My '93 has the RABS lite on when I use the brakes and the brake light comes on when I use the brakes hard.
While searching the topic I stumbled on a reference to bypassing theRABS with a piece of brake line, can anyone give me the particulars on this?
Thanks.
Hartski

 

Why would you want to bypass it. The light is likely coming on because your fluid reservoir is low.

This can occur either because there is a leak somewhere, or the brake pads are highly worn. In either case, fill the brake fluid reservoir back to full. If you have a leak, find it and fix it. If you pads or whose are worn, replace them.

Disabling RABS is a bad idea unless you intend on using the van as a race car or for off road use.

 

The pads and shoes are new, the reservoir is full, and there are no leaks.
Hartski.

 

it could be many things; One is the rear differential Sensor. A pain to get to, it is on top of the differential. It could be a bad ABS module, Ford had nothing but troubles with this rear ABS system. Even trained technicians couldn't fix them. Totally undocumented.

go to this Forum: Ford Explorer and Ranger Enthusiast Forums - Serious Explorations then find the Aerostar forum, and read the thread "How to: Solve ABS problems on a 1995 Aerostar". It's all there.

 

Another problem is when people compress the calipers and cylinders without opening the bleed screws. Forces fluid through the RABS valve backwards, which damages the valve. This is especially true if the hoses have started to break down in the inside, since the gunk will get pushed through the valve.

If the reservoir is full, and the pads and brakes are new, then either the RABS valve has been damaged, or the ABS sensor is bad. Either way, you are better off fixing it than trying to bypass the system. Aerostar can get really dangerous really fast if the system is bypassed. The problem is they must be balanced properly if bypassed, and the rear brakes which are drums, expand as the heat up, which causes them to change behavior under hard braking, where they can suddenly lock up, and that under the wrong conditions can send the vehicle into a slide. They try to front bias the brakes as a failsafe, but bypassing can mess with the bias. In either case, under hard braking, your stopping distance is much better with RABS than without.

 

It could vey well be I damaged the valves, for I didn't open the bleeder screws on any of the brakes.
Where is that valve located and how do I test it?
Hartski

 

Should be the only valve with a solenoid on it.

 

Oh, yeah, I've seen it. Perhaps I'll pull it out and give it a visual. I'm quite familiar with solenoid valves from the tractorsi work on.
Hartski

 

The RABS valve has two solenoids which control two valves an isolation valve and a dump valve.

The isolation valve disconnects (or isolates) the rear brakes from the master cylinder and the dump valve bleeds pressure from the rears after they're isolated into a reservoir on the RABS valve.

It's not a conventional ABS system where positive pressure is produced by an ABS pump.

It could be your proportioning valve is sticking. If the fluid pressure from front to back isn't in balance there is a switch inside that will trigger the brake light try pulling the connector on it and go for a drive. If the brake light stops coming on check your proportioning valve switch.

 

The brake warning system is designed so that if you pull the connector from the shuttle valve switch, it will also trigger a brake warning light. If you pushed the shuttle valve to one side of the master cylinder due to a pressure imbalance, the best thing to do is to try to push it back to the middle. You can do this by connecting continuity tester between one of the pins on the shuttle valve switch and ground, and then crack a bleed screw on either a front caliper or rear slave cylinder, and tapping on the pedal until the continuity tester shows the circuit is open. I usually use the beeping mode of a DMM.

The other possible cause for the warning light is buildup of corrosion inside the chamber that the shuttle valve switch is screwed into. You can unscrew the switch to see if crud has built on it. If so, clean it off, and re-install, and the brake warning light should stop lighting up. If not, that wasn't the problem.

 

I have the schematic in front of me and the brake light won't come on if you pull the connector from the proportioning valve switch. Maybe you're referring to a different vehicle?

The circuit is of a simple design where the switch completes the path to ground for the brake indicator light (through the brake warning diode) when the switch is activated.

Removing the connector will prevent the path to ground and if the prop valve switch is indeed triggering the light it shouldn't come on anymore.

Also since you mentioned the ABS light comes on as well you may be able to get more info by retrieving any code the ABS module has.

The RABS light is probably coming on when the fault is present and turning off when it rectifies itself. While the light does turn off the module should still have the code.

Locate the RABS test connector (it's a 1 wire 2 sided connector) it's behind the left side of the instrument panel taped to the main harness. Turn the ignition switch to run FIRST then separate the connector there will be two halves one half with a red wire and another with a black / orange wire. With a jumper wire connect the black / orange to a good ground for 2 seconds then disconnect the jumper.

The module will then flash the codes via the RABS light the codes are a series of short flashes followed by one long flash. For example short, short, short, long is code 4.

Code list:

2 - Open Isolation Valve Circuit
3 - Open Dump Valve Circuit
4 - Open Or Grounded Valve Reset Switch
5 - Excessive Dump Solenoid Activity
6 - Erratic Speed Sensor While Driving
7 - No Isolation Solenoid During Self Check
8 - No Dump Solenoid During Self Check
9 - High Speed Sensor Resistance
10 - Low Speed Sensor Resistance
11 - Brakelight Switch Always Closed or Worn / Damaged
12 - Loss Of Hydraulic Fluid For One Second Or More During Anti-Lock Stop
13 - Control Module Failure
16 - System Okay

If you wish to preserve the codes in the ABS computers memory reconnect the two halves of the diagnostic connector BEFORE turning off the ignition switch.

If you separate the diagnostic connector halves before turning the ignition to run when initially retrieving the codes or if you turn the ignition off before reconnecting the two halves of the diagnostic connector the trouble codes in memory will be erased.