- 53
- 0
- 0
- Location
- Woodbridge, Virginia
So my beloved M1079, Bessie, blew her transmission at about the 5,400 mile mark. One of the reasons I bought this truck was the extremely low miles - she only had 4,400 miles on her when I bought her.
She failed one fine afternoon at highway speed about eight miles from home. I was going up a moderate grade at about 45 mph when she downshifted from 7th to 6th gear, and then over about 5 seconds, lost power to the rear wheels. Then the dreaded "- -" appeared on the ECM. I coasted to a stop on the shoulder.
$350 later for a heavy tow truck, and she is back home in my driveway, leaving me with a giant question mark about what actually happened. The codes from the ECM indicate a failure of the output shaft speed sensor, so I test the resistance and it seems fine. I visually check all the wiring, which seems okay, too.
I read for hours about all the potential issues and do a little bit of road testing in my neighborhood. She'll drive, but only for a while. Good thing my house is at the bottom of a shallow incline because we rolled back home after a couple of failures. What I notice is a pump cavitation noise once the failure happens. It sounds very similar to a power steering pump low on fluid, but it's definitely coming from the transmission. (Yes, the fluid levels were fine) I really don't think there is a speed sensor problem.
Even though I'm pretty sure it's not the problem, I finally decide to replace the speed sensor and discover that, though the wires enter the transfer case from the front, the speed sensor is located under a larger cover at the rear. This requires draining the fluid, which has a slight burnt odor to it. I pull the speed sensor to discover the one I bought, which looks like the one in the TM, is not actually correct for my transmission. The part I bought cost $100 and it turns out the part I need costs $500. Ugh.
After consulting with a couple of Allison shops, I decide to pull the two main filters to see if there's any debris. I buy the filter change kit and do just that - no debris found.
Okay, next step is to ring out the entire transmission wiring harness from the cab back. It checks out okay. I buy a set of pressure gauges to starting checking pressures. Sadly, my set did not come with an adapter for Allison transmissions and I couldn't find anything that worked.
Finally, after about 8 weeks, I surrender. I take the truck to Western Branch Diesel in Gainesville, Virginia and place her in the capable hands of their transmission foreman, Seth, who apparently knows everything there is to know about Allison MD3070PT transmissions. I'd spoken to Seth several times during my attempts to diagnose the issue and he always listened patiently, answered my questions, and offered good advice.
Within a couple of days Seth indicates there is a pressure anomaly on one of the solenoid outputs, but that he wants to swap computers and try again. His spare is tied up in another customer's RV. I decide to buy one. Thank you Suprman for having a NOS LMTV ECM! I get the computer in the mail within two days and drop it off at the shop.
Seth calls back within a day or two and reports no change with the new computer. We discuss options and decide the next step is to drop the valve body (or TCM, as it's sometimes called) from the underside of the transmission, which contains all the solenoids, valves, and the main suction filter. The purpose is to see if there's an accumulation of debris and the second is to check all the solenoids and valves. It turns out the suction filter on the valve body is the first filter in the chain and is generally where any debris will collect.
A few days later and the call comes from Seth. The news is not good. There is a horrifyingly large accumulation of metal and friction material at the suction filter. At least one set of clutches are severely damaged and, when there is this much metal, the destruction has likely extended to other gears and bearings. His cost estimate for a rebuild is $14,000 (yes fourteen THOUSAND dollars) in parts and labor, and that's assuming that none of the large planetary gears need to be replaced at somewhere around a thousand dollars each.
I am staggered. This is bad.
Again, enter Suprman. He has a transmission from a recently running LMTV he's parting out. It's an M1093 (airdrop) and he confesses that he makes more parting them out than reselling them. I make a one day road trip from Virginia to Connecticut to pick up the thing (complete with torque converter and transfer case) and drop it at the shop the following Monday. Bessie is on her way to getting better.
We discover the new transmission is a phase one WTEC3, not a WTEC2. These were installed in later years of LMTV production and are compatible with the WTEC2 ECM using an adapter cable, which has to be ordered.
I also tell Seth to drop the valve body on the new trans to check for debris and, if found to be okay, change the filters and lube before proceeding with the installation.
Finally, after almost three months from the initial failure, my girl is home. She runs better than ever and shifts smooth as glass. I am happy she's back.
The cost? $4,500 for the replacement trans, $6K in parts and labor, and $900 for the new ECM: Total of $11,400. I considered doing a lot of the work myself, but at around 2,000 lbs, I'd need special jacks and a team of people helping to do the swap. And the truck is too valuable to just throw away. So, yes, the money hurts, but it's worth it. And I got educated along the way, too.
