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Old 11-17-2020, 06:21 PM   #1
soccercoach
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2004 L-Series 3.0L Sedan
Default ABS Light

I solved this one and am offering the details hoping it will help others.

ABS light on and traction control disabled.
Clearly an ABS problem, but where. Standard advice says read the diagnostic codes, but my Actron CP9680 does not see the ABS system, (despite being to read the ABS module on a 1999 Cadillac and other newer cars).

Guessing it was a wheel sensor, checked each sensor at the wheel by unplugging the connector and measuring with an ohmmeter. The fronts are harder to reach and required removing the wheel. Also, if the sensor is good, a volt meter on the AC range should see a signal with a solid spin of the wheel. Here is my data:

Passenger front 1580 ohms, ~ 200 mV
Passenger rear 1080 ohms, ~ 200 mV
Driver rear open ohms, 0 mV
Skipped Driver front since I found one problem and fronts are harder to check.

Replacement of rear ABS sensors requires a wheel bearing and hub assembly, while the fronts can replace just the sensor.
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Old 11-17-2020, 08:46 PM   #2
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Default Re: ABS Light

If your practical troubleshooting revealed a dead abs sensor from resistance measurements, please follow up whether or not replacing the driver rear abs sensor corrects the problem and extinguishes the abs light.
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Old 11-17-2020, 10:24 PM   #3
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Default Re: ABS Light

Ok.
It was clear that the ABS sensor reading open circuit was bad, but without any diagnostics, I hoped that was the only issue.

Resistance measurement on the replacement unit matched up with "good" side of the car.

We know the ABS computer counts the pulses due to wheel rotation, but apparently it also monitors the sensors for an open circuit condition because the issue cleared as soon as the car started. No driving or scan tool required to clear the error. Driving confirmed still no errors.
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Old 11-18-2020, 12:37 AM   #4
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Default Re: ABS Light

Many self tests can detect correct repairs, erase the error code and turn off the indicator. The acid test, if you are at all interested, is to find a dirt or gravel road to force abs activation at relatively low speed without endangering yourself, the car or property. Dirt or gravel surfaces allows abs activation at lower speed. Wet leaves on a road can be hazardous when braking and another ideal test for abs. Wet rained soaked roads or large parking lot are other places to test abs as well as icy, snow covered streets/parking lots. Dry surfaces requires a higher speed to force locking up brakes/abs activation and not advised if you've never tested brakes (with or without abs). Speed between 25-45 mph on uneven surfaces can force abs activation when deliberately standing on the brake pedal. Abs works by allowing steering control while the foot massage indicates the abs motor and valves are preventing wheels from locking up. The abs indicator should be flashing too then turn off when abs automatically deactivates or brakes are released.

My L300 is my first abs vehicle and I promptly tested it to determine how it feels and when it activates. There's a definite foot massage when its activated.
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Old 11-19-2020, 02:29 PM   #5
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Default Re: ABS Light

ABS fault can be broken wiring or connections or bad sensor. You first should get it checked to see what it says not guess what it says. There are OBII reader that can check for what ABS light actually indicates or says. So before guess the problem get it properly scanned to get the correct error code. The Error doesn't clear it is still stored until a OBII scanner clears the error code.

Last edited by Russet; 11-19-2020 at 02:30 PM. Reason: revise
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Old 11-23-2020, 10:11 PM   #6
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Default Re: ABS Light

It WOULD be nice to have an error code to narrow down the issue. As stated in the original post, I couldn't read the ABS code. My reader is a $200 consumer model that is reads the ABS systems on my other cars, but not on the Saturn.
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Old 11-24-2020, 04:12 PM   #7
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Default Re: ABS Light

I think you've done more to illuminate a diy approach to abs diagnosing and troubleshooting to what was considered a difficult to troubleshoot system ten years ago. By using a practical approach of working from each wheel sensor, reluctor and wiring to the abs unit and discovering one sensor resistance value not comparable to the other three is a hands-on common sense method almost anyone can do without a reader decoding abs errors. For awhile, Autozone and possibly other big box stores didn't have readers capable of decoding abs errors. That policy changed a few years ago as our vehicles age and demand for readers drove stores like Autozone to upgrading their equipment. The aftermarket readers/scantools are also adding abs decoding capability.
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