ABS System Operation
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Joined: Jul 2012
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ABS System Operation
I was wondering if anyone has a description of operation on the ABS system for a 1997 F150. The truck is not mine, but really hasn't been taken care of by anyone that should ever been allowed to touch tools. 
While I'm at it, I'll go ahead, and throw the truck's anomaly out there, as well; The ABS light stays on constantly. The truck used to have some brakes with the light on, and kindof operated kindof hinky, as it the pedal would occasionally go down. I hadn't driven the truck in awhile, but now the pedal goes all the way to the floor. Putting the truck in gear, the brakes have enough power to keep the truck from moving, but get it rolling, and it pretty much just slows to a stop... frightening. Before I came into the picture, the brake hoses had been changed, and I'm thinking the pads along the way, as it used to be a roadworthy vehicle (farm use now). However, the ABS light was on before that. I can hear the booster working when I push on the pedal. I'm thinking the master cylinder is ok. I don't have a lot of experience on ABS other than changing calipers or pads. A little insight to help me troubleshoot the gremlins from this truck would be much appreciated. Thanks!
While I'm at it, I'll go ahead, and throw the truck's anomaly out there, as well; The ABS light stays on constantly. The truck used to have some brakes with the light on, and kindof operated kindof hinky, as it the pedal would occasionally go down. I hadn't driven the truck in awhile, but now the pedal goes all the way to the floor. Putting the truck in gear, the brakes have enough power to keep the truck from moving, but get it rolling, and it pretty much just slows to a stop... frightening. Before I came into the picture, the brake hoses had been changed, and I'm thinking the pads along the way, as it used to be a roadworthy vehicle (farm use now). However, the ABS light was on before that. I can hear the booster working when I push on the pedal. I'm thinking the master cylinder is ok. I don't have a lot of experience on ABS other than changing calipers or pads. A little insight to help me troubleshoot the gremlins from this truck would be much appreciated. Thanks!
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Joined: Apr 2011
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By what you've said, have you bled the system? If some of the lines have been replaced, that would be my starting point. Most abs systems use wheel speed sensors to decide what wheel is moving at a different speed and adjust pressure to even them out. Check the wires going to the sensors. If that does not work, you can blindly start replacing sensors/components, or find a shop with a computer that can monitor the wheel speed while driving. My wife's caravan had the abs light on, we hooked up the monitor and drove it around the block. Three wheels turning at little variation, one stopped at zero (what the cars CPU sees) replace that sensor and all is good.
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Thanks for the response. From what I understand, the system was bled, and there is clean fluid in the reservoir. I have a code reader, I don't think it does anything other than emissions/PCM codes, though. What I'd like to find is a diagram or something that tells me what all is in the system, and how they all talk to each other. I'm thinking it might be the modulator, as it supposedly controls the brake pressure to the wheel cylinders. From what I've been reading, it's kindof a weak link/easily damaged component in the system.