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Car Forum / Ford / Ford Trucks / June 2006ABS warning light. 1999 F350, 4x4, V-10
I purchased this truck about a year ago and immediately had a problem with poor braking and the ABS warning light coming on. I had a shop test the ABS system and they told me I needed a new sensor. Replaced rear ABS sensor (this is the only one on this model) right away and still had the ABS warning. I replaced all 4 calipers and brake pad sets, master cylinder, flushed and bled the system repeatedly, checked for loose wires.... brakes are better (although they still seem to take a lot of pressure to hold on a steep hill) and ABS light still comes on after driving a few feet. Any ideas? Thanks John Well, the ABS works by detecting that one or more wheels has stopped turning, and it releases the brake pressure applied to the locked wheel(s). The system requires a sensor on each of the front tires (assuming 4-wheel ABS that I'm not sure whether you have or not), and can have one on the rear axle or driveshaft, or have one on each of the rear tires. There are are different ABS systems that apply ABS to each of the rear wheels separately (a sensor is required on each wheel), and there is a system that applies ABS to both rear wheels even if only one of them is locked up (this allows a single sensor). You said that you only had one rear sensor, and if we take that statement at face value, you have what is termed a 3-channel system. The two front tires get separate sensors, and the two rear tires share the same sensor. You could have a 1-channel system where only the rear axle has ABS, but I thought they stopped doing that a long time ago. You said you replaced lots 'o stuff, and this is generally a good practice. But, none of the stuff you replaced will turn the ABS light off. There are many reasons why the light can come on, but since you stated that it comes on after the truck begins moving, my guess is on one or more speed sensors mounted to the wheels, or a bad wire somewhere. Or, you have a bad ABS controller. >I purchased this truck about a year ago and immediately had a problem with >poor braking and the ABS warning light coming on. I had a shop test the ABS [quoted text clipped - 7 lines] > > John > I purchased this truck about a year ago and immediately had a problem with > poor braking and the ABS warning light coming on. I had a shop test the ABS [quoted text clipped - 5 lines] > steep hill) and ABS light still comes on after driving a few feet. Any > ideas? Thanks What size tires are on it and are they the size listed on the door placard? Whitelightning As long as they are all the same size it would make no difference... >> I purchased this truck about a year ago and immediately had a problem >> with [quoted text clipped - 14 lines] > > Whitelightning > As long as they are all the same size it would make no difference... Yes it can. The abs computer expects to see a voltage of "X" at "Y" mph., there is some give and take there as but if the difference is too much it can trip the ABS light on. That's why the system can be calibrated to take in tire sizes and gear ratios into account. going way large on the tires can degrade braking performance as well. Whitelightning Nope. Admittedly without referring to my FSM's yet, but going by memory from another vehicle I own with ABS: The tone rings generate an AC signal with frequencies that vary with axle rotational speed. ABS sensors are electromagnetic transducers that don't measure voltage, they measure the AC frequency generated from the tone rings. If the ABS computer measures a certain differential in frequencies generated by, say one wheel locking up (frequency dropping on one more than the others), it will activate the ABS. The system is calibrated for gear ratios/tire sizes to correct the speedometer reading. If all the tires were the same size, no matter how big they are, there would be no frequency differential between the various ABS sensors. >> As long as they are all the same size it would make no difference... >> [quoted text clipped - 7 lines] > > Whitelightning Technically, it's a pulse wave, not an AC frequency. Having said that, the subtle difference is the rise and fall time of the signal, and the pulse wave has one end at zero, where the AC wave has zero in the middle. Other than this tiny detail, your description is very good. > Nope. > [quoted text clipped - 24 lines] >> >> Whitelightning In audio we call that "square wave AC with a DC offset" and it's time to repair your amplifiers. Like I said I haven't looked at my Ford FSM but I remember an argument with Bill H. in ramj+w about ABS, he thought a larger tone ring would screw up the ABS. He couldn't understand that if it had the same number of "teeth" the size of the ring would make no difference, at the same RPM the pulse wave would have the same frequency; for example, the tone rings on a Ford truck are on the front axle's stubshafts and rear diff carrier. > Technically, it's a pulse wave, not an AC frequency. Having said that, the > subtle difference is the rise and fall time of the signal, and the pulse [quoted text clipped - 32 lines] >>> >>> Whitelightning I don't see the point in changing the tone rings, but my logic says that if they are all changed then the ABS should still work. What the ABS is looking for is a relatively longer cycle time than the other wheels. this would mean that the tire was turning slower than the others, and was approaching a lock-up condition. Changing the tone rings would not provide any meaningful benefit that I can think of, unless having a very heavy wallet is problematic -- in which case, I can help lighten the wallet and give much more meaningful feedback of the experience. > In audio we call that "square wave AC with a DC offset" and it's time to > repair your amplifiers. [quoted text clipped - 42 lines] >>>> >>>> Whitelightning Changing from say a Dana 35 axle to a 44 would necessitate changing the rings because of the 44's larger axleshafts. >I don't see the point in changing the tone rings, but my logic says that if >they are all changed then the ABS should still work. What the ABS is [quoted text clipped - 54 lines] >>>>> >>>>> Whitelightning >> I purchased this truck about a year ago and immediately had a problem >> with [quoted text clipped - 12 lines] > What size tires are on it and are they the size listed on the door > placard? Tire size will not affect the ABS, unless the tires are different sizes, then other stuff would be affected as well. Thanks for all the responses, but I still need some suggestions as to what else to look for. The tires are the factory spec size. This truck is a 4x4 with dually rear tires. To the best of my knowledge, there is only a single ABS sensor on this truck which sits in the middle of the rear axle - there are no front sensors. I replaced the entire brake system and the rear ABS sensor so its hard to figure out where to go from here. John >I purchased this truck about a year ago and immediately had a problem with >poor braking and the ABS warning light coming on. I had a shop test the ABS [quoted text clipped - 7 lines] > > John At this point, you need somebody with the diagnostic tool to look at the truck. Having said that, I'm surprised there are no sensors on the front tires. This means that you only have Rear Wheel ABS. You can look at the ABS brake controller and count the brake pipes connected to it. > Thanks for all the responses, but I still need some suggestions as to what > else to look for. The tires are the factory spec size. This truck is a [quoted text clipped - 16 lines] >> >> John |
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