Paul71LT1

Not a topic you see here everyday, but here goes:

Subject: Transmission Controlled Spark (TCS) or Combined Emissions Control (CEC) solenoid for my 1971 corvette

The subject system had been ripped off my ’71 LT1 by a previous owner, along with the AIR system, but that will have to wait for another day. I reconstituted the CEC system this past winter with a used, original solenoid(new ones are not available), and a new bracket and hoses. The theory is that the CEC solenoid activates only in 4th gear, allowing vacuum to be ported to the distributor, providing vacuum advance for fuel economy only in high gear. It also is supposed to hold the throttle slightly open so it does not close completely during deceleration, depending on how it is adjusted.

This is what I found on the car. To my good fortune, the cable and connector to the solenoid had been coiled up by the distributor, rather than cut off. Manifold vacuum had been ported directly to the distributor, providing full advance in all gears. The transmission switch was still in place with the wire connector on it. The switch works and makes contact when depressed. The 3/4 gear shaft has a cam action that depresses the switch in the neutral position, making contact, and breaks contact in 3rd or 4th gear. Therefore, the switch is also depressed in all other gears since the 3/4 shaft is then in neutral. I thought the cam on the shaft should only depress the switch in the 4th gear position? I have spoken with a tranny rebuilder and he confirmed that what I have for a cam action on the 3/4 shaft is correct and he showed me a shaft, pointing out the actuating cam, and the “D” shaped cross section that prevents improper assembly.

What I have now is a CEC solenoid that energizes, porting vacuum to the distributor as soon as the ignition is engaged. There is no effect from the transmission switch in any gear. Switch closed or open has no effect. The solenoid appears to be operating completely independent of the tranny switch. The solenoid releases as soon as the key is turned off. The solenoid should only get power when the tranny switch operates. Any idea why this is happening? Where should I look for the problem?

Any advice offered would be appreciated.

Mike Ward

Not 100% familiar with your year, but believe that you need TWO solenoids for the system to work. My '73 does.

One solenoid stops/starts the flow of vacuum to the distributor and is located on the right side of the carb. It responds to signals corresponding to what gear the car is in, and coolant temp.

The other solenoid is on the driver's side of the carb and and a plunger pushes open the primaries slightly when the ignition is on.

There is a separate wiring harness and connector for each solenoid.

Alan 71

Paul,
As you probably know the CEC system was 1 year only, so VERY few people know anything about it. Do you have the information and schematic from the Chassis Service Manual? If you do PM me and I'll give you the name of someone to talk to about the CEC system.
Regards,
Alan

7T1vette

The CEC solenoid and the TCS valve are a single unit on the left-front of my '71 SB/auto carb. The unit is powered by one two-wire connector. When activated, the CEC prevents engine-braking [which wastes fuel and makes more emissions] and the TCS allows vacuum to reach the distributor advance can.

Peterbuilt

The only information I could find on TCS is in the 1968-1983 Shop Manual page 1-10 if that helps. My car is a 74 350 auto and my emission sticker says CCS/EGR can you tell me what the CCS means, and how it should work? Thanks, PG.

wallifishrmn

Jack Humphrey who is an electrical engineer wrote the only good article I have every seen on CEC system. Google a search under Rocky Mountain Chapter NCRS on CEC and it should come up. He also had the actual wiring diagrams for the system. Those listed in AIM are incorrect. Hope this helps.

7T1vette

Is '74 the first year for a catalytic converter? If so, maybe that is the CCS reference. I had a [new] '74, but don't recall a CCS designation.

Peterbuilt

I fould out that CCS stands for Controlled Combustion System but I still can't find out where it is or what it does.
There were no catalytic converters on my 74, I think that started in 75, PG.

Solid LT1

TCS system also has a theromstatic switch on the passenger side cylinder head. Full vacuum is allowed during warm-up until operating temprature is reached. Is the connecter and switch hooked up between #6-8 spark plugs (1/2NPT switch in same fitting as the temprature sender goes on drivers side.) There should also be a circuit breaker on the firewall to power the system, I think, there was in 1970. I hope this helps you, Eric B.

wattac2

I have a '71 service manual that goes into complete detail on the system. I'm headed out right now, but can post more later tonight.

There are two relays, a "time delay" and a "reversing" shown in the diagram. Plus the solenoid that control the throttle position and the vacuum advance. the relays are controlled by a water temp sw. and a transmission swifter sw. I seem to remember that one of these relays is mounted under the dash close to the heat, but will have to check on that. More later.

Andy

Andy

PHulst

There are relays on the firewall, the solenoid on the left front of the carb, and the temperature switch that goes into the RIGHT side head of your engine. Check the green wire going to that temperature switch. I almost sounds like the switch is bad or the wire is grounded full time.

