Just in the interest of how things work..

What are the pros and cons to using a TV vs. a vacuum modulator? (The 350 and 400 use a vacuum modulator, and the 200-4R and 700-R4 use a TV cable.)

From what I can see the vacuum modulator sounds like a better deal (working off load instead of throttle position) but seems less reliable (vacuum leaks can cause problems, etc.)

Thoughts?

 

Actually the TH350 has a throttle-valve cable AND a vacuum modulator.

 

The primary reason for changing to a TV valve was to eliminate the possibility of engine age and vacuum leaks causing alteration of the shift points. It is also a little more precise in controlling line pressure than a vacuum modulator.

 

Really? My old truck that had a TH350 in it didn't have a TV cable - just a vacuum modulator. How does it have both?

 

The 350 does not have a TV cable, it's only a kickdown cable.

A vacuum modulator is far superior to a TV cable, because it measures the actual thing that the transmission needs to react to, namely engine load. With a TV cable, the transmission has to "guess" what the relationship of throttle position to engine load is.

It always makes sense to measure the thing that actually indicates the condition you want a servo system to respond to, rather than measuring some other quantity that might or might not relate to it. Rather like the technical superiority of using a MAF for measuring air intake volume (literally counts air molecules) rather than speed/density (tries to infer number of air molecules by guessing based on engine vacuum and RPM and throttle position and so forth).

 

The vacuum modulator line goes to a manifold vacuum port, and there's a kickdown cable that goes to the throttle arm.

 

That's what I was getting at. I've been digging into transmissions a bunch now and I wondered why they went to a TV cable instead of a vacuum modulator. Now that I understand what they each do (same thing) I couldn't figure out why they basically went *backwards* on it. (Other than lack of reliability on the modulator, but that kinda failure is rare these days from what I've seen.)

That being said - I wonder if converting (blasphemy, I know) a TV cabled transmission to a vacuum modulator would improve it any? I also have to wonder if that would give it a little more longevity; seeing as how it would apply a more correct amount of pressure versus load...

 

It doesn't behave the way the TV cable on the 700R4 does, but doesn't GM call it a TV cable?