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Home » ISX11.9 CM2250 - Service Manual » Cummins ISX11.9 CM2250 – Service Manual 005-054   Stall Speed Test

Cummins ISX11.9 CM2250 – Service Manual 005-054   Stall Speed Test

Stall Speed Check

 TOC

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

NOTE: It is possible that the vehicle brakes will not hold an electronically controlled transmission.

  
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 CAUTION 

Do not exceed 120°C [250°F] converter oil temperature. Overheating 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.

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The following equipment is needed for this check:

  • Stopwatch
  • Optical tachometer, Part Number 3377462 or INSITE™ electronic service tool
  • Equipment manufacturer’s stall speed and time-to-stall specifications.
  
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Place the gear selector in the highest gear or full forward.

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

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

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

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Bring the engine speed back to low idle.

  
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 CAUTION 

Do not exceed 120°C [250°F] converter oil temperature. Overheating 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].

Quickly move the throttle to the full-open position.

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

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

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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.

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If the stall speed is not within the specifications, refer to the Stall Speed Checklist at the end of this section.

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

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Time Speed Check

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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 stopwatch at the same time.

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  • When the engine speed is 90 percent of the stall speed rpm, stop the stopwatch.
  • Example: Stall speed rpm 2089 [2089 x 0.90 = 1880 rpm].

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

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

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Stall Speed Check List

 TOC

Stall Speed Too Low

Stall Speed – Too Low
The tachometer is in error.
Yes
No
The engine is up to or above 70°C [160°F].
Yes
No
The converter oil is up to 80°C [180°F].
Yes
No
The stall has been held long enough for the engine to accelerate to full power.
Yes
No
The match curve stall speed was recorded correctly.
Yes
No
The converter oil meets the converter manufacturer’s recommendation (SAE 30 rather than SAE 10, for instance).
Yes
No
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.
Yes
No
The unit is operating at an altitude high enough to affect the engine power.
Yes
No
The converter charging pressure is correct.
Yes
No
The converter blading is interfering, or in a stage of failure. Check the sump or filter for particles.
Yes
No
The converter stators are freewheeling rather than locking up.
Yes
No
The engine is set for power other than that specified on the power curve.
Yes
No
The converter is wrong due to improper build or rebuild of unit.
Yes
No
The converter is performing to the published absorption curve.
Yes
No
The engine and converter match is correct. Check the engine and converter models for the proper match.
Yes
No
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.
Yes
No
The engine power is down. Perform a cylinder performance test to find the cylinder with low power.
Yes
No

Stall Speed Too High

Stall Speed – Too High
The engine is high in power.
Yes
No
The tachometer is in error.
Yes
No
The accessory power requirements are less than 10 percent of the gross engine power.
Yes
No
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.
Yes
No
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.
Yes
No
The converter turbine element is beginning to fail and lose blades, or the converter was originally built with the wrong size element.
Yes
No
The engine and converter match is correct due to a revision in the engine rating or the converter performance.
Yes
No
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.
Yes
No
The converter is performing to the published absorption curve.
Yes
No
The converter charging pressure is correct.
Yes
No

The reasons for abnormal stall speeds listed above are some that have been encountered by Cummins representatives and probably do not 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:  16-Apr-2010

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