How can I stop the crankshaft from turning so I can torque the harmonic balancer bolt in a 1992 Buick Century

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Which one ? your vehicle has two !
Crankshaft Position (CKP) Sensor (7X)
Lower rear of the engine block, below the exhaust manifold
Crankshaft Position (CKP) Sensor (24X)
Right side of engine, at the end of the crankshaft, behind the harmonic balancer
24x replacement .
Turn OFF the ignition. Remove the serpentine drive belt from crankshaft pulley. Refer to Drive Belt Replacement in Engine Mechanical. Raise the vehicle on hoist. Refer to Lifting and Jacking the Vehicle in general information. Remove the crankshaft harmonic balancer. Refer to Crankshaft Balancer Replacement in Engine Mechanical. Note the routing of sensor harness before removal. Remove the harness retaining clip with bolt (1). Disconnect the sensor electrical connector. Remove the sensor bolts (2). Remove the sensor.
After replacement of this one you have to do relearn
24x 24

The harmonic balancer on these engines is now press fit onto the crankshaft and requires the use of Tool J-38197 for removal. The torque specification to be used during installation has also been revised. The procedure to be used when replacing the crank sensor is as follows:

REMOVAL

1. Disconnect serpentine belt from crankshaft pulley.



2. Raise vehicle on hoist.



3. Remove right front tire and wheel assembly.



4. Remove right inner fender access cover.



5. Using 28 mm socket, remove crankshaft harmonic balancer retaining bolt.



6. Remove crankshaft harmonic balancer using Tool (J-38197).



7. Remove foreign object deflector (DO NOT use pry bar).



8. Disconnect sensor electrical connector.



9. Remove sensor and pedestal from block face.



10. Remove sensor from pedestal

INSTALLATION

1. Loosely install crankshaft sensor on pedestal.



2. Position sensor with pedestal attached on Tool J-37089.



3. Position tool on crankshaft.



4. Install bolts to hold pedestal to block face, torque to (18-26 lb. ft.).



5. Torque pedestal pinch bolt to (26-44 lbs. in.).



6. Remove Tool J-37089.



7. Install foreign object deflector.



8. Place Tool J-37089 on harmonic balancer and turn. If any vane of the harmonic balancer touches the tool, replace the balancer assembly.



9. Install balancer on crankshaft.



10. Apply thread sealer GM #(NNN) NNN-NNNNor equivalent to threads of crankshaft balancer bolt. Torque bolt to + 56 degrees (104 lb. ft. + 56 degrees).



11. Install inner fender shield.



12. Install tire and wheel assembly. Torque lug nuts to (104 lb. ft.).



13. Lower vehicle.



14. Install serpentine belt.
    

see this vedio

Hi,
The sensor is located to the right of the crankshaft behind the vibration dampner (harmonic balancer). You will need a balancer puller and a special tool to space the new sensor correctly.
(can be done also with eye spacing, but the tool is recommended to ensure proper spacing as to prevent damage to new sensor and / or harmonic balancer.) You will also need a strap wrench to hold balancer pulley in place while removing and re-installing balancer to crankshaft bolt. (helper required)

