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Ford Rear Anti-Lock BrakesPosted 1/18/1999By Jamie MacFarland
The Ford Aerostar has been equipped with rear anti-lock brakes (RABS) since 1990. Though all of Ford's rear anti-lock systems are basically the same, one mild difference is the addition of a pressure differential switch (Delta P) found in the Aerostar's fluid level circuit. This Delta P switch is found in one of two places: On 1990-1991 Aerostars, the Delta P switch is found screwed into the front of the master cylinder. On 1992 and newer Aerostars, this switch is located on the combination valve assembly. In either location, the job of the Delta P switch is to alert the driver and RABS module in the event of a hydraulic problem. The Delta P switch has given many technicians headaches, mostly due to the lack of proper documentation (or complete absence of documentation) in many service manuals. Typically, if there's a problem in the fluid level circuit, customers will complain that the anti-lock warning light is on or both the red brake warning light and the anti-lock warning light are on. The bottom line on this circuit: The RABS module needs to see battery voltage at pin No. 2. And if it does not, the anti-lock system assumes that there's a problem and illuminates the anti-lock warning lamp. The problem that slows down many technicians during diagnostics when this happens is the absence of a trouble code for this fault (on pre-1992 RABs 1 vehicles). This lack of a trouble code, coupled with the often poorly documented testing procedures, has led to the unnecessary replacement of many anti-lock modules. Beginning in 1992, the RABS 2 system began incorporating a code 12 - the result of low voltage to pin No. 2 of the RABS module. Note: It is possible to get a code 12 on a pre-1992 Aerostar if the anti-lock module has been replaced because a RABS 2 module is what you'll get if you go to the parts department for a module replacement. The RABS 2 module is compatible for older models and so is used as a replacement for the earlier RABS 1 modules. The two most common problems you'll encounter in the fluid level circuit of an Aerostar is 1) a poor connection at the fluid level switch located under the master cylinder reservoir (resulting in only the anti-lock warning lamp being illuminated) or 2) the Delta P switch is grounding out the circuit (resulting in both the red brake warning lamp and the anti-lock warning lamp). This Delta P switch is designed to ground the circuit when there's a pressure difference between the primary and secondary hydraulic circuits (broken brake line, etc.). Often the Delta P switch will ground out the fluid level circuit - when in reality there is no hydraulic problem at all. To see if the Delta P switch is the cause of the illuminated warning lamp(s), disconnect the two-wire connector (on the Delta P switch) and jump the two wires together. If the light sequence returns to normal, the Delta P switch is the cause. To save time: Before you even try to pull a code, verify that the red brake warning lamp illuminates when the ignition switch is turned to the crank position. This is a bulb check function incorporated in the red brake warning lamp circuit. If you don't see the red brake warning lamp when you crank the engine over ... find out why not! Chances are good that the lack of illumination during the bulb check routine is the result of a poor connection at the fluid level circuit. And remember, on pre-1992 Aerostars, there is no trouble code for this condition!
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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