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Hello, I have a 85 with some serious drivibility issues it does not accelerate under load.I pulled code 33 ran test found 7 volts at the MAF sensor.My issue is the next diagnostic step is to dis-connect connectorA-B on the ECM and the connectors are not marked.I tried dis-connecting them one at a time and testing MAF voltage with no voltage present either time. I would like to know if anyone knows of this issue I do not want to replace the ECM if not necessary any input would be helpful and greatly appreciated.Anybody know which connector is which?
I'm not sure what diagnostic chart you are using, but mine (Alldata aka Somedata) says there should be 12v at pin E on the MAF connector. If not check the burn-off relay. Do you have a scan tool? What kind of airflow readings are you getting, code 33 means it should be reading high.
Hope this helps, Paul
Thanks I am using mitchell on line for info. I have 12v at pin e voltage across pin c and e should read 4-6 volts first test I got 6.7 volts second test five volts and third test 7.0 volts. I need to dis connect one connector on the ecm to continue should be connector a-b and they are not marked any ideas?
Alldata says check voltage between c and a (ground). Should be 4-6. I'm not sure that makes sense to me, c is signal voltage back to ecm from MAF and a is ground. You'd think there wouldn't be any voltage with the MAF disconnected. ?
The A-B connector on the ecm is the smaller one, 24 pins. I think its blue.
The ecm provides pull-up voltage on the signal wire (pin C at the MAF), via a 1K pull-up resistor so there should be voltage present with the MAF disconnected (and ecm connected).
The MAF (if it is working) pulls the voltage down by varying its resistance in response to airflow to result in a variable voltage signal back at the ecm.
The variable resistance of the MAF along with the ecm's 1K resistor forms a variable voltage divider circuit to modify the original pull-up voltage (typically 5 v).
If you measure the resistance of the MAF between pins A and C you can determine the voltage that should be seen by the ecm.
5 Volts * (MAF resistance/(MAF resistance + 1K))
At low or no flow, the MAF resistance should be very low and the voltage should also be very low. As flow increase the resistance should increase which will also raise the signal voltage to approach that of the pull-up voltage.
Since these MAFs can fail intermittently, the only real way to test it is to monitor the signal voltage while driving or monitor the MAF signal in the scan data. Static tests won't reveal much information.
The ecm must provide the pull-up voltage and also handle the A/D conversion of the signal. From your description of the problem, I don't see anything that suggests a bad ecm.
Last edited by tequilaboy; 03-07-2008 at 01:12 PM.
Don't buy an ecm just yet. I read through the diagnostic chart on Alldata, and it says at one point that if after disconnecting the a-b connector and the voltage is less than 6 between a and c replace the ecm. The voltage should be 0, because with everything unplugged there's no source of current. I agree with Tequilaboy, its more likely a bad MAF that's not showing up during the static test.
Wish I had read my replies from you guys I replaced the damn ECM with no results. You guys know where to buy a MAF at a resononable price? I replaced this one about 18 months ago for about 350.00. Patrick