Diagnostics help point you to a problem area but cannot always specify exactly what's at fault.

Use a ground wire or a (non self-powered) test light clipped to the intake manifold and, with the TPS connector unplugged and a timing light aimed at the timing marks, touch the #1 pin (orange wire) and look for a 10°-15° jump in timing as you make/break the connection. If the timing is affected, the signal is definitely getting to the ECU. You may simply have a bad ground (#2 wire) or a borderline adjustment which doesn't close the microswitch every time.

No timing change indicates the code is correct and you were on the right track.

Find a helper as you'll need 3 hands.

For '89 B23xF engines with the LH 2.4 FCU, my diagrams show the TPS wires coming in on pin 2 (throttle closed, orange wire) and pin 3 (throttle full open, red-white wire). The middle TPS wire (brown) goes to ground at the fuel rail. The TPS closed wire also goes to the EZK ICU pin 7. Your year should be almost identical.

Before you start cracking open computer edge connectors, there are a couple of simple things you should probably check. Check that the middle wire in the TPS connector cable goes to a good ground (ohmmeter should read zero). Be careful when probing the TPS connector so as not to damage the fragile terminals -probe from the back side of the connector. Find the big connector junction blocks at the right strut brace. You should find one with the TPS going through it (red-white and orange in connector pins 7 & 8). It is not unheard of to have a bad connection at this terminal block. Push in the wire terminals from the back side of both connectors to ensure a good connection. This also makes a good TPS test point if you need it.

If all the connections under the hood are clean and test out then carefully (with ignition off AND all FCU fuses removed to prevent any electrical arcing damage or better yet disconnect the battery) remove the FCU edge connector. Clean the connection with spray contact cleaner (not WD-40 and no wire brushes please), re-assemble and re-test. As for the TPS, do all probing from the back side of the conector to prevent contact damage.

I would not venture further into the FCU or ICU without the Volvo electrical diagram and a fair bit of experience and caution. In order to re-inforce this thinking, call your local Volvo parts dept and get the prices for a replacement FCU and ICU -that ought to make you think twice before attempting exploratory surgery.