On a 1971 A body. I trying to understand how the DVS works, so as I understand the key features:

1) In park, at idle, at normal temperatures, it routes ported vacuum to the distributor advance
2) In park, at idle, at elevated temperatures, it routes manifold vacuum to the distributor advance
3) In drive, and only in 3rd gear, it routes ported vacuum to the distributor advance (is this at any temperature?)

If the switch fails does it default to always ported vacuum to the distributor, regardless of gear or temperature?.

Somewhere I saw a diagram of the inside of the switch, just can't find it.

Thanks!

 

There are two separate functions in the valve. It's primary function is to control vacuum to the distributor for the Transmission Controlled Spark emissions system. The electrical connection activates a solenoid in the valve that blocks the vacuum port to the distributor in any forward gear except high (third for the AT). The system is designed to also vent any vacuum already in the vacuum advance system to atmospheric, which is why there is a connection to the top rear of the carb. The nominal source of vacuum for the vacuum advance on these cars is ported vacuum, thus the source connection to the ported vacuum barb on the carb. The thermally controlled part of the valve plays no part in this so long as the engine is not overheating.

If the engine does start to overheat (coolant temp above approx. 218-224 deg F), the thermally controlled part of the valve switches the distributor vacuum source from ported to straight manifold vacuum. This function also overrides the solenoid control from the TCS if that is active. Running full manifold vacuum to the distributor advances timing at idle and low speeds (when overheating in traffic is most likely) which increases idle speed and typically allows the engine to run cooler. Once coolant temp drops back to normal, the valve returns to ported vacuum and the TCS function is again enabled. This is all described in the Chassis Service Manual on page 6C-13 and 6K-9.

 

Thanks Joe, I missed the description on 6K-9.

So if I understand this right, if the solenoid fails, the plunger would stay in the down position, directing vacuum from port to the distributor, unless the car is overheating, going now to manifold vacuum. If the solenoid is unplugged, ported advance can now occur in any gear, unless overridden by the temperature. Page 6K-9 indicates that the plunger would block off the vent if deactivated by the solenoid. My thoughts is that I could keep it visually intact with the hoses, and just "unplug" it; instead of rerouting the advance hose directly to ported vacuum.

Hopefully that makes some sense, if not let me know,

thanks for your help !

 

Yes, that is correct. The dirty little secret here is that if you simply pull the electrical connector off the valve, it disables the TCS system and provides vacuum to the distributor at all times. You can plug it back in for shows with no one the wiser.

 

Interesting side note with this, I put in a new engine harness around 12 years ago or so (It is a 1971 350 with turbo 350). I just recalled the downshift plug for the turbo 400 was taped in and bundled with the starter connectors, so I just tied it up by the starter. At the time I thought the plug was just for the kickdown switch. I now see that this plug was a combination plug with another terminal in it for the TCS. The car had transmission swap out around 20 years ago. I just don't recall unplugging the plug from the transmission and the replacement transmission may not of even had a TCS switch in it; I need to go look.

So all this time it looks like I never had TCS completely hooked up in the first place !

What is a little confusing though is that this plug in the reproduction harness is on the driver side, while the kickdown switch/TCS were on the passenger side of the transmission. I just looked at quickly from under the car, so was hard to tell if you could unravel the tape and get the plug over to the passenger side.

Thanks again for the guidance.....

 

Yes, the harness was built exactly the same no matter which trans was installed. The TH350 switch is on the passenger side.

 

Joe,

I have a 1968 442 with original engine, and I am installing a Sniper EFI system. The EFI requires a temp sensor. I had previously installed a Rally Pack gauge and tach setup, so I removed the original sensor for the temp light and replaced it with the one for the gauge. Now that I’m adding the Sniper system, I find that my only option is to remove the factory vacuum control switch so I can use the Holley EFI sensor. My question is where should I hook up the vacuum advance line from the distributor? The EFI has a ported vacuum and a manifold vacuum port that can be used on the base of the throttle body.

Any suggestions are appreciated.

Thanks,

Bill

 

Hook the vac advance hose to whichever port makes it run better. Sorry, that's not a useful answer, but it is the truth. Not knowing what timing curves are in the distributor makes it impossible to provide an answer. The engine neither knows nor cares whether the advance can is connected to ported or manifold vacuum. All the engine sees is the ignition advance at a given set of operating conditions. The choice of manifold or ported vacuum needs to be made in the context of also selecting the initial timing, the advance curve in the vacuum canister, and the mechanical curve. This is why most EFI systems also include timing control - you can play with this 3D timing map on a laptop instead of having to change springs and weights.

 

Thanks Joe, Looks like trial and error will take a while. I ran about 150 miles with ported vacuum and was seeing issues when transitioning to WOT. Backfired a couple of times through the tail pipe and was bogging down. I have run the manifold vacuum only about 35 miles so far. It backfired through the intake one time. I reset the EFI an loaded the base map before switching it over to manifold vacuum. I am letting it go through the learning process more before I try WOT again.

It seems to accelerate fine using up to half pedal.

Bill

 

at WOT: manifold vacuum = ported vacuum, right?

 

At WOT the engine vacuum (manifold or ported) is zero.

 

Yeah, but he isn't wrong. 0 = 0

Seriously, at anything slightly past idle, once the throttle blades uncover the ports that feed ported vacuum (get it?), the two will read exactly the same.

 

Joe, I love these schematics that you either do or find....you got to be an engineer !

 

Not saying he is wrong. Also ported vacuum can vary with the position of the port in the venturi. I drew this pic from memory out of a Mopar shop manual from the late seventies. The numbers may not be accurate but close enough to illustrate ported vacuum in the venturi.

 

Ported vacuum isn't in the venturi, it is in the throttle body. Venturi vacuum is low at low speeds and increases as airflow increases with throttle opening. Venturi vacuum is primarily used to operate the vacuum secondary diaphragm on Holley carbs. Ported vacuum is zero when the throttle blade is below it, same as manifold vacuum when the throttle blade is above it, and varies in that brief operating condition when the throttle blade is right at the port. This applies to the ported vacuum used for distributor advance. There are other timed ports used for things like EGR valve or evap cannister purge that may be at different locations relative to the throttle blades depending on the factory calibration of those emissions systems.

 

There are other timed ports used for things like EGR valve or evap cannister purge that may be at different locations relative to the throttle blades depending on the factory calibration of those emissions systems.
That is what I was trying to explain and illustrate. Thank you for clarifying.