Posted: Mon Aug 12, 2013 8:31 pm Post subject: VSS signal interference
I'm having issues with the VSS signal from my RJES VSS when the alternator charge indicator wire is connected. The image below is the signal when the charge indicator wire is disconnected, which is how the signal should look.
And with the charge indicator wire connected.
I've seen the signal with a lot more interference than this and I suspect it's causing issues with the running.
This is with the standard dashboard diode in the circuit.
Anybody got any ideas how I can suppress this interference?
Joined: 30 Jan 2006 Posts: 652 Location: North Staffordshire
Posted: Wed Aug 28, 2013 7:27 pm Post subject:
Not sure what you mean by a standard dashboard diode?
Now I've seen the traces, I doubt that adding a capacitor, as discussed on the phone will solve the problem. It'll probably just smooth out the noise slightly, masking but not solving the problem.
I assume scope vertical scale is 5V / division? If so, the clean trace battery voltage appears to be about 14V, which sounds good. I'd have thought this would have dropped noticeably when you stop the alternator charging. It's difficult to tell because of all the noise, but it doesn't seem to drop much, which seems odd.
Why do both of your traces have gaps in them? That makes no sense on a 'scope trace.
As far as I know, any kind of alternator fault (apart from loose connections) such as rectifier diode problems can only give a waveform pattern which repeats at a frequency 3 times the alternator speed. As there seems to be no pattern at all to the 'interferance' in your traces, I think it is noise rather than interferance. To be interferance, it'd have to be induced by something with a similar waveform. I'd say it's more likely to be noise caused by a bad connection or similar then induced interferance.
There are a few things you can try to narrow down the cause:
1. Do a similar trace measuring voltage at the alternator output terminal or battery positive terminal to make sure you are getting a clean charging voltage when the alternator is running.
2. Repeat 1, but measuring at the 'ignition on' output from the ognition switch.I doubt that the engine would run correctly if the whole vehicle permanant positive or ignition switched positive circuit had that much noise, as you have dropouts down to 0V. They'd turn the engine management off if also present at the ECU ignition on or power supply pins, resulting in at least a missfire.
3. If test 2, above, shows a clean voltage with the alternator charging, but the noise is still present at the speed sensor signal, repeat test 1, but measuring voltage supply as close to the sensor as possible.
4. If the test 3, above, shows the voltage supply to the sensor is clean, with the alternator running, I suspect you have a bad connection or some other proiblem with the sensor or it's witing to the ECU.
5. Assuming the voltage supply at the sensor, is clean, what voltage is it? Obviously it should be at or near battery voltage.
5. Temporarily rewire the sensor completely, one wire at a time, testing again inbetween. I think I'd start with the ignition switched positive feed. Disconnect the relevant sensor wire from whatever it was connected to, and temporarily connect it directly to the main battery positive terminal. If this doesn't get rid of the noice, repeat for the 0V wire (connecting directly to the battery - terminal). If this makes no difference, temporarily rewire the VSS signal directly from the sensor to the relevant ECU wire and nothing else.
Apart from the rad fans being on at an apparent temp, as you mentioned on the phone, do you have any other reasons to suspect a voltage or vehicle speed sensor problem? _________________ Regards,
R J Engineered Solutions
Quality Subaru Powered VW Components
Just to close out this issue. After many failed attempts to get to the root cause of the problem I finally managed to fix this issue by supplying the speed sensor from 12 volt regulator that I purchased from Maplin.
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