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Discussion Starter · #1 ·
Hey all,

Yesterday, we had beautiful weather, so naturally, I got my 3000GT out of the garage. It was working completely normally on the way out. On the way home, it suddenly started shifting funny. No warning whatsoever. The first symptom, in fact, was that it wasn't really moving off the line. Once it got moving, it seemed somewhat normal.

After doing some research, I think it might be in limp mode - being stuck in third would explain why it can barely get out of its own way, and once I do manage to get some speed, it seems normal. Reverse is completely normal. Getting under the car, you can see something spinning if you look in the inspection port on the transmission. Not sure what, that's just what my brother in law said - he's familiar with cars in general but not 3S's specifically. He said all things considered it's acting like an electronic problem, not mechanical.

This naturally led me to investigate the TCU. Naturally I checked the 10A 4AT fuse and it was fine - visibly, and tested continuity with a multimeter. I got the TCU out and cracked it open, only to find... nothing. None of the capacitors are visibly leaking. None of the chips appear burnt or swollen. At least not to my untrained eye.

Any ideas? I'm posting some pics of the TCU board just in case I'm missing something. Would it make sense to order a TCU off eBay in case there's some invisible problem with this one? Do they go suddenly or do they give you some warning?

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Yes what you’re describing is classic case of Limp Mode, you could even drive it with TCU out of car (unplugged) and it should act exactly the same.

It 90% of cases it is from a bad TCU, the only thing that stands out in your pictures is what appears to be some crusty looking substance along 2 of the side edges. If that corrosion or substance (what ever it is) coverers enough on back side of board to make connection between solder joint holes, it could cause a problem with unit. If you remove the four corner screws you can raise board enough on opposite edge from the long white looking resistors that are attached to the bottom case (soldered IIRC). Those coiled spring looking strips soldered to board will flex (open up enough) to be able to see bottom of board. I actually was able to raise my board vertically to case bottom, which allowed me to replace the larger capacitor C-102 and do some cleaning of bottom of board without unsoldering the large resistor from board or case.

Those are original capacitors, which can go bad without any visual sign and most likely will shortly considering the age of you TCU if not already bad. In my case it was easy to spot because that large C-102 burnt up and burnt trace to the vertical daughter board. But after I replaced that one capacitor (none of the others) and repaired trace to daughter board, my transmission was fixed, no longer in limp mode.

If you go for a replacement board or even send yours out for repair, you can test it without installing, just prop it up against passenger side trans hump and plug in the wire connectors (they’re long enough) and test drive, case doesn’t need to be grounded it’s all done from connectors and wires.

Not sure where your located or what part # of your TCU is but below is a repair service in Texas that repairs them.
 

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Discussion Starter · #3 ·
Thanks for validating my diagnosis. The only thing that was a bit odd is it does behave differently in drive than L. In L, it still can’t get out of its own way, but starts rolling and eventually picks up speed. In D, same, but it will sometimes rev and when it does, of course, the car thinks it’s time to shift so things get dicey.

But like I said, reverse is always fine. Brother in law said that in most cars, reverse and 1 are the same physical gear, so they usually die together if there’s a mechanical issue.


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Thanks for validating my diagnosis. The only thing that was a bit odd is it does behave differently in drive than L. In L, it still can’t get out of its own way, but starts rolling and eventually picks up speed. In D, same, but it will sometimes rev and when it does, of course, the car thinks it’s time to shift so things get dicey.

But like I said, reverse is always fine. Brother in law said that in most cars, reverse and 1 are the same physical gear, so they usually die together if there’s a mechanical issue.


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Yes, that’s exactly how they act in limp mode, slow/week acceleration because you’re taking off in third gear. Ever tried to take off from a stop in a manual trans car in third gear? It’s just easier in an automatic because torque converter will do the slipping for you to get it going. By the way the thing you saw moving thru inspection hole was the torque converter.

The reverse is totally controlled by selector lever and shifting spool valve in trans valve body, it doesn’t need the TCU to work, same for park. When you put it in low that same spool valve is repositioned which causes it to act a little differently, but it still can’t make a faulty TCU shift the four electric shift solenoids mounted inside on the valve body. The trans is designed if TCU quits working to default to third gear so it doesn’t leave you stranded.
 

