Hello folks and engle...

Well while testing the surrounding wiring, I lost my 5 volts at the green middle wire to the MAF. I still have the 12 volts at pin E but am pulling 64 mvolts at pin C. What happened to my 5 volts at C? The schematic says it goes directly to the ECM as well as the other two wires used as system ground I believe. I know ECM is a taboo word but should I change it out? Thanks again in advance. Ops 1986 C4 code 33 pulled and wont go away. Replaced relays and received back my 12 volts at pin E at the MAF. The car is running fine. Just have to smog it and wanted to get rid of that code 33. Any help from youall will be appreciated....

 

Most likely you have a defective connection from MAF to ECM. The green wire is the MAF output. Low airflow, the output voltage is very low (GM doesn't say how low). High airflow, the output voltage is limited to 5 volts. Code 33 sets when the ECM sees air flow above 45 grams/sec (above 2.2 volts) for one second when:
Engine first started
TPS less than 1/4 throttle
RPM less than 2000

If the green wire going to the ECM from the MAF is open circuit, then pin B12 on the ECM will have 5 volts on it and code 33 will set. Ign off, measure the resistance from the C terminal on the MAF to pin B12 on the ECM (dark green wire) to determine if it has low resistance, it should measure the same ohms as touching the ohmeter probes together. Unplug the MAF and the ECM to insure that the ohmeter doesn't harm either the MAF or ECM.

 

thanks JFB, will do continuity test from B12 to MAF all unplugged....Also check voltage at B12. I hope theres no voltage @ B12 Id hate to have to find and rewire that open. Thanks guys and gals!

 

Ok folks this is what I got. I disconnected the battery, unplugged the MAF and unplugged the ECM. I then placed a audio probe on the dark green wire at the MAF (Wire C middle) I then got my trace wand and traced the sound to a dark green wire at one of the 2 ECM plugs. I placed a multimeter set to ohms between the wire at the ECM and the wire at the MAF. Got a full peg to 0 resistance. Which is good. Plugged back in the ECM. So I tried again to jump the G terminal to hot, turning on the fuel pump and the power to MAF. Turned ignition on. MAF not plugged in, I checked voltage at C and I got 4.99/5.00 volts. What in the world happened??? I got my 5 volts back. (scratching head). So I checked the MAF E terminal at the same time and have 12 volts. Turned everything off, plugged in the MAF, turned key back on and jumped G. Pushed into term C and got 4.65 Volts at term C. Uhg... turned off igniton and unplugged jumper to G. Checked codes and of course no code only 1.....12 or twelve because I unplugged the battery during Ohms test on the C wire. Started vehicle and up pops code 33. Now the MAF relays and burnoff relay are strictly muscle. Ive tested both and both have 12 volts at the end plugs with the center C wire having 12 volts when f pump is on and of course I have 12 volts at term E , MAF. Im leaning toward ECM.... I cant think of anything else. Any other suggestions folks? I guess I will try checking my TPS as Engle advised. Just found it funny that i lost my 5 volts and it came back. Thanks in advance again folks!!!

 

I've kinda been following this thread, and once again I'm quoting the FSM.

" The oil pressure switch or the ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay which provides the 12 volts needed by the MAF sensor". I've seen this on my 87. So, what I'd do is pull the entire fuel pump relay from the firewall and inspect the wires (back side) going into the connector for the relay. The insulation on my wires had shrunk back almost an inch (not uncommon on earlier C4s) bare wires were comming into contact with one another. This can be found on page 6E3-A-42.

Funny thing is we assume the only relays for the MAF are power and burnoff. By providing power form the battery to pin G we're simply bypassing the fuel pump circuit, including fuel pump relay. Worth a check, and you can't see the wires without pulling it off the firewall.

 

The ECM supplies 5 volts through a constant current circuit to the MAF sensor. This is why you measure 5 volts with the MAF unplugged. That you get 5 volts on the green wire, everything connected and engine idling, makes me suspect a defective MAF sensor. I'd have to measure my MAF when idling to say what voltage should be there.
The MAF output termnal is a voltage directly proportional to air flow, so at idle the output voltage should be very low. You get code 33 when the MAF output voltage is above 2.2v and the engine is below 2000 rpm.

