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The problem goes this way. The braking of the Kia spectra is normal when the road is dry and ABS does not light up even during hard braking. Braking fluid is full, and have been changed 5 months ago. The brake guide pistons are smooth and lubricated fine, I did not get any error code on the OBDII.
However, once it rained, the brake calipers are wet and things start to go weird. The steering wheel starts to vibrate on moderate braking, I can sense that the car is NOT skidding and the wheel is not locking. The ABS seems to be kicking in even though with light braking, this makes the braking distance much longer and very dangerous as the car may hit the car in front with a compromised ABS. The problem is ABS light did not light up and I have hard time trying to brake the car!
I had the workshop checked the brakes, hydraulics, brake pads and calipers and all checks out fine, and the problem will miraculously disappeared once the rain stopped and the water dried up. I could not check the ABS control unit and speed sensors as these cost a lot here and I have no means to finding out the exact cause of this problem. Can anyone give some pointers?
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Do you wait until the last minute to brake, or do you gradually brake over a distance?
If you are heavy on the brakes late into the stop, that could easily set off the ABS.
Assuming your ABS is working properly, it might be that your tires are in really bad shape. When braking, there’s a forward weight transfer which unloads the rear wheels, alowing them to lock up easily if the tires are pretty bald and the road is wet. It doesn’t take much.
The sensors may be good, but the connectors or wires to those sensors may be letting water in and shorting out. The weak signal can cause the ABS unit to raise a false alarm that the wheel(s) are locking up when you try to slow down.
On a dry day, spray a hose over and behind each wheel. Get the whole wheel and brake assembly really wet, then immediately go for a drive and test the brakes. If the problem shows up same as it does in the rain, there’s likely an electrical problem triggered by the water.
Could be an electrical problem, after all it is a 10 year old car. I do have a multimeter and could try to diagnose it before ordering a replacement parts. It is just that I does not want to get a wrong replacement part when it does not fix the issue. How can I test if the speed sensor and the abs mcu is working fine?
I know the hydraulics brake lines are alright because I am able to do hard braking on a dry day, the tires have deep thread depth, and the stopping distance is normal. But once the wheels are wet, I get the juddering effect while braking, it is kinda scary that you lose braking power with the ABS kicking in even though the wheels are not locking. The braking distance is at least twice the wet braking distance and there is tendency for the car to sway to the left.
With just a multimeter, it will be tough to test the sensors. They are passive, in that they don’t rely on external power and instead generate all their signal output.
Each wheel hub has a tone ring, a wheel with a bunch of magnet “teeth” around the outside. As the wheel turns, the magnets pass by the sensor, where a coil generates pulses of electricity from the changing magnetic fields. The ABS controller counts the pulses from each wheel, and a sudden change in the speed of the pulses indicates a wheel that has locked up or started spinning. The sensors are simple, but the signals are quite weak, so any water getting in the connectors could short it out.
An oscilloscope would let you see the pulses as the wheel turns, and maybe point out which sensor(s) are being affected by the water. If you can find or borrow one, that would be a big help in tracking down the problem.
If you can’t get an oscilloscope, take the wheels off and find the sensors and their wires. Check the connectors for corrosion or dirt and check the wires for cracks or chafing. Cleaning the contacts, taping over any broken insulation, and putting dielectric grease in the connectors to seal out water might solve the problem.
In that case, start with a visual inspection of the wires and connectors. Tape up damaged wire insulation, clean off corrosion from the contacts, and seal the connectors with dielectric grease. Careful you don’t get any grease on the brake rotors.
I checked the wires, contacts and they are fine. I measured the resistance of the front left and right speed sensor, they are 1.46 and 1.47 KOhms. According to the service manual I found, the sensors are within range. This also shows the electrical connections are good.
However, without an oscilloscope, I am unable to check if the sensor give a 130mV per rotation. With a multimeter, I could not measure any DC or AC voltages on the sensors.
Another thought from left field. Perhaps it isn’t your ABS at all, but a condition where your brake discs aren’t shedding water effectively. Are your pads slotted or drilled, and are your splash shields in good shape?
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