Jet-Jock

Ok here's one for you... perhaps a brain teaser.

Driving home from work, all seems fine turn on to my street suddenly the engine feels rough just slightly, AC goes to full max heat even though its set to 67 degrees, Service ASR light comes on and MIL (Service Engine Soon). Pull into driveway stop for a second before I pull into the garage, the engine stumbles for a second then revs itself up to (in park) like a 1000 rpms for a second, AC suddenly goes to max cold but Service ASR and MIL still on.

I think ok what now, park it in the garage get my paper clip and pull codes. Ground A to B, I get a 1-2 over and over so no DTC's there. Ok then I ground G to A, cycle through all the test modules CCM throws a H41 which is a historical loss of ECM serial data, no other codes there, move to module 4 ECM no DTCs there, move to module 9 ABS/ASR, throws a H72 which is a historical LCD Dimming output circuit shorted to ground.

HUH!!! No other codes. So I step through each module and clear each module's DTCs. Start it up, everything seems fine.

Any ideas???

Jet-Jock

Well well I'm truly amazed no one had any ideas or comments.

skateparkdave

This is a shot in the dark but were you low on fuel?? Like less than 1/2 tank? I've turned a corner when low on gas and had everything go almost dead and act funny like you describe. I have manual air so I didn't get that problem though. These cars fuel pumps dont have a very deep pickup ability and if she almost dies because of a momentary fuel starvation it can throw it into other weird conditions. Just a thought.

Jet-Jock

Hmmm, no I have about 3/4's tank right now. And it gets more strange, went out at noon to run some errands everything seems absolutely normal, coming up to a stop light just before I come to a complete stop it tries to die, then regains itself after a moment caused the MIL lamp to flash quickly twice while that was happening and went out. It recovered nearly immediately. So after leaving the pet store, I decide maybe go for a quick run down the interstate. Ran it right up to 5200 rpm well into triples runs smooth and perfect everything is indicating perfect. Hop off the exit, get back on the opposite direction repeat same process running it (at risk of course) around 95 for a few miles. Slow back up to 80's continue to the exit get off. Run a few more errands. Nothing behaving normally.

I'm wondering if it may be a bad tank of gas. She has never behaved like this. So I'm just above a half, so I'm thinking maybe I'll run over to the coast and back and burn this tank off then fill up with Mobil super instead of the BP super that is in there. I usually run Mobil but last week it was just convenient to stop at BP (Amoco).

I don't know this one has me perplexed.

skateparkdave

Is your battery voltage good?? If your alternator is going it could get simular symptoms. It can be charging enough to sustain systems at higher rpm's then drag at lower rpms under demands like when all the brake lights are lit up. Your systems require 12volt constant and if it drops below the motor can stumble or even die.

Other than that, I'm clueless. Unless you have a bad ground or corroded connections somewhere. At least if nothing else, I'm bumping your thread. Good luck.

KnightmareVette

Gremlins.

Jet-Jock

Nope alternator is about 6 months old but I thought of that and monitored voltage and it is stable at 14.1. Ground is good, and terminals are good, clean and torqued (new battery too).

No this is totally weird, I think Gremlins too. I just drove over and back to the coast about 160 miles round trip (mixture of 2 lane and interstate). She absolutely ran perfect... not a hiccup. Matter of fact best gas mileage I have ever gotten, course I think that is mainly due to the Corsa's since they were installed gas mileage has jumped from average 18.6 to 24.2 for highway driving. She runs super strong, excellent and smooth acceleration right on up through the triples... luv that on passing on two lane roads. Smooth idle.

Well I'll just chock it up as a weird one I guess.....

skateparkdave

Maybe it was just some water in the gas. I guess anything that makes the engine stumble could throw everything electronic out of whack temporarily . Oh well, glad it's ok now.

Aquaman92

Hey I am amazed you could get up to 90 on I-4

tinian

That seems pretty cool, that you can get information from your car so readily. Is this a published process? Where is A, B, G, etc?

skateparkdave

Yeah, it's published. Here ya go. Make sure you read down to description of your year model.

