VSS accuracy at low speeds

Question

Background

I recently got a P1500 code on my 98 Mazda 626 GF.

I hooked up my computer using ForScan and cleared the code. It didn't come back after three slow test drives around the parking lot.

Here is what the workshop manual says for a P1500 DTC:

Experiment

I drove around the parking lot a little faster than a brisk walk / slow jog and noticed that the dash speedometer seemed to stay on zero. The reading from the ECU bounced between zero and 7 - 9 km/h, like this:

+-------------------------------------------------------+
|time(ms)|VSS(km/h)|TP_V(Volt)|RPM(rev/min)|TSS(rev/min)|
+-------------------------------------------------------+
|63395   |6.9      |0.56      |758         |624         |
|63469   |5.1      |0.56      |755         |616         |
|63542   |3.4      |0.56      |752         |609         |
|63628   |1.7      |0.56      |752         |601         |
|63702   |0.0      |0.56      |752         |601         |
|63774   |1.6      |0.57      |753         |601         |
|63845   |3.2      |0.57      |753         |601         |
|63920   |4.8      |0.57      |753         |600         |
|63996   |6.3      |0.57      |752         |596         |
|64071   |6.3      |0.57      |752         |592         |
|64144   |6.3      |0.57      |751         |587         |
|64227   |6.3      |0.57      |755         |583         |
|64302   |6.3      |0.57      |759         |579         |
|64376   |6.2      |0.57      |763         |575         |
|64451   |6.2      |0.57      |767         |571         |
|64526   |6.2      |0.57      |766         |568         |
|64601   |6.1      |0.57      |764         |564         |
|64674   |4.6      |0.57      |763         |561         |
|64759   |3.1      |0.57      |762         |558         |
|64832   |1.5      |0.57      |759         |555         |
|64906   |0.0      |0.57      |756         |550         |
|64984   |0.0      |0.57      |754         |545         |
|65060   |0.0      |0.57      |751         |541         |
|65129   |0.0      |0.57      |751         |536         |
|65205   |0.0      |0.57      |752         |534         |
|65279   |1.4      |0.57      |752         |533         |
|65363   |2.8      |0.57      |753         |531         |
|65436   |4.3      |0.57      |751         |530         |
|65510   |5.7      |0.57      |748         |527         |
|65586   |4.3      |0.57      |746         |525         |
|65659   |2.8      |0.57      |744         |522         |
|65733   |1.4      |0.57      |742         |519         |
|65806   |0.0      |0.57      |739         |518         |
|65882   |0.0      |0.58      |736         |516         |
|65966   |0.0      |0.58      |734         |515         |
|66038   |0.0      |0.58      |737         |513         |
|66112   |0.0      |0.58      |741         |512         |
|66184   |1.3      |0.58      |745         |510         |
|66259   |2.7      |0.59      |749         |508         |
|66336   |4.0      |0.59      |750         |507         |
|66406   |5.4      |0.59      |752         |503         |
|66483   |4.0      |0.60      |754         |500         |
|66556   |2.7      |0.60      |755         |497         |
|66629   |1.3      |0.60      |787         |494         |
|66703   |0.0      |0.60      |818         |500         |
|66776   |1.3      |0.61      |850         |507         |
|66860   |2.6      |0.61      |881         |514         |
|66933   |4.0      |0.61      |898         |520         |
|67010   |5.3      |0.61      |916         |525         |
|67087   |4.0      |0.61      |933         |529         |
|67160   |2.6      |0.61      |950         |534         |
|67245   |1.3      |0.61      |954         |538         |
|67320   |0.0      |0.61      |958         |545         |
|67393   |1.5      |0.61      |962         |551         |
|67469   |2.9      |0.61      |966         |557         |
|67543   |4.4      |0.61      |960         |563         |
+-------------------------------------------------------+

Here's a graph of roughly the same data:

While researching this...

I read that vehicle speed sensors will not generate a signal at less than 3mph/5kph. This is mentioned on a site describing what to do about P0502 codes and in Tim Gilles book "Automotive Service".

My speed during most of this test was hovering right around 5kph. According to these sources, every time I drop below 5kph, the sensor will stop producing a signal and effectively read zero speed. I'm wondering if maybe this graph is actually OK?

