I have a 91 chevy sportside with a 350 engine. the tranny is a 5 speed w/OD automatic. I don't know what type exactly. I want to know what does the transmission modulator do and how do you know its time to replace.

 

It modulates the shift.
If vehicle is not shifting when it's supposed to then it may be time to check it out.Not just relace it.

 

As I recall, this is a 700R4 transmission, which is a 4 speed overdrive. It doesn't use a modulator. There is no 5 speed automatic in this vintage truck

The TV (throttle valve) cable adjustment is critical for good shifting. If it's off, the transmission becomes lunch in a heartbeat.

See the links/posts in my signature file for some information on adjustments.

Only a THM400 would use a modulator, and that is not the transmission in your truck.

 

Good call Joe.I didn't notice the o/d part of his post.