Disclaimer:
Brake work is inherently complicated, dangerous, and carries extreme risk
to life and property.
ANY USE OF THIS ARTICLE IS AT YOUR OWN LIABILITY AND RISK.
Jeep TJ Drum (rear) brakes: failure due to possible metal fatigue
During winter 2008 I noticed poor braking on wet, modestly inclined, asphalt surfaces.
Specifically, the tires (apparently front) would lock (no ABS on this vehicle) at the
slightest provocation. Since I moved from a sandy (Hawaii) to a wet (Washington state)
environment, I put blame on my desert tires (BF Goodrich All Terrain T/A) and replaced
them with more appropriate gear (Goodyear Wrangler Silent Armor A/T).
Tire size is: 31X10.5R15. This substantially improved wet-surface braking and all was well,
until the first snows fell.
In snow and ice, braking was poor. Having lived in northern Michigan and Maine, driving a
rear-wheel drive van at times in similar conditions with better results, I suspected something
wrong with the brakes. Testing revealed the following symptoms:
* Weak parking brake.
* Asymmetrical (front/rear) braking.
* Front wheel lockup during braking in slippery conditions.
* Extremely poor braking in slippery conditions, especially on snow and ice.
* Weak or no braking from the rear wheels.
* In snow and ice: locked up front wheels during braking while rear wheels rotate.
The brake light, which illuminates on differential pressure of the cross redundant hydraulic system
or application of the parking brake, illuminated only when the parking brake was applied, as normal.
Under the vehicle, I removed the drum brake inspection/adjustment covers and rotated the self-adjuster’s
star wheel. It rotated without clicking, up or down. A flashlight inspection revealed the self-adjuster
star wheel disengaged from the adjuster bar, on both rear wheels.
In the garage, lifted the vehicle, loosened the parking brake cable adjuster, and removed the back wheels
and brake drums, expecting heavily worn brake shoes. Instead, the shoes were quite good with only moderate
wear, and yet the adjuster bar hung disengaged below the adjuster’s star wheel (Figure 1).
Figure 1 (above): Disengaged self-adjuster mechanism (at arrow). Also note the good condition of the brake shoes.
Examination of the brake assembly revealed nothing amiss. This included a walk through with the factory shop manual specific to this
vehicle (1999 Jeep TJ).
Stumped and searching the Internet, I found several Jeep forums discussing this phenomenon, along with several ‘fixes’. One fix,
apparently performed at a professional brake shop, involved replacing the self-adjuster cable with a shorter cable. Another fix,
more home-grown, involved drilling new spring attachment points into the brake hardware to force the auto-adjuster components into
engagement. Yet another posting advocated placing the self-adjuster cable onto its mounting pin first, followed by the brake shoe
guide and finally the shoe springs.
Such assembly is contrary to the shop manual, and drilling into hardware that is subject to braking forces is not advisable.
Lastly, it is unlikely so many people somehow obtained the wrong size of self-adjuster cable.
Note that these are just my personal opinions on these matters.
The only thing left to do was a complete disassembly, and it was during this work that I found the problem: metal fatigue
(Figures 2 and 3).
Figure 2 (above): Drum brake auto-adjuster cable guides as viewed from the front; unremarkable at first glance, until you flip them over.
Figure 3 (above): Drum brake auto-adjuster cable guides as viewed from the back, revealing serious metal fatigue.
The metal fatigue causes an out-of-round condition, causing the self-adjuster cable to slacken as the guide works its way looser under pressure from the
spring-tensioned self-adjuster cable routed across the guide surface. The slackened cable eventually disengages the rear brake’s self-adjuster mechanisms
(star-wheel and bar).
It is apparently the case that self-adjuster (and accordingly, rear wheel braking) efficiency will gradually decrease over time until the mechanism fails
altogether, leaving little, if any, rear wheel braking.
Given that most braking is accomplished by the front wheels, this condition is particularly insidious, as rear braking fades over time. You may thus discover
poor rear braking when its too late.
Cause: unknown, though high-mileage and/or exposure to marine coastal environments may be factors.
General Rear Drum Brake Inspection Tips
Periodically test your parking brake, and don’t (as I did) consider poor performance as a simple matter of parking brake cable adjustment to be addressed at
a more convenient time. It may very well be, but it could be other things. Its easy to brush aside such issues, especially if you rarely have need to use the
parking brake.
Follow the shop (factory) manual for repairs, and qualify third-party (Internet, etc) advice against it, including any advice rendered here. The shop manual
is the final word.
When performing drum brake maintenance, remove and closely inspect the cable guides for wear and replace as
needed. Most drum kits I've seen don't supply cable guides, but they are easy to get and rather cheap
(about $2 - 3 US dollars, each).
Given the critical nature of the braking system, I personally recommend kits and parts manufactured
in the United States for the Jeep TJ.
Drum Brake Adjustment Tips
Following the shop manual’s drum brake adjustment method can be challenging, but it is by definition the correct procedure. The ‘trick’ is having the right tools.
In my experience with all drum brakes that I have ever worked on, I recommend using two skinny flat-bladed screwdrivers (Figure 4).
Figure 4 (above): I have tried brake spoons, but for me these screwdrivers always work best. Use the wider blade to engage the star-wheel
for tightening and loosening, and the longer, skinnier blade to control the self-adjuster arm when only loosening the brake shoes (used
simultaneously with the wider blade screwdriver).
Loosening the shoes is the tricky part, because you have to use both screwdrivers at the same time. Practice this on the brake assembly
before you put the drum back on, so you can better see and get a feel for what you are doing. Seemingly unwieldy at first, its easy with
the right tools.
About the direction you must turn the star-wheel: no matter what, it must ‘click’ when turned, or hold its ground (refuse to turn).
Anything else is a failure of the self-adjuster mechanism.
If you turn the star-wheel and it clicks, then the drum shoes are tightening (deploying toward the drum braking surface).
To loosen the shoes, you must then turn the star-wheel the other way. This will be impossible on a correctly functioning
brake assembly, unless you use the second screwdriver as described above (and in the shop manual) to disengage the
self-adjuster bar from the star-wheel.
For the TJ: The star wheel is in the same rotational plane as the engine cooling fan (that is, at right angles to the
tire's rotation). A clockwise rotation extends the brake shoes; counterclockwise retracts them. You MUST hear a distinct clicking
noise as you rotate the star wheel clockwise, otherwise take the drum off and correct your work.
One Last Tip (Assembly)
After attaching the shoe guide, adjuster cable, and the two shoe springs to the 12 o'clock pin, ensure the cable guide is
well-seated (flat) against its clean mounting surface. Remove the star wheel assembly and, with the now gained maneuvering
room, attach the adjuster cable. Now, put the star wheel assembly back in. Easy as pie (Figure 5).
Figure 5 (above): I know that cable eyelet just doesn’t look right hanging from the anchor pin like that, but follow the
shop manual. It specially sets the order of assembly, and the brake shoe guide goes on first. Its tempting to put the eyelet
over the inner pin because it seems to fit so well, but that’s not what the shop manual calls for. Now you can throw on the
shoe springs and, with a properly seated cable guide, assemble the auto-adjuster star-wheel assembly (not shown here).
-Mark Qu