You don't need a code scanner to get the stored codes. Your multi-meter will work fine. Here is a link that describes how to pull the codes:
http://www.dalidesign.com/hbook/eectest.html
Also, here is a cut/paste of that page but I see that the pictures in the page didn't make the transfer so you'd better use the link above for the full info:
CODE TYPES AND FORMATS
TYPES OF CODES
NOTE: Some people have problems distinguishing the different codes. READ THIS SECTION CAREFULLY.
There are drawings of the code formats at the end of this section.
FAST CODES
Fast codes contain the fault information output in the normal slow codes but are output about 100 times faster.
These are the first things output on a Key On Engine Off test.
HARD FAULTS
Hard faults are problems that the computer has located RIGHT NOW. Examples are a sensor out of range or a broken wire (open circuit).
Hard faults are the FIRST set of slow codes output in a Key On Engine Off test. BEFORE the SEPARATOR pulse.
In a Key On Engine Running test, there are ONLY HARD FAULT codes. These are output right after the fast codes.
SEPARATOR PULSE
The separator pulse is a single pulse that indicates the END of hard fault codes and the BEGINNING of memory codes.
It will show up as a code 10 on most digital testers.
MEMORY CODES
Memory codes are problems that the computer has noticed in the past. If for example there was a loose wire to a solenoid that only lost contact while driving but was making contact while testing the system there would be NO HARD FAULT CODE. The code would show up IN MEMORY. The same would happen for a sensor that only went out of range occasionally. Memory codes come out AFTER the separator pulse.
NOTE: The computer will erase the memory after a certain number of engine re-starts if the problem does not repeat itself. The number of re-starts varies from 20 to 80 depending on the year of the vehicle. The later models keep memory longer.
ENGINE ID
The engine ID in a running test is a series of pulses equal to one half the number of engine cylinders. A 4 cylinder engine ID is 2 pulses, a 6 cylinder ID is 3 pulses and an 8 cylinder ID is 4 pulses. A diesel ID is 5.
GOOSE CODE
A "GOOSE" code (also called a dynamic response test) is output during an engine running test. This is a single pulse to signal you to quickly move the throttle approximately 1/2 way down and release.
NOTE: Not all engines give a "GOOSE" code.
CODE FORMATS
Codes are output as a series of pulses. The following charts show the (approximate) timing of the various code pulses.
NOTE: Two digit codes are shown. Three digit codes have similar timing. It just takes a little practice to distinguish between two and three digit types.
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TESTS
[Top of Page]
NOTE: Vehicle should be fully warmed for all tests.
HOOKUP
See figure below. EEC IV vehicles have two connectors for self testing the system. They are located on the firewall or the left or right front fender. The large connector contains the self test output (STO) and ground (SIG RTN). The small pigtail is the self test input (STI).
KEY ON ENGINE OFF (KOEO) TEST
NOTE: On 4.9L trucks with a manual transmission hold the clutch pedal in during this test.
On Diesel engine trucks hold the throttle to the floor during this test.
1. Make sure engine is fully warmed. If in doubt, run engine at 2000 rpm for 2 minutes.
2. Turn ignition off and wait 10 seconds for system to shut off. Make sure A/C is off and transmission is in Park (automatic) or Neutral (manual).
3. Hook up light and jumper (or a tester if you have one). Turn key to ON (do not start engine).
4. Fast Codes are output (ignore fast light flashes).
NOTE: Unhook self test input jumper (or tester if used) at any time during code output to erase memory.
5. Read hard faults.
6. Separator Pulse.
7. Read memory codes.
8. See code explanations and check components as necessary.
Use FIRST CODE OUTPUT and retest after any repairs are made.
[2 digit codes] [3 digit codes]
KEY ON ENGINE RUNNING (KOER) TEST
1. Make sure engine is fully warmed. If in doubt, run engine at 2000 rpm for 2 minutes.
2. Turn ignition off and wait 10 seconds for system to shut off. Make sure A/C is off and transmission is in Park (automatic) or Neutral (manual).
3. Hook up light and jumper (or tester if you have one).
4. Make sure vehicle is safe to run and start engine.
5. Engine I.D. should be output.
6. Step on brake and turn steering wheel 1/4 turn.
If the vehicle has an overdrive cancel switch, push it.
7. If a "Goose" pulse is received, move throttle quickly 1/2 way down and release.
8. Fast Codes are output (ignore).
9. Read codes.
10. See code explanations and check components as necessary.
Use FIRST CODE OUTPUT and retest after any repairs are made.
[2 digit codes] [3 digit codes]
CYLINDER BALANCE TEST
NOTE: This test is only available on Sequential Fuel Injection (SFI) Engines.
Start an engine running test and press the accelerator lightly within 2 minutes after the last code is output. The PCM will cancel each cylinder in turn and measure the RPM drop. If any weak cylinders are noted their number will be output as a multiple of 10 (e.g. 30, 40). The output is the actual cylinder number, not the number in the firing order. NOTE: Will not always pinpoint bad injectors.
WIGGLE TEST
Engine running or engine off tests
Hook up for a self test but do not hook up the self test trigger. Turn key to on. Hook up the trigger, wait 10 seconds and disconnect. Hook up trigger again . Tap suspected sensors (be careful if engine is running), wiggle the wiring harnesses etc. IF the PCM picks up a fault the self test output will pulse and a memory code will be stored (The value of this is questionable. I NEVER located a problem with it. - JT).
MEMORY ERASE
To erase the memory disconnect the self test trigger while the codes are being output. You COULD also disconnect the battery but then the PCM forgets some important running characteristics.
OUTPUT STATE TEST
NOTE: Do not perform on E4OD diesel.
Run the KOEO test all the way through and leave the test hooked up.
Cycle throttle 3/4 open and closed while watching tester, light or voltmeter. Self test output will switch with every throttle activation. On some testers (like my old pocket testers) the light will only pulse every other push. But the outputs still switch EVERY time.
Solenoids (EGR etc.) will switch on or off with every throttle activation (push throttle they're on, push throttle they're off). You should be able to hear clicks as the solenoids switch states. The test will work for most solenoids: AIRB, AIRD, BOOST, EGR, EVR (NOTE: EVR does not click but vacuum cycles).
1991 Mustang GT has hesitation under acceleration .. what could be the cause? 
