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ABS faulting in hot weather


BeemerBerg

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My 2005 RT has a very annoying problem. Here are the symptoms:

 

When the ambient temperature is very hot (usually >90 degrees), the ABS brakes will not set. After riding for awhile, then stopping, turn on the key, and I get a red triangle warning with the slow flashing "Brake Failure" message. Sure enough, the brakes work, but not "power assist". Then if the weather is hot enough, the "Brake Failure" message goes to a 'quick flash' mode, and the brakes still work, but more much worse--like an old Honda. Again, this situation comes up with the outside temperature is HOT--it is not a function of brake usage, hard braking, or brake fluid temperature. The battery voltage seems good, at about 13.5 volts. The brakes set up OK if the temp cools down, or the bike is parked in the shade with a cool breeze to cool it off.

 

This problem has plagued the bike for several years, since it was only 10,000 miles old. I have just come to live with the problem. Several shops have looked at it: one just cleared the fault codes, and said everything looked OK. Another said that there was "too much brake fluid in the front master cylander", and bled off a little. I have flushed the entire system twice, and changed out brake pads, yet the problem persists.

 

Is there a sensor or part of the brake system that is particularly sensitive to heat--not bike or engine heat, but outside temps? Could it be an electrical-related problem?

 

Anybody else out there have this problem?

 

Thanks for any input.

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I've been over Hoover Dam riding two up several times this Summer with ambient temps over 115 in slow-n-go traffic. No ABS problems whatsoever. It did start to ping a bit, and temp gauge went up a couple of bars, but that's it.

 

I did have an ABS problem though. Solid red triangle and once a second slow flash 'Brake Failure' light. Found the speed sensor wire chafing on the rear disc. It was a self inflicted wound caused when I changed the final drive lube and didn't get the sensor wire completely in the clip.

 

A little electric tape fixed the problem.

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I had a similar problem with my 2005 RT, it wasn,t heat related in my case but eventually turned out to be water an unbrake related electrical conector that was upsetting the can bus system and causing the servo to be shut down.

My supplying dealer just cleared the codes a few times before I lost patience and took it to another dealer who sorted it fist time with a 5 pence rubber gasket. The point is that it could be any item of electrical equipment upsetting the control system, and elecrical failures can quite often be heat related. The specific fault code might point to what is causing the problem, even if it doesn't appear to be related to the brakes.

I liked my 2005 RT, but replaced it with an 07 due to the braking system. I don't like the idea of having the servo arbitarily swithced off. I didn't find the residual braking very safe.

Best of luck with your investigations.

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  • 7 months later...
BeemerBerg

update.....

 

Still having a problem with the ABS faulting when the ambient temp is over ~85 degrees. First the red triangle and slow flashing warning, then the faster flashing warning. (see OP).

 

The other day, while checking our outdoor thermometer for an upcoming ride, it hit me!! The DOT 4 brake fluid in our brake circuits act just like the liquid in the thermometer: when it get warm/hot, it expands. D'oh!!! There are actually two circuits for each brake: the 'primary' from the master cylander to the ABS pump, and the 'secondary' from the pump to the caliper.

 

Just thinking out loud here....feel free to correct any errors in my logic....

 

The master cylander-to-ABS circuit is not necessarily heated up by heavy braking, since the fluid doesn't travel directly to the caliper. But since this is a 'closed circuit', the fluid will expand as outside temps change (like a thermometer). The Master cylander acts like 'reserve pool', allowing excess fluid to return to the 'pool' as it expands, and adding fluid as needed when cool (or if there are any leaks). This is contingent on the little piston/valve in the master cylander....in the 'relaxed position' the fluid circuit from the brake line to the reserviour is 'open', allowing the fluid to equalize its pressure. Once the brake lever is pulled, this part of the piston is instantly closed off (like a check valve), and pressure is applied to the brake line, pushing fluid to the ABS unit, activating the secondary circuit & servos to pump brake fluid to the caliper, thus stopping the bike.

 

But....if that little piston in the master cylander fails to fully open/relax, the fluid has no chance of 'equalizing pressure'; and when the ambient temps go up, like a liquid thermometer, the fluid has to go somewhere, so it puts a little pressure on the ABS unit. Since the computer doesn't think you're really applying the brakes (the brake lite switch hasn't been activated), it senses that something's not quite right, and it throws the notorious ABS faults. Once in a fault mode, the only way to make it happy again is to turn off the bike, cool down the fluid or (preferably) let the primary circuit equalize as it should.

 

That's why when I took the bike into the dealer to have the problem fixed, they read the ABS fault code as "too much fluid in the master cylander", which really meant that there was too much fluid in the primary circuit, unable to 'relieve itself' into the master cylander. The shop at the time tried to correct the problem by syphoning out a little of the DOT 4 from the master cylander (it WASN'T too high in the first place!!). That in itself didn't solve the problem (the shop wasn't able to duplicate the problem, as the weather was cool at that time), and the bike continued to suffer the original complaint in hot weather.

 

I had given up trying to have the shop fix the problem, and that's why I'm trying to 'brainstorm' the solution with you guys, the "collective".

 

So...here's my thoughts: I'm going to do a rebuild of the front master cylander, which entails replacing that little piston and the spring that's supposed to push it back into the static position (washers & seals too). Rebuild kit from BeemerBonyard: $44.95.

 

Another factoid: when I did an brake fluid change a couple of months ago, I did notice that when flushing the primary circuit, the little piston took quite a long time returning to the static position after pulling on the front brake lever...perhaps a weak return spring?? It didn't strike me as an issue at the time, but now makes me think that that is the source of my problems....

 

Question: has any ever done a master cylander rebuild operation? Tips/tricks/cautions? Yea, I know the part about DOT 4 brake fluid dissolving paint, plastic, small children, etc. As I will have to disconnect the brake line at the master cylander, Is is possible to add brake fluid from the ABS unit, pushing air bubbles UP thru the master cylander?

 

Thanks for any advice and letting me think outloud here. I'll let you know how the procedure goes, and if it fixes the problem.

 

--Ken g

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A large number of these bikes have a front master cylinder that sticks on return. We (at a dealer where I used to work) thought it was the cause of some problems like you describe, but after much trial and error and replacing master cylinders, we discovered that it was not the cause.

 

You need to know EXACTLY what fault codes are being set in the ABS controller prior to doing anything with the bike. It could be something really simple when all the information is known.

 

Taking a stab in the dark and rebuilding the master cylinder will likely NOT solve your problem. I'm gonna guess here, but you may have a thermally intermittent ABS controller.

 

Really need to know the codes first.

 

Good Luck

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