1990/05/01 Caterpillar

Reference: Truck Engine News, May 1987, Page 3, "Alternator/Generator Output Test On The Engine. "

Truck Engine News; December 17, 1984; Page 3; "Starters That Remain Engaged After Engine Is Running. "

Truck Engine News, April 1989, "Page 6, "Causes And Effects Of Low Battery Voltage On Electric Starters. "

Reference: Special Instruction, SEHS7768, "Use of 6V2150 Starting/Charging Analyzer Group. "

This article replaces the article with the same title on Page 4 of the May 1987 Truck Engine News.

This is a general procedure to help aid the serviceman determine if a starter needs replacement. It is NOT intended to cover all possible problems and conditions, but to serve only as a guide. The most common 24V circuit is discussed and 12V circuits are indicated as applicable. On engines with a diagnostic connector, the 6V2150 Starting/Charging Analyzer Group helps you to do the following procedure in seconds.

General Information

Starting systems usually have four major components:

... Start switch

... Start relay

... Starter solenoid

... Starter motor

Illustration 1. Typical cranking circuit.

The only exception to this typical cranking circuit is that some small engines do not require the start relay. In this case, the start switch is connected directly to the starter solenoid.

Start switches are relatively low current devices. They are rated to switch approximately 5 to 20 amps. Because the coil of a start relay between TP2 (Test Point 2) and TP1 in Illustration 1 draws about 1 amp, the start switch can easily turn on the start relay and have long life.

The switch contacts of a typical start relay are rated to switch between 100 and 300 amps. Because a starter solenoid requires only 5 to 50 amps, the start relay can easily switch this load.

The starter solenoid has two functions: it engages the pinion with the flywheel, and it is a high current switch rated about 1000 amps that actually turns on the starting motor.

The starter solenoid has two coils: the pull-in coil (W) draws about 40 amps and hold-in coil (X) requires about 5 amps. The instant the start relay closes, both coils (W and X) receive power. Battery voltage is applied to the high end of both coils, at Test Point (3) which is the "start" terminal (S). The low end of hold-in coil (X) is permanently connected to the ground post of the starter motor. Grounding for the low end, Test Point (4), of pull-in coil (W) is momentary, and takes place through the DC resistance of the starter motor. When magnetic force builds in both coils, the starter pinion is moved to engage the ring gear. Only then will the solenoid contacts close to power the starter motor.

This temporarily removes the ground from pull-in coil (W), and puts battery voltage on both ends of it while the motor cranks. During this period, the pull-in coil is out of the circuit and draws no current. Cranking continues until power to the motor solenoid is turned off by the start switch.

The result of these switches and relays is to permit a 5 amp dash-mounted switch to turn on a 500 to 1000 amp motor for cranking an engine.

Battery voltage (power) available during cranking depends on the temperature of the batteries. See Chart A. This chart is only a GUIDE as to what to expect from a NORMAL system.

Chart B shows maximum allowable voltage drops in the high current battery circuit to the starter. These values are maximums for engines with more than about 2000 SMH. Newer engines have less voltage drops.

Illustration 2. Diagnostic flow chart for checking causes of starter problems with starter installed.

Voltage drops greater than those listed are usually caused by loose and/or corroded connections or bad switch contacts.

The entire diagnostic procedure can be reduced to:

... confirming that batteries are within specifications

... switches and cables/wiring from the batteries to the starter are not causing too much voltage loss.

The flow chart in Illustration 2 shows the entire procedure.

NOTE: If the engine is equipped with the diagnostic connector, the 6V2150 Analyzer Group can quickly be used in place of this entire procedure.

Testing The Starter

6V7070 Digital Multimeter (or 6V7800 or 6V3030 or equivalent)

8T0900 Clamp-on Ammeter (optional)

NOTE: This procedure covers a problem where the starter cranks either sluggishly (unusually slow) or not at all.

Test A. Check Battery Voltage At The Battery Posts While Cranking, Or Attempting To Crank.

NOTE: Use the posts representing 12 or 24 Volts. Do not check voltage on the cable post clamps.

1. Is voltage equal to or greater than what is shown in Chart A?

... Yes - Go to Test B.

... No - Voltage at batteries is too low. Test batteries per Special Instruction SEHS7633.

NOTE: Low batteries can be caused by the battery condition or a shorted starter.

... Optional: Check for shorted starter with 8T0900 Clamp-on Ammeter. (If voltage at battery post is within approximately two volts of the lowest value in the applicable temperature range of Chart A, and if larger starter cables get hot, this suggests a shorted starter without using an 8T0900 Ammeter).

NOTE: Chart C lists the maximum current draw for different families of truck engines.

Test B. Measure Starter Motor Voltage From TP4 To TP5 while Cranking, or Attempting To Crank.

1. Is voltage equal to or greater than that shown in Chart A?

... Yes - Batteries and starter cables down to motor are electrically within specifications. Go to Test C.

... No - Voltage drop (loss) between batteries and starter is too great. Go to next step.

... Optional: Check for shorted starter with 8T0900 Clamp-on Ammeter. (If voltage at battery post is within about 2 Volts of the lowest value in Chart A, large starter cables will get hot and confirm a shorted starter without using an 8T0900 Ammeter). Go to next step.

2. Measure voltage drops in cranking circuit per Chart B. Are all voltages within specifications?

... Yes - Check engine. Go to Test C.

... No - Repair and/or replace faulty electrical component.

Test C. "Bar over" The engine to be sure It is not locked up. Check engine Oil viscosity (weight), parasitic loads, Etc.

1. Is engine locked up and/or hard to turn?

... Yes - Repair engine as required.

... No - Go to next step.

2. Does starter crank? (Probably acts sluggish.)

... Yes - Replace starter.

... No - Listen for butt engagement (a clicking sound caused by the pinion hitting, but not engaging the ring gear). If this occurs, turn off the key switch. Then turn the switch back to "crank", and most likely the pinion will engage the ring gear and crank the engine.

NOTE: Butt engagement and open starter solenoid contacts give the same electrical readings. That is, the solenoid is activated, but the solenoid contacts can not close to turn on the starting motor because the pinion has struck the edge of the ring gear. If a butt engagement has occurred, the engine will crank on the second attempt 90% of the time.

3. Has butt engagement occurred?

... Yes - Attempt to crank again.

... No - Replace starter.