modulator ?

[Munday]

what does the modulator do in a tranny and how often should it be replaced?

 

[GNeric]

It is the load sensing device in the trans. It uses engine vacuum to raise or lower line pressure with throttle opening. Replace if you notice your fluid level going down and the engine smoking (more than usual). When the diaphragm ruptures, it will suck trans fluid thru the vacuum line.

 

[MeanBuicks]

BTW, since you didn't specify what transmission you're talking about, I thought I'd point out that a 2004R (the most discussed trans in this forum) doesn't have a modulator. It uses a cable for load sensing.

Replacing a modulator on a transmission so equipped usually involves removal of a single bolt that retains a hold-down bracket. Also make sure that the vacuum line leading to the modulator isn't kinked or leaking.