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Jim Grant Tech Tips Jim Grant Tech Tips
A collection of helpful articles based on reader submissions
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  Jim Grant's Tech TipsJim Grant's Tech Tips
Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
If you have any questions, please feel free to email us.



Lincoln - Mercury
  Jim Grant's Tech Tips

‘01 Jeep Grand Cherokee, A Lot Brake Rotors

Q: I own a ď01 Jeep Grand Cherokee, which has had several problems and I donít know if it is a lemon or if Jeeps are known for these problems. I have replaced the rotors three times, had the brakes replaced twice and also had the bearings replaced recently. The Jeep has a little over 50K miles, is a family vehicle and 80% of the driving is highway. It is not abused and is well taken care of. Could you explain why these problems keep occurring? Is it something Iím doing wrong? Also, is it common, a fluke or just a mishap that the arm under the driverís seat would break?

- Carol

A: If the arm youíre referring to is the mechanical seat back recliner lever for the seat, I have only seen one failure on a Jeep. As for bearing replacement, the wheel bearings on the front of Jeeps, which are replaced as an assembly, seem to have no rhyme or reason to failure. Iíve seen failures under 50k miles and have had others of your type that have never had a wheel bearing replaced with over 100k miles. As for the brakes, that is a different story. The reason you replaced the rotors is due to a complaint of a shake when braking. After the brake rotors were replaced, the shake seemed to go away for about 2 to 4k miles, after which the shake would become detectable and just get more pronounced as the miles went on, until you couldnít stand it anymore. In most cases, it is the front brake rotors that are the cause, but Jeep Grand Cherokees of your vintage have disc brakes on the rear as well. It seems just as you get one end of the vehicle straightened out the other end becomes a problem. So why is that? Most of the time it is the people factor that causes reoccurring brake shake. But, the vehicle is prone to problems on its own and by existing in a rust belt state, complicating these issues. Jeep has issued service bulletins (not recalls) addressing brake pulsation during light to moderate brake application. Brake pulsation can be a result of thickness variation or by lateral run out. Thickness variation can be corrected by machining the brake rotor. Lateral run out on the other hand is not easily corrected. Now part of the people factor kicks-in. The tech working on your vehicle has to determine if thickness variation or lateral run out is the cause for the brake pulsation. If the rotors have been machined and or replaced many times, yet the problem returns then lateral run out is in question. To check lateral run out all of the brake components but the rotors have to be removed from the vehicle. A measuring device called a dial indicator has to be used to check the surface of the rotor as it is rotated by hand. The allowable variation is less than the thickness of human hair! Any variation greater than .001Ē or .025mm is not acceptable. If variation is too great, the tech must reposition the rotor on the hub. This procedure is called indexing the rotor to the hub. There are 5 possible positions for the rotor to set on the hub. If the tech lands it the brakes can be assembled and most importantly, the wheel nuts must be hand torqued to 100 ft lbs. It is critical that the mounting surface/hub that the rotor sets on is clean and smooth with no rust. If the variation cannot be corrected the hub that the rotor mounts on may have to be replaced or specialized equipment can be used to machine the rotor while it is bolted to the hub. Todayís braking systems are far more neurotic/sensitive than the massive iron of the old years. Something as seemly as unsophisticated as a tire rotation, if performed incorrectly, can ruin a perfectly good brake job. To correct your problem youíll need more than brake pads and rotor slap.

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