PS2 ground / Noisy Wheelspeed

[fphil]

ECU V3.6 2xWBO2
I am using the PS2 link with the Vems speed sensor convertor to get a wheelspeed sensor. he sensor is a magnetic field Weber much in use in the 80's in conjunction with Jaeger speedometer. I had to get the logic signal from the SAF1091 chip from the Jaeger circuit through a decoupling resistance.
On the bench using a spare Weber sensor  this setup works good. On the car, engine not running, it appears that the PS2 ground  is somewhat noisy.


(Yellow: PS2GRD 50mV/div, green: PS25V, scopeGRP: MainVemsExternalGRD)

Have you observed such a noise?
Do you think it is safe to ground the PS2GRD to MainVemsExternalGRD. (looking at the schematic I was not able to see were PS2GRD is fetched)

NB EC18 pin 17 usually Ground is used in my case for the 2nd wbo2 heater - signal

[fphil]

fw 1.2.33
I appears that even if I ground PS2GRD to the main, the vems wheelspeed is noisy.

Also when Vems speed sensor convertor cable off, that is every disconnected from the Genboard PS2 connector at idle, no running, the Vems speed indicator jump 0, 1, 0, 4, 0 ....km/h.

Do you get the same?

Philippe

[VEMS]

Hello Phil,

The wheelspeed indicator moving with floating input is normal, normally a strong pullup (2k7) is mounted inside the v3 + filter in your case the ps2 option populated which means the ps2 clock is just fed straight to the cable it will therefore respond to noise injected into it (capacitive or inductive pickup).

In response to the scopeshot above its hard for me to find the volts per div for VA in that shot but since the VA seems to be 147mv I suspect the superimpose noise (/ground current) must be smaller than that, could you elaborate a bit what is connected to VA/VB and what is the volt/div for both + timebase ?

Best regards, Dave

[fphil]

Hello Dave,

Thanks for your answer,

"wheelspeed indicator moving with floating input is normal," Indeed I just check the schematic  KB_Data is simply link to the XCK0 AVR pin.
 I also tested when KB_Data = PS2GRD or PS25V the Wheel speed indicator is not moving.

On the scope the yellow graph is for PS2GRD/MainGRD and the scale is 50mV/div. 147mV is for the actual mean value of this shot, the zero of the yellow graph being that mean value.
So the amplitude of the spikes are about 200mV.

However from the above test were  KB_DATA = PS2GRD, it is not any noise on PS2GRD which causes my issue which so is not genboard related.

Doing more tests/home work ...

Regards
Philippe

[fphil]

This is to close the case 

The wheelspeed cable works with a sensor which is able to pull to ground the input Dsug9/9 of the cable circuit which is pulled up to 5V by default. This is the case of the Hall sensor chip which has an output driver.
I tried to use the cable by removing the resistance which pulls to 5V this input pin. On the bench with no noise, the cable output nicely switched 0 to 5V when the input crossed 1.3V. However this is not a linear jump and on the car even with a level of noise below 1V the output can switch the AVR clock.
The solution was given by making another cable with an the amp-op LM2904 used as comparator.

The LM2904 is a kind of Swiss knife since the supply voltage Vcc- or Vcc+ can be anything (below 32V). Of course in my case I took Vcc- =  0V and Vcc+ = 5V, this secures the output signal between these values to feed the AVR clock input.

NB It would be nice to have Webshop selling this amp-op.

[VEMS]

Hello Phil,

Could you show me a scopeshot (annotated) of the VSS speedsensor signal showing the Vpp and mean value at very low to moderate wheelspeed ?

Since the first version we have updated the VEMS VR to Hall unit to allow more configuration than previous version; This information you provide will help to catch if the required changes are already covered by these updates. Thanks!

Best regards, Dave

[fphil]

Hello Dave,

As I said I got the  speed signal from the SAF1091 chip which is used in the circuit of the Jaeger speedmeter. This is a square clean logic signal 0.2V - 5V. This is not a VR signal.

To prevent burning the SAF I used a quite large decoupling signal (270k) which reduces the amplitude signal to 0 - 1.6V when feeding the Vems ss cable input (on bench and cable without the pull up resistance), I did not scoped that signal on the running car.

Regards

Philippe