Hmm...no shift relay?? Th400 Downshift Switch

[sandrock]

Alright, I'm confused (not hard to do on any given day, but even more than usual today). TH400s are supposed to have kickdown relays and a switch on the gas pedal, right?

My '90 Burb has neither.

Doesn't mention it in the Chiltons book I have handy, but did the ECM control downshift instead?

 

[crazy4offroad]

You sure it's a TH400 and not a 700R4?

 

[sandrock]

Positive. 700R4s couldn't be had in the 3/4 4x4s from what I recalled. Plus, the gear shift only indicated PRND21.

And...it looks like a TH400, spade terminal on the driver side and all

Though I should note this is NOT the original trans. It appears to have been rebuilt, I'd say around 5-7 years ago. Case is painted aluminum, and the converter is painted black.

 

[sandrock]

Alright, found my answer....

Downshift is controlled by the PCM by closing the relay. Since my new PCM won't be able to do that sort of thing, I'll have to go to a switch on the gas pedal instead...

 

[HotRodPC]

You got it. Sorry I missed your post earlier.

 

[HotRodPC]

You've got an interesting year there Sand. You're right, its a Turbo 400, and technically, in 90 is when GM changed the naming system of their transmissions, so technically speaking its a 3L80 (3 speed, Longitudinal Install or Front to Back where FWD gets a T for Transverse install, and 80 for the strength or Torque rating of 8000 GVR)
This also when the 700r4 became the 4L60. Then in 93 the 700r4/4L60 became electronic so then became 4L60-E. Had the Turbo 400 ever became electronic, it would have been the 3L80-E, but the Th400/3L80 was replaced in 91 by the 4L80-E, and just to throw in, had the Th350 made it into 90 model vehicles it would have been renamed the 3L60. So there is that preface of info I already knew. But what I was wanting to figure out, and with help from Retro to confirm, I think 91 is also the first year of the electronic speedo, so that is all fine and well since the 4L80-E is equipped. I knew the Th400 in 87-90 gots its kickdown info from the ECM instead of throttle mounted kickdown switch, but I was a bit curious of how the ECM knew to activate it and overide it by VSS. Obviously it does not have a VSS and that is what I was trying to figure out. If there was a Th400/3L80 model will VSS in the tailshaft. Apparently, and I am just making an educated guess, the kickdown is only activated by the ECM from a read off the TBI TPS and that is it. All other function of the trans remain the same, as the governor will override the kickdown relay. Anyway, just thought I'd throw some brain exercise out there. Not that is really applies to you now that you found your answer on your own. You ever get a wild hair, convert that dude to 4L80-E. Doh, wait a minute, unless you are willing to change out the instrument cluster too, and ECM and blah blah blah, probably not feasible. If I get a 4x4 3/4 Burb, I don't think I'd have a problem running 4.10 and Th400 so long as I was running a tall tire cuz its not likley to be my cross country vehicle sucking gas at 8-9 mpg.

 

[sandrock]

Well, I can confirm my 1990 has the electronic speedo as I already have the cluster out. No cable going to it, so yup....

As to know when it would kick down....pretty sure through my experience with GM PCM tuning that it would know through the TPS. There are tables in the bin file that say when to shift (and at what pressure if there is a solenoid for that). One table is for WOT, which TPS is referrenced.

 

[HotRodPC]

So then there is a VSS output shaft for Th400 that exists.

 

[HotRodPC]

NO, wait a minute, yours is 4x4 right? So your VSS is coming off the NP241C which I was aware has the VSS. So I wonder what 90 3/4 ton 2WD used for speedo? I know the 700r4 could go with a VSS for the 1/2 ton. Maybe the 3/4 ton 2wd stayed analog cable speedo What I am trying to figure out for certain, is if a Th400 VSS output shaft exists. So far I've not been able to find one.

 

[bucket]

Yes, there is such a thing. I know a guy with a tubbed '70 Nova that has a th400 in it. I know it has a VSS on it because he was asking me what needs to be done to convert it over to cable. However, I was under the impression that you only have to swap out the "sprocket" for a regular speedo gear, and then plug the rest of the speedo bits into the hole on the side of the tailhousing.

Also, earlier trucks did have a VSS, but it was used differently. Rather than sending a signal to the ecm and electric speedo, it only sent a signal to the ecm (well, and the cruise control). It mounted on the back of the cluster, and actually got it's signal from the speedo. So, the speedo cable drove the speedo AND the VSS.

 

[RetroC10Sport]

1990 454SS had a T400 and electronic speedo.

T400 appears to have been available in any 1990 gm truck.

 

[HotRodPC]

Good to know info. I was never aware a 90 Th400 had the electronic speedo sending unit. If I understand, its a magnetic collar around the shaft itself is it not?
Or is Bucket suggesting you can swap it without chaning the output shaft? Which would mean its actually not part of the output shaft, just like speedo gears aren't. Maybe the magnetic sender slides on in place of the speedo gear??? I'll have to check into that. Beleive it or not, I've never seen it and don't even know how it functions.

 

[sandrock]

If its like any of the 4T60e/65e transaxles I'm used to, it's a toothed tonering (reluctor wheel) and a magnetic pickup. Hall Effect in a nutshell.

 

[HotRodPC]

If its like any of the 4T60e/65e transaxles I'm used to, it's a toothed tonering (reluctor wheel) and a magnetic pickup. Hall Effect in a nutshell.

yeah, that's kinda what I am suspecting. But I wouldn't be suprised by anything since I wan't even aware Th400 had a method for electronic speedo or VSS off the tailshaft.

 

[bucket]

Or is Bucket suggesting you can swap it without chaning the output shaft? Which would mean its actually not part of the output shaft, just like speedo gears aren't. Maybe the magnetic sender slides on in place of the speedo gear???

Yes, I believe this is the case.

Also, I noticed something at work today. On the wall, I have a Snap-on tranny ID chart, showing the shapes of the pans. According to the chart, the th400 was used through '93.