2JZ Speed cut issues. Have defender fitted but sometimes cuts.

[2JZR31]

So I took my R31 to the drags to try and get a 10 but I got an 11.1 @ 105mph. It should have been an 10.8 @ 129mph but the engine cut off somewhere around 2/3 track. It felt like the speed cut. (its auto)

There were no codes in the ECU. Normally I can run over 180 no probs. I have a pivot speed cut defender. Are there any cuts that apply to the transmission? When I got home I took the wheels off and put it on stands and revved it to the limiter in 3rd gear with and without the converter locked and I got readings of 218km on my pivot speed cut defender/meter. So I could not replicate the prob again. At the track I was not able to hit the rev limiter. I just got this complete cut out.

A mate of mine who done a 2JZ auto conversion to his cressida says he cant even get a normal speed cut defender to work as the ECU is reverting back to the other speed sensor when it detects the first speed sensor has had its output messed with.

Any ideas on what is happening to our cars? What have others been doing to overcome the cut with a stock ECU?

[rob]

i dont know if im dreaming this but i vaguely remember reading something about the toyota auto trans doing this to one of the 1jz cressy guys at the drags at WOT and they had to dial the throttle back so it never actually gets to WOT to avoid this.

can anyone confirm or am i bonkers?

might be an avenue for searching.

[2JZR31]

Interesting. Has anyone got any info on this? Any ideas who it might have been so I can ask them?

[CrUZida]

I think what Rob is talking about is that the ecu will not shift into top gear while at WOT with a speed cut defender.

Basically the speed cut defender limits the signal going to the ecu to about 170/175 kph (at least thats what it should do). This speed is normally just below the 3rd/4th WOT shiftpoint.
So you have to back off the throttle (to lower the shiftpoint), and when it changes into 4th nail it again.

That said, it should hit the rev limiter in 3rd if it doesn't shift into 4th, and that sure as fuck doesn't feel the same as speed cut. (speed cut is very soft)

[2JZR31]

Previously my car shifted into 4th with the SCD. I used to run 125mph with an auto shift into 4th. I was surprised it would make this shift due to the reasons you said. Maybe it depends on which of the 2 speed sensors you wire the SCD to. Mine is wired to the 3 wire speed sensor.

[PlacentaJuan]

this is interesting i have a grededy scd i have been waiting to install for a while.

but as i have only 1 speed sensor, i am hoping i dont get this problem.

[CrUZida]

Previously my car shifted into 4th with the SCD. I used to run 125mph with an auto shift into 4th. I was surprised it would make this shift due to the reasons you said. Maybe it depends on which of the 2 speed sensors you wire the SCD to. Mine is wired to the 3 wire speed sensor.

I think it more depends on your diff ratio.

From memory a 3.9:1 puts the shift point right around 180kph, which is where the problem is.

If you are running a 4.1 or something then the shift point may drop down to 170kphish, and thus not have a problem.


Then again, if you are running a 3.73 or something taller, my theory goes out the window!

[2JZR31]

You theory is right in one way and wrong in another. I have 4.11 gears in mine BTW.

Yes the diff ratio will change the actual road speed at which the shifts occur, but the diff ratio is irrelevant as the ECU does not know you have changed the diff gears. With say shorter gears the speed cut will happen at say 170, and with taller gears it will happen at say 190, but the ECU always applies it when it has calculated 180. Exactly the same will happen for gear changes. Shorter diff gears will mean it changes gear earlier and taller gears means it changes later. But the according to the ECU the gear changes always happen at the same speed.

I tested my car out on again on a private road. I didn't get the cut any more. I was able to hit the rev limiter in 3rd gear. My Pivot speed cut defender/speedo recalled 218km/h with a locked converter and 192km/h unlocked. I wish I looked at the recall when it happened at the drags.

Anyhow, I also got 218km/h with a locked converter when I had it on stands. So this means that there is zero percent slip in the converter when it is locked. This is pretty good as according to the gear calculators I have a top speed of 131mph with my 4.11 gears and my drag radials at the redline 7000rpm. That is assuming the actual trans doesn't slip.

Where does the 3 wire speed sensor connect in the trans? Would trans slip affect its reading?

