Up To Standards
- Author: Mike Weinberg
- Subject Matter: Check the whole vehicle
- Issue: Spend time up front for accurate diagnosis
The single most critical issue facing a technician today is making a proper diagnosis of the customer’s complaint. A faulty or incomplete diagnosis of the problem creates an event that wastes an enormous amount of money through lost time. Comebacks are always costly due to repairing a problem again with no cash payment at the conclusion and “lost opportunity costs” which arrive from working on a no-profit problem while being unable to perform a paying job. I am not speaking about comebacks, but rather missed or incomplete diagnosis from the beginning of the repair resulting in hours of labor wasted (money) working on the wrong part of the car. Before computer-controlled vehicles, there were mechanical and hydraulic problems to concern us.
The advent of complex, sophisticated computer-controlled vehicles has made for very involved diagnostic routines encompassing engine management, drivetrain management and a variety of associated systems such as stability control, ABS and traction control. Failure to properly diagnose the real issue that the customer is facing gets very expensive and demoralizing. Spend the time to follow the correct and detailed steps to isolate the real cause of the problem. Too many times shops are very quick to remove the unit thought to be the problem only to find nothing wrong with the trans, transfer case, rear, etc. There is never any time wasted in a correct analysis of the problem issue. Below you will find some real world problems that came from customers on our tech lines.
A shop installs a remanufactured 231 transfer case into a Jeep. The customer comes back after the repair with a complaint of binding on turns, and banging when the driveline winds up and releases. The problem was not in the transfer case but in the driver operating a mechanical transfer case in 4WD on dry pavement. The difference in road speed of tires during turns winds up the driveline because on dry pavement there is no tire slippage to equalize the wheel speed differences. Solution to the problem is to educate the owner that 4WD is never to be used on dry pavement. Snow, dirt, ice, sand all permit some wheel slippage where dry pavement causes the internals in the transfer case to lockup and release with a bang.
2005 and up Jeep Grand Cherokee models may have noise around 55 mph on acceleration, which will go away when the throttle is released. Convinced that the noise was caused by one of the differentials, shops go through the rears and find nothing wrong. The problem is in the front driveshaft CV joint, which requires replacement of the front driveshaft. Removing the front driveshaft and then driving the vehicle will make the noise go away, and lead to the false conclusion that it is the front diff. A careful inspection of the drive shaft reveals a worn CV joint as the real culprit.
2001 and up Dodge SUV and pickups equipped with CV joint front axles will be brought in with a complaint of loss of movement on turns in all-wheel-drive applications or no 4WD under load. Many a transfer case has been removed searching for the problem. A more thorough diagnosis will reveal that the front diff stub axles have worn splines and the circlip can no longer hold them in the side gears under load, so the axles pull out of the side gears. When the torque load is released, the axle will seat itself until the next turn or high-torque event takes place. If you took apart the transfer case, you lost money on this job. Inspect every part that is concerned with transferring power in the driveline before condemning a unit.
2009 and up positraction (limited slip) differential carriers made by the Dana Corp. may bind or bang on turns after installation. The part is brand new. Dana is a OEM supplier who makes excellent products. Where is the problem? Use only regular 75-90W or 90-140W lube in these diffs with a limited-slip additive. DO NOT use synthetic rear-end lube with these carriers. The coefficient of friction of the synthetic fluid will cause the clutches to chatter or lock up in turns. Regular rear-end fluid with the additive for limited slip will cure the problem.
First design GM 9.25-inch independent front suspension diffs with worn axles or/and side gears in the carrier create a parts problem. The first design parts are no longer available. The fix is to use the second-design side gears and diff stub shafts, which are available. They will fit even though there is a difference in where the circlip groove is cut to put more spline into the side gear for added strength.
2003-2007 GM SUV or pickup with 246 transfer case comes in with a problem of losing movement for no reason. Pushing the all-wheel-drive button returns movement and operation until it happens again. Many a 246 has been removed and torn down with no internal damage found. The cure is to replace the encoder motor with an OEM part. There is no detent to hold the transfer-case selector lever in position, it is held in position by an electronic “brake” inside the encoder motor. When the brake fails, the transfer-case selector lever can move on its own, causing a neutral type of condition. You should not be able to turn the encoder motor shaft without releasing the electronic brake using a 9-volt battery.
Tremec 3650 transmission in Ford products comes in with a complaint of notchy, grinding shifts, or noise from the transmission. The clutch release is good and the clutch operates correctly. Before you pull out the unit, check the fluid capacity by removing the fill plug. The fluid level should be ¾-inch below the fill plug. The original design of these units located the fill port too high on the case and if you “fill to spill” there will be too much fluid in the transmission. Overfilled units have severe consequences as these are “splash-lubricated” transmissions. Fluid circulates due to the gear train throwing the lube around the inside of the case. Too much fluid has the gear train working against the fluid as if you were trying to run on a beach in 3 feet of water.
This causes the gear train to slow down too much on shifts and upsets the synchronizer timing so the shifts are notchy, draggy, or grinding. Noise in the transmission comes from the synchronizer ring being unable to exhaust the fluid between it and the cone of the speed gear, which can make the ring rattle or flutter causing a noise.
One more thing: poor clutch release at high rpm on units with internally mounted concentric slave cylinders. An incomplete clutch release will cause notchy, grinding shifts and damage the synchronizers and speed gears. When setting up one of the units, there must be an air gap between the slave cylinder compressed at rest, and the clutch diaphragm fingers of 0.160-0.220 of an inch.
The measurement is simple: Measure the height of the diaphragm fingers to the block with a good straight edge and a depth mike or vernier caliper. Measure from the mating edge of the bell housing to block to the release bearing on the compressed slave cylinder. You may have to add shims under the slave cylinder to move it forward toward the block to get a correct air gap. Example: Cover fingers to block, 3.800 inches. Face of bell housing to release bearing contact surface of compressed slave cylinder = 4.350 inches. 4.350-3.800 = 0.550-inch air gap. Add 0.370 inch of shims under the slave to move it toward the block and creating an air gap of 0.180-inch.
Remember that you are not working on a single component such as a transmission. You are working on a complete vehicle with many interdependent systems that all have to work correctly. The time you invest to make sure where the problem comes from will be the best investment for your success in a good profitable repair.
Mike Weinberg is president of Rockland Standard Gear.