OBD II Standardization wasn’t required till 96 however each manufacturer often had its own way to diagnose problems.
Your “OBD I” plug would need a special Chrysler code reader which you Wil never be able to find in 2020 however, you can do the “Key Dance” and it will tell you the diagnostic codes.
1. Cycle the ignition key 3 times to ON-OFF-ON-OFF-ON within 5 seconds.
2. You will notice the "check engine" lamp start to flash. Count the amount of times it flashes and record it. The number of flashes represents the code. There will be a slight pause between the flashes representing the first and second digits of the code. Longer pauses separate individual codes. For example, flash-flash-flash [pause] flash-flash represents code 32.
At the very end, you will receive the code “55”. This essentially means diagnostic over. If this is all you get then you no error codes stored in the computer. This is the normal condition
Remember, disconnecting the battery for periods at a time will clear the computer.
Similarly, if you for instance had code “46: alternator output too high.” The computer will store that code for about 3-5 start ups after the last time it was in a output too high state. So if you’ve corrected the problem, and have driven the truck a bunch of times, the could be have cleared.
(Some of these codes changed during the years from 84-96) so I would reference your service manual for specifics.
CODE Descriptions
11 Timing belt skipped 1 tooth or more from initial learned value, Intermittent loss of either camshaft or crankshaft position sensor, No crank reference signal detected during engine cranking
12 Direct battery input to PCM was disconnected within the last 50 key-on cycles (normal)
13 No change in MAP from start to run
14 MAP sensor voltage too low
15 No vehicle speed sensor signal
17 Closed loop temp not reached or engine cold too long
21 02s sensor problem (oxygen sensor), upstream or down stream
22 Engine coolant temp sensor out of range
23 Intake air temp sensor out of range
24 Throttle Position sensor out of range
25 Idle air control motor circuits, target idle not reached (+/- 200),vacuum leak found
27 Injector control circuit
31 Evap purge flow monitor failure or evap solenoid circuit
32 EGR system failure
33 A/C clutch relay circuit
34 Speed control Solenoid circuits
35 Rad Fan control relay circuit
37 Torque converter clutch solenoid CKT or park/neutral switch failure
41 Generator field not switching properly
42 Fuel pump relay control ckt, Auto shutdown relay control ckt, No ASD relay output voltage at PCM, Fuel level sending unit volts out of range, Fuel level unit No change over miles
43 Multiple/ single cylinder misfire
44 Battery temp sensor volts out of range
45 Overdrive solenoid shorted or open (automatic transmission only)
46 Charging system voltage too high
47 Charging system voltage too low
51 Fuel system lean
52 Fuel system rich
53 Internal control failure
54 No Cam signal at PCM
55 End of error messages (If you get this only, no errors were found)
62 PCM failure SRI mile not stored
63 PCM Failure EEPROM write denied
64 Catalytic Converter Efficiency Failure
65 Power steering switch failure
The Allpar site has a list of al the codes
Chrysler Computer Codes, 1980s-1990s