[CrUZida]

Exactly the same will happen for gear changes. Shorter diff gears will mean it changes gear earlier and taller gears means it changes later. But the according to the ECU the gear changes always happen at the same speed.

You've slightly missed my point.

Assuming your speedo is accurate, the ecu knows exactly what speed you are doing.

Lets assume some figures.

With a 4.1 diff lets say at 6500rpm its 180kph.
The shift point at WOT would be around 6200-6300rpm, so the ecu should shift (at its only doing 170something).

However with a 3.9 diff 6500rpm will be 5% higher, so ~190kph.
The shift point will still be 6200-6300rpm, but 180kph would be about 6000rpm, so it wont shift.


Although, I've now forgotten where I was going with this, so I'll leave it there

[2JZR31]

The ECU does not know the difference. It does NOT know what speed you are going at all once you change your diff gears or tire sizes. If 6300 in 3rd is 180 on a stock car it will always assume 6300 in 3rd is 180. Even if you are really doing 170 or 190 depending on your gears. The ECU gets its speed reading from the gear box. The ECU assumes the diff ratio has not changed.

My 3 wire speed sensor goes straight to the ECU from the gearbox via the SCD. My speedo is connected to the 3 wire sensor before the SCD via 2 jaycar speedo correctors. Its a slave to the trans speed signal and has no bearing to what the ECU sees.

[cambelt]

These bloody painfull 2JZ ecu,s , I put a Blitz ecu in my car and get much higher limits but the offside to this in a 100 % road car is more fuel usage .
For all the time invested in the stock ecu by all the people on toymods it would be cheaper if we all went aftermarket ECU,s and had a few diff ecu tunes for all of us ,depending on our mods.
Dave

[wilbo666]

The ECU does not know the difference. It does NOT know what speed you are going at all once you change your diff gears or tire sizes. If 6300 in 3rd is 180 on a stock car it will always assume 6300 in 3rd is 180. Even if you are really doing 170 or 190 depending on your gears. The ECU gets its speed reading from the gear box. The ECU assumes the diff ratio has not changed.

My 3 wire speed sensor goes straight to the ECU from the gearbox via the SCD. My speedo is connected to the 3 wire sensor before the SCD via 2 jaycar speedo correctors. Its a slave to the trans speed signal and has no bearing to what the ECU sees.

Wha?

If the number of speedo pulses that gets sent to the ECU changes, then the ECU will think the speed is different! RPM doesn't come into it

If you change the diff ratio then you change the number of gearbox output shaft revolutions for the same speed.

Changing the number of gearbox output shaft revolutions changes the number of pulses you get from the speed sensor.

Unless I'm missing something?

Cheers
Wilbo

[2JZR31]

If the number of speedo pulses that gets sent to the ECU changes, then the ECU will think the speed is different!

Yes the actual speed will be different be different per pulse. But the ECU cant tell the speed is now wrong. Well not in my car where the SP1 goes straight to the ECU.

RPM doesn't come into it

Yes it does. Engine RPM and pulses from the speed signal are at a pre determined ratio to each other set by each gear ratio IN THE TRANSMISSION not the diff. Lets say the ecu looks at the speed sensor and it determines it time to change into 4th gear when it sees 13000pulses per minute. 13000 pulses equals say 6500 engine rpm.

Changing the number of gearbox output shaft revolutions changes the number of pulses you get from the speed sensor.

If we calculated 13000 pulses from the speed sensor equals 6500 engine rpm in 3rd gear this ratio will always the same irrespective of diff ratio or tyre size. The fact that 13000 pulses might now equal a different road speed becuase you changed your diff is not known by the ECU, therefore changing diff ratios will not effect the engine RPM at which a gear change is triggered by the ECU.

Unless I'm missing something?

Get it yet?

[CrUZida]

You must have missed this line I wrote earlier.

Assuming your speedo is accurate, the ecu knows exactly what speed you are doing.

Generally when you change your diff ratio, you change your speedo gears so that the speedo remains accurate (or install a speedo corrector)

[2JZR31]

No I didn't miss that. I even addressed it 5 posts ago explaining exactly how my speedo correction works.