I have a 71 convertible L48 w/TH400 and factory AC. The TCS system has long since been bypassed - the vacuum advance is hard plumbed from manifold vacuum to the distributor. But, I recently rebuilt the AC with a stock rebuilt A6 compressor and installed a POA "upgrade" which replaces it with a pressure switch that shuts off the compressor at low suction pressure. The A6 is pretty inefficient and has a significant power draw, so when it cycles it markedly effects the idle speed and even low speed cruising. I am wanting to repurpose the TCS throttle solenoid as a compressor-on boost to the Q-jet throttle opening to offset the compressor drag. From what I can figure out from the assembly manual and service manual, and checked with my multimeter, it seems the hot lead to the TCS throttle solenoid (brown on my car) is hot all of the time - it is the ground (dark blue) that controls the function of the TCS throttle solenoid through a series of relays and switches. I am thinking I can replace the brown lead to the TCS with a hot wire from the compressor lead downstream of the POA-valve-upgrade pressure switch, then rewire the TCS solenoid dark blue ground lead to the existing external ground connected to the TCS "reversing" solenoid, thus bypassing all of the convoluted TCS circuitry.

At the same time, I am wanting to install a Pertronix system to replace the points on this car. I have had that on my 65 L75/M20 and it has been flawless for 15 years, but while the 65 has an external ballast resistor that is easily bypassed to get a full 12+V to the coil and Pertronix module, the 71 has a resistor wire and I was having a hard time finding a ready source of full 12-V ignition-on-only power for it. I am thinking I can repurpose the ignition-on 12+V wire from the TCS solenoid brown wire to instead go to the coil and Pertronix module and solve two issues at once.

Does anyone see any pitfalls with my plan?

 

I would just disconnect the resistance wire at the connector in the firewall and replace it with a new wire. Lectric Limited makes an HEI wire that can easily be popped into place do proved the 12V IGN source.

 

Thanks. I had seen that as an option in an older Forum post, but I was hesitant to tear into that bundle of 50-yr-old wires exiting a 50-yr-old bulkhead connector, particularly as hard as that real estate is to get to with power brakes and no lift in my garage. I have been thinking about replacing the engine wiring harness, as all of those 50-yr-old wires and insulation are all getting old and brittle. I will probably go the route you recommend if I replace the harness.

 

TCS is transmission controlled spark. Are you talking about the anti dieseling solenoid on the carb?
If so, I just spliced a wire onto the power wire to the compressor clutch. Clutch on, solenoid on. I still have to blip the throttle out of idle but once the solenoid is out it will hold the idle at what ever I adjusted it to.
I still have the old style stock POA valve so it stays on until I shut it off.
I'm actually using the old beige solenoid wire for my electric choke.

 

Just tap in to the resistor wire with another piece of 'standard' wire where it leaves the firewall bulkhead connector. Wrap up the resistor wire and tie it to the firewall for 'posterity', or cut it off, if you wish. That modified wire will give you a straight 12vdc to your ignition system.
BTW, make sure the Pertronix system you select does not REQUIRE the stock resistor wire. Some do; some don't. Also, I had a Pertronix system that worked flawlessly...until it didn't. When they fail (and they ALL fail), it will leave you stranded at the side of the road because there is ZERO opportunity to "limp home", like a points system will provide. Just sayin'....

 

On my 71 I ran a new wire from IGN terminal in fuse box to the Pertronix ignition setup and flame thrower coil via the multi hole rubber grommet in firewall. Has worked fine for many years. I also have it switched so i can turn engine over without ignition to prime oil, also acts as crude anti theft as well.

A 71 will have a one year only CEC system not TCS but probably runs in a similar way to control advance vacuum. Have you checked if the CEC wiring is live during cranking ?

There has been lots of threads and NCRS docs on it but not all explained it well enough (in my mind) so I had an friend of mine help trace and determine how it worked and help me document it all many years back but cant recall if its live during cranking.

This was the final outcome



The full discussion is on

http://forums.corvetteforum.com/c3-t...id-wiring.html

 

Thanks all for the replies. To F4Gary, what you did with the carb solenoid is what I had in mind. I went ahead and did that this afternoon and it seems to work fine. I am debating whether to keep the solenoid energized whenever the AC is "on" or only when the compressor clutch is energized - there are pros and cons when idling in gear with the AC on, like at a traffic light. I was able to adjust the solenoid so the compressor had no effect on idle speed when I turned off the AC while idling in drive.

So I used the term "TCS" rather than CEC, but on the 71 the CEC is basically Transmission Controlled Spark. The Service Manual calls the system CEC while the Assembly Manual calls it TCS when showing the carb solenoid, trans switch, etc, . The carb solenoid is an integral part of the CEC/TCS, as it controls the vacuum supply to the distributor advance (as installed, probably not on many 71s on the road today; mine gets full manifold vacuum all the time). The increased idle speed with the solenoid energized was predominantly to reduce hydrocarbon emissions when coasting in high gear. But, it works great to increase the throttle opening when the power-hungry A6 compressor is engaged.

CraigH and 7T1vette, I like your two ideas for power to the Pertronix. After reconsidering the ampacity required, I think it will be too much for the 20 ga. now-unused power wire to the TCS solenoid. My coil measures 1.6 ohms resistance, so that is about 4 ohms at ~6 volts from the resistance wire, so that would double to 8 ohms with ~13.5 V to it. I think I need to find another power source. The brown/white wire to the TCS time delay relay looks to be about 16 ga., but I might try your ideas for tapping in around the fuse block or bulkhead. As far as power during cranking from the TCS wires, that should not be an issue if I keep the wire from the starter solenoid going to the coil - that is a full 12+ volts and only energized while cranking.

CraigH, your diagram looks about right, but I did not rigorously trace all of the lines against my understanding. I agree that the Service Manual, Assembly Manual and what I see on my car are not in total agreement. The temperature switch remains a mystery to me - I roughly understand its function as a low-temp override, but my car is missing the pigtail for that so I cannot rigorously trace it.

Yes, I agree there is a risk when the Pertronix eventually fails. But if and when that happens, that is what my Hagerty Drivers' Club towing coverage is for!

Thanks
GaryB65