As others noted, the system is dependent on a few factors including engine temperature and your gear selection. Unless your switch in the transmission is not functioning correctly (which I doubt) I'd look to the "right" as noted above.

Patrick

wattac2

"I'm baaack"

Sorry, couldn't resist the urge.

You seem to be missing the "reversing relay" shown in the manual. It "OPENS" the ground circuit to the solenoid when the transmission switch is "CLOSED". So when the tranny is in 3/4 the switch OPENS, the relay CLOSES, solenoid is energized and vacuum is ported to the distributor. In gears 1/2 the sw. is closed, the relay open, the solenoid not energized, and vacuum is not supplied to the dist. Clear as mud, but that's the way Chevy designed it.

There also was a time delay relay that allowed vacuum for about 15 seconds after the engine was started. I couldn't figure out from the text or the diagram what the functioning of the relay is/was, but it was suppose to help "drivability" after startup by supplying vac. to the dist. during this time.

The manual seems to indicate both relays were mounted on the firewall, but is not clear about that either. I've found Chevy manuals after about 1965 had less and less clarity, and a large increase in errors. I've been working Chevy's since the early '50's.

There was also an "anti-dieseling" relay(s) shown in the manual to be in the center console that controlled the throttle stop solenoid for cars with AC. It stopped the engine from dieseling by energizing the AC compressor for a few seconds after engine shutdown. My '71 was a 4-door, not a Vette so not sure of the details for this one.

Hope this helps a bit. Email if you'd like more info.

ANdy

Alan 71

Thanks Patrick!
Regards,
Alan

FB007

Nope '75

Paul71LT1

Thanks guys. I now have a clue where to go look. I will get out my service manual and find those relays as well as check the function of the right side temp switch which I know is still connected. A special thanks to Andy for explaining how Opening the switch at the tranny actually closes the relay to energize the solenoid. Sounds like a backwards way to do things but apparently it worked. I just want the vacuum function to work to the distributor. I already adjusted the solenoid (just turn the plunger with a wrench) so it can't actually reach the throttle when extended. I like compression braking! This forum is great for getting answers to technical problems. Out there somewhere in cyberspace is someone who truly has done it before.

Paul71LT1

I have checked all the pieces of the CEC system in my 71 LT-1 one at a time and here's what I found:
Temp switch between cyl 6 and 8 is working fine. Opens the circuit upon warm up.
Delay relay working good. 15 to 20 secs after energizing the ignition it de-energizes the vac solenoid.
Tranny switch is good and breaks contact in 3rd and 4th gears.
Reversing relay: NOT working within the system. I took it apart and bench tested it and it actually works fine. The problem appears to be that there is no power to the hot side of the coil in the relay. I put a meter to the connector and verified no power there. The tranny switch will make and break a ground path, but with no positive power source the relay can't function to open the circuit and therefore the solenoid never releases. I'm not sure where to go next as the wire disappears into the wiring harness and I really don't know of a good way to try and track down the problem that is preventing power from getting to the relay coil.

Any suggestions?

wattac2

I've just double checked the actual Corvette wiring diagram for a '71, and as I'd said earlier, Chevy manuals had "errors". The CEC diagram in the main part of the manual says one thing (as previously discribed), but the Corvette wiring diagram shows a completely different system.

The transmission sw ties directly to the solenoid and provides a ground when it is energized. The temperature relay (only one relay and no "reversing" relay on the Corvette diagram) is tied to the temp sensor to turn it "off" and "on", has a 12 Volt wire from the fuse block, and a third wire that ties to the other side of the solenoid. I'm assuming this single relay supplies the 12 Volts to the solenoid as needed. Again, no "reversing" relay shown in the 'Vette diagram. The diagram is not clear as to which one of the two terminals (one closed cold, one closed hot) of the temp sensor ties to the relay.

Sorry for the confusion. I should have referred back to the "Vette wiring diagram earlier. I also found other documentation "errors" in the wiring diagrams, so now I'm concerned about all the info. I've found similar problems with my '69 wiring and had to correct the manual.

Andy

Paul71LT1

I realize that to restore pure function I could jumper a 12V source to the hot side of the relay coil and everything should work as intended; however, that would not necessarily look kosher under the hood of my other wise very factory correct car. It's not an NCRS candidate yet but I try to move in that direction with any repairs and upgrades that I make. I may pull the engine next winter for a "freshening" and thorough cleanup and repaint which will make the wiring much more accessible to further pursure this problem. For this summer I plan to just leave the solenoid out of the equation and let the distributor have full vacuum as it has been for 30+ years.

Thanks for the help.