1992 Infiniti Q45 transmission Computer issue

[tonyp71]

My 1992 Q45 (production 1/92) just had a good used transmission installed. The transmission came out of a 1991 Q45...not sure of the production date but the VIN is JNKNG01C9MXXXXXX if that helps. My question is this: With some research I learned that the 1991 Q uses a transmission control unit that has a Nissan part # that is one digit off from what my 92 Q has (31036 64U10 vs. 31036 64U11. Apparently they are different? The car would not shift down to 1st gear with the newer transmission computer controlling the older transmission. So I located a used transmission computer for the earlier transmission and I hope that will solve the issue. What is your take on this? Am I on the right track? WHat are the actual differences in these TCU's between model years...and why? I am hoping this is the cause of the issue since I have been troubleshooting every other electrical and mechanical component (with the help of the excellent Infiniti Factory Service Manual) when it suddenly dawned on me that the TCU differences could be the cause of this!

Thanks!

-Tony

[Q45tech]

The software program points in the 90-92 TCU are similar.

They played with the slopes of the speed vs TPS points and torque convertor lock unlock points time duty cycles to try to fix some wear problems and customer compaints of harshness at different shift speeds.

The tcu just receives vehicle speed and 2 rpm signals [which are same on all years [90-95] and TPS [% of throttle opening] and drives 4 solenoids in transmission..........there is an accelerator mounted WOT downshift switch and a 1st gear manual switch plus the electrical switches to tell tcu what gear has been selected.

Assume that internally all 90-95 transmission are pretty much the same [excluding gear ratios and some tweeking with clutch materials and pressures]..........we have had members interchange 95 in 90 and various combos.............what you want to avoid is the U-10 TCU which tried some radical software that provide later was going in the wrong direction........higher failure rates.

[PalmerWMD]

Also keep in mind that all 90-92 CU shift into first, only on kickdown and then only below a certain speed.

So just because you dont see 1st being used in normal por even moderately aggresive driving ,doesnt indicate a defect.

If you think your TCU doesnt use first, go to a complete stop (or a low speed) and give it complete pedal to the metal.

You should see first gear, if you don,t the problem may be in the transmission itself ,rather than the control units.

Fred..

[tonyp71]

Thanks for the info guys! I'm aware of the factory programmed 2nd gear starts on these cars for smoothness. The problem is...the transmission won't start off in first gear when manually shifted down either. It's as if 1st gear is not there...no kickdown to first or engine braking either. So with specific regards to the TCU part # differences...it's safe to assume that this problem isn't TCU related then? The shop that's diagnosing it said that everything mechanically on the transmission is good and that the issue is electrical. I bought the car with a bad transmission at 125K. (see my earlier post from April 04) It's a clean one owner car. The dash diagnostic did read "TRANSMISSION MALFUNCTION" They told me I needed a new TCU. I gave them the FSM to look over and perform the self diagnostic tests. They will try the replacement TCU today and hopefully it will make a difference. What are your thoughts on the issue? Thanks again for your advice and quick replies!

[azskygod]

Just taking a stab at this, but my car displays a Transmission Malfunction too and I know it's the turbine speed sensor...

AZSKYGOD

[Q451990]

The 1st gear position in the shifter assembly is electrical (e.g. nothing changes in the linkage going to the transmission between 2 vs. 1) - so you'll want to check to make sure that the signal is going to the transmission. The transmission also has a lock-out for cold ambient temperatures for 1st until the car warms up, but that's not an issue in Tempe this time of year.

If you still have the "transmission malfunction" message let us know what code it shows with the self diagnostic. It's easy to run the self diagnostic in your garage after the car is warmed up - so keep an eye on what code or codes you get and someone here may be able to help.

Heath

[tonyp71]

I get the car back from the shop today. I had them install an inline filter and a cooler. The cooler is mounted ahead of the radiator. (I hope this isn't a concern!) I had them install the older TCU from the '91 Q and it's supposedly doing the same thing as with the original TCU....It only shifts to first when manually selected. I'll do the self diagnositic tests and let you know what I get. Thanks again for all your advice.

[maxnix]

I had them install the older TCU from the '91 Q and it's supposedly doing the same thing as with the original TCU....It only shifts to first when manually selected.

That is the way it is meant to work.

Does it not shift to first at WOT from a stationary position?

[tonyp71]

I understand that these cars are designed to start from 2nd by default. The mechanic said it will only go into 1st when manually shifted. Not under WOT from a stop and will not kickdown to 1st unless manually selected.

[PalmerWMD]

They will all go to first, w/o manual selection, if you actuate kickdown.Maybe one of the connections to/from the kickdown pedal are broke.

Fred..

PS: the TCU from the 91 is not a good one to use, if you have the parnt# Dennis warned against above.

[tonyp71]

I wasn't aware there was a "kickdown pedal" What exactly is that?I only swapped out TCU's because the mechanic said mine was bad. I think he's wrong and wasn't completely knowledgeable about these cars having 2nd gear start. I'll probably swap the original TCU back in place if the original one is still good. If the TCU did fail.. what indications and likely symptoms would there be? (besides trouble code 54 in the ECU?) Note-- it did display "TRANSMISSION MALFUNCTION" on the self diagnostic dash display. Could a faulty TCU trigger this? Oris this self diagnostic display limited to only the transmission electronic output devices (revolution sensor, TPS, speed sensor etc?) THanks again.

[DougQ45]

I speak from first hand experience and can tell you the car (91Q) will start in first IF YOU FLOOR THE CAR FROM A DEAD STOP. The pedal gets pushed very deep, so really stomp on it!!! (Empahsis added). Look out for the law, and be prepared for a big patch of rubber as the car takes off like a scalded cat! If it doesn't start in first after a couple of attempts, there must be something wrong.

[PalmerWMD]

The pedal is right below your gas pedal, when you mash it pedal to the metal ("kick-down") it gets depressed and tell the car to go down to 1st ( within a certain speed/rpm range).

Most modern automatic equipped cars have this feature.

Fred.

[Q451990]

It's basically a switch that only gets pushed at WOT... you should hear it click if you listen carefully.

Heath

[tonyp71]

Ok thanks...I'll give it a try tomorrow. ..that could very well be the problem! The one thing they overlooked(?!) The mechanic said they checked every electrical switch/sensor related to the transmission on the car. They did say the car seemed to shift better with the older TCU but I had them put the original one back in as you guys suggested. Then I did the self-diagnostic check (A/T check) and got an "OK" We'll see what happens tomorrow! thanks again.