1990 C1500 700R4 to TH400 Swap (need help & experienced advice)

[BappleApple]

Hey Guys! Been a while, but you ain't got rid of me yet

So my bestfriend & I are building his 1990 Chevy C1500 (Single Cab, LWB) that he bought as a roller.

Truck originally had a 305ci & 700R4 from the factory (700R4/4L60 in 1990, call it whatever you want but still just a 700R4)

We put a 350ci (L05) in it, and he bought a brand new TCI StreetFighter TH400 w/ the 9" Tail Shaft.

Unfortunately, he junked the original 700R4 transmission before pulling off important parts like the Speedometer Housing, Speed Sensor, or the Speed Reluctor Wheel.

I have a feeling the speedometer housing from the 700R4 may not have worked on this TH400 anyways, so if anyone can elaborate further on that for me, please shed some light.

So here's what I need to know -

- What do I need to get the TH400 all hooked up & working correctly with the truck's speedometer as far as the Speedometer Housing, Speed Sensor, Speed Reluctor Wheel? (new TH400 didn't come with them)
- Will the truck's factory transmission cross member still work?
- Will the factory driveshaft splines work in the TH400 9" Long Shaft?

You GMT400 forum members are the pinnacle of OBS enthusiasts & never have fallen short or disappointed me when it came down to receiving some commendable advice; absolute legends. Any help is greatly appreciated & a huge thank you in advance!

 

[Anubis]

I realize it is too late now because the 700-R4 is long gone but I would have rebuilt it or bought a 4L80 to retain overdrive. IMO, going to a three speed is a step backwards even though the TH400 is a strong transmission it is outdated.
To get the speedometer working you could use a Dakota Digital speed sensor

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The crossmember can be moved along the frame rails. You will likely need the drill new holes for mounting it.
You will need a new driveshaft yoke. The 700-R4 has 27 splines while the TH400 has 32.

 

[stutaeng]

Hey Guys! Been a while, but you ain't got rid of me yet

So my bestfriend & I are building his 1990 Chevy C1500 (Single Cab, LWB) that he bought as a roller.

Truck originally had a 305ci & 700R4 from the factory (700R4/4L60 in 1990, call it whatever you want but still just a 700R4)

We put a 350ci (L05) in it, and he bought a brand new TCI StreetFighter TH400 w/ the 9" Tail Shaft.

Unfortunately, he junked the original 700R4 transmission before pulling off important parts like the Speedometer Housing, Speed Sensor, or the Speed Reluctor Wheel.

I have a feeling the speedometer housing from the 700R4 may not have worked on this TH400 anyways, so if anyone can elaborate further on that for me, please shed some light.

So here's what I need to know -

- What do I need to get the TH400 all hooked up & working correctly with the truck's speedometer as far as the Speedometer Housing, Speed Sensor, Speed Reluctor Wheel? (new TH400 didn't come with them)
- Will the truck's factory transmission cross member still work?
- Will the factory driveshaft splines work in the TH400 9" Long Shaft?

You GMT400 forum members are the pinnacle of OBS enthusiasts & never have fallen short or disappointed me when it came down to receiving some commendable advice; absolute legends. Any help is greatly appreciated & a huge thank you in advance!

I don't know the answer about the speedometer.

The crossmember should work, but I believe it needs to be shifted back a bit. In 1990, the GMT400 2500/3500 still had the TH400, so there may be a set of holes for the mount to work correctly? Maybe just drill new holes?

Found this:

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Source: https://www.jalopyjournal.com/forum/threads/th350-vs-th400-vs-700r4.500280/

No, the yoke from the 700R4 (27 spline) will not go into the larger 32 spline output shaft of the TH400.

I did a 60e to early 80e swap and found this yoke: https://www.dennysdriveshaft.com/p1...l80_4l85_6l80_transmission_slip_yoke_for.html

It's the shortened version of the 32 spline, and my original U joint installed perfectly. I believe this yoke also fits the TH400.

Other thing to look into is if your driveshaft will still be the correct length

Good luck.

 

[Schurkey]

...and you'll want kickdown wiring plus the switch...somewhere. I suppose it'll go wherever the kickdown went on the GMT400s that came with a TH400.

I expect you can manhandle the cooler tubes into position, but make sure the TH400 has the right thread-in connectors on the case to fit the TH700 cooler tubes.

A TCI transmission is very expensive for a rebuilt with some premium parts in it. Don't forget to buy a REAL torque converter, not some mass-market "stall converter".

 

[Erik the Awful]

I did a 60e to early 80e swap and found this yoke: https://www.dennysdriveshaft.com/p1...l80_4l85_6l80_transmission_slip_yoke_for.html

It's the shortened version of the 32 spline, and my original U joint installed perfectly. I believe this yoke also fits the TH400.

I have a TH400 behind the 500 cid Cadillac in my Jaguar, and I had a custom driveshaft made with a Denny's yoke. Excellent piece, but know that not all 32-spline TH400 slip yokes are the same, and I had to buy their counterbored yoke. I have the stock "normal" slip yoke sitting on my shelf - it wouldn't fit all the way into the transmission.

www.dennysdriveshaft.com/p1536_2060544_transmission_slip_yoke_1330_series_32_splines_fits_t.html

 

[L31MaxExpress]

...and you'll want kickdown wiring plus the switch...somewhere. I suppose it'll go wherever the kickdown went on the GMT400s that came with a TH400.

I expect you can manhandle the cooler tubes into position, but make sure the TH400 has the right thread-in connectors on the case to fit the TH700 cooler tubes.

A TCI transmission is very expensive for a rebuilt with some premium parts in it. Don't forget to buy a REAL torque converter, not some mass-market "stall converter".

Kickdown was via ECM control of a relay (identical to a fuel pump relay) and used the TPS signal to control the TCC circuit used for a 700r4. Different ECM programming for the TH400. The 87 G20 I have started out as a TBI 305 and a TH400. Always wondered why vans had a place for a 2nd relay on the bracket.

 

[Schurkey]

Kickdown was via ECM control of a relay (identical to a fuel pump relay) and used the TPS signal to control the TCC circuit used for a 700r4. Different ECM programming for the TH400. The 87 G20 I have started out as a TBI 305 and a TH400. Always wondered why vans had a place for a 2nd relay on the bracket.

That makes things more difficult.

I bet you could still control kickdown with a throttle-mounted switch, but you'd be fabricating "everything".

A guy familiar with electronics could piggyback the TPS voltage to some solid-state magic to control a relay, but that's WAY beyond my skillset.

 

[PlayingWithTBI]

Kickdown was via ECM control of a relay (identical to a fuel pump relay) and used the TPS signal to control the TCC circuit used for a 700r4. Different ECM programming for the TH400

And didn't they use a different ECM altogether like a 1228747 (like on a 454SS) instead of a 1227747?

 

[stutaeng]

I would think any ECM from a 88-90 5.7 8-lug would work off the shelf, no? All of those had that transmission and the same engine as OP. Gearing on those trucks were most likely 3.73s, but need to make sure on that.

But I don't know much of any of the TBI trucks. I'm just assuming here...

 

[L31MaxExpress]

And didn't they use a different ECM altogether like a 1228747 (like on a 454SS) instead of a 1227747?

My 305/TH400 van had a 7747. AMUN broadcast code on the chip.