1990 300zx 2+2 auto transmission wont shift. [FIXED]

[Johnathon]

Ok guys and gals, I purchased a 1990 z32 na 2+2 with an auto transmission, shortly after the transmission decided to just not shift at all, and the pan had a ton of friction material and metal shavings, so I had it rebuilt by a reputable re builder. After installation I've been fighting the valve body like crazy trying to get it to shift. On a few occasions it'll shuttle shift like it's supposed to all the way to OD. But if i so much as cut the car off, blam, Drive and 2 are both stuck in 2, and the only way to get to 1 is to shift manually to 1. And forget about OD. Interestingly enough if i park it on a flat level surface (which is rather a difficult thing to do in this terrain) most of the time after a few hrs it'll decide to shift. I pulled the valve body yesterday again, checking each valve. None where stuck, but the 4-2 sequence valve, and the 4-2 relay valve were a bit sluggish on returning. Also the pressure regulator valve was sluggish as well, i removed them, cleaned them and the housing , reinstalled them, and made sure all was working properly before i reassembled the valve body and reinstalled it in the transmission. Oddly enough the cause of this last round was due to the o ring on the filter breaking i believe, as it was in 2 pieces, one left on the filter and the other in the bottom of the trans pan. The solenoids all check out fine on resistance, and the ECU diagnostics on the transmission controller reports no codes. I was forced to use an old o ring on the filter as the parts stores here don't have a replacement filter. And still no shift. Just curious but has anyone else had any experience with these transmissions doing this? Also to note, If i put the transmission controller in diagnostic mode, and start the engine, take off down the road, it shifts properly EVERY TIME. I thought this might be because I'm starting the engine in 1st, however i tried that without the diagnostic running and it did not shift, i cut it right back off, got the trans controller back in diagnostic mode, and it shifted properly. Could the transmission ECU be going bad?

[nissanfreak12]

Ok, you have a wall of text that is kind of difficult to understand. Couple questions...

How long ago did you have it rebuilt?

Did they install the trans or did you?

Did they not put a new trans filter on when they rebuilt it?-If not, I would call them on it, bad business practice, cutting corners is not a good thing, specially in these cars.

[Johnathon]

1. about two weeks ago.
2. I installed it.
3. There was some trash in the lines, left over from the previous transmission, and every time i pull the pan i put a new filter on, just good practice. Last time i must have pinched the o ring, causing it to burst. Not every time that I have pulled the valve body that there was valves stuck. Ive read elsewhere , after i posted, that if the revolution sensor circuit goes bad that it'll put the TCU in to 'limp mode' , causing the transmission to act exactly like it is. I checked the resistance of the revolution sensor already, it appears to be fine but I'm going to get another meter just to be on the safe side. The connector it's self is free of any corrosion, and there is continuity between the revolution sensor through to the TCU harness. I have not checked from pin 25 on the TCU to the revolution sensor as just of yet, it's snowing outside and I do need to pick up a few things ( namely another meter , and a bit of wire). What has me most confused about all of this is the diagnostic from the TCU shows no problems( all the flashes of the AT check lamp appear to be of the same length) with any of the circuits, and as I previously posted if I put the TCU in diagnostic mode the trans will shift fine. Thank you for any help/advice you can provide.

[Johnathon]

Ok, I hooked up my dvm between pin 25 and ground on the TCU, and the revolution sensor's output is on spec with the service manual. And still if I put the TCU in diagnostic mode, it will shift fine, and if its not in diagnostic mode, it will not shift. Really staring to think I have a bad TCU.

[Ace2cool]

That's what it's sounding like to me as well. Any way you can get one from another car to troubleshoot with?

[Johnathon]

@Ace2Cool Possibility. I'd have to call around to affirm. And it's a bit too early to be doing that ATM.

What all *ELSE* can put the TCU into limp mode?

[nissanfreak12]

TPS, as that controls the shift points. Make all is plugged in correctly, I did that once when I had an auto, not really what your experiencing, as mine would actually be reversed shifting. Make sure the plug the goes to the TPS without the wires off the TPS isn't reversed. Something to check. Otherwise, it may be a TCU.

Where are you located?

[Johnathon]

Nashville area. I'm going to go through all the grounds on it today, just to cover all my bases. The tps is new btw, and adjusted properly as per the FSM. And plugged in correctly... (if you pay attention to the connector there's a clip on it, both male and female side... clip side goes to clip side). I did notice that half the time when i put it in park with the OD switch on, and turn the key to run, the AT check lamp fails to come on , i have to try it a couple of times to get the TCU over to even start to get it in diagnostic mode. So, grounds first, power next, then a TCU input/output check. By the time it gets late enough to call anyone that i know that might have a spare TCU I should have all that done. I'll keep everyone posted on it though. Nothing worse than someone asking for help and never posting the resolution.

