R34 Manual Conversion Speedo Issues.

[Pure_Sincerity]

Car is an r34 4 door GT.

Was factory auto n/a now converted to manual using an rb25det neo box and running a turbo viscous LSD and not the factory open centre with sensor on the side.

Everything is sweet and it's been my daily for the last 4 months. However my speedo is only reading half as it's not connected at the speedo sensor on the box. The speedo sensor just has 2 light green wires coming from it that has been cut and it just dangling. How do I go about hooking this up ? Anyone have a Pinout for this speedo sensor ?

The car is still running the n/a engine and box loom so there is a lot of cut wires under there by the box.

Car is also still running the auto ecu.

Any feedback would be greatly appreciated.

[Pure_Sincerity]

Bump.

[GTSBoy]

I was going to reply to this the other day, but I failed to understand what you meant by "However my speedo is only reading half". Please elaborate. And are you saying that the original speedo sender was on the diff (it wasn't an ABS sender?)

[Pure_Sincerity]

I didn't do the manual conversion on this car. But the sensor in the diff at the back on the cog for shaft, if I unplug that, my ABS light comes on an my speedo goes dead. Plug it back in and my ABS works again and my speedo reads half ?

As for the speedo sensor in the box, it's not wired in, it's just 2 bare green wires hanging down.

[GTSBoy]

What do you mean "reads half?"

[xALmoN]

i'd think it was reading half his actual speed. So 50kmh at 100kmh, 45 at 90kmh so on

[Pure_Sincerity]

Exactly as it's put. It reads half. When I'm doing 60 it's reading at 30 etc.

[GTSBoy]

Given that that doesn't make sense, you can see why I'd ask.

Did the original auto box have a speed sensor that was wired up to the undercar loom?

Have you considered wiring up the speed sensor on the manual tranny to the original undercar loom?

I will point out that I don't know about the R34s autos in particular......but on an R33 manual gearbox there is just two wires from the speed sensor. These give a 0-5v square pulse that is wired direct to the speedo head. The speedo head translates that to a sawtooth (AC signal) at a lower voltage that is sent to the ECU and any other controllers that need speed signal, like HICAS, ABS etc. The speedo sensor on the R34 manual should be the same. BUT....the fact that you also have a speed sensor on the diff, that is somehow giving you a 50% pulse rate to the speedo head is a very strange thing. I'm not sure how they would have wired that up to work if it also needs a gearbox sensor. Just too strange.

On the topic of the auto ECU....how the hell can this be true? The Neo turbo ECU (if auto) has the TCU built into the ECU. The Neo NA ECUs do not, they rely on a separate TCU. You cannot run the Neo turbo engine off the NA ECU. So, if what you are saying is that you have the engine running off a proper Neo turbo ECU and you still also have the old auto TCU box in the car, then it doesn't matter. That old TCU will not be doing anything. The ECU will ignore anything the TCU tries to tell it and the TCU is not hooked up to anything it can influence (except maybe ABS or traction control perhaps).

[Pure_Sincerity]

The car is non turbo with a DET neo manual box, still running the factory n/a auto ecu.

And I don't know how the diff is giving me a half speedo reading. I don't know anything about the auto setup as it was converted before I got the car.

So I'm wandering, what's the Pinout for the auto loom speed signal and can it be wired to the manual box speedo ?

[GTSBoy]

I find it really difficult to believe that they had different clusters (different speedo heads) between auto and manual cars. Therefore it should take nothing more than hooking the two wires from your gearbox speed sender to the two wires from the cluster (speed signal input). How the hell the diff mounted sender gets involved is a whole 'nother question.

To find the correct wires you may have to get the cluster out and work out which wires go to the speedo (via tracing the circuit on the cluster). The 0-5v signal is polarised, so if you don't get it to work you may need to swap them.

And ..... aaaaah. I see about the ECU now. No turbo. Fair enough. In that case, there is nothing to worry about in that regard. The ECU receives a speed signal - it doesn't provide it to anything. It doesn't care what else you do with the gearbox, so long as it gets some sort of speed signal, which it is at the moment, even if it is strangely half what it should be.

[Pure_Sincerity]

From memory, last time I looked, there is about 6-10 bare wires under there coming from the loom.

Could I simply turn the ignition on and find 2 wires putting out the 5v signal and connect them ? Or will no signal be sent through due to it not being hooked up to the speedo ?

[GTSBoy]

No. AFAIK the 0-5v signal is generated by the sender. There is no power or signal available on the wires from the dash down to the sender. Hence why I suggested you work out which wires are the pair you need by starting at the dash.

[GTSBoy]

Wow. Notified that someone reacted to my post above (the first one from Oct 16) and realised just now that I completely hamfisted the signals. The gearbox sender makes the +/-1v sawtooth (AC) signal and the speedo head in the dash converts that to 0-5v square wave PWM signal for the ECU.

I must have been on cough medicine that day.

[Instafy]

Sorry to revive this topic again Rb20det Gearboxes are caving my head in.

So I have Rb25det With a rb20det manny trans. In my C34 auto converted . the speedo Is reading half the speed and i dont know where to start

So what ive got is the speedo in the gearbox sends an electric signal to the dash which converts it and whats happening is its only getting half ? so do i need an electric converter. Rb25det electric speed sensors are long ones so they wouldnt work with my rb20det one 

[GTSBoy]

What sort of speed sensor was used in the C34 auto? An electronic sender like in the typical RB25DETmanua l box? Because, if so, you are shit out of luck. The RB20DET speedo is usually by cable drive. I don't know of any electronic drives for that gearbox. There are for the related 4 cylinder gearboxes in Silvias etc, but I don't know if they are useful.

So, where is this odd "half" speed signal coming from? If you can identify if it's coming from a diff sensor or ABS sensor (ie, that's the original way it was on the C34) then you might be able to just use a speedo adapter box like you can get from Jaycar and various auto-electrical crowds.

[WMDC35]

On 2/2/2023 at 10:04 PM, Instafy said:

Sorry to revive this topic again Rb20det Gearboxes are caving my head in.

So I have Rb25det With a rb20det manny trans. In my C34 auto converted . the speedo Is reading half the speed and i dont know where to start

So what ive got is the speedo in the gearbox sends an electric signal to the dash which converts it and whats happening is its only getting half ? so do i need an electric converter. Rb25det electric speed sensors are long ones so they wouldnt work with my rb20det one 

Manual non turbo r33 speedo drive should sort it, electronic drive for the 20 box 

[GTSBoy]

6 hours ago, WMDC35 said:

Manual non turbo r33 speedo drive should sort it, electronic drive for the 20 box 

You know. I'm a bit dense. I should have thought of that! It's not as if I didn't fit an R34 NA box to my R32 at one point, and removed the electronic sender from it!