Anyone out there with a Th400 provide me with the top speed at which they can get a 3rd to 2nd downshift? What is the top speed for a 2nd to 1st downshift? Seems like I can only get a 3-2 at about 35-40. Is there any adjustments to the modulator or trans to increase it. I already reset the detent switch by the accelerator pedal.

 

one time i did it at 65. it will do it without me down shifting at around 60mph with no problems yet.
mike

 

I know you reset the switch, but are you sure you're getting 12V to the plug on the tranny when it's activated?

 

I have the same problem. There are two ways for the TH400 to downshift. The first I believe is based on vaccum and works at 35 to 40 even if the switch by the gas pedal is disconnected. The second uses the switch under the gas pedel and at wide open throttle it activates a solenoid in the transmission.

I don't remember where I found this but here is more detail.

On the Th400 the vacuum modulator will cause a kickdown on part throttle. The electric pedal detent switch will cause a kickdown only on full throttle if its working properly. Heres how you can checkyour pedal detent kickdown.
1. If your electric detent is working you could cruise at 35mph in 3 gear, then punch it, and the tranny will drop to 1st gear.
2. If the detent is not working, cruising at 35mph, punch it,
the tranny will only drop to 2nd.
3. On the highway at 55 or 60mph cruising in third gear, If the detent works, punch it, it will drop to second.
4. If the detent is not working, it may not downshift to second at 55 or 60mph.

The way the detent works is: When you floor the throttle, the detent switch under the gas pedal opens the circuit to the solenoid inside the tranny pan. This switch also opens the cowl in the hood. So if your's is connected you can hear the cowl open when you depress the switch.

Heres a way to check the switch. First disconnect the the single wire connector where it goes into the trans. Clip one end of a testlight to ground and put the other into the harness connection. Turn the ignition key to the on posistion. Deperss the gas pedal hard to the floor. If the test light lights the pedal switch is working and the problem most likely will lie with the solenoid mounted inside the trans pan. If the test light does not light, move the test light up to the pedal switch to check for voltage there. The key must be in the run posistion for power to be at this switch. I had to wrap black electrical tape around the pin on my pedal switch to get it to activate because there was too much play in the switch. It would only activate when the pedal was pushed very very hard. By enlarging the pin with tape, the switch would connect without such a hard effort. I also needed to replace the solenoid in the trans as well.

 

Ditto..eddadams has it correct. He explains in good detail how the whole system works. Modulator alone will cause downshifts but not at higher speeds. I also had to replace the detent solonoid inside the trans pan when I was still running the TH400. Also, rear end gear ratio will effect the speed at which the tranny will kick down. The lower ratios such as the 4.11 will likely kick down at lower speeds only compared to that of a car equipped with 3.08 rear which would still allow a kick down at a higher speed.

 

Thanks for all the input. After performing your tests, I found the solenoid to be inoperative. So, I dropped the trans pan and removed the solenoid. From the outside it looked fine. However, upon removal of the metal cover, I noticed the wire insulation to be cracked. I was grounding against the metal base. I just ordered a new one from a local chevy dealer $35. Probably could have tried to rewire the old one, but why take the chance to have to remove the trans cover again. Thanks again for the input. :) :)