is this ripple an artifact?

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is this ripple an artifact?

Postby hillp » Fri Mar 14, 2014 2:05 am

The throttle position sensor (and its 5V power supply) voltages seem to have a 50 mV ripple. Is this real? If so, does the PCM care? If it's unusual what might be its cause; PCM 5V power supply?

Thanks.
Attachments
20140313-0001 surging at low throttle angle.psdata
TPS, TPS supply, and sec ign taken while 1 Hz engine surging at low throttle angle in reverse
(856.87 KiB) Downloaded 181 times
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Re: is this ripple an artifact?

Postby Alan » Fri Mar 14, 2014 5:23 am

That looks a bit odd in 2 ways - the ripple and the (ignition) related spikes on both the supply and signal.

Are all the channels grounded? (ie black leads connected to chassis ground or battery negative)?

How many cylinders does the car have. Suggest you add a 4th channel as just battery positive & see if you see the ripple / noise on that as well.
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Re: is this ripple an artifact?

Postby hillp » Fri Mar 14, 2014 2:35 pm

It's a V8.
20140314-0001 tps voltages at idle.psdata
(1.13 MiB) Downloaded 129 times
I've added a 4th channel, the battery voltage. It has a little noise but not the pronounced ripple in the PCM-generated 5V supply or the TPS signal (derived from that 5V supply). (If a 50 mV ripple could be called pronounced.) All 3 grounds are piggy-backed on the battery negative terminal. The ripple in the PCM-generated voltages goes away when the engine is shut off.
20140314-0001 tps voltages key on engine off.psdata
(1.05 MiB) Downloaded 127 times


I don't think the "ignition (related) spikes" are such, as the spikes still occur even with the engine off, key on. No lights on in the garage. Refrigerator 20 feet away. Laptop running on its battery.
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Re: is this ripple an artifact?

Postby Alan » Sat Mar 15, 2014 5:57 am

Hi,

At a first glance your signals look really noisy, but you have zoomed in a lot. I dont think the ripple on the TPS itself will be causing problems as there will be some filtering done in the UK (turn on a 1kHz filter on each channel and the ripple is a lot easier to see).

I do wonder if the ripple though is a symptom of another problem that is causing your issue - perhaps ECU ground or power. Another thought might be the throttle servo motor if its electronic throttle control. What make / model is this?

Thx
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surging at low throttle angles Re: is this ripple an artifa

Postby hillp » Sat Mar 15, 2014 12:41 pm

It's a 1994 Chevy Suburban 5.7 liter V8 with throttle body injection (single point, really 2 of them next to each other.) The engine idles slightly roughly, and surges about 1/second only at very low throttle openings off idle, under a slight load, e.g. backing out of a driveway slowly. The throttle is mechanically controlled.

I don't have enough automotive experience with my new 4423 (though I've been using dataloggers and slower Picoscopes in other applications for over 10 years) to tell if the various waveforms I've recently collected on the vehicle are abnormal. Especially since I haven't obtained known normal waveforms on this vehicle. I understand that the current and voltage traces on the injectors can't tell how quickly the pintle is closing when shut off. It does not leak fuel when pressurized and not being commanded to open. One of the reasons I purchased the scope is that I hate just throwing auto parts at a problem.

There were some metal oxide varistors installed on the ECM's transmission inputs/outputs in the 1990's, a GM fix for horrible motorboating noises coming over the AM radio, which made it unlistenable. That fix was less than wonderfully effective, though. Wonder if these MOV's are failing? Maybe I should go back and look at the details of that installation, maybe pull them out to see if anything changes. But again, if the ripple is not a problem, I don't want to go to that trouble.
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Re: is this ripple an artifact?

Postby arjen » Sun Mar 16, 2014 4:05 pm

I don't have any experience with that particular car, but I would check alternator output and power/ground of the ecu.
About the pintle movement of the injector, in the current you should see the opening, in the voltage you see the closing of the injector. But in my experience not all injectors give you a good view of needle movement.
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Re: is this ripple an artifact?

Postby hillp » Sun Mar 16, 2014 6:48 pm

Thanks; I will check these.

arjen wrote:in the voltage you see the closing of the injector.


Wouldn't the voltage only tell you what it's being commanded to do, not what it actually does? If the closing spring were weak or broken, for instance.
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Re: is this ripple an artifact?

Postby arjen » Sun Mar 16, 2014 8:14 pm

The current ends directly when the ecu shuts of the injector. You can only see the opening in the current. The voltage contineus after the ecu shut the injector off. After the voltage peak you see the closing in the voltage trace.
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Re: is this ripple an artifact?

Postby hillp » Mon Mar 17, 2014 12:20 am

arjen wrote:After the voltage peak you see the closing in the voltage trace.
Very interesting. So the voltage trace somewhere shows the actual physical position of the pintle, sometime after the inductive voltage spike from voltage being shut off? It would show that the pintle didn't close if a piece of trash got stuck on the seat, holding it open? Any more details on this? Thanks.
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Re: is this ripple an artifact?

Postby arjen » Mon Mar 17, 2014 6:11 pm

hillp wrote:
arjen wrote:After the voltage peak you see the closing in the voltage trace.
Very interesting. So the voltage trace somewhere shows the actual physical position of the pintle, sometime after the inductive voltage spike from voltage being shut off? It would show that the pintle didn't close if a piece of trash got stuck on the seat, holding it open? Any more details on this? Thanks.


Correct.
If you select injector voltage and current in your pico software you get a nice explanation. But keep in mind that not every injector gives a good needle view in the waveform. Compare the injectors with eachother or a known good signal from the same car.
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