What do i do about the speedo... It has the correct speedo hook up, but will it work? What all is needed? M-6 mount and a spohn torque arm++++++what else? I have a couple of other th350s and 400s but they have a mechanical speedo...

 

thats actually a damn good idea

i hope more people will chime in

 

You'll need a driveshaft slip yoke for a TH400 since the yoke is larger for a TH400 than all the other GM trannys you're old yoke won't work. The output shaft is a larger diameter. Most truck yokes are a bolt on style and that won't work. The driveshaft will then need to be shortened or a new one made.

TH400 don't use detent or TV cables to downshift. It's done electrically so a switch needs to be installed on the throttle somewhere.

The speedo should work but it may need to be recalibrated for the small f-body tires. F-body is usually 26" tall and trucks are usually 28" tall. The rear end gearing will also throw off the calibration.

Since the computer needs to know what speed you're going, if it isn't calibrated properly the car may not run right.

 

Will a TH400 fit, or should i go with a th350?

 

The TH400 will fit with a couple of modifications but a TH350 is an easier swap.

 

The TH400 was available in 1990 Chevy trucks??

 

I helped my nehew install a truck 400 in his Chevelle. The standard yoke would slide in part way, then hit a boss with an O ring on the output shaft that is used to seal the bolt on yoke internally. So we popped off the tail extension and I whacked the boss with an angle grinder while my nephew slowly turned the shaft. Hairbrained, but it worked. Tranny is still running good, 4+ years behind a 454.

 

Yep, I got one in my '90 GMC.

As far as fitting it in an F-body...make sure that the larger drain pan doesn't sit too close to the ground. The trucks had a larger pan than passenger cars.

 

Yes, It came as the only tranny available in my 90 454SS truck. With a 3.73 diff no wonder the EPA sticker was 10mpg city & 11mpg highway!!

 

As I understand the TH400 uses manifold vacuum and not electric to determine gear selection. At least that how mine is set up in my 90 454SS truck. In 91 when the 4l80 was introduced it used electronics to select gears.

 

You're a lot better off with a manual valve body. Manual shifting without a clutch is quite fun, and it gets rid of the vacuum line, the governor and kick down switch.

 

1990 was the last year the TH400 was offered. Starting in 1991 it was upgraded to the 4L80E.

The 1990 454SS has the TH400 with 3.73 gears. My 91 454SS has the 4L80E and 4.10 gears. It's nice to have the OD with the 4.10 gears but I'd still rather it had 3.73 gears. I still get around 15-17 mpg on the highway even pulling a trailer.

I grabbed a few parts off a 1990 TH400 that was in a 3/4 ton GMC van. It had the electronic speedo drive so that part wasn't just offered in the 454SS trucks.

 

I have a 400 in my 96. The swap is cake. Pix on my web site. Mine is full manual with t-brake.


LJ
para.noid.org/~lj
Acceleronics.com

 

Yup we done about a dozen. The speed sender off the 90 trucks is how we make the elctric fbody speedo work. Fits FINE!

 

Thanks LT1forfun