Speedo cable vs. electric speedo differences

[RomanV]

Apart from the obvious, I am unsure what the difference is here.

On a car with a speedo cable, it obviously needs to somehow send an electric signal back to the ECU, to indicate the vehicle speed.
My understanding of this, is that the speedo cable runs straight to the dash, which creates an electric signal, which it sends back to the ECU.
Which makes sense.

However, obviously if you've got an electric speedo cable, things work differently.
The signal from the speedo drive could either go straight to your ecu and straight to your dash;
Or to your dash, via the ECU... or vice versa.

Seeing as how there is only one input/output on my ECU designated for the speedo signal, I would imagine it is an input, to let the ECU know what's going on.
So this must either come straight from the speedo drive on the gearbox, or still come back from the dash, as per the cable system.

Does anyone know which way around it works?
I am considering running a speedo cable instead of the electric drive for the moment, for the sake of simplicity.

However, this will possibly mean that there is no speed signal sent to the ECU anymore.

Could the 'old style' signal still work? (Eg, signal generated by dash)
If the ECU didnt get a speed signal, Would this matter much?
I know that it activates/deactivates the power steering, and enables the speed cut at 180ish kph.
(And is undoubtably of more importance, if you're running an automatic gearbox)
Is there anything else that will be affected?

[sergei]

The signal almost always is going to speedo (dash), then where it is sometimes converted to 5V or 12V square goes back to ECU.
All the stuff wich is requerd SPD signal feeds of the dash out put.

These conclusions are drawed from analysing the wiring diagrams off various toyotas (mostly celicas and corollas from 83 to 93).

[bluemaumau]

just to let you know i dont have a speedo hooked up at the moment. cause i cant find a e58 drive, so my question is why does the ecu need to know? because mine aint reading anything lol

[sergei]

Generally you have lower rev limit, lower idle while stationary. Some ECUs will estimate which gear you are in, and retard ignition/adjust mix while you are shifting gears.

[KinLoud]

I'm running a gen 3 3sge in my Carina (rwd).
The donor car had electronic speedo - I just connected the ecu up the the back of the cable driven dash speedo the same as the original bluetop had.
ECU seems to be happy with this setup as it shows no codes (except for oxy sensor as I don't run one).

Ken
Ham
021 408 863

[Stealer Of Souls]

Sort of a semi-related hi-jack...

The AE86 wiring diagram shows a connection from the dash cluster to the ECU for speed feedback (?).

Would an AE85 dash cluster be the same? Will it have the sensor for speed feedback in it? Assumably if it does have the speed sensor in the cluster then it'll be outputted to the same wire on the loom too?

Anyone help?

[KinLoud]

I've looked at the in relation to AA63 4age carina VS AA60 3au Carina.
The 3au Carina doesn't have the "speed sensor" in the dash at all... but looking at how the 4age dash was set up you could put a reed switch in the correct place next to the speedo magnetic drive (magnetic connection between the cable drive and the speedo needle) and run a couple of wires from it.

Ken
Ham
021 408 863

[Stealer Of Souls]

Cheers Ken.

Nah. Answer is stop being such a stinge and buy a real AE86 dash...