spegru

Hi there does anyone know how the speedometer part of the dash is connected to the rest of the wiring?
Does it work via OBD datalink connection like the tacho, or by some other method?
BTW I am not talking about the common Focus VSS gearbox sensor problem - it's about understanding how the speedo works.

Someone must have sorted this out if they used an aftermarket engine ECU such as Megasquirt on the standard Ford dash

spegru

Hi not too many responses to that. However I did swap a PM with another member (who I'll keep anonymous unless he/she wants to put their hand up!) who said (among other things)

"the Mk1 Focus cluster is a Hybrid Electronic Module (HEM). It makes a more complete use of the OBD data highway, though a few indicators are still direct switch/signal. To maintain 'legacy' compatibility, the VSS signal, which is a 12v Hall Effect goes only to the PCM. The PCM conditions it to a similar 'pulse' for use with older devices such as radio volume, cruise control,etc. The signal is also conditioned into the OBD multiplex signal. On the transverse gearboxes (MTX, IB5) the VSS uses a 40 tooth wheel that replaced the older mechanical drive. So the PCM gets 40 pulses per drive wheel rotation. The PCM can be recalibrated by Ford by a small amount to allow for wheel/tyre variations. The ST170 the VSS uses the Crown wheel directly, so the pulse rate to the PCM is ~x4 higher ! The Black Oak PCM is coded to take account, but causes confusion with a 'normal' gearbox. If you have other OBD signals on the Focus cluster then the speed should work ...... "

This got me hopeful but what then is the problem? I have a fairly sensible speed reading via OBD on my Android/Torque/Bluetooth OBD device but not on the dash.

I am left with the idea of a fault with either the ECU or two Speedos. I have just replaced the ECU and that didn't help either - so that's two ECUs and two Speedos! Seems quite unlikely that two items of the same sort should both be faulty...

I think I'll get another speedo as they're quite cheap on ebay, just in case - but I am beginning to think it is a wiring fault after all...

Just to re-emphasise - I do have a speed reading via OBD and also a nice speed related square wave pulse coming out of Pin 4 on the ECU (60pin version) that's used to control audio volume among other things. So if speed sensing on the dash is not by OBD data link then where should it be connected???

spegru

Germane to this question is whether or not all Focus Mk 1s have

a) Speed/Cruise Control?
b) Trip Computer?
c) ABS?

If any of these items are missing on some models then it's less likely they are involved in speedo operation

thanks
spegru

spegru

An update: I just managed to borrow a Mk1 Focus and plugged in a spare dash pod into the obd port via a flying lead to the plug on the back.
The speedometer works - and that's without any wires connected other than power and OBD!
So that's proved 100% that the speedo is connected via OBD

So why the heck doesn't it work in my car when everything else on the dash does? VSS Speed sensor compatibility (this is a Ford Sierra box)? - but I get a speed reading via my Torque/Bluetooth connection. Just now I even checked it wasn't getting it from GPS

Argh - so near and yet so far......

spegru

A bit of a breakthrough:
I noticed that the trip odometer has been changing very gradually.
So I wondered whether, considering the roughly 50% under reading at present, had I actually been going fast enough for the speedo to register?
So I went a bit faster.....
And the Speedo started to work!

So it's a calibration issue - of course speedos often don't register at very low speeds (the highest I'd reached as an indicated 15km/h - which is around 10mph)

So nothing fundamental then - phew!
My Sierra VSS is either faulty or just behaves differently different from a Focus one.

What I need to know now is understand the difference between a Sierra VSS and one from a Focus in terms of pulses per mile/km - whatever - and do the necessary conversion

spegru

No a very popular thread it seems but finally I have a solution involving a Rev Counter circuit (converts VSS pulses to voltage) connected to an Arduino based VCO (Voltage Controlled Oscillator) connected in between the VSS itself and the Engine ECU, thereby getting me the variable ~3x fast pulse rate that I need.
Kind of rough and ready but it essentially works