feat: abs sensor.

content/post/repairs/abs-sensor.md

@@ -0,0 +1,87 @@
+---
+title: "Tracking down a faulty ABS sensor"
+date: 2022-06-17T20:57:41+01:00
+draft: false
+categories: ["Repairs", "Engineering"]
+---
+
+Like many ageing Honda Civics our VSA and ABS lights have been coming on
+sporadically for the last few months.  Now they're stuck on all the time.
+
+First I suspected the battery, so I duly measured the no-load voltage (~12.6V:
+not great, but not awful either), the voltage dip when running the fuel pumps,
+(~11.6V) and the voltage dip when starting (briefly around 10V, but I didn't
+have a scope on it).  Yep, the battery is ageing.  What about its internal
+resistance?  Everyone did this at school, right---you put a load across the
+battery, measure the current through the load and the voltage across the
+terminals before and after; then you assume that voltage was dropped across the
+internal resistance and do R=V/I.  The trouble is that the voltage drop is tiny,
+so you need a reasonable current.  I couldn't find any decent loads, but *any*
+load will do if you have an ammeter in series, so I disconnected the negative
+lead and used the car itself.  Since the alarm promptly started going off (next
+time lock the car door!) that formed one data point, and then I turned the
+running lights on for the other.  With the multimeter in relative mode even the
+subtraction was done for me:
+
+| dV    | I    | R      |
+|-------|------|--------|
+| 0.06V | 0.9A | 67mOhm |
+| 0.3   | 6A   | 50mOhm |
+
+Neither is ideal, but both are perfectly respectable for a starting battery.
+Anyhow, it seems that measuring battery health this way is a pretty dark
+science---one paper I saw claimed that the usual means of measuring AC impedance
+and then taking the real component gave wildly different values from the DC
+resistance, so when I found the batteries had died (!) in my capacitor ESR meter
+I put it back.  It's not calibrated, so I would have had to find a bunch of
+10ohm resistors to make up a 100mOhm calibration unit, anyhow.
+
+Further confirmation that it's not the battery came when trying with a charger
+across it, and a lithium ion jumpstarter which once rescued us on a motorbike on
+the side of the road.  The warning lights refused to go away.  There's a
+procedure for resetting them without a code reader---ground the SDS pin on the
+odb connector or stuff some foil in the socket for the same purpose on the
+fusebox and play with the brake and the key.  It clearly worked, but the light
+came straight back again.
+
+Thus I turned to the sensors themselves.
+
+{{<figure src="/img/abs/setup.jpg">}}
+
+All the wheels are different, to be annoying, and I had to work one wheel at a
+time as the pavement grounds my low profile jack on one side.  I also jacked
+from the pinch welds for the first time and promptly bent them flat, but never
+mind, they'll work just as well like that.  I mostly use them for axle stands
+anyway.
+
+With some wires and a lot of annoying poking around I managed to get connections
+to both the hidden pins and the hidden sockets.  Front right worked perfectly:
+
+{{<gallery caption-effect="fade">}}
+  {{<figure src="/img/abs/dc.jpg">}}
+  {{<figure src="/img/abs/ac.jpg">}}
+{{</gallery>}}
+
+My notes claim the sensor measured 1.5 M Ohms, but on the basis that the other
+three working sensors were in the 280-300k range I wonder if I didn't misread.
+Here you can see the search protocol from the abs controller:
+
+
+{{<figure src="/img/abs/detection.jpg">}}
+
+(Apologies for the trace being upside down.)  There are two 11V peaks, which
+clearly are used to measure drawn current, and then if the sensor is detected
+the line is held high at 11v and the wheel movements are modulated on to it.
+The rear right sensor had the two peaks (so the wiring's good) but nothing I
+could do could make it stay high after that.  The sensor also measures 2 M Ohms
+(hence the comment about misreading the front right).
+
+Thus a new sensor is in the post for £10, and we'll have to see if it can be
+fitted easily.  These sensors are very simple---hall effect probes---so
+hopefully the inside of the drum isn't full of rust or something preventing it
+working.  The *outside* of the brake and the whole rear of the car certainly
+shows its age.
+
+And digital oscilloscopes are really good fun---this would have been a nightmare
+with the old crt scope.  Now I just need to learn how to take screenshots
+without the screenshot bar on the right hand side...