Transmission down shifting to 2nd Gear- Help!

[JVD]

I have a 1995 ACURA NSX-T AUTOMATIC. BLACK, 18 INCH CHROME

WHEELS, WITH ONLY 71k MILES, I AM HAVING STRANGE

PROBLEMS

(GEARS DOWN SHIFTING TO SECOND GEAR) CAN'T GO OVER 45 MPH- NEED YOUR HELP BAD!

Do you think this is a transmission prolem or a sensor problem?


This is what happened: I was entering the Interstate from a speed ramp with the car in manual drive; as I changed gears on the steering wheel from 3rd to 4th gear, the car down shifted back to 2nd, when I got off the highway the car shifted on its own up to 4TH gear and then down shifted to 2nd gear. Acura has checked all of the codes ( Code 15 Mainshaft Speed Sensor and DTC P0700) They check the signals to the TCM, fuel injectors and solenodes;they replaced the solenoids just to test them; they flushed the transmission and found no metal chippings. They checked the voltage levels and ohms (between 400-600).

The car will shift through all four gears automatically on test drives until the engine warms up and then it down shifts back to second gear. Some one please provide some recommendations about what to do about my car down shifting to second gear. Remember the car is an automatic. It shifts through all gears until the engine warms up and then it voluntarily downsifts to second. When you pull over and put in neutral for about 1 minute; drive off and it shifts through all four gears and then goes back to second. I don't think I need a new transmission or a rebuilt one. Someone out there please help me diagnose this problem. My NSX has been in the Acura Dealership shop for two weeks. Do you think this is a transmission prolem or a sensor problem?

JD

 

[jgtcnsx]

your transmission is starting to slip i would take it to get fixed or get a new one cause it will only get worse

 

[EIFFEL]

Pretty hard to speculate on a problem over the internet. Call the dealer and make sure someone is looking at the problem and not just let the car sit there. Need a seasoned tech also. Not too many techs have worked on the NSX automatics. I worked at an Acura dealer for 7 yrs. and we never had a single auto NSX problem. and this is in Atlanta.

 

[Larry Bastanza]

I agree with EIFFEL this is very hard over the internet, but I would check for a vacuum leak. Vacuum plays a key role in determining automatic shifting. For example if you have a good size vacuum leak the trans may determine a loss of vacuum as "time to downshift". Typically vacuum leaks get better with heat, but you never know.

Just a thought, not really sure where to take this one.

Good Luck,
LarryB

 

[tucsonsx]

I'd make sure the dealer has contacted Acura and not just pushed your car into the corner. I've never seen a problem that the techs from the distributor can't help the dealer's service department figure out. I doubt you need a new transmission - what you need is proper diagnosis.

 

[drew]

I've rebuilt a few auto trannys...none as complicated as the NSX (or any luxury car made in the past 15 years) that has two valve bodies and all that.

It is very difficult to determine issues between the control system and the physical system. Even seasoned transmission professionals will R&R the tranny and if that doesn't work: start replacing control system parts.

It is very difficult and time consuming to make a proper diagnosis. A lot of the time it takes more time to test a component than just to "guess" and replace it.

Since the auto trans feeds back on all the other sensors, it gets very complicated very quickly. Personally, I will go through all the sensors and build my own "test harness", but it is very UNLIKELY you will get somebody to do that...and at $85-$125/hour of shop time it may take you 10 hours to figure it out: hence the problem. It is much faster and might be cheaper to just replace everything as you go along the troubleshooting flowchart.

What you REALLY need is the factory computer harness that can manually override all the inputs and outputs. This little device costs many thousands of dollars and your dealer might not have it. Find somebody that does, find one on ebay for a couple of thousand (well worth it, just for this one job), but get one (OR just start replacing things).

I have a buddy who runs a very good transmission shop, and whenever he runs across a problem like this: he just drops the $3K-$5K on the current factory tools and chalks it up to doing business.

You are going to have to get the book out, deteremine ALL the inputs/outputs, figure out what they do and probably test them yourself (ie give hints to the dealer).

