The Th-425 is the transaxle derived from the TH-400 rear wheel drive components and is quite robust.

It was replaced(not renamed) by the TH-325 in 1978-up model cars. It is a much weaker transaxle based off the light duty TH-200 rear wheel drive components.

The 425 has a vacuum modulator but no detent (kickdown) cable....forced downshifts for passing gear are triggered by an electric solenoid.

The 325 has both a detent cable for forced downshift and a vacuum modulator.

Neither trans has what you referred to as a TV (Throttle Valve) cable.....both of these transaxles do not have a need for constant throttle position monitoring like the later overdrive 325-4L which does have a TV cable.

As far as I know, there is very little in the way of HD or Performance upgrade parts for the 325 series.

 

You sure the th325 doesn’t have a tv cable?

 

Like ericnova said the 325 was based on the thm200 trans and evolved into the th325 4L witch is a 4speed auto. My memory may be fading, but don’t recall the 325 having a modulator. Just a TV cable like the 200 and 700 transmissions

 

3W Coupe...you are likely right about the 325 not having a vacuum modular. I am much more familiar with the 400 based 425 axle.
Hopefully someone also familiar with the 325 can come in and say for sure.

 

The TH325 and TH325-4L use TV cables. I have the factory service manuals that cover these transmissions. GM moved away from vacuum modulators in the late 1970s because they were developing diesels and turbo motors that needed a different way to control the trans. The 325 is completely different from the 425 in addition to being significantly weaker. The 325-4L OD clutch pack is behind the drive chain, so there's a wart on the chain cover to accommodate it.