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Old 03-10-2013, 08:52 PM
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T-400 third gear issue

1990 chev P/U.

When I put my foot to the floor, it shifts fine 1-2-3. when I let off the throttle and hold it to maintain constant speed, it shifts back to second. Let off the throttle a bit and seems to go back into third but press throttle again just a bit and it goes back into second.

Vaccum lines all good.

I disconected what I believe is the kickdown wire (Controlled by the ECM...correct?) and it shifts early, but once I get to third it will stay there, and it still kicksdown into second when I floor it. It just upshifts early.

Whats going on??

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Old 03-11-2013, 11:16 AM
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Pretty sure that on the th-400(3l80) that the shifting is NOT ECM controlled. Pretty sure the torque converter clutch(TCC) lockup is controlled via that switch you disconnected which I think is a vacuum switch.
Most transmissions with lockup torque converters will have their torque converter clutches(TCC) unlock when the throttle is released. Are you sure the trans is downshifting into 2nd, or is the TCC simply unlocking?
A quick test is to hold teh accelerator pedal still in order to keep the vehicle at a steady cruise speed, then without moving your right foot, use your left foot, gently apply the vrake just enough to illuminate the brake lights. You should noitice that the revs come up a few hundred rpm. This occurs because most TCCs will unlock with brake application.

If you truck was 1 year newer it would have the 4l80e which is shifted via the ECM, and is also and OD trans.

peace
Hog
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Old 03-11-2013, 05:51 PM
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Need to ID this transmission for sure to provide suggestions. 1/2 ton truck? 3/4 ton truck? What size engine? Mileage on the transmission? If known.

Is there a vacuum modulator on the passenger side? Near front of trans at pan rail height.

A T-400 does not have lock up torque converter. The ECM does not control kick downs on a T-400, the kick down on a T-400 is controlled electrically usually via switch on - near the throttle.

You say it shifts 1 - 2 -3 ... DOes it shift 3 times or there are 3 gears. 3 shifts felt, would be a 4 speed auto.

If you are sure it is a T-400 , then a dropping out of 3rd gear can be weak vacuum(leak) , weak vacuum modulator(remove hose at modulator, look for oil) , weak 3rd gear clutch pack.
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Old 03-12-2013, 10:08 AM
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Quote:
Originally Posted by Crosley View Post
Need to ID this transmission for sure to provide suggestions. 1/2 ton truck? 3/4 ton truck? What size engine? Mileage on the transmission? If known.

Is there a vacuum modulator on the passenger side? Near front of trans at pan rail height.

A T-400 does not have lock up torque converter. The ECM does not control kick downs on a T-400, the kick down on a T-400 is controlled electrically usually via switch on - near the throttle.

You say it shifts 1 - 2 -3 ... DOes it shift 3 times or there are 3 gears. 3 shifts felt, would be a 4 speed auto.

If you are sure it is a T-400 , then a dropping out of 3rd gear can be weak vacuum(leak) , weak vacuum modulator(remove hose at modulator, look for oil) , weak 3rd gear clutch pack.
Thanks Crosley, my apologies, the th-400 does NOT have a lockup TC. I was thinking of my 84 GMC with the th-350-C IIRC.

If this is a 3 speed, in 1990 it's either a 3l80(400) or 3l80HD is it not?'
peace
Hog
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Old 03-12-2013, 07:06 PM
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1990 454SS to be exact. 1/2ton reg cab short box. ONLY HAS 350 at the moment.

There is nothing on the throttle body that controls kickdown. I've been told it's the ECM...maybe throught the TPS??

Vacuum to the tranny is good.

again, when I disconect the wire (theres only one wire) from the tranny, it shifts early, BUT it stays in third.

Hogg, it only shifts twice, sorry for that terible explaination. For sure a 3-speed.

I put the wire back on the tranny, if I hold the brake, it will stay in third!! let off the brake, back to second.
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Old 03-13-2013, 12:23 AM
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The ECM does control kickdown on the very last few years the TH400 was used. They quit using the carb or accelerator pedal mounted switch.

The ECM uses throttle position and MAP sensor to ground a relay, that powers the TH400 detent solenoid.

You may have a bad relay or even a signal issue from the TPS, etc.

Another thing to watch for is the governor could be worn causing early shifts, "falling out of 3rd" condition, and 2nd gear starts when warm.
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Old 03-13-2013, 07:31 PM
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Glad we got the truck Identified , with year and a T-400

Kick down also is controlled via ECM with the TBI setup on the SS454...

A buddy had one of the first built SS454 trucks.

Run a check on the systems and the switch as Jake suggests.
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Old 03-14-2013, 08:01 AM
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1990 454SS 230hp/385 lb/ft torque.
3.73 gearing 14b bolt semi floater, 3 speed auto

1991-1993 454SS 255hp/405 lb/ft torque
4.10 gearing 14 bolt SF, 4 speead 4l80e OD auto

I used to play with these truck with my 1997 rcsb,Vortec 350 255hp@4600rpm/330lb/ft torque@2800rpm. 4l60e 3.08 gears.

We would be neck and neck through 1st 2nd 3rd, then they would start to pull by. My buddies was a 1990 454ss and he had a chip and exhaust. Stock it ran a 15.5 in teh 1/4, I would run a 15.7.
I swapped in 4.10 gears and got a tune and that was enough to keep them at bay. I ran high 14's and ity pissed off the local 454SS and the Dakota 5.9 R/T rcsb trucks.
The 454SS weighed just a bit more as they had the 14 bolt vs 10 bolt, th-400/4l80e vs 4l60e and BBC vs. SBC.

The 454SS was THE truck that made me fall in love witgh the RCSB GMT 400 truck. They are THE best looking truck IMO.

peace
Hog
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