1971 CEC Question -- Help me to decode this testing procedure
 

Thanks in advance --

I am trying to fix my CEC solenoid which is always staying in the energized state.

The diagnosis flow chart in the yellow covered manual is pictured here:
https://cimg5.ibsrv.net/gimg/www.cor...725975d4e4.jpg

Here are my reversing and delay relays:
https://cimg8.ibsrv.net/gimg/www.cor...1ecea351fb.jpg

Two questions:
1. Which is the purple wire it refers to in Step 2B Part 3? I know the revering relay is 842 (in the front), but I have no purple wire.
2. Which is the purple wire it refers to in Step 2C Part 3? I know the delay relay is 843 (in the back), but I have no purple wire.

I have learned something new in my experimenting tonight... if I unplug the single unit plug from the delay relay (843, the relay in the back) the solenoid finally retracts. Of course it will not fire in this configuration though.... BUT if I ground the connector with the green wire (which fomes directly from 842) and the blue wire (I think this goes to the solenoid) it will fire -- and I can control the solenoid. I have a spare delay relay (843 littlefuse) and I tried it but it doesn't help. Does this give enough of a clue to someone whose a better electrical engineer than I??

Hi

I spent a lot of time looking at this.

In my opinion a lot of the docs is confusing, contradictory and possibly missing info.

Have a read of this...

https://www.corvetteforum.com/forums...id-wiring.html

Thank you all for your help! The diagram in CraigH's linked thread is accurate.

My problem was with both of my relays 842 and 843 and my spare 843! Now the soleniod fires for 15 seconds after start, when cold, and when in 3rd or 4th gears

My only question is: Why do you want the CEC system to be operational?

NCRS.

But, I find it runs a lot smoother with it operational. I appreciate the extra idle on start too.

The whole purpose of the CEC system was to get the combustion chamber temps up so that more hydrocarbons would burn (strictly emissions stuff). How that works is that manifold vacuum...which should feed the vacuum advance can on the distributor...is defeated under defined conditions: idling, after engine is warming up and when driving in lower transmission ranges. The system allows manifold vacuum under full throttle, in highest transmission ranges, and when the engine temp is above a certain temperature.

Basically, the CEC system forces the engine to run with retarded ignition--causing it to run too hot, and too lean. It does NOT help the engine in any way; it probably reduces engine life. If you were to defest the CEC system and put the advance can at manifold vacuum all the time AND advance ignition timing from GM spec of 8* BTDC to 12* BTDC, you engine would idle better, use less fuel, run cooler, and gain a LOT of performance.

If you want it to be just like stock, you are doing the right thing. If you want to enjoy your car and maximize engine life, make the CEC system non-functional. (Note that I didn't say to REMOVE it. Mine is still in place and "capable" of being put in working mode in mere seconds. But, I re-route a couple of vacuum hoses so that it usually runs with full manifold vacuum.)

7t1vette - I'm not sure if the idle solenoid even works and think it would easier to just leave it installed but disabled. Can you provide a simple diagram showing what needs to be done to disable the CEC?

Thanks

If your solenoid doesn't work route the two vacuum lines to it together and skip the CEC solenoid. That will show the distributor vacuum always. The way you have it now you're never getting vacuum advance.

Take the two vacuum hoses that go into the CEC solenoid and join them. doing that will show the distributor vacuum advance constantly which is what you want. As for the solenoid if it fires or if it doesn't I'd leave it alone. Disconnecting the electrical circuits isn't going to be worth the effort for little bit of idle that you get from it. if you're really concerned about the solenoid what you can do is screw the plunger in so that it doesn't move your throttle when it does fire.

Thank you

Hello -- Resurrecting my old thread to post an article I wrote which covers the full diagnosis and testing of this system. A "CEC" for dummies article if you will.

https://docs.wixstatic.com/ugd/6daab...7c.pdf#page=26

Interesting read....for those who want to keep the CEC system operational. But, that amounts very few folks: those in NCRS who want that 'original' system to function, and those who have no idea what the CEC system does and think it MUST have value or GM would not have installed it.

So, to help illuminate the latter group-- The CEC system was a "quirk" of internal combustion engine (ICE) behavior being utilized to [barely] meet EPA emissions requirements on 'base' engine Corvettes. The 'quirk' is that combustion chamber temperatures elevate when the ignition timing is retarded. And, when combustion chamber temps go up, unburned hydrocarbon emissions DECREASE! So, in their infinite wisdom [and being CHEAP], GM set about creating a simple means of retarding the ignition timing during certain stages of operation:
1. when at idle
2. when vehicle is at low speed in lower gear ranges
3. when engine is operating within a "normal" temperature band
Because running in a 'retarded' mode could be damaging to the engine and could be a safety issue if you NEED the power to avoid (or evade) a problem, there were conditions when the CEC system was NOT to be operational: with cold engine (difficult to start a retarded engine); with hot engine (possible valve damage); in highway cruising mode (upper transmission ranges); and at WOT. It could be damaging to the engine to run retarded in either of the latter two conditions.

The simple method by which GM accomplished this is to block manifold vacuum OR to send 'ported' vacuum to the distributor advance can when CEC should be operational; and switch to 'manifold' vacuum when CEC should be turned 'off'. Thus, the need for the solenoid switch on the carb. The switch on the transmission was to turn CEC system off in upper gear ranges. The relays were for the delay and to correct the 'logic' for the system.

Simple system....few components....convoluted architecture and logic diagram....difficult to diagnose faults because of so little info and even less understanding of the system.

My bottom line: If you need to have an operating CEC system, use the above info and diagnostics to fix what you have. If you do NOT need to have it operational, remove it or defeat it and leave it in place to LOOK like it still works. From an engine and owner perspective, it is nearly worthless and presents more negative results for your vehicle than any small positive benefits to the environment.