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Discussion Starter · #5 ·
Gotcha, makes sense. Ok then, I’ve got a broken TCU most likely. Next steps - clean up the board with some rubbing alcohol? Then see if it works like the grime was just short circuiting something? Or jump straight to capacitor replacement? How tough is that? I’ve soldered wiring for stereos but never circuit boards. Is it best left to a pro?


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Gotcha, makes sense. Ok then, I’ve got a broken TCU most likely. Next steps - clean up the board with some rubbing alcohol? Then see if it works like the grime was just short circuiting something? Or jump straight to capacitor replacement? How tough is that? I’ve soldered wiring for stereos but never circuit boards. Is it best left to a pro?
Yes cleaning suspect areas with rubbing alcohol and soft piece of cloth, can use Q-tips in tight areas (close together pins) but they seem to leave cotton fuzz which shouldn't cause a problem but I try to remove as much as possible.

I would for sure replace the large C-102 capacitor 47 microfarad 50 volt IIRC (printed on side) knowing that particular one is a known problem, microfarad must be exactly same as one removed. Voltage could be 50 volts or higher, as that just indicates max voltage it can take, it's hard believe it would even receive 50 volts in the TCU, I've even used 35 volt capacitor in a pinch and worked OK.

It's not hard if you have a smaller fine tip soldering iron (20+ watts or so) and something to remove excess solder like solder wicking or suction device. One thing to keep in mind is capacitors are polarity sensitive, meaning one of wire legs is positive and the other negative, if not installed with correct orientation it won't work and have been known to explode in some cases. If you look at top of that capacitor in your pictures most of the body is colored black, but small section is white that runs the length on side, that different colored area usually has rectangle shape printed in that stripe which indicates the negative leg is one at bottom under that white colored stripe in this case. Summary, orientate that different colored stripe same as now in that location, same with all capacitors even though they may be different colors.

The replacement capacitor will give you more room to work with as it will be MUCH smaller, allowing it to stand up little off the board and room to get solder to top side solder points, unlike now with that capacitor tight to board and you almost can't see under it. Which reminds me you should clean board under that capacitor when removed, could be some leaked electrolyte under you can't see now.
 

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Discussion Starter · #7 ·
Well… good news / bad news situation. I just heard from FAC in Texas about my TCU. They ran their diagnostics and the capacitors are NOT blown, and the gunk on the board is just the protectant they dip the board in to protect the circuitry (it gets grimy over time). To avoid having those caps ever go in the future, I asked them to perform the replacement anyway (it was obvious the board still had the original caps).

I’d give an excellent review to FAC - the pricing is very reasonable and my TCU was just delivered to them less than 24 hours ago. By the time I hunted down proper caps and took on the risk of damaging the board through my own inexperience, having them do the replacement to avoid future problems is well worth every penny.

But, what this means is that when I get the TCU back in a few days, my problem won’t be solved… any ideas what I should explore next? I have a call out to the shop that rebuilt the transmission in 2014… if their warranty is entirely based on mileage, not time, then it’s only got 4K more on it than it did at the time the previous owner had it rebuilt, and I might be able to have them take a look.


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........ my problem won’t be solved… any ideas what I should explore next?........
I was hoping the TCU was the problem as it is in most cases, but as you said replacing the capacitors and checking the unit was well worth the money, because those original capacitors where going to get you before long. When you get TCU back, I’d plug it in and just make sure you're still in limp mode.

There’s a long list of other possibilities, ranging from low fluid level to many internal trans problems. I’ll start with some checks that can be done from outside of the trans (below), that can also be done while you wait for the TCU. But I do have a question, before your car went into limp mode did you happen to notice not having overdrive (forth gear)?

1
) You mentioned checking 10amp fuse, I assumed #7 that feeds power to TCU (ELC-4 A/T). But there’s actually 2 feeds to TCU fuse #7 & #19 both 10amps. To assure power is getting all the way to TCU, it would be better to check wires at connectors since you now have easy access there.
.... 1a) With ignition switch ON, check for power on the Blue wire with red stripe, it will be in longest connector at 2 different pins 12 & 25.
....1b) With ignition OFF, check for power on Red wire with black stripe, it will be in shortest connector pin 39. It should actually have power with switch off or on.

