Timing belts are typically only used on overhead camshaft engines. Timing belts are used to synchronize the crankshaft with the camshaft, similar to a timing chain on an overhead valve (pushrod) engine. Unlike a timing belt, a timing chain will normally last the life of the engine without needing service or replacement. Timing belts use raised teeth to mesh with sprockets to operate the valvetrain of an overhead camshaft engine.
Whenever a vehicle with an unknown service history comes into your repair facility or is recently purchased, here are some points that should be asked to help prevent costly engine damage:
Engines, chain-or belt-driven, can be classified as either free-running or interference, depending on what would happen if the piston-to-valve timing were disrupted. A free-running engine is designed with enough clearance between the pistons and valves to allow the crankshaft to rotate (pistons still moving) while the camshaft stays in one position (several valves fully open). If this condition occurs normally, no internal engine damage will result. In an interference engine, there is not enough clearance between the pistons and valves to allow the crankshaft to turn without the camshaft being in time.
An interference engine can suffer extensive internal damage if a timing belt fails. The piston design does not allow clearance for the valve to be fully open and the piston to be at the top of its stroke. If the belt fails, the piston will collide with the valve and will bend or break the valve, damage the piston, and/or bend a connecting rod. When this type of failure occurs, the engine will need to be replaced or disassembled for further internal inspection; either choice costing many times that of replacing the timing belt.
The average replacement interval for a timing belt is approximately 60,000 miles (96,000km). If, however, the timing belt is inspected earlier or more frequently than suggested, and shows signs of wear or defects, the belt should be replaced at that time.
Inspect both sides of the timing belt. Replace the belt with a new one if any of the following conditions exist:
If none of these conditions exist, the belt does not need replacement unless it is at the recommended interval. The belt MUST be replaced at the recommended interval.
Removal & Installation
Remove all necessary components for access to the timing cover, then remove the cover.
NOTEWhen removing the crankshaft pulley, remove the 6 outer bolts only, DO NOT remove the center bolt.
- Remove the crankshaft pulley.
- Rotate the crankshaft to Top Dead Center (TDC) of No. 1 cylinder.
- The timing marks on the crankshaft and camshafts should be in alignment with their respective marks on the engine. Install camshaft locking tools (such as Saab tools KM-800-1 for camshaft sprockets No. 1 and 2 and KM-800-2 for sprockets No. 3 and 4) and a flywheel locking tool (such as Saab tool 83-94-868).
- Mark the direction of rotation of the timing belt for reassembly.
- Release tension from and remove the timing belt. Loosen the timing belt adjuster bolts.
- Rotate the crankshaft back to 60 degrees BTDC, to prevent damage to the valves.
- Remove the bracket with the upper timing belt adjuster and tensioner rollers.
- Remove the flywheel locking tool and install the flywheel inspection cover.
- Install the bracket with the upper timing belt adjuster and tensioner pulleys.
- Install both camshaft locking tools.
- Rotate the crankshaft forward to just before 0 degrees TDC and install the crankshaft locking tool on the crankshaft. Carefully rotate the engine until the arm of the tool is against the water pump flange. Be sure the crankshaft is at 0 degrees TDC and all timing marks are aligned. Remove the locking tool.
- If reusing the belt, fit the timing belt according to its marked direction of rotation and timing marks. Adjust the tensioning roller loosely by hand to prevent the belt from slipping out of the cogs. Always adjust counterclockwise.
- Measure the belt tension with Saab tool 83-93-985.
Snug the center bolts of the adjusting rollers. Turn the lower adjusting roller counterclockwise, until a belt tension of 202-220 ft. lbs. (275-300 Nm) is reached. Tighten the adjusting roller center bolts to 30 ft. lbs. (40 Nm).
NOTEThis is a preliminary adjustment of the belt tension and must not be used as a check when the belt is finally adjusted.
- Continue to carry out the adjustment by means of the tensioning roller, mark against mark. Remove the locking tool for camshaft sprockets No. 1 and 2. Carry out the final adjustment with the upper center adjusting roller until camshaft sprocket No. 2 moves 0.04-0.08 in. (1-2mm) forward.
- Remove the locking tool for camshaft sprockets No. 3 and 4 and also remove the crankshaft locking tool.
- Tighten the tensioning roller to 15 ft. lbs. (20 Nm). Tighten the upper adjusting roller to 30 ft. lbs. (40 Nm) and tighten the lower adjusting roller to 15 ft. lbs. (20 Nm).
Rotate the engine 2 complete revolutions to just before 0 degrees TDC and install the locking tool on the crankshaft. Carefully turn the crankshaft until the arm of the locking tool is against the water pump flange and tighten the locking tool. Set Saab tool KM-800-20 into position. Be sure that the timing marks on the camshaft sprockets are aligned with the marks on the tool and that the edge of the timing belt is flush with the edge of the camshaft sprockets.
NOTECheck that the alignment marks on the tensioner pulley are still aligned.
- If necessary, install the crankshaft pulley and tighten the retaining bolts to 15 ft. lbs. (20 Nm).
- Install the timing belt cover, and tighten the bolts to 6 ft. lbs. (8 Nm). Install all of the remaining components in the reverse order of the removal procedure.