C4 Corvette Diagnostic Code Recovery Techniques

In the 'In Depth' section of this site, the ECM (Engine Control Module) codes are listed and the technique for recovering those codes on the 1984 through 1993 model Corvette is explained.

This fact sheet reviews that information and also shows you how to recover ECM codes for the rest of the C4 models plus shows how to recover the ABS, ASR, SRC, DERM and RTD codes as well.

Code Recovery: 1984 through 1993:

To recover ECM codes from the 1984 through 1993 Corvette, place a short as shown below between pins "A" and "B" on the ALDL (Assembly Line Diagnostic Link) connector. This connector is located under the dash just to the right of the steering column and has space for 12 pins although not all of the cavities will be populated.

.

Turn the ignition switch to "On" but do not start the engine.

Depending on the model year, either the "Check Engine Light" or the "Service Engine Soon" will begin to flash.

As the code display sequence begins, you will see a flash followed by a pause and then two flashes. This is the indication for the number "12". The number 12 is a delimiter intended to show where the ECM code display starts and stops.

Code 12 will flash three times and then any stored codes will flash. When all codes have been displayed or if there are no stored codes, the number 12 will again flash three times.

For example, a problem with the EGR system (code 32) will be shown as 12, 12, 12, 32, 32, 32, 12, 12, 12.

This would be displayed on the Check Engine/Service Engine Soon light like this:

12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
32: flash, flash, flash (pause) flash, flash (long pause)
32: flash, flash, flash (pause), flash, flash (long pause)
32: flash, flash, flash (pause), flash, flash (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash, (long pause)
12: flash (pause), flash, flash (long pause)

The ECM code display will repeat until you turn off the ignition switch and remove the short.
Additional Codes available on the 1984 through 1993 Corvettes:

Selective Ride Codes:

On the 1989 Corvette, if you have the Selective Ride Option, any codes associated with the SRC will cause the Service Selective Ride System light to flash when you short ALDL pins "A" to "B". The sequence uses the same 12, 12, 12 delimiter as the ECM code display uses.

Some later Corvettes require you to short pins "C" to "A" to flash codes on the SSR light. It varies with the model and you will need to consult the service manual for your particular model year to find out if these are the proper pins or if there is another technique requried.

ABS Codes:

On the 1990 and 1991 model shorting pin "H" to pin "A" will cause the ABS light to flash ABS codes. (Beginning in 1992, there is another technique which is explained below).



Not all codes are displayed using this technique. There are four codes that will not display without a scan tool. Also, note that the 1986 through 1989 Corvettes will not display any ABS codes. A special test fixture is required for those models.


Sequenced Code Recovery - 1990 through 1993:

With the interior redesign in 1990, Chevrolet added additional computer capability. The CCM (Central Control Module) was added and a method was provided to display codes on the speedometer of the Corvette.

From 1990 through 1993, you shorted ALDL pin "G" to "A" and turned the ignition to "On" without starting the engine.



This action will cause the speedometer and trip odometer to become a troubleshooting aid:



The codes will display on the speedometer. If there are no codes, the speedometer will show three dashes (---) and if the CCM cannot communicate with the other computers, the letters "ERR" will show up on the display.

The module number associated with the codes will be displayed on the odometer and will show which computer's codes is being displayed.

For the 1990 and 1991 Corvette, the modules are Module "1", the CCM computer and module "4", the ECM module.

For 1992 and 1993, an additional module, module "7", the ABS (Anti-Lock Brake System) and ASR (Automatic Slip Regulation System) codes will be displayed.

The codes will show up for three seconds each and once all codes have been displayed, the speedometer will show three dashes. You can either turn the ignition off for five seconds and then back on to repeat the process or turn the ignition off and remove the short to restore normal operation.

Sequenced Code Recovery - 1994 through 1996:

In 1994, Chevrolet went to a 16 pin ALDL connector in preparation for the federally mandated change to the much more capable OBD-II (On Board Diagnostics - Version 2) system.

