Vacuum Modulator-What does it do?

[slimbelly]

I recently put a new vacuum modulator on my 93 XLT and wanted to know what the modulator does? I also want to know how the adjustment screw affects the modulator? My truck was experiencing some shift problems between 1st-2nd-3rd but now it revs up to 3000 rpm in 1st,slips in 2nd, shifts into 3rd and 4th just fine, and sometimes doesnt want to go into OD. I havent adjusted the modulator since I put it on the truck and wanted some info before I do. Thanks...M

 

[Opera House]

"The modulator works as a throttle position sensor. You must have good vacuum at the transmission, if not it will shift out too hard and high. If you have an adjustable modulator, turn the screw in to make it shift higher at light throttle. Turn the screw out to make it shift lower at light throttle."

Basically, the modulator senses load on the engine. You have a high vacuum when the engine is lightly loaded. Under load, low vacuum, it raises the speed it shifts at.

So what did you cure by installing the new one? Sounds like you lost the litle steel pin between the modulator and the transmission when removing or installing. If you drive like this for too long, you will snap the intermediate band (if you haven't allready).