Cracked Flywheel Housing - keywords 6sp 6.6L coolant crack LB7 leak LLY manual ML6 speed transmission vibration

The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Some 2003-2005 Chevrolet and GMC C45/C55 2WD Models with 6.6L LB7, LLY diesel engine and 6 speed manual transmission (RPO ML6) may experience repeated cracked flywheel housings.

These vehicles may exhibit a driveline vibration that can develop a bending mode frequency which can crack the flywheel housing. Cracked flywheel housings will manifest themselves as a coolant leak at the rear of the engine. The crack originates at the two lower flywheel housing bolts that attach the housing to the engine oil pan. The crack progresses upward until it reaches the engine coolant passage in the flywheel housing and results in a coolant leak.

If inspection reveals a cracked flywheel housing, perform the following diagnostic/repair procedure:

1. Check for loose retaining nut on transmission output shaft.
2. Dial indicate the front driveshaft tube to determine if there is excessive runout at the rear of the transmission. Follow instructions for checking driveshaft runout in SI Document ID # 802045. Maximum allowable runout at front of driveshaft tube is 0.014 in. Check as follows:

2.1.	Position the dial indicator as close to the yoke weld as possible at the front end of the driveshaft tube (toward transmission). Dial indicate the driveshaft tube and record readings.

2.2.	Mark the position of the front driveshaft yoke attachment to the transmission output yoke.

2.3.	Remove the front driveshaft, rotate 180 degrees from the marked position and reinstall driveshaft.

2.4.	Dial indicate the driveshaft tube again and record readings.

Note: If driveshaft tube runout readings are within specifications, continue to next repair step to have driveshaft(s) balanced. If front driveshaft tube runout readings are out of specifications, it will be necessary to have the front driveshaft tube checked and replaced if the tube is out of round with excessive runout and then have driveshaft re-balanced.

3. It will be necessary to have the driveshafts balanced per the following instructions:

Send the driveshaft(s) out to be checked and re-balanced. All driveshafts of a multi-driveshaft system must be individually balanced.

Note: It is recommended that only authorized Dana/Spicer Driveline Distributors be utilized for checking and re-balancing the driveshafts. The nearest Dana Distributor can be located by calling Dana Service Support at 1-800-729-3262.

4. Replace the cracked flywheel housing (refer to SI Document ID # 815738) and while the driveline components are removed from the vehicle.
5. Change the rear axle ratio as follows:

-	GL4 (11,000 lb) rear axle change to a 4.30 ratio.

-	GL8 (13,500 lb) rear axle change to a 4.10 ratio. (If ratio is not available through GMSPO please contact GM Technical Assistance)

Note: It will be necessary to reprogram the speedometer calibration for the rear axle ratio change, SI Document ID # 1212377 using the Tech 2.

6. Install all the re-balanced driveline components in the vehicle. Install a new transmission output shaft yoke retaining nut and torque to 330 Nm (244 lb ft).
7. Verify the repair by using the following method:

When the vehicle is ready to road test, hook up the Electronic Vibration Analyzer (EVA) Tool # J38792 to determine if the critical frequency range is present. It will be necessary to attach the EVA magnetic pickup at the rear of the transmission per the following instructions in order to get accurate EVA readings:

1. Select a piece of angle iron approximately 2"x2" and cut to about 2" in length.
2. Locate the clutch cable mounting bracket on the left rear side (driver side) of the transmission and remove the rear most bolt from the mounting bracket.
3. Drill a hole in the center of one side of the angle iron to accept the clutch bracket mounting bolt.
4. Install the angle iron and bolt back on the clutch cable mounting bracket so that one side of the angle iron faces up and in toward the transmission as an inverted "L". Tighten the bolt.
5. Place the EVA tool magnetic pickup on the top horizontal surface of the angle iron.
6. Road test the vehicle and record the EVA Hz and G readings from 65MPH up to the posted speed limit in 5MPH increments to determine if the critical frequency of 64 to 70 Hz greater than 1 G has been eliminated.

Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.