Hi,

I have a 1970, base 350/300hp, automatic THM400 (carb #7040202).

So I've read several posts about bypassing the TCS system (Transmission Controlled Spark) and how it can contribute to some issues... as I understand there are several components involved in the TCS system (Idle stop solenoid, Transmission switch, Temperature switch, Vacuum advance solenoid, Time delay relay), and I'm sure if one of these components are not working properly, it will affect the TCS, and that it's basically an emission control system. I've decided to bypass the TCS system, for various reasons, one of them being that for a while now, when I put the car in reverse, I found that the car would stumble, and it was quite noticeable, but in P, 3,2,1 or N, i couldn't really feel it... I did read that the TCS system is "active" in Reverse when you have a THM400, but not for the THM350... I suspect that the TCS may have been behaving incorrectly and that's why I was feeling it in reverse...

Anyhow, I've seen several guys mention to simply bypass the TCS system by putting a hose directly from the distributor vacuum advance to the carb, and not through the TCS solenoid. I am not concerned with originality, and am not having it NRCS judged.

So I did that, and the car is much smoother, and in reverse I can no longer feel any stumble.

I do have a question though... does "bypassing" the system mean removing it completely or leaving it all hooked up and simply have the hose from the distributor to the carb? Or do I need to remove the "A" and "B" hoses in the picture below? "C" is where the distributor vacuum hose used to enter the solenoid, but I have now capped that off (not clear in the photo though). My main question is whether I need to remove the electrical connection "D" or leave it connected to the solenoid (currently I have disconnected it)... does it make any difference connected or not? "E" is simply the new hose that goes from the distributor to the carb.

 

 

as long as no vacuum goes to the tcs system, you are ok leaving it in place.

 

You can leave it alone. Just make sure the carb side of your new vacuum hose is manifold vacuum, which I believe it is.
You can also pull the TCS fuse.

 

Hose E bypasses the TCS with manifold vacuum ()
I'd cap B and D right at the carb to eliminate any possible vacuum leak due to the solenoid
On a different note, did the 70 not have the check-valve in the vacuum line after the little filter? I thought they all did..
M

 

Thxs Mooser... so basically I could completely remove the TCS solenoid and any hoses attached to it, and cap off "A" and "B" (at the top of the photo), and it won't impact anything?

... and i do have a check valve, but it's simply further on the other side of the engine and not necessarily beside the white filter, but there is one.

 

Pretty much all there is to it. Take the TCS hardware and put it away in a box somewhere for "down the road" or sell it, (someone will want it for restoration) and plug off any ports not being used
M

 

Hi there,

Quick question for a 1971 c3:

_ Can we separate the tcs system from the kickdown? If yes, how ?? According to the information that I find, they are connected to each other.

_ The tcs system is really necessary? Because I don't want to plug it in.


Thank you for the answers ;-D

 

The TCS (CEC for 71) isn't necessary, it was an early pollution control system that made the engine run hotter at startup to help burn off "stuff"
71 was a unique system on it's own
The kick-down is tied to the same wiring but it was to allow for over-riding the system in certain circumstances
Here's one of the best diagrams for the 71 I've come across (Craig H made it I believe)




And a write up of how it works is in this NCRS newsletter towards the middle/end is attached

Worse comes to worst, just bypass the valve that is in the hose between the motor and the distributor will bypass the TCS/CEC system, all it did was prevent the distributor from getting vacuum advance until the engine was properly warmed up.
M

 

Thanks a lot !

In this case how to directly connect the 2 wires (an orange and a black it seems to me) which go to the kickdown (on the pedal), without going through the tcs harness?

thanks

 

I'd have to really look (mines a 72) but I believe you don't have to do anything to the wiring, they connect but are used to trip the cec into allowing advance in the case where the pedal is mashed and the transmission kicks-down. The CEC doesn't turn off the kickdown or anything
M

 

The 1970 TCS and 71 CEC system provides vacuum to the distributor and power to the Carburetor solenoid to increase RPM for 15 to 30 sec at startup. Increasing RPM by 300 to 700. Then the system is only active when the manual transmission is in ether 3rd or 4th gear allowing vacuum to the distributor.

It could also be energized by the Temperature switch in the passenger side head. If the engine coolant temperature exceeds a set amount the system is energized to increase RPM 300 to 700 to increase engine fan speed to aid in cooling the engine coolant for 1970 to 1972.

In 72 it changed again the startup is the same as 70 and 71. But it will only energize in 4th gear with a 20 to 30 sec time delay. Also the carburetor solenoid is now an idle stop solenoid to help prevent dieseling.

For Automatic transmissions it operates only in reverse and 3rd gear.

CEC is (Combined Emissions Control) verses TCS (Transmission Controlled Spark). The main difference in the CEC is that the idle plunger solenoid and vacuum control is combined into 1 unit. As the TCS are 2 separate units 1 idle plunger solenoid and 1 vacuum control solenoid.

 

CEC in 1971 has no relationship with Th400 kickdown. The article linked above in the thread, relinked here (71CEC) describes the full 1971 system in detail which is significantly different then 70 and 72 which are much more similar.

The CEC system is not necessary. It helps with cold start and overheating which is nice but it turns off vacuum advance in low gears i.e. first and second for both transmissions which most would perceive as a negative. FWIW I drive my two 71s both with CEC hooked up and fully operational without any noticed downside. But, if you want to bypass properly follow the instructions at the end of the article linked above. The key instruction is to reroute vacuum directly from carb to distributor without passing through CEC solenoid.

For other 71 CEC buffs there is a second article in that series (June 2022 newsletter) all about the delay relay testing procedure.

Also for 71 CEC buffs Jack's original article:
https://manualzz.com/doc/6300991/pdf-from-ncrs---ncrs-rmc

 

Has anyone completely removed relays and all of the CSC wires and switches? I'm moving the Alt to the pass side while the engine is out.
Thought I could get rid of these being I don't use them anymore. Won't sell, next owner with be one of my kids.

 

The electrical part of my '71's is still connected all these years even though the solenoid has long disappeared. Matter of fact I just used the bracket that holds them to the fire wall as the bracket I'm mounting my Sniper 2 fuel pump relay and fuse to. I was going to remove the TCS stuff for good but then I'd have to dress the wires to make it look good so I just left them in place. My vac advance is connected to my carb but all the relays etc are still connected and not doing anything. It runs fine. I'm a 4 speed car so it might be a little different on a auto.

TCS stuff with EFI fuel pump relay: