72 CEC System operation check- Terry Mc.

[Edward J.]

Terry, my 72 CEC system seems to be operating ,when I start the car cold engine and let it run ,I can pull the vaccum hose off the dist. advance and theres vaccum, and after running a few minutes I can hear the engine rpm's drop and the vaccum is shut off . My question is on the operations of the system with the automatic trans. it says you can put into rev. and hear the rpms increase, I dont seem to really hear a change, maybe because the cold start rpm's are higher than normal.or should the system work as well after the engine has warmed up?

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

Yes, it should work when the TH400 is in reverse. It will not reach the kind of RPM it does when cold because the torque convertor will be holding back on the engine speed. We usually judge by the rise of the rear of the car as the brakes hold back the engine power.

If you can get someone to help you check for vacuum at the rubber tube to the advance -- just like you did when the engine was cold -- but you will need someone to hold the car securely with the brakes in reverse gear. *** Do not count on the parking brake ***

BTW: CEC = Combined Emissions Control = 1971
TCS = Transmission Controlled Spark = 1970 & 1972
Transmission Controlled Spark is somewhat of a misnomer in that the system is controlled by both the transmission gear and engine temperature.

[Edward J.]

Re: 72 CEC System operation check- Terry Mc.

Terry, do you know how the tcs is turned on when put into reverse, does the netural safety switch turn it on or is the pressure switch on Transmisson ?

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

There is a pressure switch that picks up a ground using transmission line pressure. In the TH400 line pressure is high in third gear and reverse. There has recently been a thread about the internal switch and the two-wire connector. If you think the switch is not working you should be able to jumper the wire to ground at the external connector. With hot exhaust pipes it might be a challenge to get to that connector without burning yourself, but see if you can find that thread. It has a picture of the connector. My memory is that it is on the driver’s side near the rear of the transmission, just above the pan rail. You should be able to follow the wires with the engine cold. The second wire is for the throttle kick-down which is off the micro switch on the throttle linkage inside the car. Be sure to get the right wire if you are shorting to ground, and be very careful to be sure the car is firmly anchored when putting it into gear with the engine running.

Edit add: Some model years and certain engines have a time delay for the vacuum operation. I know 1972 small blocks have a 20 to 40 second (from my memory) time delay, but I don't know about LS5s. The answer is in the 1968-1972 Operations and PV Manual.

[Edward J.]

Re: 72 CEC System operation check- Terry Mc.

Thanks, Terry.

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

Here it is Ed. looks like you wee a participant.

https://www.forums.ncrs.org/showthre...TH400&uid=4723

[Edward J.]

Re: 72 CEC System operation check- Terry Mc.

I understood the 70 CEC, but my 72 the reverse thing has me questioning myself if it was working , I listened a few times after start and could not tell, I 'll try your test with it in gear and with a extra hand.

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

Just another thought: You could T in a vacuum gauge to the hose to the distributor vacuum advance unit and watch that gauge from the driver's seat. That might be easier and safer.

[Edward J.]

Re: 72 CEC System operation check- Terry Mc.

Ya, I have a vaccum gage to check it,thats a better idea.

[Dave S.]

Re: 72 CEC System operation check- Terry Mc.

Ya, I have a vaccum gage to check it,thats a better idea.

Ed,
There should be lots in the archives on this. 71 THM cars use a reversing relay to accomplish the reverse speed ups. Its either the 842 or 843 relay.

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

Dave, I am sure Ed is thankful he has a 1972 -- or he will be if he learns anything much about the CEC and LS5 with AC. Yes, I know the CEC in the thread title is misleading.

[Dave S.]

Re: 72 CEC System operation check- Terry Mc.

Dave, I am sure Ed is thankful he has a 1972 -- or he will be if he learns anything much about the CEC and LS5 with AC. Yes, I know the CEC in the thread title is misleading.

Terry,
Your right. 72 is simplier. My brain is still frazzled from getting my 71 THM and 71 4 speed ready for PV. I used Jack Humphrey's article and it was a life saver.

[Edward J.]

Re: 72 CEC System operation check- Terry Mc.

Terry, Dave, I believe I have another problem solved, I was home early today so took a look at the operation of the TCS system and it does work when cold, which according to the wiring diagram, uses the temp. switch in cylinder head for cold start, but when reverse is selected warm engine- no operation, looked at the wiring diagram for that operation, and found that ground is not coming from the pressure switch in the transmisson. and thats because at some time someone removed this switch and plugged the hole in valve body,( thats not nice ) so now I have to find this pressure switch and make up a harness( minor detail ) .

[Jack H.]

Re: 72 CEC System operation check- Terry Mc.

'842 is the reversing relay while '843 is the delay relay. There's a subtle difference in the functional silhouette of the system in the '70-72 timeframe.

In '70, the system was comprised of a vac switch only. This passed/blocked vacuum to the distributor. In '71, a single CEC solenoid provided two functions: (1) switch vac to the distributor, and (2) toggle the carb's mechanical curb idle setting.

In '72, the system had the same feature/functions as that used in '71 except the vac switch and the carb's curb idle setting solenoid were two separate components.

So, when testing the functionality of a '70, you're seeing just the engine idle RPM difference that results from passing/blocking vacuum to the distributor's vac advance. In both '71 and '72, the distributor's vac advance is being switched ON/OFF and the carb's mechanical curb idle setting (throttle) is being switched too.

The contribution of the carb's idle solenoid can be 'iffy' based on the setting of the carb solenoid plunger and the strength of the throttle return spring. In actual use, the carb solenoid would fire when the driver's foot was on the gas pedal with the throttle well off/away from its normal curb idle screw stop.

You can't guarantee there's enough solenoid force to push the carb's throttle back/off its normal stop by ONLY actuating the shift lever... Most WILL fire and push the throttle back, but some won't.

The fair way to test this aspect of the emission system on a '71 or later car is to pump the accelerator with the transmission in neutral and watch the tach needle fall to its steady state curb idle.

Now, shift into 3rd or 4th gear on a manual transmission car (foot into the clutch) and re-rev the engine and watch to see where the idle RPM falls to. It ought to be about 75-125 RPM higher.

An automatic transmission car is a bit trickier, because you'll want to shift it between neutral and reverse, FOOT ON THE BRAKE HARD, and observe a somewhat similar RPM differential understanding there IS a dynamic load on the engine due to the AT itself...

[Terry M.]

Re: 72 CEC System operation check- Terry Mc.

Ed,
That thread from a few days ago -- the one I posted the link to -- has a source for TCS switches for both M4 and TH400 transmissions.