First make sure there are no leaks that open under boost. You could be getting boost, but the leak opens up under pressure and vents it to the atmosphere. Pay attention to the intake tubes and the metal tube between the turbo and the intercooler where it passes under the over heat switch for the compressor in the top radiator hose. If the clamps here rub the tube, they will eventually wear a hole that will behave like this for a while.

Second, listen for a click from the solenoid valve when the key is turned on. The APC should send power to the solenoid and it has a permanent ground, so with the key on, one audible click should come from it. If not, check the fuses and then the wiring to the APC solenoid. One wire should have 12V with the key on and the other ground. Check each one for shorting to ground or power. It is common for them to lose their insulation and keep the solenoid from energizing. If the solenoid clicks, verify that it is actually opening its passageways, remove the hoses coming from it. You should be able to blow through the hose going to the center nipple on the solenoid ( marked C for Charge) and feel the air come out the left nipple( marked R for Return) With the solenoid closed ( key off or solenoid unplugged), this passage should be closed, no air between C and R. But you should have air flowing between C and W (Wastegate, right nipple) Check the routing of the hoses, R goes to the nipple on the intake tube, C goes to the nipple on the turbo's compressor, W goes to the wastegate actuator, below the turbo.

If all that is working and connected correctly. you can quickly rule out the knock detector by unbolting it, leaving it connected to its wiring, wrapping it in a rag, and securing it for a test drive. Do this just for the test, leaving it this way will eliminate any knock detection. If you now get boost, the knock detector or its wiring is bad. Inspect the detector for cracks in its housing. There was an update for early detectors requiring a new detector and hold down bolt, and a bulletin about interference in early harnesses to the detector that were not shielded properly. A kit is available to repair the harness if needed. Make sure to route the harness as far from the starter/alternator wiring as possible, the amps in these wires can induce voltage in the knock detector harness.

Check the pressure transducer next. You will need a pressure gauge, an ohm meter and a Mity Vac. With the key on, disconnect the vacuum line going to the transducer at the manifold. Pump up .5 Bar pressure. The APC solenoid should chatter away. If the solenoid chatters, the APC control unit and the pressure transducer are working. To get deeper into the transducer, you will need to measure its resistance at various pressures. On 82 1/2 to 83s, disconnect the wiring to the transducer and measure directly off its terminals. On 83-85, measure the resistance at pins 8 and 9 of the APC control unit with the unit disconnected. On 86 up, measure between pins 10 and 23 of the APC unit with it unplugged. At atmosphere, the spec is 5-13 ohms. Then pump up a pressure of 1.0 Bar (15 psi) and let it off to .6 Bar (9 psi). The resistance should be 88 ohms +/- .5.

If you have cruise control, make sure the cruise boost limiting switch is correct. If you have a older manual and a blue capped vacuum switch on the left fender well (upshift light actuator), the red capped boost limiting switch can plug into it and you will only have base boost if the two connectors are swapped. The wiring for the upshift light vacuum switch is gray/white and white and is a smaller gauge wire than the yellow/white wires to the boost limit switch. With cruise on, the limit switch interrupts the power to the APC solenoid(that is if the cruise vacuum lines have vacuum in them).

Make sure the APC box is not getting a brake light signal on 86 and up cars. If you apply the brakes, the APC dumps boost. Check pin 20 for voltage, if there is voltage here, the APC thinks the brakes are on.

Check the rpm signal into the APC unit. On 82 -85 it will come in on pin 7, a blue/red wire. On 86 up, it comes in on pin 25 a blue/red wire. 86s are special with their ignition amp relay, so a couple of more steps are needed on them if the signal is missing.

Check for power into the APC unit on pin 14 of the 82 (off fuse 16) and pin 14 till 86 off fuse 10. 86 and up comes in on pin 13 from fuse 10.

Check ground for the unit on pin 6 of 82-85 and pin 5 of 86 and up.

You can check the knock detector at the APC connector on 82-85 at pins 2 and 3 (pin 1 is the shield). Look for ac voltage when you tap on the block. It will be a small voltage, reason interference is so common on this signal. On 86 and up, check it on pins 16 and 17 with pin 4 being the shield.

If you go through all this and it still doesn't work, chances are the APC box is bad.

To check base boost, unplug the solenoid, attach a pressure gauge to a manifold vacuum source (one behind the throttle plate) and go drive hard up a hill in third gear. Note the highest pressure you see. The specs vary as follows