That was my first guess...the rear brake lights

 

Well, I've driven the Aero a few times and the Rear ABS idiot light is "OFF"!!! Hooray!!! The light bulb seems to have been the issue. Thank you to everyone for all of your help!!

Now we are on to working on figuring out what is making a hard "Thump/Clunk" from the front end AND what is going on with my front brakes making a terrible grinding noise. Ahhhh... gotta love owning Fords.... (fix or repair daily )

 

hard "Thump/Clunk"
1. worn out upper/lower ball joints or control arm bushings
or
2. worn out half shaft joints
3. defective AWD shaft speed sensor (2 of these gremlins) on top of transfer case or sensor wiring

front brakes making a terrible grinding noise
1. sticking pistons in calipers from breakdown/cracking of phenolic/plastic pistons
or
2. water corrosion in system blocking brake fluid return somewhere in system
3. front brake flex lines broken down internally blocking fluid return preventing brake piston pad return
4. Pads worn down metal against metal

hows the cowboy hubby holding out on the Aero repairs? glad to hear those cowkid girls are doing great.

 

1990 Aerostar E4WD 6CL 4.0L RABS

Had both warning lights on in dashboard, yellow Rear-Anti-Lock and read Brakes.

Fluid level OK, both break light bulbs work, hand break fully released.

Took the vehicle to the shop, front and rear breaks mechanically OK.

Pulled the harness of the RABS module attached to the right above the throttle pedal --> yellow Rear Anti-Lock light off. Plugged the harness back in --> yellow light on again.

At the front of the break fluid reservoir, there is black connector with two black wires. With ignition off, wiggled that, pulled/ pushed back & forth a bit, turned the key into ignition position - both warning lights gone.

With ignition off, pulled the connector off, saw greenish corrosion inside its female pins. With a tiny round metal brush (needle, safety pin or tiny screw driver should work, too), brushed the connectors' inside. With a small pair of pliers, crimped each one a bit tighter.

Pushed connector back on, ignition on, all fine.

(just as a side note: on my 1990 4L Aerostar, there is also a 3-wire-harness at the right side of the break fluid reservoir, but disconnecting/ reconnecting that one did not help)

Hope it stays that way.

 

That's great to finally fix the little problem.

That switch on the front of the master cylinder is for the shuttle valve that shuts off either the front or back fluid circuits if one of them should leak. If the shuttle valve shuttles, the switch is supposed to turn on the brake light. That switch port is not water proof, so moisture can build up in there and generate the corrosion that shorts the switch out, causing the brake light to come on. That will disable all the functions that check the brake system before working, such as the cruise control, 4th gear torque converter lock up, and (obviously) the ABS.

The other connector on the side of the master cylinder is the fluid level detector. Both switches have provisions to prevent you from disabling them by unplugging the connectors. I guess if the connectors themselves are corroded, that can prevent them from making proper contact with the switches, triggering the brake lights.

 

thanks a lot xlt4wd90 for the background explanation!

Just turned the key into ignition position to verify - both lights still off.

Just wondering, do you or anybody know the exact name of that switch - shuttle valve switch?

 

Exactly that: the shuttle valve switch. Most cars today have one in the master cylinder.

 

got the green snot corrosion in the shuttle valve switch or shuttle valve stuck to one side?

Time to inspect and replace all the front calipers and rear brake cylinders or at least their seals and completely flush the whole system. Water/moisture/water vapor contaminated. Will rust the whole system out from the inside.

 

thanks 96_4wdr! I'm selling this vehicle, so the next owner can do

 

How does the shuttle valve switch come apart?

 

The switch itself doesn't really come apart. It just screws into the front of the master cylinder.

 

Thanks for the replies but mine seems to screw in to the top of the combination valve. Should I just strip the two black wires enough to jumper and see if lights go off. Here's something else; I cleaned my ebrake lever switch again. No issues but dash red brake indicator stays on dim. I assume the shuttle switch is the same as a Delta P?

 

1. Cleaned and lubricated ebrake switch. Dash Brake light illuminates bright when set. Dim when not set.
2. Was able to pull the top connector from the shuttle switch. Jumpered the shuttle valve switch connector. RABS light and dim brake dash lights remain on.
3. Checked rear wheel cylinders, no leaks.
4. Brake fluid level fine, pumped brakes several times, could not find a line leak either.
5. Checked all brake and vehicle lights, all fine.
6. Depressed steering column red hazard switch, only works on all four corners if I have on turn right signal. No clue why this is.
7. After driving van for a few miles RABS light would go off then reappear if I set the e-brake.

Will check brake pedal switch and fuse block next.

 

To fool the warning circuit at the shuttle valve switch, you need to pull the switch but leave the connector on. If that shuts off the brake warning light, then the problem is with the switch. You can be sure by using a continuity tester between one of the pins on the switch and ground. If there is continuity, then the switch is causing the warning light. It's possible that the switch may be either tripped or it's shorting to ground through corrosion. Pull the switch and see if there is any corrosion in the hole it's screwed into. Clean those out, and any corrosion that may be on the tip of the switch. Otherwise, the valve may have shuttled, and you need to re-center it.

I've reset the shuttle valve by cracking a brake bleeder at either the front or rear brakes and tapping the brake pedal. I connect a Fluke meter with a continuity beeper between one of the pins of the switch and ground. It should beep, telling me that the shuttle valve is making contact with the switch. I tap the brake pedal to force the valve to move. If I picked the correct end of the car to open the bleeder, the valve would move. If not, the valve does nothing, and I'd have to try a valve on the other end of the car. The trick is to tap it so that it moves enough to stop the Fluke from beeping; that's when the valve is centered. If I tap it too hard, it would shuttle to the other side, and the Fluke would again beep, and I would have to close off the opened bleeder valve, and open one on the other end of the car and try again.

You said you checked all the bulbs, does that include the brake lights? I've had trouble with RABS/CC/TC lock due to burnt out brake light filaments.

If these don't switch off the brake warning lights, then the problem is somewhere else, which I have no experience with.

 

So if I pull the switch from the top of the combination valve (directly below master cylinder) brake fluid won't leak out? And yes, I check all bulbs and their operation. Any reason the dash warning lights would stay on after I pulled fuse #7?