As with the 1990-1993 model, you can display codes on the speedometer but the pins to be shorted are different:



Beginning in 1994, you short pin "12" to pin "4" and once again turn the ignition switch to "On" without starting the engine.

The speedometer will display any codes using the same protocol as the 1990-1993 model but there are some changes in the designation for the modules plus additional modules are added:

Module "1" is still the CCM module.

Module "4" is now called the PCM module (Powertrain Control Module) because automatic transmission computer control was added to Engine Control Module in 1994.

Module 7 (on the 1996 model only) is the RTD module. (This is the Real Time Dampening module which replaced the Selective Ride Control module in 1996).

Module 9 is the ABS/ASR module number from 1994 through 1996.

Finally, Module "A", the DERM (Dynamic Energy Control Module --- the air bag control module) will be requested to show any codes.

Just like the 1990-1993 display, you read the codes on the speedometer and read the module number on the trip odometer.

Again, you can cause the codes to repeat by turning the ignition off for five seconds and then back on.

Turn the ignition off and remove the short to restore normal operation.

If you need this document complete with pictures of ALDL and what pins are what letters then PM me your email adress and I can send it to you.

Dave (see next post for codes and fixes)

skateparkdave

now that you have codes pulled here's what they mean and some likely fixes.

Code Circuit or Sensor
13 Oxygen Sensor Circuit
14 Coolant Sensor Circuit (High)
15 Coolant Sensor Circuit (Low)
16 Ignition Pblms (Used on 1992-1996 models only)
21 Throttle Position Sensor (TPS) (High)
22 Throttle Position Sensor (TPS) (Low)
23 Intake Air Temperature (Low)
24 Vehicle Speed Sensor (VSS)
25 Intake Air Temperature (High)
26 Quad Driver Module Number 1
27 Quad Driver Module Number 2
28 Quad Driver Module Number 3
32 Exhaust Gas Recirculation Circuit
33 Mass Air Flow Sensor Circuit (1985-1990)
33 Manifold Absolute Pressure High (1984)
34 Mass Air Flow Circuit (1985-1990)
34 Manifold Absolute Pressure Low (1984)
35 Idle Air Control Circuit (IAC)
36 Mass Air Flow Burn Off Circuit
41 Cylinder Select Error Circuit (1985-1991)
42 Electronic Spark Timing Circuit (EST)
43 Knock Sensor Circuit
44 Lean Exhaust Present
45 Rich Exhaust Present
46 VATS Anti Theft Circuit Fault
51 PROM/EEPROM Error
52 Oil Temperature Circuit (Low Temperature)
53 System Charging Voltage High or Low
54 Fuel Pump Circuit (Low Voltage)
55 Engine Running Lean
62 Oil Temperature Circuit (High Temperature)
63 Oxygen Sensor Circuit (Open)
64 Oxygen Sensor Circuit (Lean Exhaust)
65 Oxygen Sensor Circuit (Rich Exhaust)
66 Air Conditioner Pressure Limit Exceeded
67 Air Conditioner Pressure Limit Exceeded
68 Air Conditioner Relay Fault
69 Air Conditioner Clutch Fault
72 Gear Selector Switch Fault (Start Lockout)

Possible Failures Associated with ECM Codes (From most likely to least likely)

