TH125C no 2-1 detent downshift

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TH125C no 2-1 detent downshift by OldGuyinaGT

Started on	: 11-14-2019 11:42 PM
Replies	: 9 (306 views)
Last post by	: OldGuyinaGT on 11-17-2019 08:08 PM

OldGuyinaGT
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Report this Post11-14-2019 11:42 PM

The subject line pretty much says it. I've searched this forum, and the internet in general, and can't find any discussion of this problem.

I have an '88 GT I bought about three years ago. It runs great now (for the stock drivetrain) and the TH125C runs well and does everything it should - except (since I've owned it) it has never shifted from 2nd to 1st at/near WOT. Upshifts are all good and occur at the correct speeds/throttle openings, 3-2 downshift is fine, 2-1 downshifts at 5-7 mph on coast down. I've changed the fluid and filter, tried some Lube-gard ATF Protectant, adjusted the TV linkage a dozen times, cleaned and inspected the governor. I only get a 2-1 downshift during coast down or stopping.

After studying the factory manual, I still haven't found any diagnostic information for this problem. Now, I have a shift kit to install, and I'm pretty sure it does not have anything to do with the 2-1 downshift as far as I can see. But I'd like to correct this problem and install the kit while only cracking the tranny open once. It looks like the problem may be with the 1-2 shift valve, or maybe the TV throttle valve - but I don't know. Has anyone had/corrected this problem? Surely I can't be the first.

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OldsFiero
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Report this Post11-15-2019 07:38 AM

Sounds like TV plunger is not getting pushed to full detent in VB. A common cause is TV cable bottoming out inside the casing. Try shimming the cable casing out of the case 3/16 to 1/4". Readjust cable for max TV. If this works, repair or replace TV lever or stretched link. Also, some aftermarket cables don't have enough depth inside the housing. A guage on line pressure would be helpfull as well.
Marc

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RayOtton
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Report this Post11-15-2019 08:15 AM

This will sound rednecky as hell but when I had a similar problem I added a second, slightly thicker throttle body gasket and the downshifts got much better.

Been like that for 3 years or so without anything blowing up.

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OldGuyinaGT
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Report this Post11-15-2019 11:26 AM

Thanks for the suggestions guys. I'm definitely going to try shimming the TV cable casing. I think both suggestions do pretty much the same thing as far as the TV cable goes. Also gonna order a new TV cable from Rodney Dickman to make sure that's right. I'll probably shim the existing cable this weekend and see how it does, then install the new one when I do the shift kit since I'll have the cover off anyway and will also be able to see the movement and travel on the VB end. I'll post results when I have them.

I always wanted an appropriate gauge for transmission readings; maybe now would be a good time to get one.

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theogre
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Report this Post11-15-2019 06:58 PM

TC125c and others Never downshifts 2-1 at WOT because most times your going too fast and would wreck the trans.

Three speed auto often Does downshift 3-2 even at 50-70mph but have "Rules" to do so.
First does TB/TBI/Carb reaches WOT when you floor the pedal? Many Do Not. See my Cave, Throttle Cable
Let assume TV cable is working because normal shifting is "OK..." Is TV cable adjusted right? Many cars the answer is no. Do Not try to adjust until you push the button and adjuster moves in/out easy. See my Cave, Automatic
If someone replaces TV cable w/ aftermarket "universal" ones... Take to a Trans Shop for proper adjusting because needs gauges to read trans oil pressure at one or more ports on the trans.

Even then may not downshift all the time depending on "programming" of tran's valve body, governor, and in case of 4T__ the vacuum modulator vs. engine load and car speed.

What is often confusing is what many think is a shift up/down is really TCC locking/unlocking.
ECM, Gear and Brake switches controls this.
ECM controls by engine load and car speed.
3rd Gear switch enables only in 3rd gear. (4T__ 4L__ often have a 4th gear switch too.)
Brake switch disable this when pedal is pushed or the pedal or switch have problems.

