VSS Out Issue (and MOT imminent)

[cammmy]

Hi All

Thinking I was on the home stretch of my install, I went ahead and booked my MOT for this Saturday. Unfortunately I've since found that my speedo isn't working.

It's an adjustable VDO unit, so I've set the PPM to 8000 and have set the VSS out as in the attached .MSQ (Pin: Nitrous 1, Pulse per Mile, scaling: 8000) but I get absolutely nothing. I decided to test the gauge so I switched tacho out on and put it to Nitrous 1; low and behold the speedo moved up and down with RPM.

The VSS in is definitely working as I can see the speedo in tunerstudio working fine. It's just the physical gauge isn't doing anything,

Does anyone have any idea where I'm going wrong? I'm hoping I just haven't understood a setting somewhere, so I have a chance of passing this sat.

Thanks
Cam

[Matt Cramer]

Are you using a pull up resistor on the output?

[cammmy]

THanks for the response Matt; I'm not. I asked VDO if one was required and they said no.

"Hello Cameron,

The speedometer will accept the 12V square wave. You will need to run the auto calibration procedure. "

"One more question. Will I need to use a pull up resistor"

"You do not need to use a resistor. "

Also, I should have linked the thread when I originally asked about the pin to use:

http://www.msextra.com/forums/viewtopic ... do#p457104

I'm not quite sure why it would work as a tacho and not speedo if a resistor is required? Is it not essentially the same signal?

[Matt Cramer]

If you don't have a pull up resistor, it won't send a 12 volt square wave - the output will switch between floating and ground. Try a pull up resistor and see if it helps.

[cammmy]

Okay cool, thanks. I believe I still have some from when we wired in the hall sensor for VSS.

Ta
Cam

[grom_e30]

I switched tacho out on and put it to Nitrous 1; low and behold the speedo moved up and down with RPM

was this still using the output and same gauge just seting it to tacho instead of speed out?

[cammmy]

Yup, the actual tacho is driven by the DIS unit so I don't normally have the MS tacho output switched on. I switched off VSS out and switched on tacho out (set to Nitrous 1 pin,which is wired to the speedo) and the speedo started going up and down with RPM.

[grom_e30]

if that's the case i'm not sure the pull up resistor will work as the output and gauge appear to work together. i have not used the vss output, but i wonder if its the 'scaling factor' thats the problem. on most of the vss settings your have to power cycle the ecu before making a change before it takes effect btw.

[cammmy]

That's what I was thinking. I've power cycled a bunch of times but it just refuses to work.

Does anyone have an explanation of what the scaling factor is? I've read around but haven't been able to find anything definitive. I took it to mean that setting 8000 would mean 8000ppm but is that actually the case?

[grom_e30]

The scaling factor or pulses per mile/km

thats what's in the manual so the 8000 should be correct by the sounds of it, can you test the output with a scope to see if the vss out is outputting a signal?

[cammmy]

I don't have a scope but I have a multimeter. Is there a way I can check with that?

I appreciate the replies.

[grom_e30]

a test light may work. 12v to one side of a bulb other side to the vss output and it should blink when the ecu sends a ground pulse out.

[cammmy]

I just remembered I have a stim. Will set VSS in to one of the pots on there and should be able to see the LED flash

[cammmy]

with the stim plugged in I set the VSS input to Analogue - Spare ADC and got it to read 10mph on the tunerstudio gauge. At this speed I can see individual pulses on the nitrous 1 LED.

Argh!

[grom_e30]

interesting im stumped now.

edit:
can you test it with the stim on ecu with the gauge connected up?

[cammmy]

I know. I think I'll go back out and test it again as a tacho, make sure it wasn't a fluke. I'll also try with the scaling at like 500 (the lowest the gauge supports) and then maybe 60,000.

I've got a 1k resistor on the way with someone. If I do need to wire it in, where does it go?

[cammmy]

Okay, tested as tacho again and it seems it was a fluke last time. I'm guessing I need to wire in the resistor then. Where should it go? Between the +ve supply and signal wire at the tacho end?

[grom_e30]

in that case try the pull up, between the +ve and the speed signal out from the ecu either end (gauge or ecu) does not matter but a switched live is a better bet

[cammmy]

First of all, I want to thank everyone for their input. The 1k resistor between the +ve and signal wires has sorted the issue.

The MOT guys were really cool, they let me off with a couple of things as advisories on the understanding that I sort them ASAP.

Unfortunately the starter motor has chosen to crap itself Overall a success though, I will be booking myself in for a tuning session shortly.