Confusion over brake fluids: Which is right for your car?

  • Article by: PAUL BRAND
  • Updated: January 20, 2012 - 3:50 PM

How do synthetic DOT 3 or DOT 4 fluids differ from the conventional DOT 3 or DOT 4 ones?

Q I see many brake fluids on the market are now synthetic DOT 3 or DOT 4. How does that differ from the conventional DOT 3 or DOT 4 fluid? How does that differ from silicone brake fluid? And my main question: Does synthetic brake fluid "eat the paint" if spilled on the car?

A I think much of the confusion over brake fluids is whether "synthetic" is the same as "silicone" -- and it's not. All DOT 3, DOT 4 and DOT 5.1 fluids (the numbers refer to U.S. Department of Transportation specifications) are conventional glycol/ester-based hydraulic fluids, meaning they can and will damage paint if spilled on automotive bodywork. All are synthetic in the sense that they are blends, but the "synthetic" label on some indicates they are more resistant to absorbing moisture -- a characteristic of all conventional brake fluids and the reason many carmakers recommend periodic flushing, bleeding and replacement.

Silicone brake fluid -- DOT 5 -- is silicone-based with a high boiling point suitable for motorsports, isn't sensitive to absorbing moisture and doesn't damage paint. But it is not compatible with conventional fluids.

Q I had to replace the blend door motor in the climate control system of my '97 Town Car. Everything worked well, but when warm weather arrived in the spring I started having a problem. When the car sits in the sun and you start it up, you can hear the familiar clicking sound of the blend door motor trying to seek a position -- similar to what I heard when the gears in the motor were bad. If I quickly raise the temperature setting on the control to a high value, the motor will stop doing this. As the car cools down, I can slowly lower this setting, but if it's 80 or higher outside, I can never lower it to below 70 or so. I think it's because the temperature sensor for the climate control is at fault. The air conditioning in the car works great otherwise.

A Start by using the self-diagnosing capabilities built into the climate control system. With the car's interior stabilized near room temperature, enter the self-test mode by pressing the "Off" and "Floor" buttons simultaneously, then release and within two seconds push the "Auto" button to read fault codes. Hitting the "Defrost" button cancels the diagnostic mode and deletes any codes. Codes 024/025, along with the clicking noise, indicate a bad blend door actuator. Codes 030/031 indicate an in-car temperature sensor problem, and codes 040/041/042/043 point toward the ambient temperature sensor. Codes 050/052 indicate a sunload sensor problem.

Keeping in mind the car's age, I can't help but wonder about using an aluminized dashboard sun shield in place while the car is parked in the sun. I know it would be cheap!

Q I own a 2004 PT Cruiser. Lately there has been a noise coming from the front left driver's side wheel. I replaced two tires and had all the tires rotated and balanced. As speed is increased, the "we-we-we" sound increases and can be heard to 30 miles an hour and above. I have had the vehicle to the garage twice and was assured that everything is OK. Yet, the sound still persists.

A If you and the dealer can eliminate the wheels and tires as the origin of the noise, I would focus on the left front disc brake/rotor assembly. As you are cruising along and hearing the "we-we-we" sound, maintain that speed while lightly pushing the brake pedal with your left foot. If this lessens or eliminates the noise, it's coming from the brakes. Possible causes include rust and debris buildup, a slight warping of the rotor, worn brake pads or the audible brake wear indicator tab rubbing on the rotor.

The sealed front hub and bearing assembly or the drive axle and constant velocity (CV) joint are also suspects, but focus on the brakes first.

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