How do I go about testing ECM to see if it's bad?
10-29-2000, 09:25 AM
#1
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How do I go about testing ECM to see if it's bad?
My 92 305TBI firebird has had hesitation problems after my headswap and doesn't run quite as good as it did before the swap(stock replacement torquers). The car hesitates under 2000rpms at any speed, has killed twice at speed when I WOT it. The car runs better at 3/4 throttle than full throttle because at full throttle it feels like the timing is being taken out even with it set at 0-2degrees BTDC. When I was adjusting the valves I accidentally kept the Autozone key in the A-B slots and had the car running for almost a half hour at idle and drove it around a little with the key still in there before I found it. Any suggestions or does anyone know how to check my Computer to see if a new ECM would help? Thanks
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305TBI FireBird, Holley Intake and TBI, Headers, Torquer Heads, Gears. Best E.T. 15.08 at 92.5, 0-60 6.45.(Before Heads)Summit cam purchased 204-214, to be installed in 350 shortblock "soon" Shooting for 13's, oh yeah!
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305TBI FireBird, Holley Intake and TBI, Headers, Torquer Heads, Gears. Best E.T. 15.08 at 92.5, 0-60 6.45.(Before Heads)Summit cam purchased 204-214, to be installed in 350 shortblock "soon" Shooting for 13's, oh yeah!
10-29-2000, 09:45 AM
#2
Mike,
It is permissible to have the diagnostic terminal in the ALDL connector grounded while running the engine. However, it is very important that you don't START the engine with the diagnostic terminal grounded.
As a general rule;
* You can jump the 'A' and 'B' connectors with the engine off, ignition on. This is the normal Diagnostic Mode used to retrieve fault codes.
* You can jump the terminal AFTER the engine is running. This is Field Service Mode to determine open/closed loop status and fuel offset correction.
* Your should NEVER crank or start the engine while the 'B' terminal is grounded to the 'A' terminal. This is the "ECM Auto-Destruct" mode.
What can happen is that the ECM will try to enter Block Learn Mode while the engine is cranking, forcing the integrator to write completely invalid information to the active RAM. If the engine starts, it will probably run like a Ford, Honda, or worse. It can even force the logic to attempt enter multiple modes, possibly causing permanent damage to the firmware (PROM and ROM). You can replace a PROM, but the ROM is part of the ECM.
If you are lucky, you didn't do any permanent damage. Shut off the engine, disable any Theft-Loc codes on your radio, if equipped. Disconnect power to the ECM by removing the battery cables for about ten minutes (long enough to clean the battery terminals). Turn on the lights to completely load any power from the system (I know they won't come on, but the closure of the switch will load the system so that any residual voltage in capacitors in the ECM will be completely drained.)
Reconnect battery power, make certain the jumper is removed, then start the engine. Allow it to reach normal operating temperature then drive the vehicle normally for about ten minutes or more. This exposes the ECM to a wide variety of sensor inputs and allows it to start writing new fuel and spark curve tables in active RAM.
If you're still lucky, the ECM will have reinitialized to your sensor inputs and you'll be back to solving your original problem.
If you get the ECM cleared and reinitialized, report back with symptoms of the original problem. It would help to know the manifold vacuum at idle.
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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0
It is permissible to have the diagnostic terminal in the ALDL connector grounded while running the engine. However, it is very important that you don't START the engine with the diagnostic terminal grounded.
As a general rule;
* You can jump the 'A' and 'B' connectors with the engine off, ignition on. This is the normal Diagnostic Mode used to retrieve fault codes.
* You can jump the terminal AFTER the engine is running. This is Field Service Mode to determine open/closed loop status and fuel offset correction.
* Your should NEVER crank or start the engine while the 'B' terminal is grounded to the 'A' terminal. This is the "ECM Auto-Destruct" mode.
What can happen is that the ECM will try to enter Block Learn Mode while the engine is cranking, forcing the integrator to write completely invalid information to the active RAM. If the engine starts, it will probably run like a Ford, Honda, or worse. It can even force the logic to attempt enter multiple modes, possibly causing permanent damage to the firmware (PROM and ROM). You can replace a PROM, but the ROM is part of the ECM.
If you are lucky, you didn't do any permanent damage. Shut off the engine, disable any Theft-Loc codes on your radio, if equipped. Disconnect power to the ECM by removing the battery cables for about ten minutes (long enough to clean the battery terminals). Turn on the lights to completely load any power from the system (I know they won't come on, but the closure of the switch will load the system so that any residual voltage in capacitors in the ECM will be completely drained.)
Reconnect battery power, make certain the jumper is removed, then start the engine. Allow it to reach normal operating temperature then drive the vehicle normally for about ten minutes or more. This exposes the ECM to a wide variety of sensor inputs and allows it to start writing new fuel and spark curve tables in active RAM.
