with automatic and air will have the high engine heat full vacuum feature.
The 1974 Corvette emissions system may be different. I do know that when my 70 was new and I was outside the car on a hot day with the engine running and the air conditioning on the car picked up idle speed on it's own. Thought something was wrong. In reality it was, the distributor vacuum advance was not getting any vacuum until the system got the indication from the temp switch to let it happen. From that experience I redirected engine vacuum to full time all the time. So I have a TCS solenoid that has not been used for over 37 years. An added benefit was better gas mileage using full time vacuum.

[Harmon C.]

(Message Deleted by Poster)

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[Harmon C.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Ray

The V shaped connector should plug to temperature switch with the other end to the wiring on the firewall near the heater core. With your car warmed up put the trans in 4th gear and wait one minute to see if the engine rpm goes up. It should happen in half that time. I think the timer on your 74 is on the drivers side firewall under the hood and many times needs cleaned up or replaced.
Lyle

[Dave S.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Lyle,
Is that Automatic pedal switch located under the dash at the accelerator linkage. I've noticed on my 71 Auto car that you need to tap the gas pedal to activate the 444 switch on the carb. Tells me there is something mechanical involed and it may be that switch. My 71 4 speed car activates the TCS/CEC system on it's own without tapping the gas pedal. Am I missing something.

[Patrick H.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Dave,

I find on my 72 - which is Quadrajet equipped as is your 71 - that the solenoid doesn't have enough "power" to open the throttle plates when I start the car cold. So, as you've noted, you need to turn the ignition to ON, "tap" the throttle to allow the solenoid to open the throttle plates (also engaging the choke) and then start the car. If you tap before engaging electrical power, it doesn't work and idle speed is about 450.

I suspect the setup with your Holley allows the solenoid to "do its work" without having to tap the throttle.

Patrick

[Dave S.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Patrick,
I don't know why a Quadrajet and Holley would be different. I was thinking that the 4 speed and Auto might be different but you changed that theory. Your description is right on.....The TCS/CEC switch just does not have the power to move the throttle lever. The control is working as the tap of the pedal gets things going. It may be that this is the way it is. Maybe someone else has some experience.

[Gerard Q.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

I too have to tap the throttle to achieve a fast idle on my 71 s/b auto. When I turn the ignition on I hear the solenoid click into place but as Patrick stated it doesn't have the ability to move the throttle without assistance.

[Dave S.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Jerry,
Thanks for the confirmation. The interesting thing is I have PV'd 2 shark cars and never had to tap the gas pedal on those. I thought my system was not functioning until I came across this by mistake. Others who are preparing for PV need to know this so they don't drive themselves crazy.

[Jack H.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Some CEC solenoids do have sufficient power to push against the throttle return spring and get onto high idle and some don't. It depends on the 'freshness' factor of the 1114444 solenoid, state of charge of the battery, wear silhouette of the starter motor + the specifics of the carb's throttle linkage & return spring.

BUT, the bigger picture is understanding the correct care & feeding proceedure for carb based engines of yesterday vs. today's FI cars. The owner's manual tells you to push the accelerator pedal slowly but firmly ALL THE WAY TO THE floor to set the choke, high idle cam and inject a sufficient priming stream of fuel to support cold engine startup. Those who simply 'tap' or 'toggle' the accelerator pedal aren't executing initial cold start properly....

If memory serves, Buick felt strong enough about this at one time to FORCE owners into the correct sequence by embedding the starter switch beneath the accelerator pedal! I'm going back to the 1960 era here...

On these vehicles, you climbed into the car, turned the ignition switch to its ON or RUN position (no crank/start position non the switch) and pushed the accelerator pedal TO THE FLOOR to engage the starter! Many who'd borrowed one of these cars couldn't figure out how to start 'er up....

[Dave S.]

Re: TCS = Transmission Control Spark ('74, 4spd, L

Jack,
Thanks for the clarification. Makes sense. Problem here is the reverse speed up's (Auto)and increased idle on start up (time delay relay). If we don't know enough to nudge the gas pedal to activate those functions we will think the system is non functional. Sounds like that is a common condition.