My 2000 F150 XLT Supercab came with 4 wheel ABS brakes. The 5.4 engine has about 7500 miles on it. Until this weekend, I didn't have the need to slam on the brakes. I was confident that the brakes would not lockup on the wet pavement where I was driving. However, when I slammed on my brakes, low and behold the right rear, followed by the left rear locked up! Fortunately, the traffic was very light early Saturday morning and I was alone in my truck. Needless to say, I was not a happy camper. But, dismissed it as an isolated incident.

Well after testing it on DRY pavement, all four tires locked, but I did hear the ABS kick in after about 3 seconds of skidding. This continued after repeated attempts to engage the ABS.

Is this considered "normal"?


Thanks, Jeff

 

I am not sure but I think that the abs will not work if you are going below a certain speed. I don't know what the speed is though. How fast were you going?

 

My speed was between 40 and 50mph during the testing of the ABS. I've read other posts about the ABS sensor connections being loose and damp pads/discs/drums causing lockups.

I'll have the dealer check on Wednesday when I go in for my leaf spring pack and torque converter shudder.

Thanks, Jeff

 

All 4 wheels should intermittently skid in the wet or dry; feeling like pulsations. There is still short skids. I am impressed with my truck's system. When I avoided a collision (on dry), I practically broke my wrist on the steering wheel it stopped so fast.

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2000 F-150 XL,RC,LB,5.4,4R70W,3.55LS,
Class III tow/Payload #3/Convenience pkgs.,
4-wheel disc/ABS,Chestnut/Parchment 40/60,
Ford bedliner & gas/wheel/spare locks,
3" cold air box modification,Superchip,
Dynomax ultra-flow welded 3" cat-back.

 

Max:

No short skids on mine. It is at the dealership getting checked. I'm sure it is something minor.

Thanks,

jeff

 

The ABS should kick in as soon as it detects a wheel lockup. There is no noticeable delay.

 

Close.

ABS-350:

A properly working ABS will detect impending wheel lockup and attempt to maintain the deceleration of the wheel(s) at a speed which is at or about 80% of vehicle speed.

The idea is to maintain wheel rotation while providing braking (deceleration).

A given tire's friction/traction is greatest when it is at about 80% of vehicle speed. Once a tire loses grip ('slides' or 'skids') and loses rotation, it loses directional stability (the all-important ability to steer the vehicle around another).

From a "panic"-application on even-coefficient surfaces (tire-to-ground traction similar for left and right sides of the vehicle), the instantaneous shift of weight to the front of the vehicle usually means more grip on those tires.

Herein starts the chain of events.

The rear wheels lose grip due to the loss/shifting of weight on the rear axle.

No kidding, you say.

But it's not just that simple. There is a clock that starts with the brake applicaton -- it calculates the 'theoretical' deceleration rate at which a pre-defined vehicle should be able to decelerate.

Wheel-speed sensors set up at least three modes of measurement -- the overall vehicle deceleration and individual wheel deceleration.

Any difference in wheel speeds (Differential Wheel Speed) is identified and compared against the theoretical deceleration rate.

As long as things appear 'normal', and at least one wheel is allowed to roll and it's maximum speed is used as a reference speed for the vehicle, the rest of the brakes (each take their turn) attempt to make that magic 80% wheel vs vehicle speed.

When the brakes momentarily lock -- due to myriad inputs which include a stronger than expected effectiveness of the brake(s), then the brakes are instantaneously commanded to release -- and if the wheels don't start to roll up like expected per the reference model, an adaptive delay alghorythm maintains the release until the rotation of the very slow-turning wheel(s) begin to roll up to match the wheel or wheels that have instantly become the vehicle speed reference.

A couple more things come into play -- the possiblity that the rear axle has Limited Slip -- for which the propensity to have both the left and right side slow and speed up in tandem (no DWS on that axle) -- for which the front axle speeds then become the reference speed for the ABS system.

The second of the 'couple' is the likelihood that a 'split co' (coefficient) exists -- where the left and right sides of the vehicle have significantly different traction (like when the driver crosses over into the gravel shoulder, etc). -- keeps the overall system alghorythm calculating and referencing and releasing frantically.

Most ABS's that I have compared have a point near the end of the stop where the vehicle is allowed to lock the wheels (else you'd take that 'eternal half-step' which would have you slowing at an ever decreasing rate -- only to never come to a stop) -- but that point is measured in milliseconds and feet. The added lenght of the stop at the end of the stop is more than offset by the faster than normal braking which is obtained during that theoretical 80% wheel speed early in the stop.

