3-1-1 is the ECU not getting a vehicle speed signal. It can indicate an intermittent wiring fault that, for one reason or another, is not always evident to the driver via the speedometer. I mentioned it because it has been a source of trouble with my 940's ABS.

Looking at the 1991 wiring diagram, I see the ABS computer is at the head of the line for the signal from the rear sensor. Sensor>ABS computer>signal converter>speedometer>ECU

The ABS greenbook TP-31678 has seven pages of system checks to perform with a multimeter in order to determine the source of a fault. (A little too much to retype in full)

some basic checks lifted from greenbook:

with ignition off:
-disconnect harness connector from ABS computer, and connector terminals
10, 20, 32 ,34 should show ZERO ohms to ground

ignition on:
-on ABS relay, terminal 1, 2, and 4 should show battery voltage to ground. Terminal 3 should show ZERO ohms to ground. Relay must be in connector, you've got to slide the probes up the back side of the connector and check from the rear.
-on computer connector, you should see battery voltage between terminals 7 and 9, indicating correct function of converter unit.
-should see battery voltage between terminals 27 and ground, 28 and ground, and _with brake pedal depressed_ terminal 25 and ground.
-should see .5-1.0 volts between 29 and ground.
-start engine, check for battery voltage between 15 and ground.

ignition off:
-check front sensors at computer connector. should see .9 to 2.2 kilo-ohms between terminals 4 and 6 (front left) and between 11 and 21 (front right).
-check rear sensor at computer connector. should see .6 to 1.6 kilo-ohms between terminals 7 and 9.

If you've never run tests at a harness conenctor before, be aware that the correct way to check is thru the test ports on the SIDES of the connector. There's a hard plastic comb-shaped thingy stuck in there, usually in a contrasting color plastic, which should be gently pried out to expose test ports. You DON'T want to be poking probes into the delicate spring-connectors that mate with the blades on the computer.

good luck!

john
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1989 245 245K / 1993 945 131K