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2003 chevy trailblazer_ltz
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Discussion Starter · #1 ·
The ABS kicks in with brakes applied at low speeds while stopping usually just before completely stopped maybe 5 MPH.
The speedometer will not register upon acceleration until it has reached 15MPH. Reads zero until it gets to 15MPH the jumps to 15 MPH then works fine from that speed and above.
We have replaced both front wheel sensors and they check out fine with a scan tool.

I took this to a mechanic, he tested the two sensors on the rear axle and said one tested bad. Replaced it but still have the same problem.
He still has the vehicle and will look into it more after this Holiday weekend.

I am wondering if one of the rear sensors is for ABS and one is for the speedometer?
Does anyone have any knowledge on this and if so which one is for the ABS and which one is for the speedometer?

Thanks
 

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took this to a mechanic, he tested the two sensors on the rear axle

The title reads that this is a 2003? If so there should be NO speed sensors on the rear axle. That didn't happen until 2006 on these trucks. Now someone could have replaced the rear axle I suppose but there would be no place to connect rear wheel speed sensors on a 2003 ABS system.

If I assume this is a 4WD model with transfer case there will be 3 speed sensors there. 2 are for the 4WD system and are wired to the TCCM, Transfer Case Control Module. One sensor is the vehicle speed sensor which is wired to the PCM. The PCM then forwards the signal to the EBCM for use by the ABS system.

There have been issues with the wiring to the VSS and a TSB was issued I think for the 2004 models, maybe others. I would start with the diagnostic procedure for the VSS, testing for the presence of a valid signal at low rpm. The TSB detailed how to repair any damaged wires from the VSS, noting that the wiring is to be twisted pair at 9 turns per foot.
 

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Discussion Starter · #3 ·
The title reads that this is a 2003? If so there should be NO speed sensors on the rear axle. That didn't happen until 2006 on these trucks. Now someone could have replaced the rear axle I suppose but there would be no place to connect rear wheel speed sensors on a 2003 ABS system.

If I assume this is a 4WD model with transfer case there will be 3 speed sensors there. 2 are for the 4WD system and are wired to the TCCM, Transfer Case Control Module. One sensor is the vehicle speed sensor which is wired to the PCM. The PCM then forwards the signal to the EBCM for use by the ABS system.

There have been issues with the wiring to the VSS and a TSB was issued I think for the 2004 models, maybe others. I would start with the diagnostic procedure for the VSS, testing for the presence of a valid signal at low rpm. The TSB detailed how to repair any damaged wires from the VSS, noting that the wiring is to be twisted pair at 9 turns per foot.
Now that you say it I believe he did say he tested three sensors. I think he said the new sensor he bought he tried it in two locations?
It is a 4WD.
Could you tell me at what location the vehicle speed sensor is located in?
left right or middle?
 

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Discussion Starter · #5 ·
Okay, He showed me what he replaced. The VSS and transfer case sensor look very similar. If he got the wrong part I would assume it would not fix the problem.
I see these two items on AutoZone. We would need the top item then if we are wanting to replace the VSS. I need to verify with the mechanic what part he bought.
Or maybe they both do the same thing and are interchangeable?
Font Tool Camera accessory Auto part Household hardware
 

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maybe they both do the same thing and are interchangeable?
This I do not know.

However, I will restate that it is equally possible, if not more likely, that the defect is not in the sensor but in the wiring in which case blindly replacing the sensor without proper diagnostics could be a complete waste of resources.
 
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