4.6 Transfer Cases, PTO Controls, and PTO Driveshaft Angles

4.6 Transfer Cases, PTO Controls, and PTO Driveshaft Angles

The last transmission task on the ASE T3 list — inspect, adjust, repair or replace transfer case assemblies and controls — matters on all-wheel-drive dump trucks, utility trucks, fire apparatus, and military-style vehicles. The driveshaft area adds PTO driveshafts to the angle-measurement task. This section uses Meritor's MTC-series transfer case manual as the reference design.

1. What a Transfer Case Does

A transfer case sits behind the transmission and splits power between the rear drive axle(s) and a front drive axle. A typical heavy-duty unit (Meritor MTC series) contains:

• An input shaft and helical gear train with an idler gear.
• A high/low range sliding clutch collar with high- and low-range helical drive gears. Cummins/Meritor data list a low range around 2:1 on several MTC models (for example 2.05:1).
• A front-axle declutch that connects or disconnects the front output.
• An optional neutral position and power take-off (PTO) on some models.
• A speed sensor, indicator switches, a ceramic magnet to catch wear particles, a breather (or neutral air-control solenoid), and on some models an oil pump and cooler option.

2. Air Controls and Shifting Rules (Meritor MTC)

The range, declutch, and neutral functions are air-shifted:

Shop shifting check: With the transfer case on the bench or in the truck, apply 60 psi or more to the front-axle declutch port and turn the input by hand. The front output should turn at the same rate as the rear output. Then remove the air and repeat at the high-range, low-range, and (for PTO-equipped cases) neutral ports to confirm each shift.

Won't shift or declutch: Check air pressure first (at least 60 psi), then the indicator light and sensor. Next check the air control and supply for leaks or contamination, then the shift piston, O-rings, and return spring. Only then remove the case to inspect the shift collar, engagement teeth, shift fork, and fork bore.

3. Transfer Case Lubrication and Temperature

• Oil: Meritor specifies SAE 50W full-synthetic oil in MTC transfer cases and warns not to use multi-viscosity engine oils or EP GL-5 gear oils; damage can result.
• Level: fill to the level Meritor specifies at the fill plug, on level ground. An incorrect level causes excess heat.
• Temperature: Meritor notes that MTC-4208, -4209 and -4210 cases may run above 300°F (148°C) in long high-speed line-haul or utility use. If oil reaches 350°F (177°C), stop the vehicle immediately and find the cause. An oil cooler is offered for trucks that spend long periods at high speed with overdrive transmissions.
• Magnet: inspect and clean the case magnet at oil changes; heavy metal buildup means a gear or bearing is failing.

4. Noise, Vibration, and Leaks

Meritor's transfer case vibration flowchart starts with the driveline, not the case:

1. Measure all driveline angles. Angles should not exceed 5 degrees, and the difference between any of the angles should not exceed 1-1/2 degrees. Meritor calls excessive driveline angles the leading cause of drivetrain vibration and noise.
2. Check driveline balance, U-joints for wear or damaged rollers, and the case mounting fasteners and brackets for looseness or breakage.
3. Only then look inside for worn bearings or gears.

Leaks usually come from output seals, the PTO mounting, cover gaskets, or a plugged breather.

5. PTO Controls

A PTO can be shifted by cable or lever, by air, or electrically/hydraulically:

• Clutch-shift (non-clutched) PTOs on manual transmissions are engaged with the clutch released and the gearing stopped, just like selecting a gear from rest.
• "Hot-shift" PTOs have an internal clutch pack and can engage while the gearing is turning. They are common on automatic transmissions.
• An indicator switch and light tells the driver the PTO is engaged. If a PTO does not engage or disengage, Meritor's first checks are the air connection to the PTO shift port and the indicator light and switch.
• Noise tells you about gear mesh. Muncie's rule is that a whine that rises with rpm means the PTO is mounted too tight (add a gasket), and a clatter means it is too loose (remove a gasket). Correct backlash is 0.006–0.012 in. (section 4.4).

6. PTO Driveshaft Angles, Loaded and Unloaded

The ASE task for driveline angles specifically includes PTO driveshafts and measuring loaded and unloaded:

• Treat a PTO driveshaft like any other two-joint shaft. Keep operating angles small and equal at both ends, and never at zero. Spicer's guidelines are at least 1/2 degree, equal within 1 degree, and no more than 3 degrees for vibration-free running. Spicer tightens the equal-angle rule to 1/2 degree for shafts in front of transfer cases or auxiliary devices.
• Phase the yokes correctly, keep slip joints lubricated, and support long shafts.
• Loaded vs. unloaded: as a truck is loaded, springs compress and the axle rotates, changing pinion angles. A body-mounted pump driven from a chassis-mounted PTO may also change angle as the chassis flexes. Measure at both conditions; a vibration that appears only when loaded often traces to angles that are acceptable empty but out of limits loaded.
• Guarding: exposed PTO shafts must be guarded, and the PTO must be disengaged and locked out before anyone works near the shaft.