2) You can test solenoids in trans from connector that is attached to trans mount support with wires running into trans near dipstick tube. You can see that connector looking down between MAF sensor and front of strut tower or if looking under sensor and intake tube, from front (squat down) you can see it on side of trans mount support. Just unplug harness connector from top of lower device connector portion and take ohm readings from device connector portion at the pins indicated in below test procedure When you get to the part of connecting 12volts to do sound check, the use of word Switch about ON and OFF can causes some confusion, it doesn’t mean Ign switch. It just means 12volts jumped to pin 1, 3 or 4 should make a click sound from within trans when attached and removed, it's a faint sound but it can be heard with no outside noises around. Also the solenoids are already grounded to transaxle case, so ignore that statement (already done internally).
Trans Solenoid check.JPG
 

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Discussion Starter · #9 ·
I’ll try those diagnostic steps today after work, but I figured I’d throw out there that I don’t remember a failure to shift into overdrive leading up to this issue. The transmission always holds gears for a long time when cold, then smooths out once warmed up - it was jarring at first but before this car, it had been a while since I drove an older 4AT, and I recalled the cold shift behavior in my last one (2003 Solara) was similar.

I do know for sure that the O/D has worked before, but that day I was just driving around town so I’m not sure I’d have noticed if it failed to go past third.


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.......but I figured I’d throw out there that I don’t remember a failure to shift into overdrive leading up to this issue. The transmission always holds gears for a long time when cold, then smooths out once warmed up.......
I ask because end clutch failure is one of things listed as possible cause of third gear only operation. That doesn't make complete sense to me as it mostly deals with OD and usually first thing noticed when end clutch starts to fail is loss of forth gear. I don't recall hearing of sudden lost of 1st, 2nd and 4th all at one time from end clutch failure, but who knows stranger things has happened I guess.

Do you know when last time fluid was changed in transmission? Old dirty fluid and filter could emphasize cold fluid operations, as well as cause sticking electrical solenoids. But on the other hand I doubt it would cause continuous limp mode operation, so that seems less likely the cause to me.
 

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Discussion Starter · #11 ·
I’ve only put 3000 miles on the car in 4 years of ownership - the previous owner had the trans rebuilt 1000 miles before I bought it, and the fluid has been clean and pink since I owned it so I suspect the rebuild had new fluid (why wouldn’t it?)


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I’ve only put 3000 miles on the car in 4 years of ownership - the previous owner had the trans rebuilt 1000 miles before I bought it, and the fluid has been clean and pink since I owned it so I suspect the rebuild had new fluid (why wouldn’t it?)
Yes it would've been new fluid and filter, which rules out dirty fluid as possibly cause. But with that few miles on transmission since rebuild it does add another possibility, if valve body wasn't cleaned to a sanitary state (almost operating room type cleanliness), a small piece of trash could cause one of the sliding spools to stick and not allow shifting. That could also cause sudden failure in shifting while fluid is still clean and pick (reddish), but would be hard to identify without removing valve body and actually finding a stuck spool valve.
 

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Discussion Starter · #13 · (Edited)
I have a call out to the shop that did the rebuild… my brother in law said that a lot of places warranty that type of work on miles alone, not time, so maybe I’ll get lucky. It was rebuilt in 2014 so I doubt they’ll honor it, but it’s worth a shot. UPDATE their warranty is 3 years, 36k… so I’m way over.

Actually, really lucky would be if I get the TCU back and the problem is magically fixed (like it was a loose connection or something). If I have time tonight I’ll be checking the other items you recommended though.

Basically what you’re talking about though is crud from the previous transmission failure getting dislodged after all this time and plugging a line in the valve body, right? Would it make that scenario less likely if I told you that for the past 4 years of ownership, every drive has basically been driving it like I stole it?



