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Not a rare problem
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 2 Aug 2007 07:42:15 Share Post by Email
In Reply to: Re: rare abs problem, MathieuXX, Wed, 1 Aug 2007 13:47:05
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Actually, your problem is the ABS pump NOT running.

The ABS pump is designed for intermittent operation only. In stop and go driving, the pump has to run quite a bit. And in hot temperatures, the underhood temps are quite high, and in stop and go driving, you get minimal airflow into the engine compartment. The ABS pump has a thermal shutdown feature, so if it gets too hot, it stops running. When it stops running, the pressure in the system drops as you use the brakes, and eventually the lights come on.

After the lights come on, you've got about 10 good stops before you lose brake boost. Not a happy situation. It 'fixes' itself after it has a chance to cool down.

If the accumulator is bad, that will make the pump run more. Yes, yours was replaced. However, it won't hurt to do the accumulator test anyway. Start the car, and wait for the ABS pump to stop running - this will be a few seconds AFTER the ABS and Brake Fluid lights go out. Once you know the pump has stopped, Stomp the brake pedal, hard and fast - the Kill The Spider stomp. See how many pumps it takes for the ABS and Brake Fluid lights to come on. 12-15 is good; 10 or less not so good; less than 5 or 3, the accumulator is still bad.

Assuming the accumulator is OK, the problem is just the high temps under the hood and an aging pump that isn't as efficient, so it has to work harder and longer. It's not a great design - it's one reason Saab went back to a vacuum system for brake boost in later years, and left the pump just for ABS.

So, what can you do? Well, anything that keeps underhood temps low helps. If you don't have an 82 degree C fan thermoswitch (stock is 89), install one. This isn't the thermostat, but the switch that runs the fan. The fan will come on sooner, and that airflow will help keep the ABS pump cool. If you don't usually run the air conditioning, do so. The AC makes the engine run hotter, yes, but it also makes the AC fan run. From the ABS pump's point of view the engine is still hot, but the airflow helps more to keep underhood temps down.

Lastly, if that doesn't help, go back to an old trick from the 99 and two stroke days. Install a switch in the dash that will jumper the radiator fan switch. If you get stuck in traffic on a hot day, just flip the switch and let the radiator fan run all the time. This will keep the ABS pump cooler. Make sure you jumper the switch - that controls the relay and the relay will take the current load. If you wire the switch to drive the fan directly, it'll burn out the switch.

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