Okay, Dianne’s not feeling good enough today to check data,but I’ll try to do a live data store on my tester to capture fuel trims . The data has to be hand entered into a spread sheet as the tester data down load isn’t compatible with my home computer.

vic

 

Some thoughts. If I remember correctly you have a scope and understand the frequency output of the MAF. Your freeze frame reading of 1.9 g/s is so far out of the normal idle (5-7 g/s) that any anomaly will be easy to spot. I would back probe the voltage at the connector (power and ground), hook up the scope to the MAF signal output and see what goes wonky when the scanner says 1.9 g/s. This way you will know if it’s:

1. Power or ground
2. The MAF
3. The wiring to the PCM or the PCM

George

 

George,

you asked me to run the truck without the MAF connector plugged in. Does this force the pcm to remain in open loop drive mode?

 

It forces the PCM to convert to speed density mode to compute the initial air fuel mix based on other sensors like MAP, TPS and ECT. It takes the MAF, its power/ground, wiring to the PCM and the PCM interpretation of the MAF output out of the equation for a test period. I dont know if this test prevents the computer from going closed loop, I have never forced this vehicle into speed density mode. Let me know on that. That however is a different issue, if the problem changes between open and closed loop, that would help condemn or eliminate the O2 sensors and that feedback loop for air fuel delivery. You can see if the problem changes on either side of that transition starting a cold truck and watching live data for closed loop while observing the behavior.

George

 

I recorded this data a few days ago, I think it got lost in my inexperience with with iPads and forum websites. I hope this helps. I also did some other data recording that showed some really absurd data like map jumping from 20 inches manifold pressure down to “3 inches” between 2 adjacent samples.while accelerating.

I also retested the ground connections from to and through the Maf to Pcm. Since the pcm drives many different sensors, I’m wondering if the 5 volt regulator in the pcm is dropping out or going into current limit under some weird load conditions.

I ordered a rebuilt pcm, it won’t be delivered until Oct 2. If that doesn’t solve the problem it has to into a shop, because my smog check is due before Nov 3 .

Frustrating!!!

 

In addition to unregulated 12V fused power coupled with frame ground, you have two regulated power supplies: 12V (high ref) and 5V (lo ref) with their associated clean conditioned grounds. If either of those conditioned power circuits are messed up then all manner of chaos can ensue. It is critical to monitor them back probed, fully loaded between the ref voltage and conditioned ground. When one of these is disturbed its almost always one of the sensors wreaking havoc on the "party line". Once a corrupted reference voltage is identified then you pull each sensor one at a time until the clean voltage is restored. Then you go to work on the wiring and sensor for the suspect circuit. Its almost never the PCM/VCM/ECM but they get condemned here all the time.

George

 

Is the +5 volt regulator sourced from the regulated +12v? I saw the map go way low in one data recording. Are the ho2sensors powered from reg 12 v or raw bat 12 volts? This might give a clue as to a possible ho2 sensor problem. I have a clamp on current meter, but it doesn’t read accurately below about 5 amps, is it possible to clamp onto each wire on an o2 sensor?

 

The O2 sensors are powered from unregulated 12V and frame ground. I don't know if the 5v regulated supply is derived from the 12 regulated supply or generated in parallel. I'm not sure why we care about this. Why are we putting a clamp meter on the O2 sensor wires, and do you mean all 4 wires?

George

 

This is the data where absurd readings occurred.


Do you think this data would indicate a sensor may be loading down either regulated power supply? Where ever it is, it’s so transient that it only seems to occur for milliseconds. I’ve designed circuits with absolute pressure transducers and they all use +5 volt power.

I don’t have even a block diagram of the pcm, got any sources that would give me more information?

vic

 

What are the column headings, are those freeze frame events?

George