TCS--What is it's function?

[D S.]

Okay, I have a TCS solenoid on my 1970 Corvette that is wired to a relay on the firewall and has vacuum hoses that goes to various places. There is a harness that plugs into the switch on the side of the TH400 transmission. What does this do? How does that affect the transmission and operation of the car?

Thanks,
Scott Sims
Texas Chapter

[John H.]

Re: TCS--What is it's function?

Basically, TCS (Transmisson-Controlled Spark) denies manifold vacuum to the vacuum advance unit on the distributor unless the car is in high gear, hasn't warmed up yet, or is overheating. It's one of the many emissions "crutches" used in the days before catalytic converter technology. There are other control mechanisms (relays, delay timers, etc.) depending on model year application; it should be explained in your Chassis Service Manual.

[Pat M.]

Re: TCS--What is it's function?

Basically, TCS (Transmisson-Controlled Spark) denies manifold vacuum to the vacuum advance unit on the distributor unless the car is in high gear, hasn't warmed up yet, or is overheating. It's one of the many emissions "crutches" used in the days before catalytic converter technology. There are other control mechanisms (relays, delay timers, etc.) depending on model year application; it should be explained in your Chassis Service Manual.

John - that's the best brief explanation for this complicated system I've ever read. Well said!

[D S.]

Re: TCS--What is it's function?

Thank you. From an operation standpoint when I start my 1970 Corvette and drive it while it is still cold what does it do as opposed to when it is fully warmed? Performance differences? Or will I notice anything at all? Does having it completely operational cause the car to perform better?

[Jim T.]

Re: TCS--What is it's function?

Scott the vacuum going to/from the TCS on my original owner 1970 Corvette was disconnected about 2 weeks after I bought it, still disconnected. Full time vacuum was connected with just a new piece of vacuum hose. Better gas milage having full time vacuum in first and second gear with my turbo 400. Engine ran cooler in Dallas stop and go traffic.
What made me change to full time over 38 years ago was when I was driving the car, it was hot. I got out to talk to some people I knew and left my car running with the air conditioning on. The idle speed really picked up some RPMs and I thought something was wrong, there was, the engine reached a high enough tempertature that the TCS switched to full time vacuum to cool it down. After reading my service manual I ordered right after purchase of my 70, I disconnected the TCS from controlling vacuum to my distributor.
So the TCS solenoid it still on the engine, just not used.

[Terry M.]

Re: TCS--What is it's function?

If the TCS system is properly functioning -- and that is a HUGE if -- when the ambient temperature is below about 75-80 degrees the idle will be at about 1200 rpm until the engine temperature goes above 80 or 85 degrees as measured in the passenger side head. When the head temperature exceeds that, the idle will drop to normal (depends on engine and transmission combination).

All that assumes the ambient temp is NOT cold enough to set the choke. If ambient is cold enough to set the choke, the high idle setting will mask the temperature portion of the TCS system and you will not know if it is working or not.

There is a high temperature portion of the TCS system that above I believe 230 degrees (working from memory here) it will allow full vacuum to the distributor to advance timing in an effort to cool the engine.

The real differences in performance is that the distributor does not see full vacuum until 3 or 4th gear (depending on the year of the Corvette. 1970 is 3 & 4, other years are 4th only) in a manual and high gear in TH400. Thus you are not running full vacuum advance through the gears and performance -- however you measure it -- will suffer.

As John mentions there are other delays depending on the year and engine in your car. I spent a lot of time on the 1968-1972 Operations and PV manual explaining this system for each year and power train combination. I think I got it right, but check it out and tell me if I screwed up some of those details. There are a lot of them

[D S.]

Re: TCS--What is it's function?

Thanks, Jim. I'm going to drive it a while and see what it does or doesn't do before deciding to disconnect it. Having a big block with AC gas mileage is the pits anyway. I reside in the Dallas area.

[Stuart F.]

Re: TCS--What is it's function?

It bears mentioning that the standard vacuum tap (source) in the QJ is above the throttle plate, and to get full advance at idle, you must either connect it to another manifold vacuum source or modify the port in the carburetor as I used to do. The chamber is deep enough to open it up with a little grinding so it is drawing from below the plate. We did it that way because the polution police at the State Inspection knew the trick of connecting to the choke pull off vacuum source with a "T".

Stu fox

[Jim T.]

Re: TCS--What is it's function?

For full time vacuum you could use a tee connected to the vacuum connection on the intake for the vacuum modulator connection of the turbo 400. If your TCS is functioing, changing to full time vacuum should reduce your 454's engine temp in stop and go traffic with or without the A/C on. Back in 1970 my gas mileage in Dallas went from 11 to 14 mpg in city driving with full time vacuum with a 350/300 turbo 400.

[Bill C.]

Re: TCS--What is it's function?

As stated above -- The factory vacuum source is ported. You will not pull any vacuum until you are at RPMs over 1100.

With a working TCS system - this does create an issue (I have this on my car) , in 4th or "D", there may be some surging while driving. This seems to happen becuase the vacuum signal is not strong enough to hold the advance plate in one position.

Only way I have found to fix this is to "T the TCS source into manifild vacuum. Then when you go into 4th/"D" - you get sufficient signal to move the advance plate stable.

Wonder if anyone lse has ever had to do this ????


Bill

[Terry M.]

Re: TCS--What is it's function?

I have never had that issue, but then I don't have a Rochester either.