C-3 Spark Control Solenoid

[D S.]

What exactly does the spark control solenoid do? I have a '70 LS-5 with TH400. The carburetor is a rebuilt 7040204. I assume there's a relay on the transmission and two wires from it goes to the solenoid which mounts on the intake manifold in front of the carburetor. The solenoid has three ports, one in and two out. I think I know where the 'in' comes from and where one of the 'outs' goes but according to my assembly manual one 'out' goes to the top rear (fresh air/no vaccuum?) port of my Q-Jet but I don't have a tube there. There is a small hole. Can someone enlighten me on this?

Thanks,
Scott

[Jack H.]

Re: C-3 Spark Control Solenoid

This is the TCS (transmission controlled spark) system. There's a SWITCH in the transmission that's triggered when the MT is shifted into either high gear position (3rd or 4th). There's a control relay that monitors the switch and 'fires' the TCS solenoid according to what gear the car is in along with other considerations (a little simplification here).

When the TCS solenoid is 'fired' it passes vac from the carb/intake to the distributor to activate spark vacuum adance (high gear and highway cruising) and when the relay doesn't fire the solenoid, vac is denied to the distributor and it runs solely off its mechanical/centrifical advance control.

The idea is to reduce engine RPM and retard the spark for more complete combustion during engine idle. System presumes the car is used primarily for acceleration to highway cruising speed through 1st and 2nd gear and therefore doesn't need vac advance as the centrifical advance dominates the timing map.

Bottom line, there are FOUR components in the system:

(1) Tranny switch
(2) Control relay
(3) Vac solenoid
(4) Wiring harness

Oh, there are three ports on the vac solenoid. One is the input path from the carb/intake, the other is the output path to the distributor and the third provides a vent to the distributor to allow vac stored in the advance canister to escape.

[Cary Lewis]

Re: C-3 Spark Control Solenoid

Jack I'm not trying to hijack the thread here but you seem to know a lot about these solenoids and I have a problem hooking mine up on my '74 LS4 4 speed car. I was able to find a NOS solenoid but my assembly manual shows a single port fitting out of the intake maniflod behind the carb in one location and a double in another. My car has a single and it is connected to the line that goes to the filter and then to the splitter and so on. My vacuum advance from the distributor attaches to a connection in the passenger side of the carb. My new solenoid has 2 ports, one small and one larger. What connects where?
Thanks for any help that you may provide!
Cary Lewis

[D S.]

Re: C-3 Spark Control Solenoid

Does this apply to TH400s as well?

[Tony H.]

Re: C-3 Spark Control Solenoid

Carl,
Here's a link to the set up I found on Dr Rebuild's website. What you describe appears to match the configuration detailed in Doc's catalog. I love Doc's catalog. It's the best in my opinion, chock full of information.
Tony




74 LS4 Manual Carb vacuum lines

[Cary Lewis]

Re: C-3 Spark Control Solenoid

Tony I looked at that but if you do that where does the line that goes to the filter and then to the splitter go, it is now hooked to the single fitting coming out of the intake? Would it then go to the passenger side of the carb where the distributor advance line is now located?
Cary Lewis

[Robert E.]

Re: C-3 Spark Control Solenoid

Tony,

Did you get that picture on Doc's website to load? I've never had any success with his safeimage pictures loading.

Robert

[Tony H.]

Re: C-3 Spark Control Solenoid

Yes, I got it to load. At the front page of docrebuild.com, there is a note that says to add www. to the link to get the picture to load. Did the picture load for you when you pressed on the link I left in the above message? It should have. See where the www. is added. Compare that link to the one in Doc's website. Add www. right after the http:/

[Tony H.]

Re: C-3 Spark Control Solenoid

Cary,
I see your point. According the diagram on the link I attached, it doesn't show the filter. I believe there should be a filter in place coming off the manifold. I would have to look into it further. I would have to guess at this point. Some more knowledgeable would have to help you here. If I find something, I'll let you know.

[Dave S.]

TH400 - Reverse only

I believe the TH 400 engages the TCS system in reverse only. The 4 speed cars engage the system in 3rd and/or 4th depending on the year. The chassis manual gets real specific on this function. It's a better source of information than the NCRS TIM & JG or NCRS 68-72 Operations manual in my opinion.

[Jack H.]

Re: C-3 Spark Control Solenoid

Can't comment on the way the TCS 'plumbing' was installed in '74 as I don't have an AIM to take a peek at. But, the earlier cars derived source vac from a port on the carb and not from the intake's main tap... If that continued to be the case into '74, you ought to see the line being connected in the AIM on the page in UPC-6 where the carb is being installed to the intake.

[Terry M.]

Re: C-3 Spark Control Solenoid

Cary,
I can't comment on the specific vacuum hook up details, but the one 1974 we had occasion to work on had a Temperature Control Spark. There was no connection to the transmission -- it worked on temperature only. That was a TH400, however. We figured it out using the Chassis Service Manual. In that case the vacuum hook up was not an issue -- we had to figure out how it worked for performance verification.

[Michael H.]

Re: TH400 - Reverse only

Dave,

Not 100% sure about this but I seem to remember the TCS applying full vac adv for reverse and high. (3rd gear) I think the switch was activated by direct clutch/accumulator apply oil which would have been engaged in either/both of those ranges. If I hapen to run across my TH400 oil schematic, I'll check it out.

[Dave S.]

Re: TH400 - Reverse only-I stand corrected

Michael,

You are right and I was wrong. Jack H. properly corrected me on this and provided me the write up paragraph from the Chassis manual. I learned something today however illogical it seems. The NCRS Operations manual mentions reverse only as that is the only practical way to test the fumction.

[Michael H.]

Re: TH400 - Reverse only-I stand corrected

Good point on the test/judging procedure. I suppose trying to get the car into 3rd speed on the show field would be a bit impractical.

Tapping into the direct clutch oil circuit to signal the TCS was the most logical approach but it also included reverse as that clutch is in operation at that time.