I have an 02 trailblazer with 235k miles. The speedometer fluctuates back and forth tripping the abs sensor. In addition a brake light comes on and off. I have replaced the speed sensor with 2 aftermarket sensors and 1 gm original sensor. It has been a slow progression to worse. Now it affects how the transmission shifts. I replaced the right front hub with an aftermarket hub and sensor. In talks with people, I have been told to get a gm wheel speed sensor for the hub I replaced or change the trucks computer. The engine is not idling well either. I am thinking the computer is dying.

First you need to have EBCM - electronic brake control module check for DTC'S - diagnostic trouble code's . If the light is on there is a code stored . I think most part stores can do this and for free . After you find out what code is stored , looking up that code in service repair information for diagnostic trouble shooting chart would be next step . There are a few codes too.
DTC C0035 or C0040
DTC C0045 or C0050
DTC C0110
DTC C0131
DTC C0161
DTC C0186
DTC C0196
DTC C0201
DTC C0240
DTC C0241 or P0856
DTC C0244 or P1689
DTC C0245
DTC C0283
DTC C0287
DTC C0290 or C0292
DTC C0455
DTC C0550
DTC C0558

Find the code do the diagnostic's , fix the problem , wiring issue's , a sensor , bad module ,what ever it is . The light will go out then . Here is a sample of code discription .
• DTC C0035 Left Front Wheel Speed Sensor Circuit
• DTC C0040 Right Front Wheel Speed Sensor Circuit

The traction control uses the ABS wheel speed sensors (1 on each wheel - usually in the wheel bearing assembly). The speed sensor is for your speedometer. Do you have an ABS light on too?

look listen and dance not matter how you performance keep it calp and trust it

Sounds like your speed sensor on the rear end. Could just be a loose connection check the harness is securely pluged into to sensor. I know on my 02 ram I have abs sensors that go in to my hub bearings, if you have this that could also be your problem. when i installed my new hubs I bent the tab on the connector to far and my light came on and off during driving. I had to tap down my connector.

SPEEDOMETER TROUBLESHOOTING

Can be loose speedo wiring connectors under instrument panel or its issue with speed sensor on transmission.

I suggest you to go through help links mentioned below with appropriate solutions to confirm the problem :------

How to trouble shoot speedometer?

http://schematicsdiagram.blogspot.com/2011/06/speedometer-not-working.html

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1996 fiat bravo speedometer not working?

http://technoanswers.blogspot.com/2012/01/1996-fiat-bravo-speedometer-not-working.html
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Speedometer works but odometer not working?

http://technoanswers.blogspot.in/2012/01/speedometer-works-but-odometer-not.html

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Speedometer runs backward?

http://technoanswers.blogspot.com/2012/01/speedometer-runs-backward.html

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Warning lights on and speedometer died?

http://howtobyme.blogspot.com/2011/08/warning-lights-on-and-speedometer-died.ht

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Speedometer not working on 1993 Ford F350?

http://repairhelpcenter.blogspot.in/2012/02/speedometer-not-working-on-1993-ford.html

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ABS & Brake lights warning indicator steady on dash and speedometer not working for Dodge Ram?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

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Vehicles Cluster panel Not reading Miles?

http://schematicsdiagram.blogspot.com/2011/12/vehicles-cluster-panel-not-reading.html

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Odometer works properly but speedometer works intermittently?

http://technoanswers.blogspot.com/2012/01/odometer-works-properly-but-speedometer.html

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These will help.

Thanks.

Can be loose speedo wiring connectors under instrument panel or its issue with speed sensor on transmission.

I suggest you to go through help links mentioned below with appropriate solutions to confirm the problem :------

How to trouble shoot speedometer?

http://schematicsdiagram.blogspot.com/2011/06/speedometer-not-working.html

-----------

1996 fiat bravo speedometer not working?

http://technoanswers.blogspot.com/2012/01/1996-fiat-bravo-speedometer-not-working.html
------------

Speedometer works but odometer not working?

http://technoanswers.blogspot.in/2012/01/speedometer-works-but-odometer-not.html

-----------

Speedometer runs backward?

http://technoanswers.blogspot.com/2012/01/speedometer-runs-backward.html

-------------

Warning lights on and speedometer died?

http://howtobyme.blogspot.com/2011/08/warning-lights-on-and-speedometer-died.ht

-------------

Speedometer not working on 1993 Ford F350?

http://repairhelpcenter.blogspot.in/2012/02/speedometer-not-working-on-1993-ford.html

----------

ABS & Brake lights warning indicator steady on dash and speedometer not working for Dodge Ram?

http://howtobyme.blogspot.com/2011/11/abs-brake-lights-warning-indicator.html

-----------

Vehicles Cluster panel Not reading Miles?

http://schematicsdiagram.blogspot.com/2011/12/vehicles-cluster-panel-not-reading.html

-------------

Odometer works properly but speedometer works intermittently?

http://technoanswers.blogspot.com/2012/01/odometer-works-properly-but-speedometer.html

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These will help.
Thanks.

look under the truck at the ****** and check the wiring to the sensors if good may be a bad sensor,also check the wiring and connections to the abs wheel sensors [ located from frame to the hub behind the rotors ] the abs also acts as speed sensor if problem was with an abs sensor at the[ hub] wheel usually set the brake system to normal power brakes [if u hit brakes hard will lock up or skid wheels] with the abs light coming on if the speed sensor at ****** malfunction can and usually causes shift problems

This is most likely 2 problems. The ABS/Brake light and speedo issue will be the speed sensor located in the read differential housing. The temp guage issue (and I'm assuming this is at idle) could be thermostat or temp sending unit. The fluctuation is causes by the electric cooling fan cycling on and off to control the temp. However, most of the time this fluctuation should be from about 195 - 220 degrees.

There are two problems here. First you need and abs control unit. They are very expensive and it is possible to rebuild them. A couple of places to call: ATE, Auto Truck Electronics, they are in Florida, and BBA Remanufacturing near Boston, MA. I have used both companies and they charge much less to rebuild them than it costs to purchase a new one. That should take care of the speedometer, brake light, and ABS light. As for the check engine, that could be anything and the car will need to be scanned. ETS light can be remedied by a trip to the dealer. There is a campaign to install new software to the Electronic Throttle Controller. This is no charge to the customer and I believe the car has to be under 200,000 miles.

You may have a faulty electronic speedo unit.