Wobbly speedo & holding back

[S2central.net]

I've been delving in the back of my speedo recently to see what is at the root of my missing VSS signal... Plan is to get in there today with an oscilloscope and see the signals coming from speedo sensor (G22) and then the speedo output. The speedo itself is working fine (touch wood) - its just that it doesn't provide the VSS output to ECU and gives me an annoying fault code.

I am expecting a somewhat square wave from G22 with 50% duty cycle and a frequency that is four times the axle speed - thats the theory... I dunno the voltage range yet - nor do I know if VSS is just a buffered version of G22 or whether it is amplified or re-shaped in someway... I do know that the ECU is happy to receive the raw G22 signal.

There is a small active circuit on the rear of the speedo module. I wasn't brave enough to attempt its removal on my own healthy unit - and I have started buzzing out the circuitry so as long as we can find a source of replacements for the one chip on that board then we should be good. Pretty sure its an amplifier of some sort - but once I have characterised the input and output then it will be easy enough to create an alternative circuit.

I have decided that the 1st phase of my 'virtual engine' project will include a G22 simulator and all the other inputs that get sent to the instrument cluster - then I will have a test rig for clusters and reliable instrumentation when the full virtual thing is running.

[S2central.net]

Here's the quick math...

Lets say 10mph - thats roughly 16kph which translates to approx 4.5 meters per second

Assume 2m rolling circumference on the wheel and we get 2.25 axle rotations per second

Multiply that by four from the sensor and we should have 9 pulses per second for every 10mph

[S2central.net]

I was flat unable to get a decent connection for my scope in a moving car for the raw G22 signal, but after stripping and rebuilding the cluster I now have a good healthy VSS signal getting into the ECU. Result...

My suspicion is on the four little pins on the cluster PCB that connect to the speedo module - I made these a little tighter fitting when rebuilding the cluster - just by teasing the tongue down gently with a fine probe.

A scope on the VSS signal at ~30mph looks a bit like this...

21ms between pulses with a negative pulse width of approx 2.5 - 3.0ms which equates to duty cycle of ~85%
Amplitude seems to be approx 4V - I have a couple of pics from scope which I can post later.

The numbers don't match the theory but then I was making a sweeping assumption with 50% duty cycle from the G22 sensor.

I have ordered some timer circuits and other gubbins to emulate the raw G22 signal into the speedo for tests & such.

I will have to hook up something more robust to try and probe the G22 signal for comparison - I think my earth point was bad. Then I will know better if VSS is materially different from G22 - I suspect it is not.

[S2central.net]

Been out again and the theory was right along... well some of it was...

Raw G22 signal is 50% duty cycle and the period of 40ms between pulses stacks for a tootle at 20something mph (the speedo was disconnected during the test)

So this means that the speedo does reformat the raw signal from G22 into something with different shape for VSS... Will need more tests on a bench to ascertain exactly what but for now it would seem that the ECU at least isn't so fussy about this as long as it gets a stream of pulses to indicate the car is moving.

[S2central.net]

I'm on fire today...

Attached is the datasheet for the chip on the speedo module.

I haven't read it yet, but what a great find for future reference.

[mcandmar]

My suspicion is on the four little pins on the cluster PCB that connect to the speedo module - I made these a little tighter fitting when rebuilding the cluster - just by teasing the tongue down gently with a fine probe.

Those pins are usually the cause of intermittent or dead speedometers, quick clean with IPA and 9/10 times they burst into life again.

Interesting how the ECU signal is tied into the clocks, i tested my VSS out after reading your thread and found a dodgy connection where i spiced the Cruise Control module in

Quick rewire and i have Cruise Control again

[S2central.net]

I built a G22 simulator yesterday - and it works... Next step is to correlate the VSS signal from that ITT chip and see if I can clone that as well - haven't tried to see if those chips can still be purchased but I am confident that something can be cooked up. Photos to follow... I should probably create an appropriate thread.

[Greg_S]

Just been having a read of your simulator thread too and makes for interesting reading. Will take speedo out and give the pins a clean and check for any dry joints if the wobblyness returns.

[S2central.net]

I'd wager it is the spring loaded tongues in those 4 pins where the speedo connects with the cluster - that is the only thing I did on mine to make any material difference and luckily it worked a treat.