N14 Base Engine Troubleshooting and Repair Manual – 005-054   Stall Speed Test

Stall Speed Check


Converter Transmissions Stall Speed

The stall speed is the engine speed (rpm) obtained at full throttle when the converter output shaft is locked.

NOTE: It is possible the vehicle brakes will

hold an electronically controlled transmission.



Do not exceed 120°C [250°F] converter oil temperature. Overheating which can result and converter damage can occur. If the oil temperature exceeds 120°C [250°F], put the transmission in neutral and operate the engine until the oil temperature is below 120°C [250°F]. Check the converter oil level.

The following equipment is needed for this check:

  • Stop watch
  • Digital tachometer kit, Part No. 3375631, or optical tachometer, Part No. 3377462
  • Equipment manufacturer’s stall speed and time to stall specifications

Install the tachometer on the fuel pump.


Place the gear selector in the highest gear or full forward.


Make sure the vehicle has good brakes and air pressure in the brake system.

NOTE: The brakes

prevent the vehicle from moving when the engine is at full throttle. Engage the vehicle brakes to keep the vehicle from moving.


Operate the engine until the converter temperature is 80°C [180°F] or above.


Bring the engine speed back to low idle.



Do not exceed 120°C [250°F] converter oil temperature. Overheating and damage to the converter can occur.

Quickly move the throttle to the full open position.


Check the engine speed (rpm) at the point of stall:

  • Always hold the speed until it is stable
  • Take several readings
  • Make sure the readings are accurate


Check the speed (rpm) against the specifications for the equipment, converter, or automatic transmission.

NOTE: The stall speed for the engine and converter/transmission can vary ± 8 percent from the manufacturer’s specifications.


If the stall speed is

within the specifications, refer to the stall speed check list at the end of this section.

Check the equipment manufacturer’s troubleshooting procedures for other reasons for stall speed problems.


Adjust the throttle shaft plunger screw, if necessary, to change the stall speed. Refer to Procedure



increase the fuel rate unless it is the true cause for the low stall speed. Overfueling of the engine and poor durability can result.


Time Speed Check


Perform the previous Stall Speed Check procedure through the “check the engine speed (rpm) at the point of stall” step then:

  • Quickly move the throttle to the full open position and start the stop watch at the same time


  • When the engine speed is 90 percent of the stall speed rpm, stop the stop watch
Example: Stall speed 2089, [2089 X .90 = 1880 rpm]

NOTE: The type of unit and the stall speed rpm can make the stall speed time a maximum of 10 seconds.


Check the equipment manufacturer’s specifications for the time to stall or the acceleration time.

If the time is excessive, check the fuel pump AFC for an air leak. Refer to Procedure

006-047 and the Stall Speed Check List at the end of this procedure.


Stall Speed Too Low

  Yes No  
1.     The tachometer is in error.
2.     The engine is up to or above 70°C [160°F].
3.     The converter oil is up to temperature 80°C [180°F].
4.     The stall has been held long enough for the engine to accelerate to full power.
5.     The match curve stall speed was recorded correctly.
6.     The converter oil is to the converter manufacturer’s recommendation (SAE 30 instead of SAE 10 for instance).
7.     The engine driven accessory power requirements exceed 10 percent of the gross engine power. Check for abnormal accessory horsepower losses such as hydraulic pumps, large fans, oversize compressors, and so on. Either remove the accessory or accurately determine the power requirement and adjust accordingly.
8.     The Air Fuel Control (AFC) is properly adjusted.
9.     The unit is operating at an altitude high enough to affect the engine power.
10.     The converter charging pressure is correct.
11.     The tailshaft governor is interfering with and preventing a full throttle opening. Disconnect the tailshaft governor. Do

not exceed the manufacturers maximum output speed.

12.     The converter blading is interfering, or in a stage of failure. Check the sump or filter for particles.
13.     The converter stators are free-wheeling instead of locking up.
14.     The engine is set for power other than that specified on the power curve.
15.     The converter is wrong due to improper build or rebuild of unit.
16.     The converter is performing to the published absorption curve.
17.     The engine and converter match is correct. Check the engine and converter models for the proper match.
18.     The engine is matched to too large of a converter. If this condition is believed to exist, please report the engine-converter-accessory information to the factory.
19.     The engine power is down. The engine torque rise could be less than shown on the standard engine curve. See the fuel setting adjustments and the turbocharger air manifold pressure check.

It is sometimes easier to change the engine fuel rate than to determine the true cause for low stall speed, but the customer ends up with an over-fueled engine which will also negatively affect durability. Do not increase the fuel rate as a “cure-all”.

Stall Speed Too High

  Yes No  
1.     The engine is high in power.
2.     The tachometer is in error.
3.     The accessory power requirements are less than 10 percent of the gross engine power.
4.     The converter oil is aerating or foaming. Check for low oil level, air leaks in suction line, lack of foam inhibitor in the oil, or suction screen or filter. Would be accompanied by a noticeable loss of machine performance.
5.     The converter is being held at full stall. Check for slipping front disconnect clutch or a rotating output shaft. On the converter-transmission package, this can be impossible to check.
6.     The converter turbine element is beginning to fail and losing blades, or the converter was originally built with the wrong size element.
7.     The engine and converter match is correct. Due to a revision in the engine rating or the converter performance.
8.     If the oil level is too high on the transmission-converter units with the oil sump in the transmission, it can cause severe aeration due to parts dipping in the oil.
9.     The converter is performing to the published absorption curve.
10.     The converter charging pressure is correct.

The reasons for abnormal stall speeds listed above are some which have been encountered by Cummins representatives and probably do

include all possible causes. The correction of the problem is either covered in the vehicle service manual, the converter service manual, or is self-explanatory.

Last Modified:  12-May-2003