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| Cam Position Sensing w/o a Cam Position Sensor https://slantsix.org/forum/viewtopic.php?t=3254 |
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| Author: | Bob D [ Fri Feb 15, 2002 2:04 pm ] |
| Post subject: | Cam Position Sensing w/o a Cam Position Sensor |
My cousin owns a '96 Saturn that I service for her. Last week the "Service Engine Soon" light came on. According to my OBD II scan tool the fault was "Cam Position Sensor Circuit Range/Performance". Well, tracking this problem down has led to a very interesting piece of information that could be applicable to a sequential EFI conversion for the SL6. After much investigation I think that I've finally determined what is happening. Apparently the Saturn doesn't have a Cam Position Sensor! Instead the computer measures the resistance of the spark plug/wire combination and in this way determines when the piston is in the compression stroke. This is effectively a cam position sensor without actually having a physical cam position sensor. If the spark plugs or wires are worn (high resistance) then this will "confuse" the computer and trigger the fault code. This is how I THINK this system works: The Saturn uses a distributorless ignition and as such has a crank trigger. The ignition fires each spark plug when its respective piston is near TDC during both the compression and exhaust stroke. It actually fires two cylinders at once, one piston will be on the compression stroke, and the other will be on the exhaust stroke. Remember that a crank trigger can't tell whether a piston is on the compression or exhaust stroke. It takes much more voltage to fire the spark plug during the compression stroke because there is more resistance to overcome in the spark plug gap. By monitoring this voltage the computer determines when a cylinder is on the compression stroke, and in this way eliminates the need for an actual cam sensor for the purpose of firing the injectors sequentially. The GM V6 engines went distributorless in 1986, but I don't know when (or if) they went to this system that doesn't use an actual cam sensor. As for me, I subscribe to the KISS philosophy, and I'm going to stay with the batch fire system. . I've owned both batch fire and sequential V6 cars and quite frankly couldn't tell the difference. I just thought this might be an area worth investigating by anyone who is determined to do a sequential EFI conversion. Bob D BBobbias@aol.com |
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