This is from my 2000 XJ (yeah, I have a '99 and a '00)
I bought it a few months ago and had to replace Evap stuff (they were actually missing).
Been driving it a few hundred miles since then with no problem.
Yesterday, all of a sudden while driving down the road the speedo and tach both drop to zero,
CEL comes on and the airbag light too.
Speedo & Tach would come off / on intermittently all the way home.
At times the LED odometer would display NO BUS.

I know that the code are indications that the ECM / PCM is either sending false signals or going bad.
The odometer ad 375K on it but the PO said that the engine has been swapped and only has 120K on it.

Where is the PCM located any way? Is it accessible to replace it?

 

The PCM is next to the airbox. Dont mess with that just yet. I Pulled this from another forum.

P1694 (M) No CCD Messages Received From ECM Bus communication failure to PCM.

P1698 No CCD Messages Received From PCM Bus communication failure to PCM. A "Companion DTC" was set in both the ECM and PCM.

Possible Causes
CCD Bus (-) circuit open between PCM and TCM.
CCD Bus (+) circuit open between PCM and TCM.
Transmission CCD bus wiring damaged.
Faulty TCM.

Diagnostic Procedure
1. Using scan tool, clear TCM DTCs. Start engine and let idle for at least 2 minutes. Using scan tool, retrieve TCM DTCs. If DTC P1694 returns, go to next step. If DTC P1694 does not return, go to step 5.

2. Check instrument cluster gauges for proper operation. If fuel gauge and voltage meter operate properly, go to next step. If fuel gauge and voltage meter do not operate properly, test the CPS.

3. Turn ignition off. Disconnect Gray PCM harness connector. PCM is located at left front of engine compartment. Disconnect TCM harness connector. Measure resistance of CCD BUS (-) circuit (White/Black wire) between TCM harness connector and Gray PCM harness connector. The 97-01 TCM is under the dash near your right kneecap. If resistance is more than 5 ohms, repair open in CCD BUS (-) circuit (White/Black wire). If resistance is 5 ohms or less, go to next step.

4. Measure resistance of CCD BUS (+) circuit (Violet/Brown wire) between TCM harness connector and Gray PCM harness connector. If resistance is more than 5 ohms, repair open in CCd BUS (+) circuit (Violet/Brown wire). If resistance is 5 ohms or less, replace TCM.

5. Conditions necessary to set DTC P1694 are currently not present. Inspect related wiring and harness connectors. See WIRING DIAGRAMS. Repair as necessary. If no problem is found, check for broken, bent, backed-out or corroded terminal pins. Check for chafed, pierced or partially broken wires inside insulation the wire harness to the NSS. Repair as necessary.

CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to the 10K-or-20K scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.