M1078 Transmission Problem

[OD Green]

Hello Troopers!
My M1078 A0 has been pretty good, 12K mi, but has developed a transmission problem. Starting out or downshifting to 2nd it will often stay in 2nd- the left range indicator blank. Shutting off the main power resets it and it will often be ok for a while. I have had codes 25-11 and 22-16 and 13-13. The 13-13 just started and might be from my testing. I replaced the speed sensor but it didn't fix it. I have some AC noise on the DC that I wonder might be the problem. I used the TMs and tested the harness from the xfer case to the front connector behind the grill, both wires tested ok, and none go to ground. I'm hoping it's not the turbo sensor...really don't what to drop the pan. Could this be bad diodes in the alternator?

 

[Ronmar]

If you can reliably replicate the problem, take the alternator out of the equation. You can pull Relay K11, that will take the alternator off the line…

80mv seems a little high, but I have not put a scope on mine. Where did you sample that voltage?

 

[OD Green]

If you can reliably replicate the problem, take the alternator out of the equation. You can pull Relay K11, that will take the alternator off the line…

80mv seems a little high, but I have not put a scope on mine. Where did you sample that voltage?

Hi Ronmar-

I pulled the relay earlier and took a ride, it still locked out but I then had low voltage code 13-13 son it wasn't a great test. I have 3 volts of AC on the DC, the scope was at the posts in the "glove box" I ordered a new voltage regulator hoping that's the issue but it might be the diodes in the alternator...but it charges ok- Thanks for the help!!

 

[Ronmar]

Your batteries OK? You should have been able to drive for a while on good bats with the alt offline without getting a low voltage code. Have you gone over all the grounds and checked the connections to the controller? Have you measured the voltage at the controller connections?

3V? I didn’t see that in your scope shots. It was kind of hard to see but it looked like the close view was 2v per division and the ripple was less than a whole division. The automated measurements on the right was reading 80mv pk-pk…

So with the codes cleared, take it for a drive. As soon as it does it, stop and read the codes…

 

[OD Green]

Hi Ronmar, you must be as old as me! Divisions? Check out the digital results on the right for 12VDC- 4 volts peak to peak, that's AC on the DC- but also a min max- this is at an idle- the event may be fast and not on the display. I cleared the codes and they came back- 22-11, 25-16, 13-13 The 13 is new- Have not tested at the controller, pain to get to it. I'll have the new regulator next week and see if it helps. I'm worried it's the turbo sensor on the transmission pan...not looking forward to dropping the pan. That must show up at the cable end under the grill- plan to find the pins and ohm it before too long. Hate the thought of an Allison shop and rates... really need to figure this out. Thank you!

 

[roverchef]

Just starting from the easiest and working backwards....
When was the last time you did fluids/filters in the transmission?

 

[OD Green]

Just starting from the easiest and working backwards....
When was the last time you did fluids/filters in the transmission?

I changed to original oil to full synthetic Delvac 2 years ago, filters at the same time. Cracked a cooling line this summer and lost about 2 gallons or so but it never gave me trouble after and I've driven it hundreds of miles since. I did notice that it's now hanging in 3rd when down shifting sometime...for about a few seconds-8 seconds. That's new. I took it out this AM, after a while it locked out and threw a 25-11 code. Wonder if the new after market speed sensor it's working right- the code is for that sensor

 

[Skyhawk13205]

What are your output speed sensor OHM readings? It sounds like your turbine and engine speed sensors agree, the 25-11 code is only produced if turbine and engine speed sensor are the same speed with output speed at 0 when it should be at the specific speed ratio. The output speed sensor is inside the transfer case you can measure the OHMs directly at the 6 pin plug attached to the front side of the transfer case.

 

[Ronmar]

Well if they are coming right back something is going on.

I have trouble shot alt issues where the folks have drove for days without an alt and not had low volt issues… the fact that the codes returned when you took the alt out of circuit indicates it is not the issue anyway…

 

[OD Green]

What are your output speed sensor OHM readings? It sounds like your turbine and engine speed sensors agree, the 25-11 code is only produced if turbine and engine speed sensor are the same speed with output speed at 0 when it should be at the specific speed ratio. The output speed sensor is inside the transfer case you can measure the OHMs directly at the 6 pin plug attached to the front side of the transfer case.

Output speed sensor is around 300, it's new, Have not checked the engine speed sensor- maybe that's the problem? Also when running the TM test between the passenger side transmission speed sensor connector A&B to the connector on the xfer case 6 or 8 pin- I get no continuity, Not sure if there is a break, if the test is wrong for my truck, or if I'm doing the test wrong. Earlier this summer the neutral safe lockout stopped working and I had to hotwire the starter circuit. Maybe related? Seems hard to break a wire in the cable unless something hit it- I suspect the main cable from xmission to grill is ok. There are no wires shorted to ground. This is a tough one!!
Thank you!

 

[OD Green]

News! I scoped the balanced pair at the transfer case and saw lots of noise on the line when in drive. I appeared that perhaps there was an intermittent open in the output speed sensor or associated wiring. I drained the transfer case and opened the inspection plate- inside I couldn't visually see a problem- looking back I should have had an ohms meter connected to the pins- I did some wiggling, unplugged the solenoid and plugged it back in, bent the harness away from the inspection plate- and put it back together. Since that moment I've not had an error code- about 25 miles. I would normally have seen a code either right in my driveway or within 5 or 10 mins. I'm cautiously optimistic and may be soon looking to source the sensor without the harness and high price. Anyone have a source for just the sensor? PAHZZ 73342 29505601 SENSOR,SWITCH

 

[TomTime]

Nice truck!

 

[OD Green]

Nice truck!

Thank you!

 

[OD Green]

Your batteries OK? You should have been able to drive for a while on good bats with the alt offline without getting a low voltage code. Have you gone over all the grounds and checked the connections to the controller? Have you measured the voltage at the controller connections?

3V? I didn’t see that in your scope shots. It was kind of hard to see but it looked like the close view was 2v per division and the ripple was less than a whole division. The automated measurements on the right was reading 80mv pk-pk…

So with the codes cleared, take it for a drive. As soon as it does it, stop and read the codes…

Hi Ronmar, I'm wondering about the batteries- will need to spend some time with them at some point, as to the scope- the noise varied and I should have done a better job at logging my tests. There is something making noise with the alternator and regulator 100% out of circuit. The biggest revelation was the loss of pulses and big noise on the output sensor balanced line. I bought some needle probes that pierce the wire for testing to avoid cutting everything. When testing the output speed sensor the noise was 4 or 5 times the signal at times- looked to me like the sensor went open. After futzing around in there it's calmed down- I don't expect forever. I need to find the sensor for cheap and replace it. Thanks for the help!!