1974 TCS Configuration

[Paul L.]

This post is similar to the recent one on the 1973 BB transmission controlled spark (TCS) system. The TCS on my 1974 L-48 TH400 has been removed (I have it) but I have recently found an NOS #1997425 solenoid switch, on its way. I have looked through the AIM and at Dr. Rebuild's diagram but it is not clear to me how this is set up. There are three vacuum spouts along with the electrical connection (still intact and tucked away).

It seems that one spout is for the vacuum advance, one for ported vacuum on the carb (driver's front), and one for manifold vacuum (passenger side) on the carb. I have a "new" #7044206 carb. A mix of ported and manifold vacuum confuses me.

Does anyone have a diagram or picture of the setup?

This is the old solenoid.



This is the "new" carb. You can see manifold vacuum near the choke and ported vacuum on the driver's side higher up.

[Edward J.]

Re: 1974 TCS Configuration

Paul, my 72 L48 auto car appears to look as yours, one port to the pass. side of carb. and the other to the vaccum advance. the other is just a vent.(The vent is the one with the short piece of hose)

[Paul L.]

Re: 1974 TCS Configuration

Paul, my 72 L48 auto car appears to look as yours, one port to the pass. side of carb. and the other to the vaccum advance. the other is just a vent.(The vent is the one with the short piece of hose)

Thanks Ed,

I took that unit off two years ago and went to straight manifold vacuum (that's the choke area spout). But I am trying to get back to original.

That short hose on the right side of my old unit is just plugged with Goop? Not even vented. I am having a hard time understanding this system.

If you look at this diagram it suggests that one end of the solenoid hooks up to manifold vacuum and the other to ported? That is hinted at in Dr. Rebuild's diagram as well.



The car runs well on manifold vacuum with a slight off-idle stumble. I am tempted to just install the solenoid with blocks (Joe Lucia's BBs). The Smog Patrol here looks for the visuals but does not sniff.

Does anyone know how this system connects and works?

[Paul L.]

Re: 1974 TCS Configuration

I don't like TTTs but does anyone have a clear diagram or pic for me? There must be someone with a thoroughly stock 1974 L-48.

[Robert S.]

Re: 1974 TCS Configuration

I read my 1974 Chevrolet Service Manual and as best I can tell from the system description it is as Edward Johnson's reply suggests. Ports are for full vacuum, connection to the vacuum advance unit and the third is just a vent with some sort of filter. I have a 74' with what I believe is the original TCS solenoid but none of the hoses are present (system was bypassed by a previous owner - no surprise). Hope this helps.

[Paul L.]

Re: 1974 TCS Configuration

There is no filter on that single side spout. That short hose is just a plug. But I'll keep plugging away at this and write to GM Engineering if necessary. I've tried also at CorvetteForum but the replies are few in number as well.

[Harmon C.]

Re: 1974 TCS Configuration

Paul
If you are leaving the plate under the carb. the solenoid will be under it. The picture of the solenoid you posted the short hose is to the carb. the long hose to the vacuum advance. Look at the TCS solenoid as a switch that opens the two ports on the end so you have vacuum flow.If you are not going for judging leave it off. The metal tube at the rear has no hose and could have a filter. The picture above shows this.

[Paul L.]

Re: 1974 TCS Configuration

Paul
If you are leaving the plate under the carb. the solenoid will be under it. The picture of the solenoid you posted the short hose is to the carb. the long hose to the vacuum advance. Look at the TCS solenoid as a switch that opens the two ports on the end so you have vacuum flow.If you are not going for judging leave it off. The metal tube at the rear has no hose and could have a filter. The picture above shows this.

Thanks Lyle,

While I have a new #1997425 (1974 L-48, TH400) solenoid on order I will install it and probably just "dummy" it up. Then go with/keep my current vacuum advance to manifold advance setup. It works well.

The Smog Police here just look for appearance. And being 25-year-olds they don't really know what they are looking at. But they can impose $365.00 fines if visually all is not right. I've basically given up on trying to understand the TCS system.

I'll post on 1974 AIR pumps next.

[Edward J.]

Re: 1974 TCS Configuration

Paul, In the diagram 3.93 it shows the filter, it is generally just a piece of rectangle gray foam that just keeps the mositure and debris from entering the solonoid. when the solonoid is turned of the vaccum needs to vent the solonoid, other wise the vaccum advance is staying applyed.

Hope this makes since to you. Ed

[Michael W.]

Re: 1974 TCS Configuration

I think Paul has got a good point. The TCS for a '74 L48/THM is different than that used on a 4 speed. The former takes a 3 port solenoid, the latter a 2 port.

Doc Rebuild's catalogue shows three tubes going to the THM solenoid, as compared to two on the 4 speed version. Unfortunately, the drawings are not clear enough to determine where the mysterious third tube leads to.

I'm stumped.

[Paul L.]

Re: 1974 TCS Configuration

Mike,

The best I can figure from his diagram is that the third hose goes to ported vacuum on the driver's side. So we seem to have vacuum advance connected, manifold vacuum near choke assy connected, and ported as well. How this all comes together is beyond me.....In April I will try that and see what happens.

The solenoids for four-speed and automatic SBs are different. The former is GM #1997410 and the latter #1997425. I have ordered the ...425 version.

[Harmon C.]

Re: 1974 TCS Configuration

I think most of the difference of the part numbers is the bracket. I have had the bracket removed and some have a different part number stamped on the inside of the bracket.

[Paul L.]

Re: 1974 TCS Configuration

I think most of the difference of the part numbers is the bracket. I have had the bracket removed and some have a different part number stamped on the inside of the bracket.

Lyle,

I have to disagree. The bracket just mounts to the intake manifold bolt whether four-speed or automatic. And the bracket in integral to the "innards". I'll pull my old one apart and we can see what is inside. I'll post pics.

This subject is taking on a life of its own.

*Edit* - These are the innards of a 1974 TCS solenoid for a SB TH400. I have no idea what I am looking at.

[Harmon C.]

Re: 1974 TCS Configuration

GM made lots of part numbers of the TCS solinoid. I have had a few apart and when you reverse the bracket it makes another part that fits a different car. Not all brackets have two part numbers. The round metal piece with tapered ends changes the vacuum flow to the two plastic ports. If I found a nice donor TCS solinoid the only thing I had to do was change the bracket off the original part to the donor.

[Robert S.]

Re: 1974 TCS Configuration

In reference to my previous reply, I went back and reread my 1974 Chevrolet Service Manual regarding the TCS system. I think I found the answer to the confusion. The Service Manual discusses, in some detail, the theory of operation and troubleshooting of the TCS system. The TCS system has a TCS solenoid with two ports and a vent with a filter. The Service Manual says this system is used only on manual transmission Corvettes.
The Service Manual says that automatic transmission Corvettes use a Thermo-Override system. The Service Manual does not discuss, at all, how the Thermo-Override system is intended to work and provides no troubleshooting information. The Service Manual, however, does show a picture of the Thermo-Override solenoid and the three attached hoses. The hoses in the picture are labeled: 1)full vacuum, 2) ported vacuum and 3) the hose going to the vacuum advance unit.
The above also agrees with what is shown for hose routing in the DR. Rebuild diagrams.

Bottom line: Two hoses on manual transmission solenoids and three on automatic transmissions solenoids.