Need help with an ATEC / DDEC I problem

[kerlyn]

I'm trying to help get an '87 Prevost back on the road.  The vehicle (DD 8V92) just failed to start one day and we traced that to a bad DDEC I EDU (unlike later DDECs, this one is a two-box system with the ECM up front and the EDU mounted on the engine).  After we got the vehicle running again, it is showing a solid "Check Transmission" light but fails to flash out a code when the transmission test switch is thrown.  (We do not have any sort of reader for this system and instead have relied upon flashing codes for diagnosis.)

Now when we install a jumper in the DDL socket to read DDEC codes, it apparently interrupts the flow of data on the Engine to Transmission Data Link High (ATEC ECU 240/ DDEC ECM 533) and Low (ECU 238/ ECM 532) lines.  This latches a code 66 in the ATEC ECU that we can get to flash out by throwing the transmission test switch.  This seems to verify that the Check Transmission (ECU 215) signal and its associated (normally closed) bulb relay are working properly.  We can clear the "66" code by removing the DDEC test jumper, throwing the transmission test switch, and putting the transmission in reverse.  After that, with the test switch off and no jumper installed in the DDL socket, the Check Transmission stays lit even though there is no code.

The function of ECU 215 is described on the ECU Wiring Diagram as "Grounded normally open circuit when alarm conditions are present".  I take this to mean that the line is grounded when all is well (thus energizing the bulb relay) and floating when it isn't.  Oddly, when the DDEC test jumper is installed in the DDL socket (grounding ECM 451) the Check Transmission light goes out.  I suspect this is due to loss of Engine to Transmission Data flow but haven't verified that is happening.  The Engine to Transmission Data High / Low lines are the only connection from the ECM to the ECU.  Of course there is a pair of Transmission to Engine Data High / Low signals running in the opposite direction along with the Speedometer signal (ECU 205 / ECM 556).

I have the DDEC I and ATEC Troubleshooting Manuals.  The latter is SA 2048A and has no test procedure for the condition I'm describing.  Any ideas?  I should add that the transmission appears to shift normally (although we haven't tried excessive throttle) and the "Do Not Shift" lamp is NOT lit.

Thanks, -K-

[luvrbus]

If that is a 755 Allison the lite will stay on if the transmission is in the diagnostic mode fwiw

[kerlyn]

It is an HT 755 Allison.  I agree that completing a successful self-test should extinguish the lamp.  The ECU has been out of the bus and off to Jultech in MN for a bench test, where it received a clean bill of health.  Is it now necessary to drive the bus in order to clear the lamp?  I read in the ATEC Troubleshooting Manual that there's a self-calibration procedure for the throttle sensor.  In this installation, the TPS is connected to the ECM and I assume throttle position is sent over the Engine to Transmission Data pair.