She failed one fine afternoon at highway speed about eight miles from home. I was going up a moderate grade at about 45 mph when she downshifted from 7th to 6th gear, and then over about 5 seconds, lost power to the rear wheels. Then the dreaded "- -" appeared on the ECM. I coasted to a stop on the shoulder.
$350 later for a heavy tow truck, and she is back home in my driveway, leaving me with a giant question mark about what actually happened. The codes from the ECM indicate a failure of the output shaft speed sensor, so I test the resistance and it seems fine. I visually check all the wiring, which seems okay, too.
I read for hours about all the potential issues and do a little bit of road testing in my neighborhood. She'll drive, but only for a while. Good thing my house is at the bottom of a shallow incline because we rolled back home after a couple of failures. What I notice is a pump cavitation noise once the failure happens. It sounds very similar to a power steering pump low on fluid, but it's definitely coming from the transmission. (Yes, the fluid levels were fine) I really don't think there is a speed sensor problem.
Even though I'm pretty sure it's not the problem, I finally decide to replace the speed sensor and discover that, though the wires enter the transfer case from the front, the speed sensor is located under a larger cover at the rear. This requires draining the fluid, which has a slight burnt odor to it. I pull the speed sensor to discover the one I bought, which looks like the one in the TM, is not actually correct for my transmission. The part I bought cost $100 and it turns out the part I need costs $500. Ugh.
After consulting with a couple of Allison shops, I decide to pull the two main filters to see if there's any debris. I buy the filter change kit and do just that - no debris found.
Okay, next step is to ring out the entire transmission wiring harness from the cab back. It checks out okay. I buy a set of pressure gauges to starting checking pressures. Sadly, my set did not come with an adapter for Allison transmissions and I couldn't find anything that worked.
Finally, after about 8 weeks, I surrender. I take the truck to Western Branch Diesel in Gainesville, Virginia and place her in the capable hands of their transmission foreman, Seth, who apparently knows everything there is to know about Allison MD3070PT transmissions. I'd spoken to Seth several times during my attempts to diagnose the issue and he always listened patiently, answered my questions, and offered good advice.
Within a couple of days Seth indicates there is a pressure anomaly on one of the solenoid outputs, but that he wants to swap computers and try again. His spare is tied up in another customer's RV. I decide to buy one. Thank you Suprman for having a NOS LMTV ECM! I get the computer in the mail within two days and drop it off at the shop.
Seth calls back within a day or two and reports no change with the new computer. We discuss options and decide the next step is to drop the valve body (or TCM, as it's sometimes called) from the underside of the transmission, which contains all the solenoids, valves, and the main suction filter. The purpose is to see if there's an accumulation of debris and the second is to check all the solenoids and valves. It turns out the suction filter on the valve body is the first filter in the chain and is generally where any debris will collect.
A few days later and the call comes from Seth. The news is not good. There is a horrifyingly large accumulation of metal and friction material at the suction filter. At least one set of clutches are severely damaged and, when there is this much metal, the destruction has likely extended to other gears and bearings. His cost estimate for a rebuild is $14,000 (yes fourteen THOUSAND dollars) in parts and labor, and that's assuming that none of the large planetary gears need to be replaced at somewhere around a thousand dollars each.
I am staggered. This is bad.
Again, enter Suprman. He has a transmission from a recently running LMTV he's parting out. It's an M1093 (airdrop) and he confesses that he makes more parting them out than reselling them. I make a one day road trip from Virginia to Connecticut to pick up the thing (complete with torque converter and transfer case) and drop it at the shop the following Monday. Bessie is on her way to getting better.
We discover the new transmission is a phase one WTEC3, not a WTEC2. These were installed in later years of LMTV production and are compatible with the WTEC2 ECM using an adapter cable, which has to be ordered.
I also tell Seth to drop the valve body on the new trans to check for debris and, if found to be okay, change the filters and lube before proceeding with the installation.
Finally, after almost three months from the initial failure, my girl is home. She runs better than ever and shifts smooth as glass. I am happy she's back.
The cost? $4,500 for the replacement trans, $6K in parts and labor, and $900 for the new ECM: Total of $11,400. I considered doing a lot of the work myself, but at around 2,000 lbs, I'd need special jacks and a team of people helping to do the swap. And the truck is too valuable to just throw away. So, yes, the money hurts, but it's worth it. And I got educated along the way, too.
