70 LT1 TCS Troubleshooting

[Mitch S.]

All, anybody know of the adjustment required on the trans to get the TCS to work in both 3rd and 4th gear? Works only in 3rd now and before I put it on a lift to investigate, I am looking for some insight on any adjustments.

Thanks in advance, Mitch

[Richard T.]

Re: 70 LT1 TCS Troubleshooting

The TCS only works in 3rd gear.

[Nick M.]

Re: 70 LT1 TCS Troubleshooting

I give up. What is TCS?

[Mitch S.]

Re: 70 LT1 TCS Troubleshooting

Nick, stands for Transmission Controlled Spark, you have similar on your 72 LT1......it helps control emissions when driving in 3rd and 4th gear since the the lower 2 gears have the timing retarded for emission control. It advances the timing in higher gears.
On the 72 I think it is called CCS or ECS...does the same thing, hope this helps, Mitch

[Richard T.]

Re: 70 LT1 TCS Troubleshooting

TCS. Transmission controlled spark solenoid. Designed to help control emissions for around town operation by retarding the spark advance thru a vacuum line on the vacuum advance valve (pot) on the distributor. The TCS reduces vacuum to the valve (pot) by venting it thru a plastic vent on the top front of the TCS. The TCS is hooked up in series with the vacuum line to the advance valve (pot) on the distributor. The electrical plug applys voltage to the solenoid when the transmission is put into third gear. Latter 71 to 82 Corvettes had a throttle position stop on the TCS. These solenoids are a different design from the early 1970 TCS units. Some club members feel that the unit, if left hooked up causes a MPG reduction, but I have not noticed this on my very early 1969 built 350 1970 Corvette. I have also not noticed a loss of power. Follow GM tune up instructions when adjusting the idle or spark advance. Hope this helps.

[Harmon C.]

Re: 70 LT1 TCS Troubleshooting

All, anybody know of the adjustment required on the trans to get the TCS to work in both 3rd and 4th gear? Works only in 3rd now and before I put it on a lift to investigate, I am looking for some insight on any adjustments.

Thanks in advance, Mitch

Some years they work in just 4th,and some may work in 3-4 or just 3rd I'm not sure but the control for when they work is a flat spot on the 3-4 shaft in the transmission. If the transmission has been rebuilt and that shaft replaced it could be the wrong one. Make sure you have the correct information as to what gears the TCS should work in before you change the shaft as some manuals may have bad information.

[Jim T.]

Re: 70 LT1 TCS Troubleshooting

TCS is Transmission Controlled Spark. According to my 70's owner manual the TCS only permits full time vacuum to the 70's vacuum advance when the transmission is in high gear.
I have a 70 with turbo 400 and can verify that my car when bought new from the dealership functioned this way, no vacuum advance untill the turbo 400 shifted to third gear.
I "thought" the 4-speed cars had full time vacuum allowed by the TCS when in third or fourth. However I have stated what my 70 owners manual says on page B3, so according to the owners manual it is 4th or high gear.
Another instance in which the TCS will allow full time vacuum in any gear and when in park at idle. The temp switch in the right head will tell the TCS when the engine is to hot and allow full time vacuum through the TCS to help cool the engine.
Upon of discovery of the temp switch function with the TCS I disconnected the vacuum hose from the my 70's carb to the TCS solenoid and ran a direct rubber vacuum hose from the carb to the distributor vacuum advance. Result was cooler running engine and better gas mileage when driving involved starting and stopping using 1st and 2nd. It has been 38 years since I made the modification.

[Terry M.]

Re: 70 LT1 TCS Troubleshooting

Mitch has the right information. For 1970 with manual transmission he should have full vacuum to the distributor in BOTH 3 and 4-gear. For the other years it is 4th-gear only, and TH-400 is high (third if you wish) gear only. Some of the years, and some engines have a time delay, but that is not the case in 1970. And Lyle is right some sources will have the wrong information. I can vouch for the accuracy of what is in the 1970-1972 Operations and PV manual, but that is the only one I am responsible for.

