Should be the A-2 hole. The B-box is for the A/C codes if I remember right. I guess you're asking about the vehicle's speed sensor? Should give you a 3-1-1 code from the A-2 socket. What kind of other symptoms are you seeing?

Here's what the book says:

Code 3-1-1 (Vehicle Speed Sensor Signal)
1) Test drive vehicle. Using Volvo Scan Tool (998-8686)(if you happen to have one of the $3000 jewels), read off VSS signal. If speed is zero MPH, fault is permanent. Go to next step. If speed is okay, fault is intermittent. Go to step 6).

2) Test drive vehicle and ensure speedometer functions. If
speedometer shows zero MPH, speedometer or one of its components is
defective. If speedometer is okay, determine if speedometer is VDO (most likely VDO) or Yazaki style. If speedometer is VDO, go to next step. If speedometer is Yazaki, go to step 4).

3) Turn ignition on. Check if Code 1-3-1 is stored. If Code
1-3-1 is stored, check wiring between instrument cluster connector
terminal A7 and ECM terminal B18 for a short circuit to voltage. If
Code 1-3-1 is not stored, go to next step.

4) Connect breakout box between ECM and ECM connector. Shift
transmission to Neutral. Raise and support front of vehicle. Turn
ignition on. Connect voltmeter between breakout box pins No. 18 and
28. Rotate front wheels. If voltmeter reading varies between .5-5
volts, source of code is poor contact in ECM connector. Repair as
necessary. If voltmeter shows a steady reading of about zero volts,
check wiring between instrument cluster connector terminal A7 and ECM
terminal B18 for a short circuit to ground. If voltmeter shows a
steady reading of greater than 5 volts, on VDO speedometer, check
wiring between instrument cluster connector terminal A7 and ECM
terminal B18 for an open circuit. On Yazaki speedometer, go to next
step.

5) Turn ignition off. Connect an ohmmeter between breakout
box pins No. 18 and 28. If ohmmeter reads about 500 ohms, check wiring
between instrument cluster connector terminal A7 and control module
terminal B18 for a short circuit to voltage. If ohmmeter does not read
500 ohms, check wiring between instrument cluster connector terminal
A7 and control module terminal B18 for an open circuit.

6) Turn ignition on. Check if Code 1-3-1 is stored. If Code
1-3-1 is stored, check wiring between instrument cluster connector
terminal A7 and ECM terminal B18 for an intermittent short circuit to
voltage. If Code 1-3-1 is not stored, check instrument cluster circuit
for intermittent faults. Check ECM and instrument cluster connectors
for an open circuit. Also check wiring between instrument cluster
connector terminal A7 and ECM terminal B18 for an intermittent open
circuit or intermittent short to ground.


Hope this helps. Mike