Diagnostics

Computer Command & Control system diagrams



84 Corvette Wiring diagram
Diagram part 1
Diagram part 2
Diagram part 3
Diagram part 4

Q:My check engine light comes on, how do I figure out the codes?
A:Connect a jumper wire or paperclip between the Ground and Diagnostics terminal, watch for code 12 three times, then actual error codes flash three times each. Each code is the first digit of the number flashed, a pause, then the second digit of the number flashes. When all codes have been displayed, code 12 will begin to flash again. Read the codes from the chart

The ALDL connector in the 82 Vette





Q:This ALDL port in my car, does it put out serial data that the handheld and PC-based readers can read? If so, which ones work?
A:Yes, there are several items that can be used to read the data. The Auto X-Ray is a handheld unit that costs around $300-$400 but displays limited information on it's small screen. Diacom from Rinda Technologies is a program for your PC/laptop that will display all information on one screen. It costs around $300 for the basic package, and more for an advanced version that does graphing. The best bet  however, is to use Jonas' software called "WinALDL" that only requires a small circuit to be built, and reads data in. It works for both the original ECM's and the upgrade '7747 ECM.


Q:What else can I read from the ALDL port?
A:You can put a test light between +12v and any of the three terminals to check for TCC engagement, Fuel Pump, and A.I.R. valve engagement. Also with grounding the diagnostics terminal to ground you can observe the operation of the O2 sensor and the switching from lean/rich by the ECM. To do this ground the diagnostics terminal and start the engine. The check engine light will flash rapidly when the engine is cold (open loop). When the engine is warm (closed loop) each time the light flashes the O2 sensor detects a lean/rich condition and the ECM reacts to this. (This is called "Cross Counts" - the ECM counts how many times the reading crosses the ideal mixture point.)


Q:My check engine light comes on but when I try to read the diagnostics codes nothing happens, what's wrong?
A:There once was a fable that the ECM would drain the battery, but this is bogus. Some people disconnected the ECM's orange lead from the distribution block in the drivers side rear compartment. Re-attaching the lead will restore power to the ECM's memory. NOTE THIS!!! If this was done to your car the fuel pump will most likely be running all the time with the ignition on. This is done because otherwise the pump isn't controlled by the ECM. Normally the ECM controls the fuel pump when the ignition is turned on. If the engine isn't cranked it will shut off after 3-5 sec. If there is a new lead, from the ign-terminal on the fuse box going to the fuel pump relay, remove this wire and reconnect the original wiring.


Q:My tachometer choke light never lights up, what is wrong with it?
A:This is a non functioning control light. It was cheaper for GM to use the previous year's tacho face with the choke light instead of making a new one without it. The only thing they changed is the added white Cross-Fire Injection text.




Q:I know what my ECM is but where is that thing located in my car?
A:That depends on the car; in the 82 vette the ECM is in the driversside rear storage compartment behind the battery. In the 84 Corvette and the Camaro Z28's, firebirds and trans-ams the ECM is mounted under the dash on the passenger side.

Location of the ECM in the 82 Corvette:(the metal box behind the battery)






Q:I know where my ECM is but what does it do?
A:The ECM is the "brain" of the fuel injection system, it monitors the operation of the engine through the input of different data from sensors and controls the injection system, the transmission and the emissions systems with output to the different actuators/solenoids in the system.

The '82 ECM is the 1225550, the 84 manual is the 1226430 and the 84 auto is the 1226026.

The ECM is devided into three major sections:

1. The ROM (READ ONLY MEMORY.)
2. The PROM (PROGRAMMABLE READ ONLY MEMORY) also known as CALPAK or simply "the chip".
3. The RAM (RANDOM ACCESS MEMORY).

The ROM contains the basic set of instructions for the ECM to follow. The programming in this chip cannot be erased or changed. The ROM is NON Volatile.

The PROM works along with the ROM to fine-tune the functions of fuel and timing control. The PROM can be replaced by a "power" chip (also a prom) or an EPROM (or even EEPROM) from wich the data can be erased and changed. The PROM is NON Volatile.

