How a loose five-cent nut turned into a Mack MP7 engine repair

Categories: Engines, Shop Talk, Transmissions

Rear Main Seal Discovery

When we removed the transmission to replace the rear main seal, we saw damage on the seal and rear engine housing. How did this happen? This is due to the bull gear nut that became loose. This resulted in the nut hitting the rear engine cover. Believe it or not, something as simple as a loose five-cent nut can wreak havoc on your rig. In this case, this particular issue is the common cause of failure on Mack MP7 engines.
This is the story of a truck that came in with glycol failure in the transmission and what was required to understand the problem, and, most importantly, to fix it.

The Symptom: Won’t Shift Up

The truck wasn’t shifting up, so our customer brought in the rig for us to take a look. We ran a basic diagnostic and:
  • The transmission oil level was low
  • We checked for warning lights and found the check engine light was on
  • We checked for codes on the engine and found an active code for coolant level low
  • We checked for codes on the transmission and found code P0720 – output speed sensor circuit and code P0776 pressure control solenoid 2 (PCS2) stuck off

Leaks and Loose Bolts


We inspected the transmission for any leaks or damage and found a leak coming from the left filter cover. We checked the bolts. They were loose so we torqued the bolts to the proper specs and moved on.

Next, we inspected the drive line carriers and the u-joint. They were still in a good shape, but we noticed the engine’s rear main seal was leaking. We also observed the coolant filter was loose and not fastened all the way in. It was leaking. So, we tightened up the coolant filter back to spec. When we swapped the transmission drain plug, we noticed the oil looked milky and the drain plug magnet was covered with clutch material.

We performed a GLYCOL TEST and got the diagnostic message: FAIL, TRANSMISSION IS CONTAMINATED WITH COOLANT.

So, we performed the clutch test and sure enough, the clutches were slipping. This makes sense because the transmission fluid was contaminated with glycol. This meant that the transmission at least needed a new cool and the engine needed a new rear main seal.


Pulling the Transmission

We pulled the transmission. The rear main seal was leaking, and when we replaced the seal, we could clearly see that it had been ground down on one corner.

Rear Engine Housing

Upon further inspection, we found that the gear above the crank gear which mesh together was completely loose and had ground down the seal and the rear engine housing.

After discussing with the customer, we began the process of removing the rear engine housing to inspect the damage and replace what was necessary.

To remove the engine rear housing, the valve cover and oil pan both need to be removed. Removing those also required the removal of a multitude of coolant hoses, charge air cooler hoses, and other miscellaneous hoses and fittings.

After careful documentation and zip tie labeling, we began removing everything restricting the valve cover access. Eventually, we removed enough to be able to access the air compressor bolts. We removed all three bolts and strapped the air compressor to the frame to keep it out of the way. Then we removed the valve cover. We put the truck up in the air and removed some coolant and air lines from the air compressor. We removed the oil pan and everything connected to it.

We disconnected the wiring harness on the rear of the engine to make space to remove the timing cover. Before removing the timing cover, we had to remove the internal injector harness as there was no disconnect to separate it from the cover. We disconnected all of the injectors and engine brake then pulled it through the timing cover. We removed the timing cover and then started on the rear cover.

Once we removed the rear cover, we were able to get a thorough inspection of the double idler gear and the rest of the geartrain. The nut holding the double idler gear was hanging on by only a thread or two which allowed the gear to slip back and forth on the bearings. This is what ground into the rear main seal and ground very badly into the rear cover. The damage was bad enough to require a new bull gear assembly.

We have seen similar failures in other cases where the gear falls off completely and destroys the remaining gears in the geartrain.


Reusing Gears

Thankfully in this case all of the surrounding gears were not damaged and could be reused. So we removed the double idler gear and then the bearing mounting shaft. Once all of the parts had arrived and we had thoroughly cleaned up the gear plate surface, we installed the new double idler gear which came as a pre-torqued unit. We torqued all of the Allen bolts to spec. Then we prepped the new rear cover.

We swapped all of the studs from the old one over and applied Volvo silicone to all of the required surfaces. We lifted it into place using studs in the rear of the engine and torqued all of the bolts to spec. We installed a new gasket into the new timing cover and applied silicone to it. We installed it, making sure it was flush with the valve cover seating location, and torqued it all to spec.

We installed a new gasket on the oil pan and then installed the oil pan onto the engine. We noticed the oil level sensor inside the pan was damaged and installed a new one. Then we installed the internal injector harness. We installed the valve cover with a new gasket. Afterward, we installed new o-rings in the air compressor and the fuel pump. Then we installed all of the various hoses and fittings to complete the cooling and air system.

Once that was complete, we could install a new rear main seal which would enable us to install the new transmission.


Completing the Repair

We already installed the new transmission cooler so once the transmission was in, we could attach the driveline with new straps. Then, we filled the truck with transmission fluid, engine oil, and coolant. We took the truck on a long test drive and it drove very well.

Prevention as the Cure

The point of this story is two-fold.

First, all the rattling on the road will eventually loosen bolts, wires, screws, and fittings. It’s unavoidable. That is a key reason to schedule preventative maintenance. That little five-cent nut that became loose in this recent repair triggered much more costly repairs and put that owner’s truck out of production longer than the time and cost of time in the shop doing preventative maintenance.

The second point of the story is that once the domino effect starts happening, the driver may have no idea until a symptom shows up. In today’s example, the symptom was the inability to shift up. If preventative maintenance isn’t a part of your company budget, get regular feedback from your drivers. Chances are they are noticing something not feeling right or that the rig is running a little off. Right when something off is reported is the time to schedule preventative maintenance and hopefully catch that little five-cent nut from costing your business downtime and can help you avoid complete replacements.