ECU sees many times the speed than speedo does (R34 swap)

[Kapr]

Hi. This is just my curiosity because iam sure the tuner figure something out...but either way i want to ask.

The "problem" is that the Nistune ECU (Stagea FW and RB25DET NEO from Stagea) sees many times the speed than the speedo does(he says the speedo show 120 km/h - correct speed and the ECU shows like 510km/h )

I told him that this car/tranny getting speed from rear diff.
If iam not mistaken the RB25DET NEO "need" to see tranny speed sensor? (my tranny has the plug for it).

So couple of "question"
If he connects/wire the speed sensor to the tranny...are the signals gonna intervene and if...what if he disconnects the rear diff one and the speedo stops working? Or the "new" sensor from the tranny "take over" and will not going need the one from diff?

As i said i know he figure/get that done...iam just curious to know what are the solutions 🙂 

[GTSBoy]

25 minutes ago, Kapr said:

RB25DET NEO "need" to see tranny speed sensor?

Correct.

25 minutes ago, Kapr said:

If he connects/wire the speed sensor to the tranny...are the signals gonna intervene and if...what if he disconnects the rear diff one and the speedo stops working? Or the "new" sensor from the tranny "take over" and will not going need the one from diff?

Um. I dunno. I haven't cared enough about the way that the NA cars work to know for sure. But.....

The 33/34 turbo manual cars have an electronic speed sensor in the gearbox that outputs a +/- (ie, sawtooth AC) voltage signal. That is connected to the speedo. The speedo then outputs a 0-5v square wave (ie, PWM) signal that the ECU (and any other CU on the bus) sees. The speed sensor is NOT directly connected to the ECU.

So here's the problem. Your new ECU expects to see the PWM signal, but must somehow be getting a direct signal from the diff speed sensor. Which would suggest that the wiring of the NA car is not the same as the turbo cars.

I think you will need to spend some time with (hopefully the wiring diagram for the car) and a multimeter to see what is connected to what. Then, presuming I am correct**, you would then want to separate the ECU speed signal input from the rest of the car's wiring, and probably either buy a speed signal converter, or build one using an arduino (or similar). That would take in the speed sensor signal and output a scaled (and suitably rearranged) signal for the ECU.

** We shouldn't presume that I am correct here, because there might be something else crazy going on.

I don't think you could convert the speedo to be fed from the gearbox sensor, because the pulse rate from that sensor is probably different to the diff sensor and then the speedo would read wrongly. And this also wouldn't fix the ECU's problem either, because the ECU doesn't want to see the gearbox signal direct either (assuming that they are all on the same wiring, for some odd NA related reason, see above caveat!)

Does this help? Probably not. Can you make it work? Almost certainly. With the above work.

You should buy a handheld oscilloscope from Aliexpress so that you can view these signals directly. Connect up the probes and drive the car. Show photos of the screen when drving at known speeds and connected to different places, and we'll see what we can learn about it.