obd1 code 48 maf not reading - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums
 
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post #1 of 10 (permalink) Old 07-27-2009, 10:46 PM Thread Starter
 
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obd1 code 48 maf not reading

I have a 1995 trans am. I am getting a code 48 that says it is not reading the mass air flow sensor. I have replaced the MAF twice and the MAF connector. I have no idea now lol. Any ideas?
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post #2 of 10 (permalink) Old 07-28-2009, 06:32 AM
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Check for a 5 volt reference, check for a ground, and check for a signal from the MAF. Those are what the three wires do in that connector. Just leave the MAF hooked up running and probe the wires w/ a digital volt meter and see what each wire does while grounding the black lead to a good ground.
post #3 of 10 (permalink) Old 08-17-2009, 11:47 AM Thread Starter
 
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apparently i was not getting a ground. I am now getting 4.4 volts on 1 wire around 12 volts on the other and now that it has a ground i am getting a much much smooter idle and its not running as rich.
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post #4 of 10 (permalink) Old 08-05-2014, 11:32 PM
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This is whats happening with my car. Can you please tell me what I need to do in order to fix this? Keep in mind that where I am somewhat mechanically inclined I am not a full fledged mechanic so in layman's terms please...i.e. I use the Chiltons manual for everything and sometimes uTube
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post #5 of 10 (permalink) Old 08-08-2014, 10:13 AM
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CamaroGirl, a scan tool would be very beneficial since you can see the MAF readings as you check out the system. Without that, you probably want to verify that the connector is clean. Visually inspect the MAF element to make sure that it is not dirty or has debris on it. Be careful at this step and don't break it. Many parts stores sell a MAF cleaner. To do the checks stated by previous members, you need a multimeter which you can pick up at Radio Shack. With the car turned on, but not running, unplug the MAF. Verify 12 volts across the two pins in your chiltons manual that supply voltage to the MAF. If this is good, reconnect the sensor. To measure the MAF output, you will need to pierce the signal wire (not power) to the MAF sensor. Use the sharp point of the multimeter probe to pierce the insulation. Connect the positive lead to the sensor output, and the negative lead to battery ground. Start the car and observe the voltage on your multimeter. Vary the throttle, thus varying the airflow and verify the voltage changes. Hope this helps.
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post #6 of 10 (permalink) Old 08-08-2014, 01:52 PM
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Ok heres something weird. I thoroughly cleaned the maf as well as the connectors. Didnt change. Got another one from salvage yard cleaned it nothing changed. When i completely unplug the maf and drive it it runs alot better. Not perfectly but way better. Any ideas?? Am buying the 12 to 16 pin wire tomorrow and a usb cable so i can make my own wire to attach to my laptop to run the codes. Im confused at this point. Will it hurt my car to run it w the maf unplugged???
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post #7 of 10 (permalink) Old 08-09-2014, 11:20 AM
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1- unplugging MAS switches the PCM into Speed Density mode which bypasses the MAS completely. It would would run well in SD mode if MAS was defective. However the DTC 48 should also switch pCM to SD mode and car would also run OK. Is that the case ?
2- The ALDL protocol is 'Serial Data' all ALDL cables are either a) old style PC Serial data to ALDL OR b) anything newer has a Serial Data to USB chip embedded in the cable. Can't tie USB directly to the PM..

The MAS circuit is fairly simple ref this url http://www.mifbody.com/DTCs/DTC%2048.jpg
- As stated earlier you need +12 V ignition between pins B ground and C 12v ign.. Pull the connector w/key on engine off check at the connector end.
- Unfortunately pin A MAS output to PCM is a square wave with variable frequency depending on the air flow. So you need a oscilloscope to verify it. Only thing to do on the yellow wire is to verify continuity to the PCM. Pull connector at MAS key on / engine off using a DVM (Digital Volt Meter) you should have 5 Volts between pin A yellow wire and pin B ground . If no +5 the yellow wire is open, shorted or the PCM +5 bias voltage is defective.

I'm Bob.. 1994 Z28, A4, Stall, CAI, Cam, RR's, 342 rear, LT's , CatBack, Tuned
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post #8 of 10 (permalink) Old 08-09-2014, 06:32 PM
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GaryDoug has posted a much better image of dtc 48 troubleshooting. Look at post #9. If you click on the link in bobdec's post it will show you the circuit diagram.

Last edited by cocobolo95; 08-09-2014 at 07:16 PM.
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post #9 of 10 (permalink) Old 08-09-2014, 07:07 PM
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Coco, I have never been able to post anything but a small image in attachment mode. Attachment mode has severe limits on physical size (height and width). I can't read the diagram you posted even if saved as a file. I always upload the image to my library and reference the link in the post.


98 Trans Am, LS1/A4, all stock, original owner
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post #10 of 10 (permalink) Old 08-09-2014, 07:09 PM
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That looks much better, I will delete mine.
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