Merry Christmas All!!

I have a 72 coupe with an auto transmission that will not down shift. This is a fresh build with 50 miles on it. Shifts great other than when I go to get on it. I can manually down shift and it works fine. I've confirmed that I have good vacuum to the modulator valve. The modulator valve is new as well. Here is my question. Is there some kind of electrical switch that does this operation? I see there is a 2 prong switch on the Lt side of the trans. I also see there is a switch on the accelerator. Neither are not hooked up currently. Is this the switch or actuator to down shift the transmission? If it is... Is there a separate harness that runs from the accelerator switch to the transmission? I don’t see either switch in my wiring schematic and I don’t have any wire connectors unhooked in the vicinity of the switch on the accelerator. I should tell you that the car has all new wiring as well. Not sure that matters...

On the firewall side, I do have a 2 blade connector with a tan / orange wire that is not hooked to anything. This would be close to the accelerator switch. Again not sure if it actually goes to anything on my car. This aftermarket wire harness seems to have some additional connectors that don’t get used. I don’t know how it would go through the firewall anyways since there is only approx 6-8 inches of wire exposed from the loom or harness itself.

Any guidance you can provide would be awesome!! I just recently got my vette on the road from my lengthy 16 year restoration. The car runs and drives great so far, just need to figure out the transmission issue!!

 

A pretty common problem is the connector not being all the way in on the trans (or falling off completely)
Maybe start there and work your way back.
The switch at the accelerator is fed by the tan (one of them) right back to the main fuse block.
HIH
M

OH
This diagram is from the chassis manual for the kickdown and is confusing since they show two separate tan wires connecting to the TCS solenoid and then connected together. The solenoid connections are both #141, they are actually the same (one single) wire. but that shouldn't affect your kick-down in any way shape or form.
M

 

And of course make sure your go pedal is actually tripping the kickdown switch, (carpet, mats etc bound up under it)
You should actually be able to hear the trans solenoid click (if your right under there with the engine off and someone trips that switch, of course a test light is better but....
M

 

Use an ohmmeter to check switch continuity at the accel pedal. The switch must be good and the pedal must 'trip' it to ON position for the kickdown to work. If you wish to wire it up as was done at the factory, then use the diagram provided above. Otherwise, just feed 12 vdc through that kickdown switch and on to the connection on the side of the trans housing.

Inside the trans (mounted with 2 small bolts to the underside of the valve body [channel plate] assembly), there is a solenoid valve which, when activated, dumps pressure and causes the downshift. That solenoid valve could have failed...or be plugged with 'gunk'; or the feed wiring to the trans or inside it could be bad; or it could just be that it isn't hooked up at the accel pedal.

Good luck with your diagnosis and with your repair.

 

More then likely the kick down switch on the gas pedal linkage is stuck or worn out and not making contact internally. Mine had a bad switch at 130k miles on it. Another time the throttle linkage needed to bent a little to contact the switch arm.

 

Thank you for the schematic!! I do not have the accelerator switch hooked up or the switch at the transmission either. I believe I have located the connectors as shown in the photos below for both. It will be easy to hook up the round connector with orange / black wires to the trans but the flat connector with orange / tan wires is on the engine side of the firewall. Am I missing a jumper harness or something here? Or do I not have the correct connectors? I don't see anything that comes out of the fuse box area that appears correct to attach to the accelerator switch. There is roughly 8 inches of wire or so on the flat connector (orange/tan) but I don’t see where the connector would fit through the firewall. Where would this pass through the firewall? The tan wire has 12v with the key on as well.

 

The bottom two pics are the connector for the transmission

The top pic certainly looks like the one for the switch on the accelerator



But as you say, it's on the wrong side of the firewall.
I'll see if I have any pics of where it came through, it came through the hole right beside the throttle cable.
Do a continuity test between that plug and the one for the trans.

Worse comes to worse, pull the wires out of the flat plug and pass them through the grommet and re-install on the inside

M

 

Strange that on mine the orange wire looks like a larger gage than the tan while on your they look the same...

 

I confirmed that the orange wires in both connectors are on the same circuit via a continuity test this afternoon. It looks like I found my issue here. I plan to work on the car this weekend and see how it goes. Hopefully both switches are good!

As for the different gauge wire, I imagine there is some difference as I have replaced every wire harness in the car.

Thanks for the help!!