Questions

Is it normal for the VSS data to look like this at very low speeds (brisk walking / slow jog speed) or is this an indication of a problem with the sensor?
If it's not normal, where should I start looking beyond just testing the sensor itself?

EDIT 26/7/2017

I've still got this problem, and thought I'd point out that every once in a while the ABS warning light will come on. Also, I actually checked the transmission, and there is just a plastic place holder where the VSS sensor is supposed to be. So I'm assuming that the ECU must be using the signals from the ABS wheel speed sensors in place of a VSS sensor.

Which of course makes this a little more trouble to diagnose. I'd really like to diagnose this rather than going out and buying new ABS sensors at $55 each including shipping.

@Moab Care to expand that into an answer with an explanation? Could this be an ECU issue ( WSM also lists that as a cause for this DTC ), and could this cause the vehicle to stall? See my other related question: http://mechanics.stackexchange.com/questions/35843/98-mazda-626-with-intermitant-stall-and-restart-problem?noredirect=1&lq=1 — Robert S. Barnes, Aug 31 '16 at 12:55
that doesn't look normal. scope the vss it self while driving may be a bad ground. — Ben, Aug 31 '16 at 13:02
is it a hall effect sensor? if it is voltage drop the ground and wiggle test the harness. if its a ac signal it may be converted to a square wave at the instrument cluster and sent to the pcm from there. — Ben, Aug 31 '16 at 13:44
You show the readings at your walking speed, does it continue to fluctuate as you increase into 10-20-30-40 ranges. As the VSS feeds speedometer microchip, powertrain control module, and cruise control are you seeing effects on these. for instance cruise control at 35MPH fluctuations. Speedometer needle jumping. — spicetraders, Aug 31 '16 at 14:36
@spicetraders No, once I'm over say 10 km/h it seems to be fine, and looking back over some other recent logs I don't see any strange fluctuations. It only seems to be a problem cruising at speeds less than 10 km/h. — Robert S. Barnes, Aug 31 '16 at 16:00
@spicetraders I think that while they're related, they are separate questions. — Robert S. Barnes, Aug 31 '16 at 16:58
Well if I was looking at one for a solution I would most certainly want to know the other issues and look for a common point of fault. — spicetraders, Sep 01 '16 at 00:12
@Ben According to the wiring diagram in the Haynes manual it's a two wire sensor, which according to what I've read would mean it's an AC pickup type. — Robert S. Barnes, Sep 03 '16 at 20:06
@RobertS.Barnes it may be, i've seen 2 wire hall effect sensors before as well though without the fsm it'd be hard to say unless someone else knows off hand. — Ben, Sep 03 '16 at 20:29
@Ben In the WSM it say I should see 1 volt AC at idle, 6.5 ACv at 44km/h and 6.4 ACv at 88 km/h on ECU pin 58. — Robert S. Barnes, Sep 03 '16 at 21:09
If your model is anything like the US model 626 (which it may be, you referenced the VSS signal on pin 58 of the ECM. It's the same in the US wiring diagram.) The two problems are probably unrelated and you need to locate the VSS. On US models it's located by the dipstick tube. I'd still suspect a wiring problem seeing as it's somewhat intermittent. — Ben, Jul 27 '17 at 00:11
@Ben There is no VSS. The European / Worldwide GF models uses the same tranny as the US 2.5L, the GF4A-EL. I have the ATSG manual for the tranny, and in the spot where the VSS should be there is just a plastic plug. That's why I'm assuming the ECU is using the ABS wheel speed sensors in place of the VSS. — Robert S. Barnes, Jul 27 '17 at 06:12
They must be different then. Even on the US 2.5l engine with the GF4A-EL transmission it shows a VSS on the center of the transaxle. https://www.dropbox.com/s/hqxpdvugsolm51q/Untitled.png?dl=0 and the wiring diagrams are almost identical. — Ben, Jul 27 '17 at 22:18
@Ben Then does it make sense that in the absence of a VSS, the ABS wheel speed sensors are being used instead? — Robert S. Barnes, Jul 29 '17 at 18:39
I've never seen it done like that on a Nissan, but it's possible. Hyundai uses the RF WSS as a speed sensor signal. So maybe. You'd have to consult our wiring diagrams. — Ben, Jul 29 '17 at 18:42

VSS accuracy at low speeds

Background

Experiment

While researching this...

Questions

0 Answers0

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