1970 Transmission Controlled Spark (TCS) Operation

[Richard J.]

I have been restoring a 1970 LS5 coupe and have been working on the Transmission Controlled Spark (TCS)system. According to the operations section in the JM it states that the system should activate with the engine running and shifting the trans into 3rd and 4th gear. I see no switch on the transmission or in the console... How does this system know then the transmission is shifted into 3rd or 4th gear? I see nothing in a wiring diagram and nothing in the assembly manual... Anyone got help for me here? Thanks, Rick

[Joe L.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Rick------

There should be a switch on the transmission. It is located on the sidecover on the boss for the 3-4 shifter shaft (the forward boss). An extension of the boss (at about a 7 o'clock position on the shifter shaft boss) is drilled and tapped for the switch which screws into it.

The correct side cover with provisions for the switch is GM casting #3952648. If you have any other sidecover casting, it's been changed and won't suppoort the TCS switch.

[Pat M.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Rick - there is a switch at the transmission that activates the TCS solenoid in third and fourth gear when the car is at normal operating temperature. I can see the switch on the driver's side of the transmission on my 1970 4-speed, but I'm not sure where you can locate a good wiring diagram. The 1970 Chevy Shop Manual describes and shows this switch, but not as it would appear on the car.
Check the archives - there's a bunch of really good threads describing how the TCS works. Good Luck, Patrick Moresi #45881

[Patrick H.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Rick,

The switch is located on the bottom of the forward shift lever shaft on the transmission in this eBay auction:

Patrick




eBay Muncie auction

[William C.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Keep in mind that the switch is activated by a flat machined onto the shaft going into the trans. One flat for 4th gear only versions, and two flats for third and fourth gear versions.

[Stephen B.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

What's the part number on the TCS switch that mounts in the transmission cover?

[Wayne M.]

Some 3952648's not drilled and tapped

Bill, that's interesting on the 3rd gear activation. I remember my new '72 convertible the first time I took it on the highway. Thought something was wrong with the motor when the switch activated after shifting into 4th.

This pic is on an M22 over-the-counter, 1971 assembly. Note the boss is there on the 3-4 shaft, but no switch. Also note the fine-threaded studs; it also has a "584" tail with the 27-spline output; not sure if 10-spline input -- I've never had it out of my '65 in 19 years that I've owned the car. I believe they were making several versions of OTC trannys back then, so you could simply swap the earlier versions.

[Joe L.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Bunky-----

The original part number was GM #3961567 for 1970-71 and GM #3996286 for 72-74. These are discontinued, though. There may be replacement part numbers but, if there are, I can't find them.

[Stephen B.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

Thanks Joe. I checked my '70 LT-1 transmission, and it is missing the correct side plate and switch. My '70 L-46 has the correct side plate and switch.

[Terry M.]

Re: 1970 Transmission Controlled Spark (TCS) Opera

If memory serves 1970 had the 3-4 actuation, and 1972 had 4th only. I can't remember 1971 without researching the Operations manual. There is a difference in the electrical connection between 1970/71 switch and the 1972, and that accounts for the part number change Joe cited. The difference in which gear the TCS functions in is due to the number of flats on the shift fork shaft, as Bill and others have mentioned.

[Dennis D.]

from service and repair *NM*