Air Line to Trans Modulator

[ChuckMC8]

Air Throttle Installation, continued: 
   Got my air trans modulator and it has a fitting for 1/4" air line. My main air line from treadle to the governor is 3/8" per specs- From my air line "tee", does the branch line down to the air modulator require downsizing to the 1/4" line? thnks

[JackConrad]

Chuck,
  When I ran the line to my modulator I used 3/8".  Same as from the throttle pedal and to the actuator on the govenor.  If I remember correctly, my modulator had a 1/4" FPT (female pipe thread) in it. I purchased a fitting that was 1/4" MPT (male pipe thread) X 3/8" compression. Make sure the 2 lines from the TEE to the actuator and modulator are the same length.  Jack

[ChuckMC8]

Thanks Jack. My modulator is WM-775-B from MCI......the instructions/service bulliten that came with the modulator isnt very clear. I'm wondering if I have the wrong modulator. It does say on the enclosed paperwork that it is supposed to have a 1/4" ID line to the modulator....but the modulator has the fitting that would take a 1/4" OD...a size smaller than the 3/8". On top of that, the threads in th end of the modulator is 1/8" pipe, rather than the 1/4" PT that Jack describes in his. The paperwork indicates that this is the correct part # for the HT74o trans.
   So....I called the tech support at Williams (The MFG) and the guy was no help at all. He said that all the piping needs to be 1/4". So I asked him if that was ID or OD and he didn't know........gosh. Whaddya do then? Just thank the man and hang up.
    Maybe my modulator was incorectly packaged. I can adapt the 3/8 line to the 1/4 at the modulator if thats the way its suppose to work, but that doesnt seem right.

[Stan]

I don't understand the need for same size or same length of line to the actuator and the modulator.  The volume of the actuator and modulator will have more effect on timing than the size of the line, or do these units have matched volume and equal travel?

The normal practice in air control systems is to use large lines when you need a large volume of air and use a small line when you want a rapid pressure change with small volume.