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Discussion Starter · #14 ·
1[/B]) You mentioned checking 10amp fuse, I assumed #7 that feeds power to TCU (ELC-4 A/T). But there’s actually 2 feeds to TCU fuse #7 & #19 both 10amps. To assure power is getting all the way to TCU, it would be better to check wires at connectors since you now have easy access there.
.... 1a) With ignition switch ON, check for power on the Blue wire with red stripe, it will be in longest connector at 2 different pins 12 & 25.
....1b) With ignition OFF, check for power on Red wire with black stripe, it will be in shortest connector pin 39. It should actually have power with switch off or on.

View attachment 294849
Just out of curiosity, should those pins read +12 with the ignition in the ON position but the engine not running? I have the ECU unplugged because I removed it to access the TCU. I just tested all three pins (pin 39 on the short connector I checked both ON and OFF), and all of them read 0 (grounded to the center console frame, which should have continuity with the radio, which is working). The battery is fine and reads around 12.8V


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Basically what you’re talking about though is crud from the previous transmission failure getting dislodged after all this time and plugging a line in the valve body, right?
Yes crud from previous trans failure. But more complicated than plugging a line, as the valve body is mostly a bunch of passageways with sliding spools to direct the fluid thru those passages. The clearance between those round sliding spools and their bores is minute and it only takes a near microscopic particle to cause one of many spools to stick in bore and not allow fluid to be routed the way it needs to be. See attached picture, the different length round things are what I refer to as sliding spool valves, they are ground to close tolerances to fit snugly in round bores of valve body but still move freely as directed by spring, solenoid or piloted fluid pressure. That’s why rebuild services disable the valve bodies and clean to near sanitary conditions and reassemble after any transmission failure, even though the valve bodies rarely receive any physical damage. This picture is not from our transmissions, just a picture I located for you to see what I meant by spool valves and passageways.
294856

Would it make that scenario less likely if I told you that for the past 4 years of ownership, every drive has basically been driving it like I stole it?
:ROFLMAO: No driving it like you stole it won’t usually cause a valve body failure, more than likely clutch packs, bands, etc. failure but not valve body. In fact in my very earlier years when I started running blown nitro burning hemi engines 3500+ h.p. before the lenco transmission were available, I ran HIGHLY modified TorqueFlite automatic transmissions with HIGHLY modified valve bodies. Which I can assure you where driven like I sole it every pass down a drags strip, though I destroyed many clutch packs, bands and even and occasional planetary case, after rebuilding trans I just made sure valve body was completely clean.
 

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Just out of curiosity, should those pins read +12 with the ignition in the ON position but the engine not running? I have the ECU unplugged because I removed it to access the TCU. I just tested all three pins (pin 39 on the short connector I checked both ON and OFF), and all of them read 0 (grounded to the center console frame, which should have continuity with the radio, which is working). The battery is fine and reads around 12.8V
No you don’t need engine running but you should plug the ECU back in. Without looking at prints again I don’t think it would stop power going to TCU, but since it’s simple to just lean it against trans hump and plug the connectors in, just do that and not wonder if it might.

I hope you mean meter negative lead grounded to console frame and positive meter lead to connector pins :unsure:. If you meant connector pins grounded to console frame you probably just blew both #7 & #19 fuses, as that would have been a direct short to ground:eek:. If you plug ECU back in and do the checks on pins from wire colors of above post #8, steps 1a & 1b you should read ~12volts at pins 12 & 25 (with meter) with ignition switch turned to ON position. You should read ~12volts at pin 39 (with meter) with switch ON or OFF.

If you in fact are not getting power to those pins, that confirms you’re not getting power to the TCU which will certainly cause limp mode. If so check those fuses again, because other than them being blown, would be fuse connection problem in fuse box, wiring problem in circuit or no power to the fuses. You might pull fuses out and check that one socket clip has power for #7 with switch ON and #19 all the time no matter switch position.
fuse box.JPG
 

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Discussion Starter · #17 ·
No you don’t need engine running but you should plug the ECU back in. Without looking at prints again I don’t think it would stop power going to TCU, but since it’s simple to just lean it against trans hump and plug the connectors in, just do that and not wonder if it might.

