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Old 03-06-2012, 12:21 PM   #1 (permalink)
BRH
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Default ABS failure had cheap, quick fix

I have a 2002 Subaru Outback with 113,000 miles. The ABS light went on a few months ago and more recently the Check Engine (OBD II) warning light. The ABS light first went on when the alternator failed, a problem I took to a mechanic. While replacing the alternator the mechanic said that a new ABS control unit would cost $1200. Now I had two warning lights, plus weird behavior in the dashboard when I stepped on the brake (some dashboard lights would get brighter, others dimmer).

I scanned the OBD and got code P0703 "Brake switch b input circuit". I verified that the brake lights worked fine. I poked around the internet and found how to read the ABS codes which were
28 - rear left ABS sensor
52 Abnormal motor and/or motor relay
54 Abnormal stop light switch

I also found the Subaru diagnostic flow chart (or whatever it's called) for checking out ABS problems. I went through the flow chart for 52 and 54, since they were easiest. 54 didn't tell me anything but going through 52 I found that the ABS control unit wasn't grounded well. (The unit is by the right headlight in my Outback and the ground wire connects where the bracket for the power steering reservoir bolts to the frame.) I cleaned up the connection, cleared the ABS codes and everything looked good. (The Check Engine light remained on, but I figured it would go out on its own eventually.)

Later the next day, when I flipped the headlights on the ABS warning light popped on again. I read the ABS codes again, and there was just one, 54.

At this point I want to comment that the ABS system will only retain *three* codes. My guess is that 28 was a red herring. With a bad ground (problem 52), *everything* would appear to fail, including all four ABS sensors. Since the system can only report three codes, it only reported one of the four sensors.

I repeated the check-out procedure for 54, but this time both with the headlights OFF (as per the instructions) and with the headlights ON (since that caused the problem). The checkout procedure involves testing the voltage of pin #2 on the ABS control unit connector, which should be 10-15 volts when the brake pedal is depressed. The voltage is 0 when the brake pedal is released. Everything was fine with the headlights off, but with the headlights ON, the voltage was 6.5 V with the pedal released. Since the problem involved the brake circuit and the headlights/running lights, I figured there was something wrong with the tail lights. I believe there are a total of 8 bulbs back there, and I started removing them one-by-one, then testing the voltage on pin #2. Pretty quickly I found that one bulb was the culprit. I guess it had some sort of short circuit, though it still illuminated.

I replaced the bad bulb and cleared the ABS codes. The Check Engine light immediately went out and now everything's fine.

So what I feared to be a bad ABS system (carrying a $1200 bill) was just a bad ground and a bad light bulb. Total cost to repair, $5. So I recommend going through those diagnostic procedures if electric anomalies appear in your car. It seems you can save a lot of money.
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Old 03-06-2012, 12:45 PM   #2 (permalink)
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That's a great write-up, especially for your first post -- welcome to the forum!

Incidentally, the brake and tail lights are in one bulb, and it's not uncommon for one of the filaments to break and fall onto the other. One effect is, for example, the clearance lights glow dim when the brake pedal is pressed.

I hadn't heard of the interaction with the ABS, but can see it happening if a clearance light filament crosses with the stop light filament. With the clearance (parking) lights on, some voltage will get back to the brake switch and from there to the ABS controller. The controller will see that voltage when there shouldn't be any, and see that as a fault in the brake switch. Makes sense, in hindsight.
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Old 03-06-2012, 01:32 PM   #3 (permalink)
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Quote:
Originally Posted by plain OM View Post
That's a great write-up, especially for your first post -- welcome to the forum!

Incidentally, the brake and tail lights are in one bulb, and it's not uncommon for one of the filaments to break and fall onto the other. One effect is, for example, the clearance lights glow dim when the brake pedal is pressed.

I hadn't heard of the interaction with the ABS, but can see it happening if a clearance light filament crosses with the stop light filament. With the clearance (parking) lights on, some voltage will get back to the brake switch and from there to the ABS controller. The controller will see that voltage when there shouldn't be any, and see that as a fault in the brake switch. Makes sense, in hindsight.

good post. I have seen the dual filament problem on a Honda years ago.


I wouldn't discount the possibility that he's got a poorly rebuilt alternator or has additional ground, battery cable or other 'connection' problems.
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