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ABS: Electronic Tips

Monday, September 1, 2008 - 00:00
Brake Tech Brake Tech Series brakes antilock brakes ABS antilock brake systems antilock braking systems anti-lock brakes hydraulic control unit HCU repair shop repair shops technicians automotive aftermarket

What sets modern antilock braking systems (ABS) apart from conventional, hydraulic brakes is their electronic controls. Often, barely-measurable amounts of current can make the difference between full antilock brake systems (ABS) functionality and a tough-to-diagnose problem. Here are some tips — some general, some specific — that are worth remembering when servicing ABS electronics.

See the light

Stop lamps aren't just for letting following drivers know a vehicle is decelerating. In some ABS control schemes, the microprocessor finds out there's a "brake apply request" by detecting a voltage drop as current begins running through those lights. But voltage won't drop in an incomplete circuit — like when bulbs are burned out. Some systems set up this way monitor voltage drop to the stoplights in the taillights; others work with the center high-mount stop lamp (CHMSL). Some use them all. In any event, make sure all glow.

There's a related scenario when a seemingly brake-unrelated vehicle modification can cause antilock brake glitches: adding a rear spoiler with an additional third brakelight. It could cause voltage drop to vary beyond what the ABS computer is looking for. (As if that weren't enough, federal regulations frown on having redundant CHMSLs — even though you see cars with an extra third brakelight every day.) You and the customer may have to decide which light to disconnect, especially if the extra current load causes the ABS to malfunction.

Speaking of lights, the telltale lamps on the instrument panel do more than let the motorist know their foundation or antilock brakes need service. How and when they glow can help you troubleshoot malfunctions.

Hard or soft?

A hard code indicates a present condition and will remain (or recur) even if you clear it, unless you fix its underlying cause. A soft code indicates a problem that may have existed before, but isn't there currently. However, clearing a soft code doesn't necessarily mean you'll never see it again. If the condition it represents is intermittent, the code will come back when the condition comes back.

Once you've repaired a condition represented by a diagnostic trouble code (DTC) and cleared the code from memory, interrogate the system at least once more. Here are two good reasons why. First, you'll verify that the code is gone; and also, because some systems are capable of displaying only a limited number of codes at once, you may see one or more additional DTCs that weren't evident previously. They're not new, they just didn't become accessible until codes further up the conga line were displayed, their causes fixed, then cleared.

Bad information

Computer-controlled systems will do something they shouldn't if they get bad information from their sensors. If that wrong info seems plausible to the computer, it's going to do just what it was programmed to do, without setting a DTC, since it won't think anything is wrong.

Often, this is exactly what's going on when an ABS modulates one or more of the brakes unnecessarily; sometimes this only causes a noise and/or throbbing sensation in the pedal. In some cases, it can lead to pulls on deceleration. In either case, the ABS controller is causing the condition to occur because it thinks the wheels are decelerating too quickly. That's what the wheel speed sensors are telling it.

Does this mean the sensors are bad? Not necessarily. An excessive gap, either because of poor adjustment or a rust buildup that has moved the sensor away from its exciter, can cause false wheel speed sensor (WSS) readings. A damaged exciter can also cause a WSS to send false (but plausible) information to the controller.

Sometimes it's almost routine for a WSS to provide bad information. For example, Ford found that front WSSs on 1995-97 Explorers and 1997 Mountaineers could sometimes cause false ABS activation because of weak signal output. A TSB OK'd changing these trucks over to 1998-spec front WSSs — in pairs, of course.

Watch the picture show

Today's electronic tools are capable of graphically illustrating what's going on with ABS components; use them to "look" at parts you can't see.

For example, while on some vehicles you can easily eyeball a WSS exciter ring, they're impossible to see on others — at least directly. But depending on the application, a lab scope or scanner can show you the shape of that tone ring by mapping its output. You can actually get a clear enough picture to find nicked teeth or cracked rings, and this saves lots of needless disassembly.

RABS and RWAL

Whether it's called Rear Antilock Braking System (RABS) or Rear Wheel Anti-Lock (RWAL), the rear-only ABS used on many trucks until recently are essentially the same Kelsey-Hayes system. Before checking trouble codes or consulting a vehicle-specific flow chart, begin your diagnosis with a simple circuit check.

There are some variations, but also many similarities. Here is a typical GM RWAL test routine:

Block the wheels, release the parking brake, avoid stepping on the brake pedal and turn the ignition key on.

If the warning light illuminates about two seconds then shuts off, the ignition cycle is OK. Step on the brake pedal; the indicator should not come on. If it does, suspect the combination valve.

Check the ABS ECU circuit fuses if the indicator glows steadily after the two-second self-check. Your next step will be to consult the "light on" diagnostic flow chart.

If the indicator either flashes or does not illuminate after the two-second self-check, consult the "light flashing" or "light off" flow chart.

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