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Discussion Starter #1
Long post alert! :)

My car is an US TT, automatic. I’ve destroyed this transmission several times, it was last rebuilt less than 5K miles ago.

I’m not looking for any real power anymore, it is a single conversion but I have the boost so low I doubt it’s much higher power than BPU.

The last couple of months I’ve been daily driving the car and it has a problem that I’m at my wits end over.

The transmission will NOT hold 2nd. If I leave it in D and accelerate normally, maybe 15% throttle, it’s a perfect, buttery smooth, at every 2500-3K RPM - 1,2,3,4,lockup. No slipping, no harsh engagement. You could not ask for anything better.

I can leave it in D, but then accelerate at a little more throttle, maybe 25-30%, still under no boost, sometimes it will almost instantly shift as quickly as it can 1-2-3-4, I can feel the box shift and watch it lug the RPM. It’s the equivalent of going to 1st to 4th gear with a manual at 3K RPM. I have to be careful and not lean in to much when pulling out in front of traffic.

I can let off the throttle or manually pull it down to L and it recovers under low throttle.

If I attempt to manually shift the box from a dead stop, light throttle or full throttle it will stay in 1st until I either shift to 2nd, hit rev limit or let out of the throttle. But the instant I shift up in to 2nd, it instantly shifts to 3rd. Even while the selector is still in 2nd. If I’m going slow and low throttle it will do this every time at 3K RPM. Even while it’s still in the 2 position.

If I have it under load (boost) its even more odd, it does not feel like it’s in neutral but almost like it’s in between gears, engine is doing what it should be but the transmission pretty much checks out at this point. I can coast till I fall to around 3K RPM and it will select 2nd.

As most of you know, this transmission is 100% electronically controlled. It’s going to do, within its capacity, what the ECU tells it to do.

I replaced both of the VSS’s on the trans, the two sensors that look like modified cam position sensors (magnetic). Calling them VSS’s is kind of wrong because one measures the forward drum speed (input?) and the other measures output shaft speed. There is a third sensor that is a driven shaft and I guess it’s a backup to the speedometer if one of the other VSS’s fail. I have the new third sensor but it’s not really used for a signal to the ECU to determine transmission control. I’m going to install it but I don’t think it will help.

Replacing the two VSS’s did appear to make the shift points more exact and the transmission shifts smoother. None of the hard engagement when going from park to reverse anymore but the refusal to stay in second is still there.

I pulled the throttle body and calibrated the TPS’s, I still have factory TRAC on the car, still works. The TRAC TPS (sub) was replaced years ago because I broke it pulling the engine but the main TPS is 26 year old Toyota. Checking it with a meter, it checks fine. Throttle response and idle are perfect.

I’ve found that TPS, at least on OBD1 MKIV’s, as long as it knows idle and a reasonable amount of change, it’s good enough for these cars. The ECU will compensate for a lot. But according to the TSRM mine is spot on.

Since this is throttle/RPM related even though it checks good, could this be a TPS issue?

ECU is stock. Had swollen caps but none were leaking, they were all replaced. I still wonder though if the ECU could be screwy since it’s telling the trans what to do.

Wiring (ground) issue? Could it actually be a problem in the transmission? Valve body? Solenoids? TRAC?

I’ve taken any piggy back ECU’s, etc. off the car to get it back to stock as much as possible. I will likely never sell the car, I just want to drive it and enjoy the targa out days and let anyone who wants to race, drive by and wave.

I’ve always needed to buy a built A340e and I don’t mind doing it but for what, if I get it, install it and it’s still stupid because it’s doing what it’s being told to do, I’m no better off because I’m not making any real power at the moment anyways.

What am I missing? I’m open to any ideas and I’m hoping someone has seen this before.
 

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Mind if I do a J?
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3,249 Posts
Your effort writing this thread is more than your trans is trying to hold your power.
 

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Discussion Starter #3
Yeah, I type too much. This issue is driving me crazy though.

I’ve blown enough of these up to know when it’s letting go due to power, this is something electrical.
 

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iSPOOL
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Who rebuilt the transmission? Have you consulted with them yet? I've blown my fair share of auto transmissions and had complications but haven't had an issue like yours.
 

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Discussion Starter #5
A small local shop that I’ve used forever.

I’ve talked to them and they are who wanted me to change VSS 1 and 2. After that didn’t resolve everything they think it’s something with TPS/ECU/Electrical.

I called and talked to a guy at ATF Speed, really nice guy, spent 30 minutes on the phone talking to me about it and he said he had a MK4 in there once doing the same thing and it was slew of electrical issues to resolve it.
 

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Discussion Starter #7
No, but it’s been on my mind. It has the factory 93 ECU. All of the capacitors were replaced last spring. None were leaking but several were domed out and about to fail.

I would like to try a known good ECU to see if it helps, otherwise I need to buy a ProEFI 128 and be done with it. From what I understand they can control the auto quite well.
 

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Had similar problems with the stock ecu. I had two auto ecus and had all kinds of shifting issues, I even had both rebuilt. I currently have a AEM v2 tuned for my 1995 twin turbo and no issues. Also I had speedo issues so I replaced the capacitors.
 

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Discussion Starter #10
I had speedo issues before I replaced the caps. Sometimes I would be doing 50mph and it would be stuck at 25mph and hang there a while even after I stopped.

I’m sure that is likely the problem. Tonight it felt like it was going in and out of overdrive while I was driving at the same speed and throttle.

Are you running straight AEM? Direct to the harness with the factory ECU removed?
 

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At what speed does that happen? I used to have that between 45-50mph. Its AEM V2 only connected straight to the harness, no Toyota ecu. Its pretty good but it isnt proefi. Bought it off Titan Motorsports with a basic auto bpu setup, then had it locally tuned. First to second manual shift mode is kind of aggressive on full throttle at 17psi. Ive been running it for two years Im eventually going Proefi.
 
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