[nissanfreak12]

Sounds like the TCU as well. Good luck, let us know.

[Johnathon]

Hey everyone, i picked up another TCU, and indeed it WAS the tcu bad... 23 year old TCU ... about time it went i guess. Shifts like a dream now.

Let me recap all that i done for those that may have the same issue in the future.

If your automatic transmission has these symptoms:

AT check lamp does not come on every time you turn the key on with the shift leaver in park and OD switch on
AT does not shift into 1st gear when placed into drive, starts out in 2nd gear, and refuses to shift.
AT diagnostic mode reports no problems.
AT WILL shift if placed into diagnostic mode, and engine started and drove down the road.


1st. Check revolution sensor, Pin 25 should have 1volt AC or greater at 19 mph. This can be accomplished with a small piece of wire and a DVM with sufficiently long enough leads. Use the wire to probe Pin 25 , and ground the other lead, place the meter within view, and drive down the road.

2nd. If 1st is OK, as if the revolution sensor will throw the TCU into limp mode, and exhibit most of the symptoms above, check for battery voltage on pins 4 and 9. Pay careful attention to the pin out of the TCU connector, pins 4 and 9 are right next to each other.

3rd. Check grounds on TCU connector, pins 15 and 48. Should be straight ground , 0 ohms.

4th. run a Self Diagnostic. If Diagnostic turns up nothing, and AT check lamp does not come on EVERY time you turn the key on with the gear selector in park with the OD switch on... and 1 through 3 are good, replace the TCU.

With all that said I apologize for not posting sooner, i got the new TCU yesterday, however it was rather late by the time I got home. Went out today and rechecked all the pins for power and ground before I installed the new TCU before I hooked it up and took off down the road. And it's amazing what a difference it made. shifts 1-2 , 2-3, and 3-4 are all pretty firm. And synchronized perfectly. Thank you all on the Nicoclub forum for your help, assistance, patience, and time. I'm learning my car as fast as I can, and if I can help anyone here that's new to the 300z I will be glad to help.

[nissanfreak12]

That is wonderful and thanks for giving the results. Most people do not post results.

[moobz290]

Quick question will a 96 300zx tcu work on my 1990?

[dannysfairlady]

Quick story i bought a 1990 300zx NA 2+2 auto transmission and my transmission too is having problems shifting. It'll rev all the way to about 5k rpms and then jump into the next gear. my problem is i don't want to have to buy another transmission so cold it be the tcu? ALSO how do i even replace it?

[bignycatx]

Hey guys, I'm having similar problems with my 300Z 2+2. Sometimes I'll start it and the AT check light will come on and go out within 5 seconds and everything works fine. Sometimes the AT check light will not light up and I'll only have second gear. I've even had the AT check light come on one time 5 miles into my drive home and then it started shifting properly.
(AT check lamp does not come on every time you turn the key on with the shift leaver in park and OD switch on..... check)
(AT does not shift into 1st gear when placed into drive, starts out in 2nd gear, and refuses to shift......check)
(AT diagnostic mode reports no problems......cannot get it into diagnostic mode)
(AT WILL shift if placed into diagnostic mode, and engine started and drove down the road......cannot get it into diagnostic mode)

Here's what I get when I do the checks above.
Step 1 above, Pin 25 on the revolution sensor I get no more than .4 volts at 19 mph. For some reason I was checking in DC but I went back out and did another check in AC today. The book shows the revolution sensor is good at 500-650 ohms which is what I get on mine when disconnected at E38 pins 2 and 3. Just for S and Gs I decided to check voltage at pin 25 while revolution sensor was disconnected and got 5volts DC.
IS THAT NORMAL???

I went on the check the rest..
Step 2 above, TCU pins 4 and 9 to ground has 12vdc to ground on each.

Step 3 above, TCU pins 15 and 48 0 ohms to ground.

Step 4 above, cannot get AT check light to come on in diagnostic mode.
I tried another TCU but same results,,,it was from a junkyard so I don't trust it.

I'm thinking a short in the wiring somewhere is giving me the 5volts DC at pin 25.

[bignycatx]

Ok, went back at it again and found out I had connection problems at E38 and that's why I wasn't getting the proper voltage from the revolution sensor. Did some re-wiring and now I'm getting over 4 volts AC at Pin 25. Is there a way to reprogram the TCU or do I just need to replace it?

[bignycatx]

Lucked out and found one on ebay for $50. AT check light comes on every time the ignition is turned on and it shifts through the gears like it's supposed to. Thanks Jonathan, good write up.