ABS Activating Un-necessarily

[outrag1]

1999 2dr Tahoe 4X4

Recently I've been getting false ABS activation. No ABS light comes on, but when coming to a slow stop, the ABS kicks in (for no good reason). I removed the ABS fuse and it no longer happens, so clearly it's something to do with the ABS system. After reading through the archives, I think I've narrowed it down to bearings or speed sensor. My scanner did not pull a code however if it was the sensor.

My question is, how do I determine which side may be at fault, so as to
minimize my expense in parts. The truck has 34K orig miles on it so I'm
leaning more towards the sensor than the bearings, but not sure. It looks like the whole hub assy (bearings) comes with the sensor for approx $245 and the sensor alone is approx $100.

Thanks for any input.

-Bob D

[GMMerlin]

Check the resistance of each wheel speed sensor...one should be out of range

[outrag1]

How do I check that... I'm not a skilled elec tech.

[GMMerlin]

You will have to disconnect the sensor connector and use a mutli-meter set on the Ohms scale to read the resistance

[tyrant33]

I am having the same problem with my 2001 Tahoe. Are these sensors hard to replace? I have mechanical ability but never chenged one yet.
Thanks in advance

[outrag1]

I will check the resistance. Can you tell me what "out of range" numbers would look like, i.e. what would be normal resistance so I can determine if it is within that range or not.

Thanks

[GMMerlin]

Depending on the temperature in your area 855 ohms to 1230 ohms

[FCFFUZZYFACE]

Don't rely only on the resistance measurement. GM has instructed me that we can fail a sensor by resistance measurements but we cannot pass a sensor by resistance measurements. I've had many bad sensors check in spec by the ohm meter. A scanner with a live wheel speed readout is the most reliable and quickest way to find the problem. Typically you will find one sensor that does not start reading wheel speed as soon as the other sensors I.E. three sensors will show 5mph and one will show 3mph (weak sensor). If the wheel speed is ok then check the toothed ring to see if it has a crack. Have fought this problem many times and there will be no trouble codes set and resistance will be in spec.

FCFFUZZYFACE

[GMMerlin]

Originally Posted by FCFFUZZYFACE

Don't rely only on the resistance measurement. GM has instructed me that we can fail a sensor by resistance measurements but we cannot pass a sensor by resistance measurements. I've had many bad sensors check in spec by the ohm meter. A scanner with a live wheel speed readout is the most reliable and quickest way to find the problem. Typically you will find one sensor that does not start reading wheel speed as soon as the other sensors I.E. three sensors will show 5mph and one will show 3mph (weak sensor). If the wheel speed is ok then check the toothed ring to see if it has a crack. Have fought this problem many times and there will be no trouble codes set and resistance will be in spec.

FCFFUZZYFACE

This is true, but the original poster does not have a scanner to perform the diagnosis.
If the resistance check is within specs, next you can check the AC voltage output of the sensor..this will show if a sensor is dropping out at low speeds. Since the sensor sends an AC signal to the EBCM, you will be reading the same input that the EBCM does and will not need a scanner to read the signal.
Once it is determined which sensor is giving the problem, then you can check for cracked reluctor rings.
One diagnostic step at a time.....

[outrag1]

My fault for not clarifying, but I do own an Auto Xray 6000 Scanner. It's not pulling any codes, but there are no specific parameters that I can see for the front wheel sensors, only vehicle speed in general.

I also own the Helms manuals and it looks like I can check the resistance starting at the wires off of the ebcm going down to the sensors????

Thanks

[outrag1]

I took my multimeter and read from the 4 wire connection at the EBCM relative to the left and right front wheel speed sensors. Here is the resitance data:

Left Front - 1.024 Kohms
Right Front - .980 Kohms

The truck was cold and not driven all day with regards to temperature of the sensors.

Both appear to be within the 855 to 1230 range....

Not sure where to go at this point, aside from bringing to a dealer for scanning,

Thanks

[outrag1]

I took my multimeter and read from the 4 wire connection at the EBCM relative to the left and right front wheel speed sensors. Here is the resitance data:

Left Front - 1.024 Kohms
Right Front - .980 Kohms

The truck was cold and not driven all day with regards to temperature of the sensors.

Both appear to be within the 855 to 1230 range....

Not sure where to go at this point, aside from bringing to a dealer for scanning,

Thanks

[GMMerlin]

Next I would check the A/C voltage from each sensor as it spins..one should drop out to near 0 acV

[outrag1]

<<Next I would check the A/C voltage from each sensor as it spins..one should drop out to near 0 acV>>

Thanks for the reply. Given what you're suggesting, how do I go about testing this exactly. Is the sensor unplugged from the harness when I test it? I'm not sure what you mean by "as it spins". Do I need to have the wheel in movement somehow when testing?

Thanks again.

-Bob