M1078A1 Won't crank after initial start.

[troyken]

We are having an issue with our new( this week) to us 2003 LMTV. It will initially crank and start with no problems, If the truck is shut off via ignition switch and restart is attempted the truck will not crank at all. This can be cleared by turning the main battery switch. off and then back on. This seems to reset the transmission ecu and the truck will crank and restart. It has the cats eye error indicator on the transmission shift display when it won't crank. I notice that the led at the neutral relay in the breaker /relay PDP panel is active when the truck will crank and inactive when the truck won't. I found that the signal to the neutral relay coil is at 12 v when the truck will start and about 2.5 volts when it won't. The battery switch routine restores the 12v signal to neutral relay when ignition is turned on again. The neutral relay contacts supply a 24v signal TO the CAT engine ecm . I assume this signal tells the engine ecm that it is ok to crank.

The neutral relay and alternator excite relays have been replaced. Without the neutral relay active, the alternator excite relay and then the starter motor solenoid are not powered. Steady 12 and 24 v is present at the PDP panel.

The neutral relay coil signal comes FROM the trans ecu. Is this a failure that anyone has seen? The problem happens every time and does not appear to be intermittent. We have replaced the trans shift panel and the wiring between that and the trans ecu is good. Truck runs and shifts fine otherwise. There doesn't seem to be any codes at the shift panel.

Does this point to a bad trans ecu? The CTIS was removed (not by us) by the way. Maybe I'm missing something obvious or some interlock?

It is a Wtec III Allison 7 speed transmission.

Any input would be welcome.

Thanks,

Ken

 

[Ronmar]

Are there any codes stored in the transmission. With the switch on, press both the up and down arrows to enter diagnostic mode. The display will say D1 Followed by two sets of numbers, and repeat the sequence. Write the numbers down and note if the mode indicator is lit or not.

press mode button to step thru the 5 memory locations D1-D5. Note the numbers listed for each location and any mode lights(mode lights indicate an active fault)…