Walt's '57 ECM WoesPosted 7/1/2008
By Jeff Bach
Awhile back I was paid a visit by two fellows who were trying to pick my brain or possibly get me to come across town to help them figure out a problem they’ve been having. It concerned a technical malfunction in one of their powertrain systems.
Dave’s powertrain operates fine with his ECM, but Walt’s engine would not run smoothly and they couldn’t get communication to a scan tool. Now these are not your average try-a-few-things-and-throw-in-the-towel sort of fellows. They have been through the street rod performance people’s technical support and tried all of their suggestions. They then sent the whole wiring harness and ECM back to the company and had them retest their work. They got word that the ECM was no good and sent the company a new ECM, which Dave got through GM with his discount. Upon reinstalling the harness with all the proper power and ground hookups, they now have a different problem! Walt’s car idles smooth but won’t accelerate. The scan tool now communicates with the ECM and receives codes. Dave had pretty much concluded that the ECM was again at fault but couldn’t get the technical support guy to agree with him. It seems that they wanted him to do a relearn of the system to clear.
This is where I came in.
When I ran out of things to tell them to try and hoops for them to jump through, I finally packed up my scope, scan tool and road bag and headed over to the body shop where the car was being assembled. The body shop guy doesn’t want to put the car any further together until they get the powertrain issue resolved since part of his job will be to bury all the wiring beneath polished aluminum and chrome covers.
I hooked up the scan tool with a CANdi module and did a code check. I found a U0101 TCM data code, a p2138, and a p060D, which are both TAC-related codes, and a P0315 crankshaft position (CKP) system variation learn code. The p060D code did not show up in the code chart for the specified powertrain, and I have never encountered this code before. I assumed it could have been related to a programming issue. These ECMs have to be programmed so that they don’t need pass key data and aren’t looking for the usual inputs from places such as ABS, TCS, and IPC modules. When you take on one of these projects you are accepting that certain standards are no longer in place. The ground beneath you can feel a little shaky at times. I attempted to move on to the other TAC code and proceeded to diagnose it using the factory trouble tree, which turned out to be a short-lived effort in futility. I was able to verify the wiring from both the APP sensors and the TAC module was intact. The P0315 code is normal for a new ECM and requires you to hold down the throttle until the engine hits the rev limiter for the ECM to “learn” the variations in the crankshaft and the crankshaft sensor. Of course, since this is not possible with the TAC situation, that code will have to be dealt with later.
This image came from the TPS sensors at the TAC module. It looked a little scary to me at first but after doing a little studying I realized that it’s quite normal. This was obtained by going wide-open throttle and release. The spikes you see on sensor 2 coming down are actually pulses put there by the ECM every 157 mS to determine the integrity of the circuits. It looks to see if they show up on the other sensor. If they do, it assumes the two circuits are shorted together. You can see this a little better in Figure 5.
Next, I switched to Walt’s ECM and took the same readings, which are displayed in Figure 6.
This image shows the TAC TPS signals start to move normally toward each other then change and go back to below reference as the ECM drops control of the TAC motor. It is at this point that the codes set and the throttle valve is left at idle no matter where you put the pedal. Clearing the code and starting again is the only way to get the throttle to start to move again. In Figure 7, I look at the APP sensor and the TAC TPS signal to see if they coincide.
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