Checking ECM Code

[ponytrekker]

Like many before me, I am getting the check engine light when I start my 94 Q45, 122K and well maintained.

It goes on a few seconds after the system check and stays lit, the car runs rough, like a cylinder (injector?) is misfiring.

Can someone describe the process of checking MAF connections and/or how to view ECM codes, to help me begin to troubleshoot the problem.

Thanks

Jay

[DAEDALUS]

I could have sworn the procedure was listed in a couple threads, but I just did a search and I can't find it.Keywords: ECU check engine light code codes

Disconnect the battery first. The ECU is behind the small carpeted kick panel right in front of the passenger-side door speaker. There's a 10mm nut holding the panel down down, as well as a couple metal clips under the weather stripping.Once the panel's out, you can see the ECU and a monster harness attached to it. The ECU is held in with a couple screws (large phillips). You probably have to remove the harness first to access the top screw. Loosen the bolt all the way till the harness comes off clean. Be careful not to bend any pins on the ECU. Once the ECU's out, you have to attach the harness back up to it. Tighten the bolt down snug, making sure the connector goes in evenly. Reconnect the battery.Put the key in the ignition and turn it to the "On" position. The dash lights should come on, and so should the LED on the back of the ECU. Next to the LED is a slotted set-screw feature. Use a screwdriver to turn the screw clockwise till it stops. Wait at least 2 seconds, and then turn the screw back to where it was. Start counting! The LEDs will start blinking, almost like morse code. Long blinks followed by quick short blinks. There may be more than 1 code, so keep track. They'll repeat a few times, so don't worry if you don't get them the first time. Codes are: (long blinks)*10 + (short blinks)

Once you have the codes, you can reset the ECU. Turn the screw again clockwise, wait at least 2 seconds, and then turn it back. LED should be on constantly. Remove the key. Disconnect the battery. Remove the harness connector, and reinstall everything.

[DAEDALUS]

For MAF connections, the manual says you can roll back the insulation on the MAF harness connector. Roll it back, and check the voltage with the engine at idle and at 2000 RPM.

[Q45tech]

Just remember the MAF voltage is very non linear, in that from the 1.2 volts at idle to the maximum of 4.5 volts represents a change from 6 grams per second to 255 grams per second.....in only 3.3 volts.

Think of a graph where X voltage is raised to the 3.5 power [exponent].

2000 rpm unload rev in neutral uses nothing more than 3 times the air flow just due to more rpms per second plus a tiny bit more.

The general rule of thumb is 1.2-1.25 grams per HP created........4-6 to overcome the friction at idle, 20 or so to run 60 mph [ac off], 25 HP ac on.

[daegrigg]

I am not getting the check engine light. Can there be codes without the check engine light? My car runs badly under 2000 rpm. The voltage at the MAF should go from min to max in the 600 -2000 range, right? So maybe I should check the voltages at my MAF.

[DAEDALUS]

Codes can be present without the CEL--Knock sensors.2000 RPM isn't even close to redline. Voltage wouldn't be above 2V at that speed, 1.4-1.9V. The MAF sensed should approach 4.5 volts around 7000 RPM.

[maxnix]

The voltage at the MAF should go from min to max in the 600 -2000 range, right? So maybe I should check the voltages at my MAF.

No, the airflow increases as RPM increases until redline, thus so must voltage.

Around 4.4 V MAX sticks in my mind, but I urge you to search for all posts by Q45tech with the word MAF in the body, current and archived, for a more complete and competent explanation of MAF operation and diagnosis.