It SOUNDS like following:
The reversion back to 2nd gear is probably "limp mode"

It may be that the clutch packs are worn. When the fluid warms up it becomes less viscious and allows slippage. You would not believe how close the tolerance the clutch pack is. If the clutchs are slipping, then the main and drive shaft do not spin as expected and it probably throws it into limp mode.

71K miles is enough to need a "soft parts" rebuild.

Why "don't you think" you need a rebuilt transmission? What other information do you have???

Good Luck

Drew

 

[JVD]

Does an automatic NSX have a clutch? You said the clutch pack could be slipping? I don't think I need a rebuilt transmission because sensors or soft parts tell the gears to change. The transmission consist of the hardware. I know you understand that better than I do. If there were a mechanical problem, I think there would be mental chippings, noises, jerks etc.. I think something is producing false signals between the TPS and TCM. What would you do? Thanks.

JD

 

[drew]

It could very well be the sensors.

But you need diagnosis tools, you plug a special harness into the transmission harness and drive around testing things. It is very expensive, but with it you can figure things out pretty quickly (2-4 hours). Without it: you are into many hours of diagnosis...and it is cheaper and faster to guess and replace things.

I find that most dealers won't spend the money to buy all the neat tools (which make zero money...in fact it is considered an expense). They would rather have their tech spend [billable] time replacing [billable] things and make money off of the inability to do a precision diagnosis.

Yes, an automatic transmission has a clutch...it should have about 5-7 of them called a "clutch pack"...those little clutches "friction rings" are considered a "soft" part. Along with the seals, o-rings (that also get hard and allow hot fluid to slip by)....

**

Your constant shifting into second gear is probably limp mode. The computer figured out that something is wrong and that is where it tells the transmission to stay. Since it also states the main shaft is not happy...it sure sounds like a slipping clutch pack. The fact that it doesn't slip on cold fluid, points (at least to me) that the higher viscosity is masking the problem.

That is my "best guess" and if it was my car I would start testing for that very likely possibility.

**

As for metal chipppings: that is for a MAJOR problem when bearings and gears let go...

Believe it or not: automatic transmissions have normal wear parts. They don't make noise or jerk...just refuses to go when you rev the engine. You rev high enough, it will jerk into gear...but with "limp mode" it might not even let you get that far.

**

It could even be a faulty engine coolant sensor that is giving bad info to the tranny, which might be trying to compensate for a problem that doesn't exist. The ECM and tranny computer are so interlinked that any thing can be telling the tranny to act the way it does.

If you want a PRECISE diagnosis your deals needs the proper tools, which I doubt they have.

**

If it were me: determine if the dealer has the proper tools. If they don't: find one that does. If you can't do that: spring your car from the dealer. Take the car home and become an expert on the car and fix it yourself.


Professional means that you do it for a living, it doesn't mean that you know what you are doing.

Good Luck

Drew

 

[Larry Bastanza]

Some very good points here. I never asked: Do you have an engine check light. If so, have you pulled the codes?

HTH,
LarryB

 

[JVD]

Check engine light is on. Pulled a code 15. Mainshaft Speed Sensor.

 

[Larry Bastanza]

Well that certainly narrows it down.

Has this been checked by your tech yet? Have they checked all the wiring and/or swapped the sensor with a new or known good one? That is what I would do next.

HTH,
LarryB

 

[TaurusThug]

sounds like time for a tranny rebuild... the auto tranny in the transam we had was fine and didnt do anything odd untill one day on the highway it just revved up and we knew that the "bands" or clutch packs were shot. maybe honda built in a limp mode in order to keep it from destroying the engine if this was to happen. do you have any kind of engine management??? that could tell you some stuff if you have it setup for it.

 

[JPS Europa]

Check engine light is on. Pulled a code 15. Mainshaft Speed Sensor.

I bet that this is the culprit. I replaced a Vehicle Speed Sensor in one of my other cars and it sure was acting very much like the way this car is acting. The other car was OBD II though, so it was easy to see while driving around with the reader in the car.

 

[drew]

JVD can you tell us the results of solving the problem?

That way you can help other folks that might have the same problem or questions.

Drew