Your Chevrolet’s speedometer gives important readings about your current speed. If you are experiencing problems with yours, don’t fret -- these issues are common and easily fixed (usually). In fact, TrailBlazer speedometer problems are one of the most common topics discussed on popular Chevy online forums. If you are experiencing speedo issues for any Chevy model, here is some advice that might help you.
Most Common Problematic Chevrolet Speedometer Symptoms
Inaccurate readings after changes to your Chevy: This is common with truck and SUV owners who change their wheels and tires, but can also happen if the wheel/tire combo is changed on a car. Larger combos, or those with different dimensions, will cause an inaccurate reading and the speedometer must be re-calibrated to regain accuracy.
Your Chevy speedometer is dead: A dead speedo is probably the result of one of following three problems:
1) In the older models (pre-1990), a break in the cable which runs from the transmission to the speedometer is a common cause.
2) On vehicles made after 1990, the speed sensors may be faulty, causing inaccurate readings at the speedometer.
3) A faulty speedometer head*, a more serious problem, may also be the culprit.
* Please note that if the speedo head is bad, it needs to be diagnosed by an expert to confirm.
The “check engine” or “service engine soon” light came on and the speedometer stopped working: With digital speed sensors, if they malfunction, they often stop sending data to the Chevy’s computer and this triggers a warning light to illuminate on the dash. Replacing the bad speed sensor will solve this issue.
Also, if speedometer in your Chevy stops working or reads erratically, test the cruise control out. The cruise control uses the same sensors as your speedometer. If the cruise is working and the check engine light is on, the problem is likely with the speedometer itself.
Also keep in mind that anytime a warning light comes on, your ECM should be scanned right away as this might indicate a very serious problem.
Jerking or bouncing needle on the speedo face: When the speedometer fails to settle and moves between speeds, it’s almost always a wiring or cable issue -- sometimes it is also a speed sensor issue. In almost all cases, the wiring or cable needs to be replaced, or the speed sensor needs to be re-calibrated.
Here’s how to get into your Chevy and really check things out according to the advice on the problem above.
Removing and replacing the speedometer cable:
* Take off the engine cover
* Look where the cabling enters the main body of your vehicle
* A thick black wire will be leading into the instrument panel, this is your vehicle’s speedometer cable
* Remove the bolt from under the dash so you can remove the cable
* Remove the nut on the transmission to pull the cable out
* Remove clamps on the transmission and in the engine bay that hold the cable
* Once the cable has been removed, replace it with a new one
* Fix everything back into place
Replacing speed sensors - front wheel drive Chevys:
* Locate the speed sensors behind the engine, above the trans-axle
* Remove the hose, air cleaner, and connector from above the sensor
* Remove the bolts holding the sensor in place, twist and take out the sensor
* Use a new o-ring, clean the flange and housing, and install new sensor
* Replace bolts and components
Replacing the speed sensors - rear wheel drive Chevys:
* The speed sensor will be behind the transmission on the driver’s side
* Lift the vehicle and remove the wheel
* Disconnect connectors on the sensor and remove the bolts
* Carefully remove the sensor and put a new one in place with a new o-ring
* Replace connectors, bolts, and other components
Not every speedometer problem calls for a component replacement, as discussed prior. Additionally, you should consult a professional if you cannot identify the specific problem or have doubts about the diagnosis or your ability to repair it on your own with the resources available in your own garage.