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)…

 

[troyken]

We have checked for codes with this method and found none stored. That's another part of the mystery.

 

[Ronmar]

Hand over hand all the transmission cabling and inspect all the transmission connectors. If Allison is not giving you Neutral Start, then Allison is not happy…

also confirm that proper power is reaching it when under load/ign switch turned on. This could be a case of garbage in = garbage out. The transmission controller has two power inputs, one is battery power and the other is ignition power. The Allison uses two schemes to control external relays and indicators. One is to provide a path to ground for an already powered relay or indicator to energize that device. The second is to provide output power to a grounded relay or indicator and energize it that way. the output power for relays and indicators using this method is sourced from the battery power input. So bad batt input would result in poor relay control.

this could be as simple as a bad connection somewhere between batteries and the trans controller under the passenger dash. A quick check would be to sample the power at the TCU battery fuse. If y9u still have a circuit breaker in that ckt, pull it and install a standard automotive blade fuse. On either side of the amp label on the fuse you will see two little exposed metal areas. These allow you to measure voltage with the fuse installed, and are a great way to measure circuit voltage with the load a-plied.

THE ONLY VALID TEST FOR VOLTAGE Is when the circuit is loaded.

 

[GeneralDisorder]

I would start by cleaning all the grounds for the cab and power distribution panel, and heck the battery terminals, etc also. Check for voltage drop from cab ground to chassis ground.

And replace all the relays and replace all the circuit breakers with blade fuses.

After you have done that, report back results. There's a HIGH probability that this will fix the issues.

 

[troyken]

Thanks. Relays have been replaced. Blade fuses... I hadn't considered those as breaker CB77 checks good. I will inspect grounds. Does the large cannon plug and cable at transmission run direct to trans ecu or is there another connector in line somewhere before ecu connectors( blue, black, grey)?

 

[Ronmar]

There should be a cab pass thru connector for the trans harness. On the A0 they put it on the drivers side tunnel at the front forward of gas pedal/behind grill. Not sure if they relocated it on the A1…

yes all the CBs can be replaced with blade fuses…

the chassis grounds on these trucks were trash from the factory(star washer biting thru paint) and should all be stripped to bare metal. Starter to frame, alt to frame cab to frame inside cab to dash, ect.

 

[troyken]

Romar- I knew I saw that pass thru connector somewhere the other day and I could not remember exactly where. I will check that tonight as well as grounds. I noticed that they did not seem to be to bare metal, at least not now anyway. We have only had this truck a week so I'm learning it as I go.

Thanks.

 

[Ronmar]

Romar- I knew I saw that pass thru connector somewhere the other day and I could not remember exactly where. I will check that tonight as well as grounds. I noticed that they did not seem to be to bare metal, at least not now anyway. We have only had this truck a week so I'm learning it as I go.

Thanks.

Yep, strip them to bare metal, throw the star washer in the trasn, the coat them with dielectric grease or paint over them after y9u re-assemble them)…