1975-77 Tech Manual & TCS

[Tom R.]

Did TCS (transmission control spark) take a hiatus from 1975-77 or is my Summer 2000, 2nd edition of the 75-77 tech manual missing this section?

[Paul B.]

Re: 1975-77 Tech Manual & TCS

Tom, right off of my head I do not think with HEI, Catalytic Converters & EFE(plus A.I.R. in some instances), I don't believe there was TCS(temperature OR transmission)contolled spark in these years.

[Duke W.]

Re: 1975-77 Tech Manual & TCS

Prior to catalysts, "engine out" emissions had to be controlled to achieve the test bogeys. With catalysts, engine out emissions could be higher even though test bogeys were lower, so more conventional and efficient ignition and fuel calibrations were possible. The result was generally higher fuel economy and better driveability, but peak power was down due to the additional exhaust restriction added by early catalyst designs and GM's decision to use only one even on performance oriented cars like Corvette.

Duke

[Joe L.]

Re: 1975-77 Tech Manual & TCS

Tom-----

TCS didn't take an "hiatus" for 75-77, it was "terminated" for good. The last year for TCS was 1974.

[Tom R.]

Re: 1975-77 Tech Manual & TCS

Joe:

The 78-79 and 80-82 tech manual references TCS for some L48 applications. Not that tech manuals are always reflective of what rolled off the factory floor. I've got an L48 but with only TVS which ports to the EFE actuator. Another ports to the EGR valve and Canister. It seems like I've seen two...color coded (off thermostat housing) but can't recall where they went...perhaps it was EGR? But if not EFE and TCS...then what?

The 78 AIM references a 1242101 switch on 7A page 14...but then the sheet shows a cancel. Not sure is this is switch or not.

[Joe L.]

Re: 1975-77 Tech Manual & TCS

Tom----

I don't think that the 7A, sheet 14 involves anything to do with TCS.

For TCS to exist, there has to be some sort of transmission or linkage mounted switch or relay. None were used after 1974 so I don't see how TCS could have been used.

[Tom R.]

Re: 1975-77 Tech Manual & TCS

I have seen this before with our late model manuals...and probably equally so for earlier manuals but systems are included on the basis of conjecture. Judges overlook it because it does not get seen on the judging field and somewhere along the way...a member gets motivated to find resolution to the quirk and finally the recommendation gets attention, earns credibility and gets adopted. This may be the case for TCS. But tell me what sources are you referencing that indicate it did not exist from 75 up?

[Joe L.]

Re: 1975-77 Tech Manual & TCS

Tom-----

I can't find reference to TCS or related parts in selected 75+ Chevrolet Service Manuals, I can find no reference to it in selected 75+ AIMs, and I can find no record of any SERVICE parts for components of such a system beyond the 1974 model year.

[Tom R.]

Re: In other words

Once catalysts and converters were introduced...the emission technology landscape changed. Exhaust emissions could be increased through richer carb settings and higher timing to improve fuel economy and driverability. This would have increased performance but offset by the effect of the converter which restricted exhaust flow and therefore performance.

So why did Corvette not see ThermAC until 76 when it was first introduced in 1968? From what I've seen, GM "tinkered" until converters were available to take on NOx in 73 and in 75 with EGR and then catalysts. I realize from 73 through 75 Corvette introduced (or a return) to cowl induction.

[Duke W.]

Re: In other words

Correct. I'm not familiar with the specific use of heated inlet air (I think GM refers to this as Controlled Combustion System.) on Corvette, but as least for the base L-48 engine, Corvettes could have higher proportional emissions than the other cars that used the same basic engine because Corvettes were lighter and required less power to run through the test where absolute emissions were recorded in grams per mile.

The cold air induction only became active at WOT, and the certification test did not require the use of WOT, so WOT emissions were not measured and had no effect on certification.

The first catalysts were "two-way" type that only oxidized HC and CO. NOx had to be controlled by timing and EGR as required for each specific application. Three way catalysts that added NOx reduction weren't used until 02 sensors with feedback air-fuel ratio control systems to maintain stoichiometric mixture were added as TWCs require the exhaust gas compostion of a stoich. mixture to both oxidze and reduce.

Duke

[Tom R.]

Re: In other words

So 81's were our (Corvettes) first TWC converter?

Thanks...insightful thread on these late model C3 emissions.

[Duke W.]

Re: In other words

I'm not sure when GM began using a three way catalyst, but it would have corresponded with the use of "electronic feedback" carburetors and O2 sensors.

You can tell what kind of catalyst you have by looking at the emission control codes on the tuneup/emission decal in the engine compartment. "OC" is oxidizing catalyst. "TWC" means a three way catalyst that oxidizes HC and CO and reduces NOx.

Duke