th350 to th400 questions

[lmarinukjr]

Ok first of all im extremely new to this so be gentle. I bought a 1980 pick up 4x4 and the trans went i call the guy i bought it from he told me it was a th400 in the truck so i bought one from a yard get it home and its a th350 in the truck. So i already have the trans and t-case. what else do i need to do the swap. Is there some kind of write up?

[VileZambonie]

You'll need to wire up the detent solenoid and get the switch that goes on the gas pedal.

[oldredtruck]

 isn't their a diff in driveshaft lengths? If I remember correctly - I did a TH350 swap to a TH 400 a couple decades ago and had to have the driveshaft shortened by an inch and a half...so this would mean your driveshaft would need to be lengthened??? ( if I am reading this correctly.)
but my application was a pass car - so I don't know about the situation involving a T-case in the mix and how that affects things...

[lmarinukjr]

is there any write ups on this because like i said im completely new to this i dont know where to find the other parts or harnesses someone told me i have to move the crossmember and there is something about a vaccum module. the only thing i know is if i cant get this done in the next 2 or three weeks the truck is going to the scrap yard and the engine is going on ebay

[VileZambonie]

There's not much to it. It's a pretty straight forward swap. There is a single wire connector on the side of the trans. If you want proper automatic downshifts you hook up a wire to this blade terminal. That wire goes to a switch you buy at the autoparts store for your truck. The other end goes to your fuse panel. You already have a vacuum modulator on your TH350 so just hook up the same hose.

[lmarinukjr]

my truck has a cable on it not vacuum.where does the switch go?the new trans has a vaccum hook up on the side where does that run? how do i move the cross member? is there another truck i can get the front and rear drive shafts from or do i have to get custom ones made?

[VileZambonie]

If you have a th350 you have a vacuum modulator and a detent cable. The Th400 has a vacuum modulator and a detent solenoid. Crossmember location should be the same. The mount may be different. Once you have it bolted in there you'll be able to see.

Here is a TH350 and you can see the vacuum modulator


Here is a TH400 and you can see the modulator

[HAULIN IT]

 I'm having a little trouble following your post without the punctuations & capitals, ect. Did you get the transfer case with the TH400? Is it the same looking as the one in your truck? If you have one gear & one chain drive you are going to have trouble with driveshafts for sure.
 As Vile mentioned, the TH400 does not use the cable for passing gear, it uses a switch mounted on the gas pedal under the dashboard...you would need to get one, install it & run the wire. Both trannys use a vacuum modulator, both are on the Right side of the tranny, the TH350 is on the back corner & the TH400 is on the side (see Vile's pic. The Gold, round thing is the modulator) a manifold vacuum source is all you need to make that work.
 As far as the crossmember, if you need to move it, just unbolt it & slide it, the holes should be there to re-attach it. Hope this Helps, Lorne

[lmarinukjr]

The th400 came with a transfer case it doesnt appear to be the same. I understand im going to need a custom driveshaft. The switch that goes on the gas peda how does it attach? I dont see a vaccum module on the 350 thats in the truck. I was just asked by a friend if the 350 and 400 have the same shift pattern because he said it was reversed on the 2wd transmissions? is that true?

[Donut]

There "should" be a vacuum modulator on the back of the trans (your old one).
Shift patterns are the same on all of them, (tbh350,400 2wd or 4wd)

[VileZambonie]

Take a pic of your old trans for us.

[lmarinukjr]

Its currently still in the truck as soon as i pull it out ill take a pic.

[Lt.Del]

If it is a TH350 or TH350c, it has a modulator to adjust the shift points.

here's mine

 

[lmarinukjr]

OK so i just had another issue come up it may be bad but im not sure. I just ran the numbers on the trans i bought from the yard its out of a 88 chevy pick up not an 85 k5 blazer like i was told. Will this be a problem?

[VileZambonie]

No problem but you may have to swap the speedo gear in place of the VSS if was so equipped. Post a pic of the output where the speedo cable would go.