View single post by Mark Rosenbaum
 Posted: 07-13-2005 12:53 am
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Mark Rosenbaum



Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
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 Top Dead Center or TDC is the reference point from which all of the events in a cylinder's cycle of operation are timed.  You will often see references to Bottom Dead Center or BDC, but BDC is always exactly 1/2 crankshaft revolution or 180 crankshaft degrees away from TDC, so this, too, can be considered as using TDC for the reference point.  In a multi-cylinder engine, it is convenient to have a baseline from which to begin any description of its operation, and the point where the #1 piston is at TDC is an obvious point to use.  Consequently, most engines have a built-in means of determining when the engine is at that point.

In the JH engine, the distributor shaft can only mate with the auxiliary shaft in the oil pump in one way.  However, the actual opening of the ignition points and thus the spark depends on the position of the distributor body with respect to this shaft.  It is therefore most convenient to remove and replace the distributor when the engine is at #1 TDC.  Should the engine be turned while the distributor is out, one merely restores the engine to #1 TDC before reinstalling the distributor.

If you don't already have a copy of the shop manual, you will almost certainly wish to buy one.  The reprints are usually a better choice as they generally have updates and additions not present in the original.  While written for the professional mechanic, and plagued with a few inaccuracies, the shop manual is an invaluable resource for working on a JH.

To find TDC on your car, begin by examining the front of the engine.  Down towards the bottom you will see the crankshaft pulley, which drives the belt that operates the water pump and alternator.  There is a large nut at the front end of this pulley which may be used to turn the engine.  Immediately adjacent to the rim of this pulley is a protrusion from the engine's front cover that has several teeth.  There is a mark on the rim of the pulley which, when aligned with the most clockwise of these teeth, indicates that two of the engine's pistons -- #1 and #4 -- are at TDC.  Details of the teeth and pulley mark depend on which engine version your car has.

Moving up to the top of the driver's side of the engine, you will see the two large alloy cam gears.  Each of these has an index line on the forward face of its rim.  (Most cam gears will have two index lines, but the only ones that count are the one marked IN on the upper, intake cam gear, and the one marked EX on the lower, exhaust cam gear.)  When these two index lines are aligned, the two camshafts are in the position where all of the valves in the #1 cylinder are closed.  Since the relationship between the camshafts and crankshaft is fixed, this alignment occurs only when the #1 piston is at TDC.

In other words, to find #1 TDC, not only must the crankshaft index be aligned with the most clockwise tooth on the front cover, but the two index marks on the cam gears must line up.

If your vacuum capsule is in an inconvenient location, you can rotate the body of the distributor 90^ or 180^ until the capsule is in a better location.  However, doing this means that the distributor rotor will no longer be pointed to the right spark plug wire turret, so you will have to unplug the wires and reconnect them as described in one of my posts.

It's possible but not likely you've damaged the starter or its solenoid.  If you can get the starter to work reliably, I wouldn't worry about it.

According to the 'Members' function of this board, there are at least two JHPS members in or near Dallas.  You may wish to send them emails and make their acquaintance.  Most JH owners seem to be very friendly folks.

Finally, you may wish to peruse the information at
http://www.web-masters.com/gms/index.html
You'll find a treasure-trove of JH line drawings there, along with parts lists for many sections of the car.