[Scott65]

Have any of you retrofitted a TH400 with a speed sensor? I see bits and pieces of parts available to add a speed pick up in place of the factory speedometer gear. Seems the factory sensor may no longer be available though. Was wondering if any of you had done this, perhaps for traction control. I want it for other reasons right now, but eventually for traction control too. Any experience, advice, or even a link to a kit you know of would be appreciated. I can piece together something that would work, but figure maybe there's a simpler solution.

[Sirrotica]

GM used the T400 until 1990 in their 3500 series trucks and it had a electronic speedo with a reluctor wheel. The sensor system ran the speedo, the ABS system, and cruise if so equipped . I don't know if this will help you out or not, at least these parts are readily available.

Here's a link to the sensor: https://www.discountbodyparts.com/ca...gNbw&gclsrc=ds

And a link to the 40 tooth reluctor ring: https://www.jegs.com/i/FTI+Transmiss...sFpw&gclsrc=ds

[Scott65]

Thanks, that is helpful information. I guess even though GM supposedly discontinued the sensor, the aftermarket keeps on keeping on. Appreciate the info

[antique69lemans]

That sensor in the link , application shows 4 spd auto, probably 4l80e. In a 3500. Something is goofy because the pic isn't a 4l80e style plug. So one of those is incorrect .

[Scott65]

Good catch. I'm not sure it matters for my use, but I'll have to do some checking. I think as long as it outputs a signal I can probably find a way to use it. In my case the signal will go to an aftermarket ecu.

[Sirrotica]

The 4L80E wasn't introduced until 1991, and 90 was the last year for the T400 no overlaps that I'm aware of. I owned a 1990 G3500 and it was definitely a 400. I had the tranny out twice. Pretty sure the reluctor may be used in the 400 and the 4L80E because the 4L80E is basically a 400 with factory OD added. Roughly 75% of the parts interchange between the two transmissions.

I do know that a speedo drive gear from a 400 can be used if you want a mechanical speedo in a 4L80E with a different aftermarket tailshaft housing, so it stands to reason the reluctor can be used in either transmission.

[Scott65]

I think it should work. Gonna see if I can research what type signal it outputs. As long as its something my ecu reads, it'll work.

[Sirrotica]

Hopefully it works for you, and factory/aftermarket parts that are still available.

[Scott65]

Yeah it's a plus to have readily available parts. I'm already gonna need to keep some spares of some stuff that would be hard to get in a pinch. Anything standard is good...

[krisr]

Just use an autometer sensor (#5291) and calculate your drive/driven gear ratio inside the box. Its a 16 pulse/rev hall effect so will work straight with the Megasquirt. That's how I got it setup in mine.

[Scott65]

Kris, do you use that to read a 40 tooth gear like the one in the link above?

[Scott65]

Never mind, I see what it is. I dont think I have the stock gear set up anymore. I know it's not in the transmission anymore. I'll have to look around... If I have it I might could put it back. But this 40 tooth gear arrangement with the pickup is about half the cost of the autometer stuff. Guess I'm gonna need to do some research still.

[krisr]

Nope, the autometer sensor screws onto the end of the driven gear assembly in the side like a normal speedo cable - it even has a pass through for a speedo cable if you don't run an electric one. I've got the normal plastic gear on the output shaft in the back of my turbo 400 (can't remember the tooth count) as I used to run a cable speedo. My tooth count in the MS3 works out at "6.01" but I can't remember how I got it there as I've got no notes on what the speedo gears were.

[Scott65]

Yeah, I pulled the drive gear off when I rebuilt the transmission years ago and never put it back. Installed a brass freeze plug in the housing. On the plus side, I've had zero leaks for the last 18 years...

[ponjohn]

Additional price for the parts may be worth it if you don't need to pull the trans.

[Scott65]

I'll have to pull the extension housing either way unfortunately.