1. Preparation:
* Consult your owner's manual or a repair manual: This will provide specific information about your Blazer's transmission and speedometer gear ratios. You need to identify the correct replacement gears based on your tire size and axle ratio. Incorrect gears will result in an inaccurate speedometer reading.
* Gather tools: You'll need a socket set (likely metric), screwdrivers (Phillips and flathead might be needed), possibly a gear puller (depending on how stubborn the gears are), pliers, a grease gun, and a flashlight.
* Park on a level surface, engage the parking brake, and disconnect the negative battery cable. Safety first!
2. Accessing the Speedometer Cable and Drive Gear:
* Locate the speedometer cable: This cable runs from the transmission to the speedometer instrument cluster. You'll likely need to access it from under the vehicle. Some Blazers may require removing the lower dash panel to access the cable at the instrument cluster.
* Disconnect the speedometer cable: Carefully disconnect the cable from both the transmission and the speedometer. It's usually a simple matter of pulling it apart after some manipulation.
* Access the transmission's speedometer drive gear: This is typically located on the top or side of the transmission case. You may need to remove some surrounding components to get good access (consult your repair manual). The location varies slightly depending on the specific transmission type (e.g., TH350, TH400, SM465).
3. Removing the Old Gears:
* Carefully remove the old drive gear: This often requires a gear puller, especially if it's been in place for a long time. If you don't have a puller, you might try carefully using a screwdriver to pry the gear off but be extremely cautious not to damage the transmission case or the gear itself.
* Identify the gear: Before removing it entirely, make a note of its tooth count. This will help you select the correct replacement.
4. Installing the New Gears:
* Install the new drive gear: Ensure the new gear is the correct replacement for your application. Lubricate it with transmission grease before installing. Press it firmly onto the transmission output shaft.
* Reconnecting the Cable: Carefully reconnect the speedometer cable to the transmission and the instrument cluster.
5. Testing and Final Steps:
* Reconnecting the battery: Carefully reconnect the negative battery cable.
* Test the speedometer: Take a short test drive to verify the speedometer's accuracy. If it's significantly off, you likely have the wrong gears.
* Reassemble: Put everything back together, making sure all bolts and components are secure.
Important Considerations:
* Tire size: Your speedometer's accuracy depends heavily on your tire size. Using larger tires without adjusting the gears will cause the speedometer to read slower than your actual speed, and vice versa.
* Gear ratios: Incorrect gear ratios will result in inaccurate speedometer readings. Consult a repair manual or a parts specialist to ensure you have the correct gears for your specific vehicle configuration.
* Transmission type: The location and type of speedometer gear will vary depending on the transmission in your Blazer.
This information is for guidance only. It's highly recommended to consult a repair manual specific to your 1985 Chevy Blazer and its transmission type before attempting this repair. Improperly installing the gears can damage your transmission. If you're not comfortable with this task, it's best to take it to a qualified mechanic.
