# TCS Trouble (Transmission Controlled Spark)



## Herding Goats (Sep 16, 2017)

Hi All, 
Working on my '70 4-speed car and having trouble getting the TCS system working. The system does seem well documented in the manual. The original M-21 was replaced with a '74 M-20 out of a Grand Prix. The '70 switch was bad so I purchased a replacement from an aftermarket parts supplier but it seems to operate backward of what it should. I have full vacuum advance in N,1st, 2nd, & 3rd and none in 4th. This, of course, causes the engine to race at idle. Is there a possibility that the interlock is different in '74? Does anyone know if the switch should be normally open or normally closed for a '70? My last question is if anyone that has eliminated this system would you recommend I do that. I really wanted to avoid modifying the original carburetor which does not have the appropriate vacuum provision, but given the trouble with the system perhaps now and in the future, it would be one less thing to break and potentially improve performance? Thanks in advance for any input.


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## geeteeohguy (Feb 2, 2008)

The Transmission Controlled Spark was designed as a band aid to control NOx emissions at cruise. By retarding the timing at cruise. While this DID lower the NOx emissions, it also tended to lower fuel economy. I installed a ton of Echlin and STP kits 35-40 years ago when these cars were still in the CA smog program, and the kits eliminated ALL vacuum advance. They gave you a sticker to place on the dash not to exceed 55 mph. Those were the days. Reality today is you should have your car set-up as designed to be legal in CA on the streets. That said, nobody looking under the hood will know the difference. As a licensed Smog Check inspector/adjuster, I cannot recommend removing the part. I can tell you that the car will run better and get better fuel mileage with straight manifold vacuum advance and the switch bypassed. Your car, your choice. Glad mine are '65 and '67's and don't have any of this stuff on them!!!


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## Herding Goats (Sep 16, 2017)

Hi Geeteeohguy, Thanks for the info and feedback. Regarding the elimination of the TCS system, for off-road use only, I believe I would need to find a different carb housing which would allow me to add a ported vacuum connection. Right now with the un-ported manifold vacuum at idle the engine races. I'll need to do some further research on this carb modification but would love to get the TCS system working properly until I can address that. For a system that is either on or off, this TCS sure has me baffled. My understanding of the early (1970) system is that the solenoid is default open and should be powered until the transmission is shifted into 4th gear, blocking all vacuum advance to the distributor from N-3rd. Then providing vac advance only in 4th gear (at cruise). I have verified that my solenoid, which I believe to be original, operates this way. For some reason, I am getting the opposite effect overall. Could the switch be defective? I have seen lots of problems with repro parts so that wouldn't surprise me. I can't imagine that the transmission (which has been changed) would have any internal changes that would cause the switch to operate in reverse? My original switch has failed and does not complete the circuit in any position so I cannot tell how it should work. Anything you see that I may be missing?


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## geeteeohguy (Feb 2, 2008)

The switch could be reversed....any way to connect it differently? How many wires, etc.? As for the ported vacuum, most Pontiacs ran off of manifold vacuum. Some from the carb below the throttle plate, others from a hollow carburetor stud. You want to run manifold vacuum. And lower your idle speed to compensate. You will have a cooler running engine with better throttle response off idle. No need to change out the carb at all!!


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## Herding Goats (Sep 16, 2017)

geeteeohguy said:


> The switch could be reversed....any way to connect it differently? How many wires, etc.? As for the ported vacuum, most Pontiacs ran off of manifold vacuum. Some from the carb below the throttle plate, others from a hollow carburetor stud. You want to run manifold vacuum. And lower your idle speed to compensate. You will have a cooler running engine with better throttle response off idle. No need to change out the carb at all!!


After two builds, finally got this engine running and wanted to share an update. I was unable to get the TCS system working and went with a manifold vacuum source (for off-street use only of course). I installed a vacuum advance corrector (basically a limiter for the stock distributor) by Lars Grimsrud and Henry Olsen which seems to have tamed the high advance I was getting at idle. Thanks for the input along the way!


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## geeteeohguy (Feb 2, 2008)

You can take any information Lars provides to the bank. Glad you got it sorted.


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