The 90 service manual gives a routine to pull RABS codes. When the light comes on, don't turn the truck off before reading the codes. Put it in park, chock both front & rear of the wheels, note whether the red brake light is lit also, then momentarily ground the test connector.

On my 90, the test connector comes out of a large loom under the dash, coming through the firewall, near the parking brake. On mine there are 2 similar connectors in that area, the correct one has only 1 black wire with an orange tracer running to it, & was hidden on top of the loom. The correct one is hanging down on the bottom right of the 90 pic below. 92 has the test plug on the passenger side. See jermil01's 92 pics below.

After you've grounded & ungrounded the wire, the yellow abs light starts to flash. Count the flashes. It said to not count the 1st series, because it may start in the middle. The last flash of the series is a long flash. Then it repeats over & over, until you turn the key off.

Count all of the short flashes & the long one together (So 4 short & 1 long is code #5). If I remember right, it should be a number between 2 & 15 or so. In Steve83's Brakes & Hubs album, he has the code definitions & other info for RABS-II near the bottom.

There are also more testing instructions in the manual that I can look up after you know what the code is, & whether the red brake light is on or off.

C Testing Procedure from the 1990 Ford Truck Shop Manual. Use this section when both Yellow & Red lights are on.

C1
-Low brake fluid

Brake fluid level ok = Go to test C2
Brake fluid level low = Check for fluid leaks & repair. Fill system

C2
-Master cylinder float -Check float for buoyancy
1- Remove cap from master cylinder.
2-Using a clean steel implement, push down on float in reservoir.

Float moves down = Go to test C3
Float does not move down (Sits at the bottom of the reservoir) = Replace master cylinder reservoir.

C3
-Diode/resistor element -Check for proper functioning of the diode/resistor element.
1- Turn ignition key to the on position.
2- Check parking brake & release if applied.

Both ANTILOCK & BRAKE warning lamps go off = Replace RABS diode/resistor element.
Both ANTILOCK & BRAKE warning lamps stay on = Go to test C4.

C4
-Diode resistor element continued -Continue to check for proper functioning of the diode/resistor element.
1- Remove the parking brake switch & the diesel low vacuum switch, if so equipped.

Both ANTILOCK & BRAKE warning lamps go off = Replace RABS diode/resistor element.
Both ANTILOCK & BRAKE warning lamps stay on = Go to test C5

C5
-Antilock valve switch
-Obtain the flashout code as described in Diagnosis & Testing in this section.
(See the section above for pulling RABS codes)

Flashout code is obtained = Refer to Flashout Codes Charts
ANTILOCK & BRAKE warning lamps stay on steady = Go to test C6

C6
-Master cylinder switch
-Check for the proper functioning of the master cylinder level indicator switch.
1- Remove the connector from the master cylinder.
2- Connect a jumper wire between the 2 purple/white wires in the connector.
3- Turn the ignition key to the on position.

ANTILOCK & BRAKE warning lamps stay on = Go to test C7
ANTILOCK & BRAKE warning lamps go off = Replace the master cylinder reservoir.

C7
-Brake Light Wiring
-Check for shorts in brake light wiring.
1- Disconnect module harness connector from module
2- Turn ignition key to the on position.

Antilock light goes off & Brake light stays on = Check for short to ground in the 977 circuit. Refer to wiring diagram in this section.
Both ANTILOCK & BRAKE warning lamps go off = Replace module.

D1
-Obtain the Flashout Code as described in Diagnosis & Testing in this section.
(See the section above for pulling RABS codes)

Flashout code cannot be obtained = Go to test D2.
Flashout Code is obtained = Refer to the Flashout Code charts.

D2
-Master Cylinder Connector
Make sure master cylinder connector is fully plugged in.

Master cylinder connector is not fully plugged in = Plug in the master cylinder connector.
Master cylinder connector is plugged in = Go to test D3.

D3
-RABS 20 Amp Fuse
-Remove & inspect the RABS 20 amp fuse.

Fuse is OK = Replace fuse. Go to test D4.
Fuse is blown = Short to ground between the fuse & the module wiring harness. Repair short in the 601 circuit & replace the 20 amp fuse. Refer to the wiring diagrams in this section.

D4
-Shorts in Module Harness Connector Wiring
-Check for wiring short to ground.
1- Turn ignition switch to the "on" position.
2- Remove the module harness connector from the module.
3- Observe the REAR ANTILOCK light.

Light goes off = Go to test D5.
Light remains on = Check for a short in the 603 circuit. Refer to the wiring diagram in this section.
D5 Section "D" is incomplete at this time.

4a
-Check for closed RABS valve switch
1-Disconnect RABS valve harness connector from valve connector.
2- Place ohmmeter on the 20k ohms scale.
3-Check resistance between valve connector switch pin & valve body.

Valve connector -Pin View

. . . . . . . . . . . .. . . ._____
. . . . . . . . . . . . .. ./>. O.>.\
Valve switch pin ---l-O . >.O>l
. . . . . . . . . . . . . .l . ..O. .. .l
. . . . . . . . .__l__. .l________l
. . . . . . . . . . . .____
. . . . . . . . . . . . ._

Resistance is greater than 10k ohms = Go to test 4b.
Resistance is less than 10k ohms = Replace RABS valve

4b
Check for short between RABS ground switch & valve ground lead
1- Set the ohmmeter on 20k ohm scale.
2- Check resistance between the valve connector switch pin and valve solenoid ground pin.

Valve connector -Pin View

. . . . . . . . . . . .. . . ._____
. . . . . . . . . . . . .. ./>. O.>.\
Valve switch pin ---l-O . >.O>l
. . . . . . . . . . . . . .l . ..O----l---- Valve solenoid ground pin
. . . . . . . . . . . . . .l________l

Resistance is greater than 10k ohms = Go to test 4c.
Resistance is less than 10k ohms = Replace RABS valve

4c
Check for rabs valve switch wire shorted to ground or module
1- Disconnect battery.
2- Set the ohmmeter on the 200k ohm scale.
3- Disconnect the module harness from the module.
4- Check the resistance between harness connector pin 6 and chassis ground.

Module harness connector -Pin view

__________________________
l.........................................l
l...14 13 12 . . . . . . 11 10 9?..l
l...O . O .O . . . . . . .O . O O...l
l ........................................l
l...O . O .O . O .O . O .O . O. ..l
l...8 . 7 . 6 . 5 . 4 . 3 . 2 . 1....l
\ ....................................../
..\______________________/


Resistance is greater than 100k ohms = Replace computer module.
Resistance is less than 100k ohms = Repair short in 535 circuit, valve switch wire from valve to computer module.