What You Can Buy For $350

For $350 you can buy a small laptop with touch screen

LaptopTouch

…or a modern game console

xbox-one

These would be quite advanced pieces of technology with state-of-the-art hardware loaded with complex software and doing a whole lot of useful or just pleasant things.

But when it comes to automotive electronics then all you can get for $350 could be just this

Sensor

This gizmo is a wheel speed sensor for ABS. There is nothing high tech about it, it is an inductive sensor made from few meters of very thin copper wire wounded around small iron core, enclosed in plastic casing with couple of feet double insulated flexible wire and connector on the other end of it.

Its not like something I could make up myself in the shop but certainly it is not even close to laptop or game console in terms of complexity and precision.

This sensor got its plastic cap pulled off upon removal, this is why you can see what is inside it – nothing magical, no gold or diamonds there either

20160302_121421

Not every ABS sensor is that expensive but on the truck I’ve worked on another day this was the price that local dealer quoted to me. Apparently this truck had some rare configuration of the brake system so the sensor was somehow very specific for this application. Add an hour or a bit more of labor on top of that (new sensor is not any good for you if it is not installed where it should be) and resulting bill will give you much less fun than a new game console with couple of good games for the same amount of money.

Lets cut that bill in half then and make it an example of simple diagnostic process. I love doing these things!

So here is the concern: when braking at the low speeds ABS intermittently starts to operate even if the road is not slippery. For unprepared driver this could be quite a scary experience when all of a sudden on a good dry road brakes just go, disappear at the last few meters of braking distance no matter how hard you press on a pedal. Repair story that customer shared with me have been even more scary. Mechanic first said that brake job needs to be done to fix that concern. Done the brake job but concern was still there. Then customer have been told it is a brake servo at fault here and it needs to be replaced. Good part got replaced with new one but the problem remained. So only then that mechanic realized there was something wrong with ABS, but since he did not have an “expensive computer” that would tell him which sensor to replace than he only had an option to replace all of the sensors that were “$500 each”.

I could not believe that idiots people like this actually work in this trade…

facepalm_picard1

After having spent two grand already for brake repairs customer didn’t quite like an idea of spending another two grand on these sensors (he was told that there are four of those sensors) so he just removed ABS power fuse and disabled ABS altogether. Not the best thing for the winter but at least the truck had been braking OK in the summer.

The truck came to my shop for another repair when I noticed the ABS light stays on all the time. Eventually I got “go ahead” from the customer and proceeded with diagnostics. My scan tool (aka “expensive computer”) only found a code for ABS power relay stored in ABS module memory which obviously was caused by ABS power fuse removed. This code cleared as soon as fuse was re-installed and no more other codes were detected. So my “expensive computer” (I’ve paid $5000 for it by the way) is not telling me what sensor to replace, now what?

The thing is, there is no computer that would tell mechanic what part to replace. No matter how much you are willing to pay for it. Scan tool is just that, a tool, an instrument, you need to know how to use it and reading codes is only a beginning of diagnostic process. Somewhat like getting a readings off a volt meter for example.

Like I said it was simple diagnostic. In the first five minutes of a test drive I got a fair idea of what is going on and where, the rest of the diagnostic time was only needed to back up my idea with 100% proof and figure out a repair method.

ABS indeed have been operating almost every time when braking gently from the low speed down to a full stop. I pulled on the scan tool screen wheel speed data and ABS valves operation data (aka PIDs as for Parameter Identification Data) for all three ABS channels (this truck actually only had three sensors, not four)

ScanTool

Right away I noticed discrepancy in Left Front Wheel speed data: while other two sensors are showing that wheels are rotating as vehicle starts moving this sensor still shows zero speed for a short while like if the wheel is not rotating at all. And when it slows down speed signal drops to zero just seconds before truck stops moving and isolation valve (lowermost graph) turns on at the same time indicating ABS operates and cuts off braking pressure to the left front wheel brake caliper. Notice how the rear wheel speed sensor reading goes up at the beginning of each acceleration cycle – this is because road was slippery and rear wheels have been spinning on take-off.

This data is easier to analyze when downloaded to the PC where I can pull out more graphs on the screen simultaneously. Here is the data for all wheel speed sensors and all three isolation valves and again I can see that only left front isolation valve operates on slow down just before the truck stops moving

ScreenScan

Therefore there are strong indications that ABS module can not “see” a signal from the left front sensor at the low speed. Should I replace that sensor now?

No. Next step would be to check resistance of the sensor.

Resistance

This resistance should be compared to the resistance of another sensor, not to the data in service manual. Why? Because in the manual there would be a range of acceptable sensor resistance values and this resistance greatly depends on the temperature of the sensor. I could just grab the tip of the sensor and observe how resistance reading climbs up because of the warmth of my fingers. Sensor on the other side would have about the same temperature and therefore should have the same resistance. Resistances in fact were very close. If the left sensor would have noticeably higher resistance that would indicate problem with its inductive coil or broken wire and explain why the signal have been weak (I am going a bit ahead of the topic here, this is because weak signal was what I suspected right away from the beginning).

Since both sensors had the same resistance than they should have been producing the same signal. It also means that new sensor would hardly fix the problem here – it would be just the same. In fact its an OEM sensor that should, and most likely would be just the same. Alternative sensors from aftermarket parts suppliers might be not made properly and could have different resistance. It would be handy if aftermarket parts suppliers would supply detailed data about their products but they don’t or are very scarce on that data.

Now is the time to check how the signal from sensors looks like. Inductive sensor produces alternating voltage and it is a classic example of a signal to read with a scope. To get a correct reading scope probe should be connected to the signal wire, not the return wire of the sensor. I could have used a wiring diagram to determine which wire on the sensor connector is for signal and which one is for return

Wiring

Although practically it was quicker just disconnect the sensor and check which of two wires that are going to ABS module have low resistance to the ground. Wire with low resistance to the ground is sensor return, wire with high resistance to the ground is for the sensor signal, this is how electronic module works.

With two scope channels connected to the front sensors signal wires and wheels spinning while truck is on the lift the difference in signal amplitude is very apparent – left sensor signal is weak (blue waveform).

Waveform

If you have a sharp eye for details you can also see the small difference in sensor signal frequency – this is because of the differential operation and resistance in CV shafts wheels are rotating at slightly different speeds, left wheel is a bit slower. Although it is not twice as slow that would otherwise explain the difference in signal amplitude.

So here comes the result of diagnostics: sensor itself is OK but the air gap between sensor tip and signal ring in the wheel hub is excessive. The rest of the repair is technicality: remove the sensor, extract sensor tip cap seized in place because of corrosion, glue it back to the sensor, return the air gap back to the proper specs and re-check sensor signal to verify the fix.

 

The resulting whole cost of repair and diagnostics is only a half of the cost of replacing just one sensor, not even mention “four of them” :-). Quite a good savings isn’t it?

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