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OldGuyinaGT
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Report this Post11-16-2019 12:43 AM

I should check the throttle cable, and will do so.

When I adjusted the TV cable, I used the procedure in the FSM - more than once. I also tested with a notch or two each way. Got differences but not a 2-1 downshift. The 2-1 downshift tests that currently fail are in the FSM. At/below 20 mph with gear selector at "2" sudden WOT should result in a 2-1 downshift. That isn't happening. TCC passes FSM functional check. I will replace the solenoid when I have the cover off anyway.

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OldGuyinaGT
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Report this Post11-16-2019 06:59 PM

Today I fabricated a spacer to place between the TV cable and the transmission housing. I made it out of 1/4" plastic (actually a scrap of Swanstone composite countertop material (hey, it's what I had that was 1/4" thick and should hold up). I meant to take a photo (somehow didn't capture it) of it but basically it's the shape of the end of the cable housing, with a 1/4" hole for the 10mm bolt and a 3/4" hole for the nose of the cable housing to fit. I cut a small slot into the 3/4" hole so I could slip it past the cable without having to disconnect it. I installed it and readjusted the TV cable. Interestingly the TV cable adjustment is almost exactly as before - within a notch at least (I had marked it when I adjusted it before). I expected it to change.

Anyway, I just got back from a short test drive, and lo and behold, I can get 2-1 downshifts now! Like the FSM says, they won't occur much above 20 mph, but they do happen. I verified by putting the shifter in "2", starting off, letting the tranny upshift 1-2 , then coasting back down to 20, then standing on it. Under these conditions, I do get the 2-1 downshift. I also get 2-1 in D if I've started from a stop, trans shifts 1-2, and I accelerate hard if still below 20 mph. This isn't going to convert my Fiero into a race car, but It will make it better to drive under some conditions.

Thanks to OldsFiero and RayOtten for their inputs. I hope to eliminate my spacer when I get into the tranny and inspect the lever and change the cable. But for now the spacer works.

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OldsFiero
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Report this Post11-17-2019 05:20 AM

Glad you got it! The spacer was intended to be temporary to prove out the problem. All GM transmissions that are cable only controlled will WOT 2-1 at low speed. In fact, it is a good way to verify that you have enough pressure rise in the absence of a gauge. Happy motoring.
Marc

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theogre
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Report this Post11-17-2019 10:39 AM

Is a crap fix because TV cables often is Not your problem.

You should have check Throttle cable and pedal and related parts First.
Any problems w/ them and TBI and TV cable never reaches WOT. On AC cars, ECM won't shutoff the compressor when pedal is "floored" either.
Most claim Fiero motors are weak have same problem.

You need help to push pedal w/o forcing it down and look at throttle or use ECM scan to read TPS angle %/° Either takes minutes to check.
But now you likely need to Pull your spacer first because TV cable and trans valve spool may fight reaching real WOT on engine. Might even break the TV or spool.

Pedal, arm and plastic bearing bolted to floor wears.
Throttle Cable can stretch.
some Fiero versions have more parts that wear too as shown in cave.

[This message has been edited by theogre (edited 11-17-2019).]

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OldGuyinaGT
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Report this Post11-17-2019 08:08 PM

I said that I would check the throttle for WOT, and I did. By the way, the AC compressor does shut off at WOT and has for the entire time I've owned the car.

The "crap fix" is the only thing to date that has gotten my TH125C to detent downshift 2-1. Also raised the 1-2 and 2-3 part-throttle upshifts in a way I most definitely like. So for now it works, I like it, so it stays. And I will (as I also said) check it all and reevaluate it when I pull the cover, change the TV cable, change the filter and the TCC solenoid, and install the shift kit. If I can get proper operation without the spacer, I'll remove it.

Do me a favor and allow for the possibility that I may have some knowledge too and don't use such a condescending tone in your replies.

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