If you're still lucky, the ECM will have reinitialized to your sensor inputs and you'll be back to solving your original problem.
If you get the ECM cleared and reinitialized, report back with symptoms of the original problem. It would help to know the manifold vacuum at idle.
------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0
10-30-2000, 11:07 PM
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I cleard the computer by disconnecting the battery and put on the lights and it still acted up. I punched it from a 40mph roll and it almost killed then "caught" itself and pulled decently(still not as strong as before headswap). I guess I need a scantool huh? I have the GM catridge but coundn't afford the ScanTool itself, does anyone know where I could get the ScanTool itself for cheap? Is there anyway to check the ECM with a Voltmeter or something? Also how do I check the Vacuum and how much is the gauge or whatever I'd use? Thanks for the post Vader.
10-30-2000, 11:27 PM
#4
Mike,
Vacuum is tested with a vacuum gauge connected to an intake manifold vacuum port or fitting. A vacuum gauge can be had for as little as $25.00.
Before you head out to buy a scantool, use the diagnostics built into the ECM to help you find the problem. Scan the ECM for error codes. Even though you may not have the SES light on the dash warning you of a problem, there may be stored error codes. There is an article in the Tech Section of this site outlining the procedure.
Since you recently changed heads, the intake must have been off. If you're certain you don't have vacuum leaks at the intake, you might want to take a close look at the EGR valve. Any leaks through the valve can cause poor idle/no idle, rough running, no power, and poor economy.
Make sure the TPS outputting correctly and the IAC is funtioning. Double check all vacuum hoses and the PCV valve and hoses.
Other common TBI and SD-TPI problems are incorrect readings at the MAP sensor, erroneous feedback from the oxygen sensor, and failing IAT/CTS sensors. Even though your TPS is non-adjustable, there is a maximum specification for voltage at idle position. If yours is above 0.8 V, you have a problem. A dead knock sensor will kill all your spark advance.
One of these should get you closer to the problem.
------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0
Vacuum is tested with a vacuum gauge connected to an intake manifold vacuum port or fitting. A vacuum gauge can be had for as little as $25.00.
Before you head out to buy a scantool, use the diagnostics built into the ECM to help you find the problem. Scan the ECM for error codes. Even though you may not have the SES light on the dash warning you of a problem, there may be stored error codes. There is an article in the Tech Section of this site outlining the procedure.
Since you recently changed heads, the intake must have been off. If you're certain you don't have vacuum leaks at the intake, you might want to take a close look at the EGR valve. Any leaks through the valve can cause poor idle/no idle, rough running, no power, and poor economy.
Make sure the TPS outputting correctly and the IAC is funtioning. Double check all vacuum hoses and the PCV valve and hoses.
Other common TBI and SD-TPI problems are incorrect readings at the MAP sensor, erroneous feedback from the oxygen sensor, and failing IAT/CTS sensors. Even though your TPS is non-adjustable, there is a maximum specification for voltage at idle position. If yours is above 0.8 V, you have a problem. A dead knock sensor will kill all your spark advance.
One of these should get you closer to the problem.
------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0
11-02-2000, 05:14 PM
#7
Just for future reference... starting the engine with the A&B terminals connected toghether is not a terminal mistake. The ECM will work even with the terminals shorted. Just try to shut it off when you notice it though. I've done several times to my ECM, by mistake, and I've yet to see it smoke. LOL
P.S. ROMs and EPROMs and read only memories. Therefore no permanent damage can be done to them since you can't write anything to them, only read. All in all our ECMs are pretty rugged
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92 Z28 L98 350
---------------
Ported and polished heads, ported stock TPI base, ported plenum, Comp Cams XR270HR-10 cam (lift .495/.502 duration 218/224 lobe separation 110), Edelbrock TES headers, LT4 valve springs, Crane AFPR, Flowmaster catback with LT1 style tips, MSD coil & wires...
"Take that auto, drop it in first, hold the brakes, stomp the gas and grin from ear to ear! :-)
P.S. ROMs and EPROMs and read only memories. Therefore no permanent damage can be done to them since you can't write anything to them, only read. All in all our ECMs are pretty rugged
------------------
92 Z28 L98 350
---------------
Ported and polished heads, ported stock TPI base, ported plenum, Comp Cams XR270HR-10 cam (lift .495/.502 duration 218/224 lobe separation 110), Edelbrock TES headers, LT4 valve springs, Crane AFPR, Flowmaster catback with LT1 style tips, MSD coil & wires...
"Take that auto, drop it in first, hold the brakes, stomp the gas and grin from ear to ear! :-)
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