Limp-Home in Third: Automatic Transmission Diagnosis on a Refuse Truck
A 2020 rear-loader refuse truck with an Allison 4500 RDS 6-speed automatic has 78,000 miles. The transmission uses a push-button range selector and drives a hydraulic packer pump through a PTO. At the end of a route the selector displayed a flashing 3 and the check-trans light came on; the truck would not upshift out of 3rd and had no 6th gear. The driver checked fluid level on a 4 percent sloped street with the engine running in neutral and said it was at the HOT mark. The fluid is dark and smells burnt. Stored codes are P0733 (gear ratio incorrect) and P0715 (input speed sensor). The pan has not been removed. The fleet wants to know whether to top off, service, or replace the transmission.
1. Was the fluid level check valid? Describe the correct procedure and explain what the fluid condition adds to the diagnosis.
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The check was not valid. A 4 percent slope shifts the sump level enough to read high or low by a significant amount, and an overfilled unit can aerate and foam, which itself causes poor shifts and overheating.
Correct procedure:
- Vehicle on a level surface.
- Engine running, transmission in neutral or park as the OEM specifies, parking brake set.
- Fluid at normal operating temperature, roughly 150-200 F.
- Cycle the selector through the ranges, return to neutral, then check the dipstick on the specified side and wipe/reinsert per the OEM procedure.
The fluid condition matters more than the level here. Dark color and a burnt smell indicate overheated fluid and likely clutch distress. Topping off will not correct that, and a service may only hide the symptom if clutch material is already circulating. The next step is a pan inspection, not a decision based on the dipstick alone.
2. Explain how the TCM sets P0733, and how P0715 could be either the cause or a consequence. What would you test before condemning the transmission internals?
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The TCM calculates actual ratio from input speed divided by output speed and compares it with the ratio commanded for the current gear. P0733 sets when the calculated ratio deviates beyond a threshold, usually with the converter clutch applied and after a commanded shift has had time to complete.
P0715 is an input speed sensor circuit fault. It can be the cause: if the numerator of the ratio calculation is missing or erratic, the TCM may set a ratio code and default to a limp range. It can also be a consequence: debris from a slipping clutch can damage the sensor, or a failed sensor can mask a real slip.
Before condemning internals, test the input speed sensor signal with a scan tool while rotating the engine or during a road test, check the harness and connector for chafing and fluid intrusion, and compare input speed to engine speed with the converter locked. Only after the speed signal is proven good should a clutch or valve body fault be pursued.
3. The pan is removed and contains a moderate amount of dark clutch material but no large metal chunks. What decision do you make and why?
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Moderate clutch material in the pan is enough to change the decision from service to repair. Clutch material means a friction element has been slipping, and that material is also circulating through the valve body, solenoids, and cooler.
- A fluid and filter service would remove some debris, but the worn clutch pack remains and will continue to slip. The fresh fluid often accelerates the failure by cleaning varnish from the remaining friction surfaces.
- A cooler flush and line check are required, because trapped debris will contaminate a replacement unit.
- If the debris is only varnish and fine fuzz on the magnet with no friction material, a service, relearn, and road test with live ratio and pressure data is reasonable.
The PTO circuit should also be checked. If the packer pump is loading the transmission at low engine speed and line pressure is marginal, that can contribute to clutch slip and should be corrected before the unit is returned to route duty.