I hope you mean meter negative lead grounded to console frame and positive meter lead to connector pins :unsure:. If you meant connector pins grounded to console frame you probably just blew both #7 & #19 fuses, as that would have been a direct short to ground:eek:. If you plug ECU back in and do the checks on pins from wire colors of above post #8, steps 1a & 1b you should read ~12volts at pins 12 & 25 (with meter) with ignition switch turned to ON position. You should read ~12volts at pin 39 (with meter) with switch ON or OFF.

If you in fact are not getting power to those pins, that confirms you’re not getting power to the TCU which will certainly cause limp mode. If so check those fuses again, because other than them being blown, would be fuse connection problem in fuse box, wiring problem in circuit or no power to the fuses. You might pull fuses out and check that one socket clip has power for #7 with switch ON and #19 all the time no matter switch position.
View attachment 294857
Hahahaha…… yes, I had multimeter positive lead to harness and multimeter negative lead to console frame. ECU plugged back in. The pins are a bit bigger than the other ones I’ve tested using paper clips, so I wouldn’t swear that I got a solid connection, I’ll try again tomorrow. Got 0V on pins C46-12, C46-25, and C47-39.

I pulled fuses 7 and 19… 19 got about 12.5V regardless of ignition, 7 got the same but only when ignition is on. First time I tested, I got 0V but I think I just was fumbling around and had a bad connection. I’ll try that again tomorrow as well, to rule out a flaky fuse panel, but I’m pretty confident it was my error and the panel has power reliably going to those circuits.


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I’ll try again tomorrow. Got 0V on pins C46-12, C46-25, and C47-39.

I pulled fuses 7 and 19… 19 got about 12.5V regardless of ignition, 7 got the same but only when ignition is on. First time I tested, I got 0V but I think I just was fumbling around and had a bad connection
OK… Just to be sure because it’s hard to identify pins at a unplugged connector, make sure it’s a Blue wire with a red stripe in C-46 connector, NOT a solid blue wire or one with different color stripe. Also make it’s a Red wire with a black stripe in C-47 connector, NOT a solid red wire or one with a different color stripe. You've got to have power on those two wires for TCU to work correctly.

I did double checked prints and can’t see anyway the ECU unplugged should stop that from happening.

Also if #7 fuse is outputting power to circuit with switch ON, if you flip ECO/PWR switch to PWR that indicator light in tach face should light up. Also if you push OD bottom for it to pop out, the OD light should display in tach face. Neither should need the TCU hooked up, but dash lights rheostat needs to be turned up enough that they show.
 

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Discussion Starter · #19 ·
OK… Just to be sure because it’s hard to identify pins at a unplugged connector, make sure it’s a Blue wire with a red stripe in C-46 connector, NOT a solid blue wire or one with different color stripe. Also make it’s a Red wire with a black stripe in C-47 connector, NOT a solid red wire or one with a different color stripe. You've got to have power on those two wires for TCU to work correctly.

I did double checked prints and can’t see anyway the ECU unplugged should stop that from happening.

Also if #7 fuse is outputting power to circuit with switch ON, if you flip ECO/PWR switch to PWR that indicator light in tach face should light up. Also if you push OD bottom for it to pop out, the OD light should display in tach face. Neither should need the TCU hooked up, but dash lights rheostat needs to be turned up enough that they show.
Interesting, thanks for pointing that out - I actually had the transmission in PWR mode the whole time I was driving that day, and the last few times I’ve driven the car. The dash light turned on and off with the switch, so #7 is working.

All of which means one of two things… either I’m going to get extremely lucky and FAC was mistaken, and the car works fine when the TCU is hooked back up…

Or this is going to hurt.



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Interesting, thanks for pointing that out - I actually had the transmission in PWR mode the whole time I was driving that day, and the last few times I’ve driven the car. The dash light turned on and off with the switch, so #7 is working.
Don't jump to conclusions to fast, even though it leaves fuse 7 on same wire, the PWR mode is feed from different branch of that wire than the TCU. Which means it needs power from its branch of that wire to pins 12 & 25, to work like PWR indicator. Just stating facts, not trying to dampen your spirit.
 
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