 

An idling car should only flow around 6-8 gm/sec, so the MAF voltage in a warm idle would be expected to be in the 0.5-0.7 volt range.

Below is a copy of the MAF flow vs voltage tables from my original 88 ABTR bin. All the 86-89 MAF cars share the same calibration as far as I'm aware.

Note: The 2.2 volts mentioned for setting code 33 is really the 45 gm/sec MAF high diagnostic threshold. You can see from the table that at 2.2 volts the flow is greater than 45 gm/sec, so it is close enough for discussion purposes:

Mass Air Flow Table #1

Volts gm/Sec
0.00 8.4
0.18 3.2
0.37 4.5
0.55 6.2
0.73 8.4
0.91 10.9
1.10 14.0
1.28 17.7
1.46 22.3


Mass Air Flow Table #2

Volts gm/Sec
1.46 22.3
1.55 25.0
1.65 27.6
1.74 30.6
1.83 33.8
1.92 37.1
2.01 40.7
2.10 44.5
2.19 47.7


Mass Air Flow Table #3

Volts gm/Sec
2.19 47.7
2.29 51.3
2.38 55.2
2.47 59.1
2.56 63.3
2.65 67.9
2.74 72.4
2.83 77.3
2.93 82.1


Mass Air Flow Table #4

Volts gm/Sec
2.93 82.1
3.02 87.4
3.11 92.6
3.20 98.4
3.29 104.2
3.38 110.5
3.47 116.8
3.57 124.7
3.66 134.2


Mass Air Flow Table #5

Volts gm/Sec
3.66 134
3.75 142
3.84 150
3.93 158
4.02 167
4.11 177
4.21 185
4.30 195
4.39 206


Mass Air Flow Table #6

Volts gm/Sec
4.39 207
4.43 213
4.48 219
4.53 226
4.57 233
4.62 240
4.66 248
4.71 255
4.75 255
4.80 255
4.85 255
4.89 255
4.94 255
4.98 255
5.03 255
5.07 255
5.12 255

 

Ah yes, I actually had that problem a while back. I actually lost my overdrive because a wire slipped off the OD relay which is located near the Fuel pump relay. I bought a load of new connectors, cut off the old plugs and recrimped all. Most wires were melted back and exposed. All new plugs and I got my OD button on the shifter working again but you do have a point. I will push into the middle wire on the MAF relay and E pin on the MAF to check for 12 volts while idling. Easy check. Thanks Lambert!!!

 

I will also check voltage at idle...Thanks JFB and stay tuned

 

Well does sound like MAF malfunction. At idle I cant be at 248 gm/sec at 4.68 volts. But im getting ahead of myself. Ill check the car at idle and get back with you all. Thanks again. This is getting interesting

 

This is an old post from way back. Below are images of the readings I took back in '07 off my '89. The reading boxed in yellow was my problem! To get these results you must have a jumper wire connected from battery's + terminal to Pin "G" on the ALDL plug...and the ignition key in the run/on positon but car is not running. I used a sharp noid probe in series with voltmeter to achieve these measurements. You may want to measure your voltage this way as well so you can see what is going on. Your wires and pin location may be different on your '86...so consult you FSM if you are in question. All of the car's components (including the MAF and ECM) were plugged in at the time of testing.





The reading on B12 (dark green wire circuit 998) is too low....in your case too high. Mine was ~ 30mv
I unplugged the MAF sensor and the voltage shot up to 5v (it is supposed to do this!). This was a bad MAF sensor that was brand new out of the box. Your MAF may be bad as well or you have a wire or connection problem inbetween the ECM and the MAF it self.

 

Hello, Yes I did check with G terminal jumped and ignition on. Also Noid light in series with positive of volt meter. got 12 v at E and 4.65 at C. Just checked it started and got 14 volts at E , which is reg battery voltage, and same 4.65 v at C. Quite an impressive test log !!! Im leaning toward a bad MAF new out of the box gees unbelievable. I also see on the schematic a system ground from MAF. What do you think???

 

Well guys and gals, its FIXED!!! No more code 33. I swapped out the new MAF with another. It was the second new MAF and one of the relays. Its fumbling a bit but i think its relearning. Hasnt had a MAF hooked up for a couple years now. Thanks again for all your help. I think this place just saved me 500 dollars!!!! Pizza and PEPSI is on my tab