The RAM has three primary functions:

1. To act as scratchpad (temporary memory) in case of a mathematical calculation.
2. To store BLM (Block Learn Multiplier) information when the engine is off or the ECM is in open loop.
3. To store fault (trouble) codes. These codes are stored for 50 starts or whenever the power (orange wire on juction block for '82 vette) is disconnected. Unlike the ROM and PROM the RAM is volatile.

The ECM controls the injection system, it fuctions in either one of these different modes:

1.Shutdown Mode..
When the ignition is off for 2 or more seconds. Only the RAM (trouble codes) and the BLM remain powered.

2.Startup Mode.
When engine is cranked. Timing is locked at initial starup and air/fuel ratio is enriched. This mode is on for about two seconds after starting to ensure the engine will continue to run.

3. CLOSED LOOP.
When the engine is warmed up, the oxygen sensor is heated up properly and a certain amount of time has passed. The ECM will read all the data coming from the different sensors and will regulate the engine with this information.

In closed loop mode the ECM has 2 differend modes;

a. Enleanment Mode<.br> When the throttle is closed.

b. Enrichment Mode.

At wide open throttle.
High engine load.
High engine RPM.
Overheating engine. (the cold fuel is a great contributor to engine cooling)

The ECM is normally in enleanment mode, it constantly lowers fuel addition. When the oxygen sensor detects a lean condition enrichment mdode is entered untill the oxygen sensor doesn't sense a lean condition anymore. Than enleanment is entered again. This (theoretic) gaurantees an "ideal" air/fuel ratio.

4. OPEN LOOP.
When the engine hasn't warmed up, the oxygen sensor isn't warmed up properly or a certain amount of time hasn't passed. The ECM is controlling engine variables through the BLM function of the RAM.

5. Fuel cutoff Mode.
When the throttle is closed and the MAP sensor notices a negative load (deceleration) untill about 1500 RPM..

6. Clear flood Mode.
When throttle is depressed more than 80% while cranking. Injectors are turned off or air/fuel ratio is lowered to 20:1.

7. Limp Home Mode.
When there is major trouble with any of the critical sensors/actuators.


 

List of ECM trouble codes for the 1982 and 1984 L83 Cross-Fire Injection engine

 

This is a table of the trouble codes used in most GM cars. They do NOT apply to cars using OBD-2 (ON Board Diagnostics system 2)

CODE

DESCRIPTION

12

No tach signal to ECM

13

Oxygen sensor

14

Coolant sensor circuit voltage low

15

Coolant sensor circuit voltage high

16

NOT USED

17

NOT USED

18

NOT USED

19

NOT USED

20

NOT USED

21

Throttle position sensor voltage high

22

Throttle position sensor voltage low

23

Manifold Air temperature high NOT USED

24

Vehicle speed sensor

25

Manifold Air temperature low NOT USED

26

NOT USED

27

NOT USED

28

NOT USED

29

NOT USED

30

NOT USED

31

NOT USED

32

EGR system malfunction

33

MAF/MAP sensor too high

34

MAF/MAP sensor too low

35

Idle speed control error

36

MAF burnoff problem (BOSH) NOT USED

37

NOT USED

38

NOT USED

39

NOT USED

40

NOT USED

41

Interruption signal tach to ECM

42

EST (Electronic Spark Timing) problem

43

ESC (Electronic Spark Control) problem/Knock sensor

44

Lean exhaust condition

45

Rich exhaust condition

46

NOT USED

47

NOT USED

48

NOT USED

49

NOT USED

50

NOT USED

51

Defective/improper installed Prom

52

CALPAK (MEMCAL) missing

53

High battery voltage

54

Low fuelpump voltage

55

ECM defective

56

NOT USED

57

NOT USED

58

NOT USED

59

NOT USED

60

NOT USED

61

NOT USED

62

NOT USED

63

MAP too high (2.8L genII only) NOT USED

64

MAP too low (2.8L genII only) NOT USED



Location of the different sensors in the engine compartment.




