ABS Tech Tip

John Technician Sarasota, Florida Posted   Latest  
Tech Tip
Braking
2017 Chevrolet Malibu LS 1.5L (T LFV) 6-spd (6T40)—1G1ZB5ST5HF181412
C0299 - ISO/SAE Reserved
False Abs Activation

Hello all. Maybe others have seen this before, but was new to me. 17 Malibu comes in from a body shop we deal with with a false low speed ABS activation. Vehicle was hit on the right front and repaired, including a new hub, bearing and WSS. Upon road testing the vehicle I can feel both front wheels activating when stopping at speeds below 10 MPH. I graph both front WSS and they are working fine. Go into ABS data on the scan tool and the dump valves for both front wheels are cycling when the problem happens. At this point I go back to the only code I have stored in the ABS module, C0299, which is basically a code for low vacuum at the power brake booster. Vacuum is provided to the booster via a cam driven mechanical vacuum pump and the booster has a pressure sensor built into the line. Sure enough the pump is only making about 4 inches of vacuum. Also to note the brake pedal felt fine and was not hard from the low vacuum. Replacing the pump fixed the low vacuum code AND the false ABS activation. Unless I missed it when reading code description in SI, There was no mention of the module making the front wheels pulsate when the low vacuum code sets.... Hopefully this will save someone some time if they encounter a problem like this in the future. 

+8
Paul Owner/Technician
West Collingswood, New Jersey
Paul Default
 

Did you scope the sensor

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John Technician
Sarasota, Florida
John Default
 

Hi Paul. No, I didn't scope it with a lab scope. If the problem persisted I would have done that. I only graphed the PID on the scan tool. Once I saw that both front wheels were doing it and due to the year of the car and very low mileage I went looking in a different direction. I do realize that the scope will catch many fast glitches that the scan tool would not. Thanks for the reply.

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Paul Owner/Technician
West Collingswood, New Jersey
Paul Default
 

Seem like GM having issues with mechanical vacuum pumps. I had 2017 escalades with similar problem. I notice they redesign the pumps

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Nathan Owner/Technician
Hillsboro, Ohio
Nathan Default
 

That is pretty odd. However, that does not surprise me. I looked in shop key and the only reference I see does not indicate what effect this code will have, but that it can affect things in the engine control system, not in the ABS. Maybe someone else can chime in and explain.

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Bob Owner/Technician
East Longmeadow, Massachusetts
Bob Default
 

Hi John, Did you check to see if all the tires are the same size? A mismatch could cause false activation.

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