TH400 question

[380tom]

My transmission seems to shift solid but when I drop it into second gear (auto trans) it seems to free wheel. If I press on the gas it engages to second gear but when I let off the gas it seems to free wheel. Like it's in neutral. i.e. no engine braking
First gear holds solid on or off the gas pedal but second doesn't
But as soon as I touch the gas pedal it drops down and engages???
What's going on here??? Any ideas??

 

[diesel4me]

I've had bad modulator valves do that on TH350's and 400's before--if the diaphram in it ruptures it'll feel like it coasts in third or second when you let off the gas,usually it refuses to upshift until its racing the engine some too also,wont upshift until 35+ mph into third...if the modulator is getting no vacuum due to a rusted steel line or rubber hose failure to the intake manifold it'll act that way too...having the modulator hose hooked to the wrong place (like on the carb to a ported vacuum source instead of manifold vacuum at the intake like it should be on) can cause this trouble also..

There are other things that can cause this,but my troubleshooting chart isn't handy--in most Motors and Chiltons books they have tranny diagnosis charts,look under "Lack of engine braking" and you'll find more things that can cause it--but all of the ones I had do that were cured by fixing the bad hose or replacing the modulator...its a 5 minute job to replace one,its on the passenger side of the bell housing and only 1 bolt holds it in...if you pull the hose off it and see ATF inside the hose,or any pours out of the nipple after removing the modulator,its junk....new one is about 15 bucks..