So I ran across this old thread and discovered that my new to me 74 small block has this system. the old thread makes mention that there is a switch on the accelerator pedal under the dash that activates a flapper on the hood and it also makes the transmission down shift at WOT.

Other than driving it, can anyone tell me the best way to test this system, and if it's not working, is it worth fixing? The car currently has the rubber seal between the air filter housing and the hood removed, actually it's missing.

https://www.corvetteforum.com/forums...-system-3.html

 

I have a 75 and would be interested in hearing how to test also and maybe pictures of it. I guess it is under the area on the underside of the hood where the bolts are.

 

Here's a good description.
To see if yours works open the hood and with the key on press down on the accelerator pedal and see if the flap under the hood opens.

http://www.73spot.com/index_stats_files/cowlintake.htm

 

Thanks Peterbuilt

 

I have a 1975 Corvette, that should have the 'Ram Air' or 'Cowl Induction' option. At least it has the air filter housing with the rubber seal on the top. There is a grate at the top of the hood, but I don't see any sign of a switch under the accelerator, or a 'butterfly' anywhere under the hood that opens (aside from the carburetor of course). Is it likely that the air cleaner was taken from another Corvette with the option, or is there a way to alter the current system to conform with the option? Thanks for any help. Seff

 

The switch is NOT under your accelerator pedal..it is attached to it. and when you press your accelerator pedal..the metal rod pressed against the arm of the switch that is mounted on the accelerator mounting assembly.

IF your flapper assembly is not there...BUT the air cleaner and rubber seal is there. Your engine is getting fresh air from the cowl all the time. Which is NOT a bad thing.

IF you put everything back as like the factory designed...like 'Peterbuilt' wrote...the ONLY time the flapper will open is when you have your foot planted to the floor to make the switch make the flapper solenoid open it.

DUB

 

The fresh air set up was standard, all 73 thru 75's had it.

 

If anyone is interested I have one of these switches NOS never used. PM me.

 

So the only difference with the solenoid and the door would be you would get that kick when you hit the switch, instead of a gradual curve of added power. I can live with that. Thanks DUB.

 

Find a 12v source and ground under the hood and test the solenoid that way. That's how I found mine wasn't working.

 

YES...the solenoid is either OPEN or not. There is no gradual curve of it opening in relation to your accelerator pedal position.

If my memory serves me correctly..the switch that is mounted on the accelerator pedal assembly is the same switch used for an automatic for the kickdown. So on an automatic car the switch serves two purposes.

DUB

 

That switch also causes the THM-400 transmission to downshift. There is a (12vdc) solenoid valve mounted to the valve body channel plate in the tranny sump. When that switch is tripped, the ram air door opens AND 12vdc is sent to the trans (via a plug in the left side of the trans case) so that the tranny downshifts. Even if the ram air door is always open (the way it is now), the trans won't downshift at WOT without that switch (or a substitute added to the accelerator pedal linkage).

 

That's got me thinking.

I have a 75 auto and I recently noticed that there was a strange wire running down my hood to a plug in front of the radiator that wasn't plugged into anything. Seems that's my Cowl Induction. Peering into the hood intake, it looks like my ram air door is closed, so I'm guessing I never get the extra air or tranny downshift at WOT.

To be far, I rarely go WOT, but on the occasions I have I've definitely reached a dead zone around 3000rpm and surging that might be the tranny topping out.

Thoughts?

 

Just dug around and found the fwd lamp wiring harness the induction cowl solenoid is supposed to be connected to. Hooked it up and what do you know, IT WORKS. My induction flap opens at WOT as it's supposed to.

I love this forum... I went a year of ownership without even knowing that existed, and had it not been mentioned here I may have gone many more years with it never working!

 

Does you transmission downshift when you plant the accelerator pedal to the carpet???

AS you might have figured out...the switch on your accelerator pedal assembly as I wrote has a double duty....so you air flap is working (GREAT!) ...but that does not mean that your transmission is kicking down when requested.

I will not 'say' that this 'topping out' is directly due to the transmission. Other factors can cause this....AND

If you want to get this issue repaired...you need to start a thread about it...so we do not hi-jack this thread.

DUB

 

Thanks DUB,

Yes you're right of course, two separate functions. I don't want to hi-jack the thread, but it does seem the OP is done with it and I can't see the harm in adding more information to it (relevant to cowl induction system). But yes, if it's not the accelerator switch - new thread time.

As the OP asked, what is the best way to test the downshift function is working from the garage? Perhaps find that plug on the side of the tranny and run a 12v test light to it?

 

Yes. Use a test light or multimeter to see if 12vdc signal is reaching the wire going into the side of the transmission. That will tell you that the downshift switch and wiring is working as expected.

But, if the downshift function still does not work, the problem will be that the solenoid switch on the valve body is either plugged up with gunk or is just defective. You can buy replacements for those solenoids for significant money. (I remember when GM used those switches for another purpose and the actual cost was about $1.75 each!)

 

YEP...you were SPOT ON..the wire going to the transmission as mentioned can be checked with a voltmeter. I do not like to use a test light because it can light up even if LESS voltage is going to the end of the wire and give a false result. But a voltmeter tell you actually how many volts are getting to the wire end.

And one TIP. IF a person is checking this with floor mats in..take them out. IF it still does not work...BUT you can reach up and manually move the slider on the switch and get it to work. I have cut a section of 3/8" fuel hose and slid it on the portion of the switch that contacts the accelerator pedal arm and moves...thus...giving the switch more 'range'.

Also...and many of you MAY be amazed....if you remove your air cleaner and press the accelerator pedal ALL THE WAY to the floor...and look at the ending position angle of the linkage on the carburetor....and then...manually go out and rotate the linkage on the carb so you go to FULL THROTTLE...you might find that your accelerator pedal...even though it is on the floor..it is NOT allowing the carb to be at FULL throttle.

I can not tell you HOW MANY accelerator pedal arms I take out and adjust to get the accelerator pedal to go to FULL THROTTLE....and I do this ALSO if they have floor mats.

And often times...I disconnect the accelerator cable at the carb to make sure it is not hindering the carb from being able to go to full throttle and adjust/modify accordingly.

DUB