Procedure for replacement:
Remove negative battery cable.
Set parking (emergency) brake.
Release tension on serpentine drive belt and remove belt from harmonic balancer pulley. ( large pulley at bottom of engine)
Break loose lug nuts on passenger side front tire, do not remove bolts yet.
Raise and support vehicle on passenger side, remove wheel.
Remove plastic splash guard (cover) from passenger side wheel well.
Remove dampner / balancer center retaining bolt (counterclockwise, it is torqued very tight.) Use a strap wrench around pulley and have a helper hold that in place while using a 1/2" breaker bar and socket to turn pulley bolt out of crankshaft. Be careful not to rotate engine in a counterclockwise direction while breaking the bolt loose.
Using balancer puller, remove dampner / balancer pulley from engine.
You will now see the crank sensor to the right of the crankshaft bolted to the timing cover.
Disconnect sensor wiring and remove retaining bolts, remove sensor.
Put new sensor in place and screw in bolts, do not tighten them yet.
Reconnect wiring to sensor.
Now use special tool to properly space sensor. If you cannot find one at the parts store, you can space the sensor with the balancer pully held in place (not installed yet) and space the sensor evenly with the veins on the hub of the balancer pulley. Take your time and be sure it is as evenly spaced as possible.
Tighten sensor bolts.
Clean balancer hub free of any oil and dirt deposits using brake or carbeurator cleaner.
Reinstall balancer to crankshaft by tapping around the inner surface of pulley with a rubber hammer.
It is a press fit, so be sure you have it started evenly and progress slowly in a circular motion to ensure you are keeping it aligned straight with the crankshaft.
Reinstall balancer bolt using the same method as removal, torque bolt to 110 foot pounds, then tap around balancer again twice with rubber hammer, turn the engine 2 full revolutions clockwise using the balancer to crankshaft bolt, then tighten bolt an additional 1/4 turn.
Replace splash guard, put wheel on tightening lugs to finger tight and then wrench tight in a star pattern.
Lower vehicle until wheel makes contact with floor, tighten lug nuts to 115 foot pounds in a star pattern using a torque wrench.
Fully lower vehicle, remove jack.
Re-install serpentine drive belt
Re-connect negative battery cable.

End of procedure....Hope that helps!.....Mike

Behind the large pulley at the lower part of your motor. You have to remove the belt and then loosen the large bolt that holds it in place. Use a pulley puller to remove the harmonic balancer. With that off, loosen the screw that holds the bracket to the mount. Position the new bracket on the mount and tighten. Be sure that the gaps in the new sensor are postioned correctly. Replace balancer and belt and you should be good to go.

to remove the balancer you will need to remove the belt, the remove the bolt off the balancer then use a puller to remove the balancer,to install line up the key way slide balancer back on and then tight bolt,

Yes this is known as the harmonic Damper and your problen is your missing a part thats why its spining a piece of metal that goes in that area that keeps it from spinning on the crank shaft you need to talk to a mechanic for hands on work

You can find out prices at autozone online. If you give me the engine size and what your problem is in detail I will try to he;p you for free.

REMOVE OR DISCONNECT

1. Serpentine belt from the crankshaft pulley.
2. Raise the vehicle on a hoist.
3. Right front tire and wheel assembly.
4. Right inner fender access cover.
5. Using a 28 mm socket, remove the crankshaft harmonic balancer retaining bolt.
6. Crankshaft harmonic balancer using tool J 38192 or equivalent.
7. Sensor electrical connector.
8. Foreign object deflector.
9. Sensor and pedestal from block face.
10. Sensor from pedestal.

INSTALL OR CONNECT:

1. Loosely install the crankshaft sensor on the pedestal.
2. Position the sensor with pedestal attached on special Tool J 37089.
3. Position the special tool on the crankshaft.
4. Bolts to hold the pedestal to the block face, torque to 20-40 Nm (14-28 lb.ft).
5. Torque the pedestal pinch bolt to 3-4 Nm (38-40 lb.in).
6. Remove special tool J 37089.
7. Place the special tool 37089 on the harmonic balancer and turn. If any vane of the harmonic balancer touches the tool, replace the balancer assembly.
8. Balancer on the crankshaft.

• Apply thread sealer (GM 1052080) or equivalent to the threads of the balancer bolt.

1. Torque the crankshaft bolt to 140 Nm + 56°(104 lb.ft + 56°).
2. Foreign object deflector.
3. Electrical connector.
4. Inner fender shield.
5. Tire and wheel assembly, torque to 140 Nm (104 lb.ft).
6. Lower the vehicle.
7. Serpentine belt.

Check out this link: http://www.autozone.com/addVehicleId,1602402/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c152800cc1e1. This is for a 6 cylinder in your year. This gives you the specs for all of the bolts associated with the crankshaft damper. Good Luck!