Bottom line: On dry pavement at roadway speeds a full pedal brake application would be expected to merely haul down that truck with no wheel lock apparent -- to the point that the brakes (clean, dry, aggressive pavement) would give you the impression that they were 'weak' due to the lack of skid or noise. An alert person (most are not when surprised) would be aware of the whirring of the hydraulic motor in the ABS -- which may continue to turn for as much as 2 seconds after the vehicle came to a stop.

There is a completely different alghorythm that crosses over if/during extremely slippery traction (ice/snow/water) situations -- and there are many failsafe (love THAT term) operations that attempt to decide whether the truck is stopped or merely has all of the wheels locked up (like on ice) -- and this part of the process is what separates the good from the great AB Sytems.

Think back on that brake application where you were aware of first one rear wheel then the other rear wheel locked up. If you were empty -- and the truck was snubbing very VERY aggressively, the deceleration rate could have been such that the deceleration ramp (actual) and the calculated deceleration rate for the tires remember that 80%) merely reached an itersection at the point where you became aware of the wheel skid (could they have still been turning -- only turning at a rate which allowed a noticeable squeel?)

If so, this is a LONNNNNNnnnng way around the barn to say that your system might have been working very acceptably.

The ABS 'black box' maintains a recording of your stop -- and can and will be used against you in a court of law, etc..

End of ABS-350

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Y2K™

Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"

 

Y2K 7700 4x4:

Thanks for the ABS explanation. BTW, the dealership determined that the rotors were "dirty" so they cleaned them. Have not tested them yet.

Thanks, Jeff

 

When my truck was new I had to hit the brakes hard once in the rain the abs worked fine but lately the front wheels seem to lock up on hard braking but the warning light for the abs doesn't come on when I had lock up so I guess everthing is working right.

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2000 F150 XLT Reg. cab
4.6 Triton Engine
4x2 3.55 rear end gear
sliding rear window
automatic keyless entry
white styleside short box

 

Y2K 7700 4x4,

Thanks for a GREAT (and time-consuming) post. Very educational. Never thought about how limited slip influences ABS.

 

Glad you liked it. I used to teach defense attorneys how abs worked -- since, it seems that when a truck crashes, the driver either: a) couldn't stop or b) couldn't steer -- We sell brake and steering subsystems.

I was intimately involved in the development and testing of abs (we called it Skid Control in the early 70's) for air-braked on-highway vehicles. Our product not only utilized an analog controller (think about Differential Wheel Speed and Adaptive Delay without memory banks and look-up tables common to computers -- It's like the wizardry of automatic transmission control before computers -- only ever-so much faster), the relay valving utilized pneumatic logic which remembered prior cycle information and helped with adjustment, application, and air conservation.

'Black-Box' internal recorders were not used yet -- early controllers were analog (then digital gave way to computer) -- and recorders could have saved a lot of trouble.

As for ABS - There's quite a bit more to it -- but for a mid-level discussion -- the above's reasonably on target.

 

OE812: My ABS kicks in at ANY speed. I tested mine just for fun all last winter in our snow and ice around here. Five mph or any speed, it should work and you should know it is working by the "pulsing pedal vibro foot massage".

The tires may lock but only for a micro-second before the brakes release. On dry pavement, you should hear a "chirping" sound if you stand on it at higher speed. You should NOT hear a screech.

I've owned lots of performance cars (including Volvo, BMW and Porsche) and I think Ford's 4-wheel disc, 4-wheel ABS system on these F-150s is EXCELLENT. Great pedal feel! So, don't settle for anything less than a well-working system.

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Black 2000 SC short-box XLT. 4.2V-6, 5-speed, 3.55 limited-slip. CD, remote entry, factory side steps, FORD box liner.

My third F-series: '73 F-100 Custom 240-cube 3-speed, '98 F-150 XL-SC 4.2 5-speed, and '00 F-150 XLT-SC 4.2 5-speed. GREAT trucks!

 

Dear Shifter:

Of the three trucks -- which had the best engine/tranny/axle setup -- and why?

 

The ABS brakes on my 99 4X2 SuperCab have also locked up at slow speeds, only to work perfectly during a panic stop at high speeds. Must be the nature of the beast.

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1999 F150 SuperCab, 4.6 (Romeo)