13 Check wiring and connectors. Bad Sensor
14 Check wiring, connectors, thermostat. Monitor actual engine temperature. If within limits, and wiring/connector is OK, change thermostat and/or sensor.
15 See above, particularly thermostat
16 Direct Ignition (DI) Fault
21 Sticking or Misadjusted TPS. Also check wiring/connectors. Adjust/replace TPS.
22 Sticking or Misadjusted TPS. Also check wiring/connectors. Adjust/replace TPS.
23 Measure sensor resistance with Digital Ohm meter. Must not be 0 ohms or infinite ohms. Replace if it shows one of these readings. Check wiring/connector of sensor. If OK, replace sensor.
24 Only valid if vehicle moving. Check connections at ECM Check TPS setting. Possible ECM failure.
25 Measure sensor resistance with Digital Ohm meter. Must not be 0 ohms or infinite ohms. Replace if it shows one of these readings. Check wiring.connector of sensor OK.
26 Check EGR, Canister Purge and AIR pump relays with a digital Ohm meter. A resistance of less than 18 ohms indicates a bad relay. If OK, potential ECM failure.
27 Potential ECM or on a manual transmission car, potential upshift relay problem. Check relay, replace if less than 18 Ohms using a digital Ohm Meter.
28 Air conditioning Clutch relay and/or cooling fan relays. Check with digital Ohm meter, replace if less than 18 Ohms. If relays OK, potential ECM failure.
32 The most common cause of this code is a bad or intermittent EGR switch which is located on the EGR pipe between the exhaust manifold and the intake manifold. Replace this switch first when you get this code. Next, check electrical connections at EGR valve solenoid and then the ECM. Check all vacuum lines for leaks especially around the EGR valve. Possible ECM failure.
33 (1985-1990) Inspect intake system for leaks, Inspect for vacuum leaks, Check MAF connector and wiring, Check MAF for open using digital Ohm meter. Possible ECM failure.
33 (1984) Check vacuum hoses. Check wiring to sensor. Change sensor. Check connections at ECM.
34 (1985-1990) Clean the throttle body. Check MAF connections. Replace MAF relay. Replace MAF Sensor. Possible ECM failure.
34 (1984) Check Vacuum hoses associated with MAP sensor. Check wiring and connections, particularly at ECM. Replace the sensor. Possible ECM failure.
35 Check fuel pressure, injectors, leaking throttle body. Change the IAC valve.
36 Check connections at MAF, MAF relay and MAF Burn off relay. Check resistance of MAF relay and MAF burnoff relay with digital Ohm meter. replace if less than 18 Ohms. Possible ECM failure.
41 Check wiring at ECM. Possible PROM failure, or incorrectly seated PROM. Reseat PROM. Possible ECM failure.
42 Check wiring at ignition module. Replace ignition module. Possible ECM failure.
43 Check ECM wiring. Replace knock sensor
44 Check wiring.connectors at Oxygen sensor. Check fuel pressure. Replace Oxygen sensor.
45 Check evaporative charcoal canister for smell of fuel (which normally comes from filling fuel tank to full). check fuel pressure regulator. Possible leaking fuel injector or sticking/bad EGR valve. Possible bad Oxygen sensor.
46 With negative battery lead disconnected and using high pressure, non residue contact spray cleaner, spray into area where ignition key inserts followed by inserting and removing key several times. Check for open/short on harness from steering column VATS ignition key to ECM. Possible defective anti-theft module.
51 Faulty or incorrect PROM in the ECM. Change PROM with correct version for automobile.
52 Check connections at the oil temperature switch. If OK, replace switch.
53 If voltage is more than 17.1 or less than 10 volts, this code will be set. Check battery leads, alternator drive belt for tightness and have electrical shop check alternator output. Voltage with engine off should be 12 volts. Voltage with engine running should be 14-15 volts. Use digital volt meter for checks and measure at the battery terminals.
54 Using digital Ohm meter, check fuel pump circuits for shorts or opens.
55 This code is set when there is not enough fuel when accelerating. A possible fuel pump failure or insufficient fuel pressure due to a fuel line restriction is indicated.
62 Check wiring associated with Oil temperature switch. Replace switch.
63 Check wiring and connections to Oxygen sensor.
64 Check wiring and connections from Oxygen sensor to ECM. Check ECM ground terminal and battery ground. Check fuel pressure and fuel pump. Replace Oxygen sensor if all of above OK.
65 Check evaporative charcoal canister for fuel fumes. Replace if contaminated. Check oil for presence of fuel. Check fuel pressure regulator, fuel pump, check for leaking injectors, Check for stuck/defective EGR valve, Replace Oxygen sensor if all above OK.

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tinian

Thanks SkateParkDave! This is pretty cool. You know I'll be doing a health check on my 1994 today.