*UPDATED* cool info * Which TH400 modulator do I need? green stripe/blk screw/blank?

[uglychevyZZ4]

damn gm! this is the best they could do? seems there is 3 kinds, green stripe, no stripe, or one with a black screw WTF! mine is all rusted out, and GM nor schmucks says theres no other way to know.. i have 15 minutes til they close

i have 78 GMC k20 with 350/th400/203

 

[uglychevyZZ4]

 

[dyeager535]

I can't help you with the factory stuff, my books don't go back that far.

I have a feeling the dealership is lazy and just not checking, if it's even possible anymore. GM wouldn't have just thrown on whatever they had lying around (or else they'd all be the same, no?) so that means there was a method to the madness.

AFAIK all TH400 modulators fit the same, the only difference being (apparently) how much vacuum the engine produces at various throttle positions, and adjusting the modulator to tailor the shifts to the engine/vehicle.

I'd go buy an adjustable one (has the screw) and play around with it. No vacuum means maximum line pressure, so you'd want something in there, but short of no vacuum line, I doubt you'll hurt one with whatever modulator you put in.

 

[diesel4me]

I don't get it either!..

Been here!--when I was a parts counterman,I was driven batty by listings in parts catologs that wanted to know what color stripe was on the OEM modulator,or the trucks gear ratio,etc..when one brand listed ten different numbers, another brand listed very few options!..

All the tranny rebuilders here have ONE "universal" modulator for TH350's and TH400's,they dont ask a bunch of unanswerable questions before selling you one!..(it's pretty rare to see ANY paint stripes on a modulator in this area--usually the can is rotted right off the dam thing! )..

I usually buy a TM-15 Standard brand modulator--they fit all TH350's and TH400's,and are a "wide range adjustible" unit..and I have swapped many modulators from TH400's and TH350's,and never noted any difference in shift points or firmness..having one that WORKS is all you need!..

 

[uglychevyZZ4]

well found out hours later that GM and schucks cant decide of the 3 which you need..heres what i learned for ya'll

1st-- option 1 and 2 will look alike and also perform alike, the vacume nipple sits a tad lower on one. acted the same.
2nd-- option 3 is larger and for higher altitude like colorado, for thin air, sucks at sea level.

3rd.. GM cant de3cide which one of the 3 you need for ONE tranny, when B&M makes ONE modulator for th350 and th400 together, same part # LMFAO!! i got that one, works great problem solved, im happy and on the road,

thanks everyone and nice to see some of you again not that I am re-chevy'd