OK. I'll answer my own thread. As i suspected replacing the ABS surge protect under the passenger seat was the fix (my car is a 92 build near the 91/92 transition). Getting one was hard - there were none on ebay globally. I got one from someone parting out a car on Craigslist.

I opened up the old one (which was difficult) and the contact was not moving when I teased it with 12V. I reflowed the solder joints with no change. It seemed like the coil perhaps had blown although the fuse was intact. If was impossible to access it enough to really see. The whole unit seemed rather overengineered. It basically seemed to be a 12V/10amp relay with resistor. In case anyone needs the definitive ABS test procedure I've included it below.

ABS FAULT TRACING


1990-93 Volvo 240 (and probably others of the same era e.g. 740, 940)
The Control Unit (CU) contains a monitoring circuit to detect any internal faults within the control unit, as well as electrical faults in the sensors, solenoids, modulator unit, etc.
If the monitoring circuit detects a fault, the control unit will disable the ABS system and light the warning lamp on the dashboard. If the light comes on during vehicle operation, perform the following checks. All tests must be performed, in order.
1. Remove the soundproofing under the left side of the dashboard. Inspect the 10 amp fuse on the transient surge protector, located adjacent to the ABS control unit.

2. Check all connectors, wires and ground connections for the ABS system. Inspect the connectors at each component.

3. Insure that the ignition is switched OFF .
a. At the CU under the dashboard, depress the lock spring and swing out the connector, disconnecting it from the CU.

b. Remove the cover from the connector. Remove the white protective moldings from the sides of the connector.

c. Use an ohmmeter to check ground circuits. Terminal numbers are stamped into the side of the connector. The ABS wiring grounds at the left A-pillar. Test between ground and terminals 10, 20, 32 and 34.




Never check connectors from the front or terminal side; damage may be caused. Always check through the holes in the side of the connector without using excessive force to make contact.
d. Resistance should be 0 ohms in all cases. If any other reading is encountered, check for damaged wiring or improper connections. Wires are grounded on the left A-post.

e. If a fault is found at terminal 32, replace the solenoid relay on the hydraulic modulator and retest.



4. Check the transient surge protector:
a. Turn the ignition switch ON .

b. Connect a voltmeter between ground and terminal 1 on the control unit connector; 12 volts should be present.

c. If no voltage is present, measure voltage directly at the transient surge protector connector. Terminals 1, 2 and 4 should be energized and terminal 3 should be grounded.

d. If only terminal 1 and 4 are energized when terminal 3 is grounded, the transient surge protector has failed and must be replaced.



5. Check the power supply to the control unit connector:
a. Connect the voltmeter to a known good ground. Depress the brake pedal and at the same time, connect the meter to terminals 25, 27, 28 and 29.

b. The voltmeter should read 12 volts at all terminals, except terminal 29.

c. Voltage should read 0.5-1.0 volt at terminal 29.



6. Start the engine. The voltmeter should read 12 volts at terminal 25.

7. If no voltage or incorrect voltage is found in the above tests, proceed as follows. For a problem at:
a. Terminal 25: Check brake lamp switch and replace if needed. Inspect brake light bulbs and replace as needed.

b. Terminal 27: Replace defective solenoid relay.

c. Terminal 28: Replace defective pump relay.

d. Terminal 29: If reading at 27 is correct, voltage at 29 should be 0.5-1.0 volts. If not, replace solenoid relay.



8. Turn the ignition switch OFF .

9. Check voltage to the hydraulic modulator:
a. Remove the cover from the hydraulic modulator. Detach the connector from the hydraulic modulator.

b. Switch the ignition ON .

c. Connect the voltmeter to a know good ground and to terminals 6, 7, 10 and 12. Voltage in all cases should be 12 volts.



10. If there is no voltage or improper voltage to any terminal, proceed as follows. For a problem at:
a. Terminal 6: Inspect wiring for shorts and/or poor connections.

b. Terminal 7: Attach the connector to the hydraulic modulator with the ignition OFF . Turn the ignition ON when connected; the ABS warning lamp on the dashboard should come on. If not, replace the warning lamp bulb.

c. Terminal 10: Transient surge protector failed.

d. Terminal 12: Inspect wiring for shorts and/or poor connections.



11. Turn the ignition OFF ; attach the connector to the hydraulic modulator.

12. At the control unit connector, use an ohmmeter to measure the resistance of each wheel speed sensor.
a. Test the left front sensor between terminals 4 and 6. Test the right front sensor between terminals 11 and 21.

b. Resistance for the front sensors must be 900-2200 ohms (0.9-2.2 kilohms). If the resistance is not within specifications, unfasten the harness connectors in the engine compartment and measure resistance directly at the sensor. If the readings still differ, inspect the wiring and/or replace the sensor. Also, check the pulse wheels for defects or damage; maximum radial run-out is 0.006 inch (0.15mm).

c. Measure resistance of the rear speed sensor by testing at terminals 7 and 9. Resistance should be 600-1600 ohms (0.6-1.6 kilohms).

d. If resistance is not correct, detach the sensor connector on the fuel filler pipe in the trunk. It will be necessary to break the connector seal; do so without damaging the wiring. If readings are still not within specification, inspect wiring and/or replace the sensor.



13. Check the wiring to each sensor.
a. Raise and safely support the vehicle.

b. Connect an ohmmeter to the pairs of terminals use in Step 11.

c. As each pair is tested, have an assistant turn the correct wheel on the vehicle including the rear. Rotate the wheel(s) about one revolution per second; the resistance should vary as the wheel spins.



14. Test the hydraulic modulator solenoid valves: Connect one ohmmeter lead to terminal 32 on the control unit connector. Connect the other test lead to terminal 2 (LF solenoid), then 35 (RF) and then 18 (rear); Resistance should be 0.7-1.7 ohms.

15. Test the pump relay in the hydraulic modulator:
a. Turn the ignition ON .

b. Connect a jumper between terminal 28 on the control unit connector and ground. The pump should run.




Do not maintain the connection longer than 2 seconds; damage can occur.
c. Repeat the test. Simultaneously, measure the voltage between terminal 14 and ground. With the jumper grounding terminal 28, 12 volts should be present.

d. If the modulator does not start, inspect the wiring and connectors. If no fault is found, replace the pump relay and retest.



16. Test the valve relay in the hydraulic modulator:
a. Connect the voltmeter between terminal 32 on the control unit connector and ground. Use a jumper wire to connect terminal 27 to ground. The valve relay on the hydraulic modulator should activate (listen for distinct click) and the voltmeter should show 12 volts.

b. If the relay does not energize, or the correct voltage is not present, inspect the wiring and connectors carefully.

c. If no wiring fault is found, replace the valve relay.



17. Turn the ignition switch OFF . Disconnect all test equipment.

18. If no faults were found during testing, replace the ABS control unit and retest.

19. Reinstall the hydraulic modulator cover and the soundproofing at the left side of the dashboard.

20. Road test the vehicle and confirm proper system operation.