4

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:

DTC 1500 indicates that the VSS input signal was intermittent. Possible Causes - Damaged VSS • Damaged ECM (PCM) • Intermittent VSS connections • Intermittent open in VSS harness circuit(s) • Intermittent short in VSS harness circuit(s)

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:

Speedometer Readings

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

  1. 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?
  2. 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.

  • No this is not normal. Bad sensor or wiring issue. – Moab Aug 31 '16 at 12:09
  • @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: mechanics.stackexchange.com/questions/35843/… – 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
  • @Ben Only thing I've got is a cheap multimeter. – Robert S. Barnes Aug 31 '16 at 13:41
  • 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

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