Q:How do I test my TPS sensor:
A:Get a test tool from one of the major parts suppliers, connect it between the tps and the harness and attach a digital multi meter to the red and black wires, measure for DC voltage in the 2v range, set your TPS to .525v +or- .025v at idle position. The reading should be around 4.5v at WOT. Also you can check the resistance "curve" with the blue and black wire.







Q:How do I test my MAP sensor:
A:Disconnect MAP connector with engine running, idle should increase. If not, replace it.
The map sensor is the right one, the other is the hood louvre solenoid relais.




Q:How do I test my Coolant sensor:
A:Hook up DMM and measure the resistance, tne check if it reads the corret value for a given temperature (will insert table later) A little trick Twinnie always uses is to remove the coolant sensor, measure the resistance between the 2 poles and put the sensor in a cup of hot water. The resistance should increase.




Q:How do I check my EGR valve?
A:Start the engine and apply the throttle and feel if the diaphragm (you can feel this from under the EGR) moves DO NOT PUSH IT UP YOURSELF. If not disconnect the vacuum line and feel for vacuum. If there's no vacuum check the functioning of the switching solenoid. If that's okay your vacuum line is clogged or disconnected somewhere. If you do have vacuum shut off the engine and try to push the diaphragm up with your fingers. If it's stuck replace the EGR valve.

Here you can see the EGR valve mounted on the manifold with the EGR solenoid to the right, below the throttle body.





Here's another pic of the EGR solenoid:






Q:How do I check my PCV valve?
A:Remove the valve and shake it. If you hear a rattle the valve is okay otherwise replace it.








Q:How do I check the crankcase evap active carbon filter in my air cleaner?
A:Remove the clamp on the inside and take out the filter. If it looks messy (don't be fooled, the black porous stuff is active carbon) try to blow through it. If it's very hard to blow through replace it. These filters are very hard to find. Twinnie removed his filter and the bent tube. He then installed a normal breather cap on the valve cover. This can be tricky since the A/C compressor and pressure lines can be in the way.

Here you see the EGR valve and the active carbon filter mounted on the aircleaner



Q:How do I test my IAC valve(s):
A:The IAC motors can be tested with a special test tool, the tool isn't for sale but can easily be made with the following diagram (ken, diagram). If you suspect the IAC valves are bad simply replace them. The IAC actuating of the ECM can be checked with a simple method:
- Start engine, allow to warm up.
- Disconnect one of 2 IAC connectors (doesn't matter which since they are wires parallel) and ground one of the 4 terminals with a 12V test light, one at a time. The light should flash on all 4 connectors. If not measure the resistance between terminal A&B and C&D, it should be higher than 500 ohms. If not the ECM is shot and should be replaced. Also look into the idle air passages (the kidney shaped holes) with a flashlight and look if you can see the IAC valve's pintle. Now apply the throttle, the pintle should move in (opening the hole) if not check if the IAC valves are stuck and replace them if necessary. Also check the actuating of the ECM with the above described method.








Q:How do I clean my IAC valves and IAC passages
A:Just clean the pintle with a cloth and spray the spring with carb cleaner. DO NOT TRY TO UNSCREW THE PINTLE. To clean the IAC passages spray an amount of brake cleaner in the passages (with the engine running)

Here's picture of the IAC valve (or stepper motor) with it's pintle.




Q:How do I check the functioning of the Knock sensor and the ESC (Electronic Spark Control) ?
A:Connect a timing light to the no 1 ignition wire. Start the engine and notice the timing, then with a hammer strike the engine block and check if the timing retards. If not replace the knock sensor. The ESC almost never goes bad. If this still doesn't help replace the ESC.





Q:How do I test the operation of the auxiliary fan?
A:Turn on the ignition and take off the connector from the passenger side coolant temperature sensor in the cylinder head. Ground the 2 poles on the block. The fan should kick in immediatly.





Q:How do I test the oil temperature and coolant temperature gauges?
A:Take off their respective connector, driversside coolant temp sensor for coolant temp gauge and oil temp connector next to oil filter for oil gauge. Then ground them on the engine block and watch what the gauges do, they should fully deflect.