The signal from the transmission is from the shaft of the 3-4 shift selector -- as Lyle states, and his suggestion that this shaft was replaced is likely the source of the problem -- or the whole transmission was replaced with a unit from a different year. 1970 has a unique 3-4 selector shaft with two flats on it as Lyle stated.

[Richard T.]

Re: 70 LT1 TCS Troubleshooting

I know that the TCS is for emission control but at over $4 a gallon for gas I like all of you would like the best MPG possible and performence. I have a 1970 coupe, 300 HP 350, four speed, standard ratio posi, electronic ignition. The book estimate is 14 MPG high way, I get 15 with the TCS hooked up. I have run the car with this unit bypassed and I cannot feel any difference in performance; I cannot tell if the TCS is working or not; the car has plenty of power in all gears. I leave it hooked up for show judgeing. How do other owners feel about this? To me the more MPG the better. Will the car perform better with TCS hooked up or bypassed? 15 MPG, leave well enough alone? Sincerely: Rich

[Terry M.]

Re: 70 LT1 TCS Troubleshooting

The answer to your MPG question depends on how much time you spend in first and second. If your system is working right, you are getting full vacuum in third and fourth gear. If you spend a lot of time in 1 & 2 you might see some improved fuel mileage with a full-time advance -- but I doubt the difference will be noticeable to most folks.

To use the full-time advance you will have to re-adjust the carburetor -- and there is a possibility the primary plates will wind up in the wrong position in relation to the fuel delivery slots. One can makie the fuel mileage worse while trying to defeat the TCS system.

[James B.]

Re: 70 LT1 TCS Troubleshooting

I know that the TCS is for emission control but at over $4 a gallon for gas I like all of you would like the best MPG possible and performence. I have a 1970 coupe, 300 HP 350, four speed, standard ratio posi, electronic ignition. The book estimate is 14 MPG high way, I get 15 with the TCS hooked up. I have run the car with this unit bypassed and I cannot feel any difference in performance; I cannot tell if the TCS is working or not; the car has plenty of power in all gears. I leave it hooked up for show judgeing. How do other owners feel about this? To me the more MPG the better. Will the car perform better with TCS hooked up or bypassed? 15 MPG, leave well enough alone? Sincerely: Rich

On my '72 454 w/ 4-speed, I drove with the TCS both ways: hooked-up and disconnected and noticed a slight performance gain with it disconnected (and with vacuum hose connected directly to the distributor). Also noticed lower engine temperature when idling and driving at slow speeds (with TCS disconnected).

[Mitch S.]

Re: 70 LT1 TCS Troubleshooting

FIXED!!!!
Terry M and Lyle C were right with the description of the 3-4 shaft and the shape of the shaft. When I removed the switch to confirm that it was operating correctly, i determined that the switch was too deep into the cavity and not disengaging completely when shifted into 4th gear. I removed the switch and put a shim/spacer in place and re tightened the switch so as to make the correct contact and dis-engagement and it works fine now in both 3rd and 4th gears.

Thanks all for the responses and help, Mitch

[Terry M.]

Re: 70 LT1 TCS Troubleshooting

Now this brings back memories Mitch. Take a look at the AIM. There was a gasket added to the switch during the 1970 model run. I thought at first that was to stop leaks, but I think now it might have been to shim the switch out a little bit.

I'm glad you got it, and it was simple.

[Mitch S.]

Re: 70 LT1 TCS Troubleshooting

Terry, probably both! It obviously was missing and was there for a specific reason, actually it was leaking a tiny bit too....................hmmm

[Terry M.]

Re: 70 LT1 TCS Troubleshooting

How early is your 1970. 2-18-70 is the date in the AIM for that gasket/shim. It was available from Chevrolet years ago when I went down this road, but I think mine was just leaking. Part #3929530