# Help: 94 Olds 88 - PCM program experts

• posted on January 14, 2004, 5:37 pm
CAR: 1994 Olds 88, 3800 V6, auto PCM: 16183247 BHDU
PROBLEM: intermittent MIL lamp, EGR pintle position error.
ANALYSIS (so far): using laptop PC with PCM scan and control software, retrieve
historical data. EGR problem observed. Clear PCM codes. Perform PCM relearn procedure. Scan active data during three seperate 15 minute road tests. Observe EGR problem has reoccured, and that no other codes were set.
Chart (plot) the three EGR related values from PCM against time: ADEGRPOS- EGR actuator voltage DSEGRPOS- desired EGR actuator position EGRPOS- EGR actuator position
The chart shows that the current EGR pintle position is way out of wack from the desired position. The following table is an excerpt from the data stream showing the EGR data activity. Example: in time frame 945, the EGR control voltage was 2.22v, the current EGR position is at 24.3% open, and the PCM desired position is 23.5%. If I am correct, then the PCM is pulling B+ voltage (that originates from the 15amp cruise control fuse in the relay center) down to 2.22v at pin RA16, and 2.22/4.65=.477 or 47.7%. But the desired EGR position is 23.5%! In frame 946, you can see that the EGR position then moved to 40.4%.
FRAME    EGR volts     EGR %POS    Desired EGR %POS 945        2.22        24.3        23.5 946        1.82        40.4        21.2 947        1.08        12.5        30.2 948        1.06        4.31        37.3 949        2.27        47.8        38 950        2.18        48.2        38 951        1.71        43.9        39.6 952        1.69        27.8        39.2 (NOTE: the min and max EGR control voltage observed during the run was .53v and 4.65v, respectively)
ACTIONS TAKEN: continuity test all wires from PCM to EGR, ok. Check PCM harness female pins for good contact with PCM backplane male pins, all good and tight. Swap EGR valve with known good unit, repeat ANALYSIS, same results.
QUESTION #1: what is the program logic in the PCM for controlling the EGR?
QUESTION #2: what kind of circuit is in the PCM that sends it output control voltage to the EGR from pin RA16?
QUESTION #3: Is this a normal failure point in this model/year PCM?
QUESTION #4: anyone know where on the internet I can get the schematic for the PCM?
You can see the graphic plot here: http://www.glocktech.net/b
TIA Bill
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• posted on January 15, 2004, 12:03 am
Bill,
As a former GM tech., I can say that you are reading too much into this. If you state the error code, I can give you some information on how to diagnose the problem. It's not rocket science. In all of your information, I did not see the error code stated anywhere. What is the PCM code?
GMdude

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• posted on January 15, 2004, 1:12 am
Oops, it was right at the top. must have lopped it off. EGR pintle position error - P1406

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• posted on January 15, 2004, 12:26 am
Bill,
One more thing. I know where you are coming from. You have a strong understanding of computers, and you have provided much data. But, GM doesn't provide this data to their techs. They go by the error code, and then go down the information ladder, to solve the problem.
I'm a Master Certified Tech., but this information is somewhat useless to me. A GM engineer may have fun with it?
I have no way of retrieving engineering documents from GM.
I too, have a strong understanding of computers. I built the machine I'm sending this message on. But, the documentation is not there for advanced problems, with GM.
GMdude

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• posted on January 15, 2004, 1:18 am
GMDude,
Yes, that's true. I came from the digital process control industry, senior software engineer. We used a lot of analog to digital and digital to analog equipment to control everything. I figured that the PCM has to have this circuitry built into it in order to convert the 0-5V to counts.
Bill

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• posted on January 15, 2004, 3:44 am
I agree with GMD; you're making it a bit harder than it should be. If, however, you wish to delve deep into GM ECMs, I might suggest the diy-efi mailing list (www.diy-efi.org). They are *not* a group of production car troubleshooters, but they are devoted to understanding, manipulating and developing ECMs on this level. There is a list specific to GM ECMs, I think, and you can probably learn quite a bit from there.
Mark
On Wed, 14 Jan 2004 10:37:53 -0700, Bill wrote:

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I fart in Darl McBride's general direction.

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• posted on January 15, 2004, 4:32 am
Thanks for the tip. I will investigate that link tomorrow.

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• posted on January 15, 2004, 3:57 am
I'm not certain, but quite likely the PCM EGR control output is pulse-width modulated, so measuring the voltage is not going to provide much useful information.
I think most likely your EGR valve's pintle mechanism is worn out or sticking and not responding properly or fast enough to the changes in desired pintle position. This seems to be something of a pattern failure on some GM vehicles - see this page:
http://www.obdiicsu.com/Studies/2001CaseStudies/StickyEGR.html
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• posted on January 15, 2004, 5:07 am
Robert Hancock wrote:

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• posted on January 15, 2004, 4:52 pm
Robert,
Thanks, that was an excellent white sheet on the EGR. However, I don't believe it is the EGR valve at this time. I replaced the EGR valve with a known good unit and got the exact same results. I need a way to pin point the PCM as the source of the problem before getting a new PCM. I picked up a used PCM from a junk yard and will test it tomorrow. I can't afford to install a new PCM and find the problem didn't go away.
Bill
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• posted on January 16, 2004, 12:41 am
GMdude,
Thanks for the schematics and test procs. I should mentioned before that I had access to Mitchell's. Sorry. I did run through these procs, and that's how I elimiated the EGR valve as the culprit. But having said that, I am going to repeat them just to be sure.
Bill

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• posted on January 16, 2004, 2:36 pm
GMDude
There was one additional driveability problem reported that I did not experience: the car occasionally stalled when coming to a stop. Turn key off/on and restarts ok. I added that info to the web site.
Bill

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