By Richard McCuistian

The Central Sequential Fuel Injection Cam sensor on late model Chevys (except the new 5.3L), is mounted in that peculiar little plastic distributor housing and produces one pulse per camshaft revolution. The Chevy shop manual says the loss of this signal “may not affect the driveability” of the vehicle, but that signal loss will affect the type of control the VCM has on the fuel injection system.  With no CMP signal, according to the GM shop manual, the Vehicle Control Module (VCM) will pulse the injectors bank to bank instead of each individual injector at a time.

NOTE: I saw a 1997 S10 Blazer just today that had a hard shift into overdrive because one of my students didn't plug the cam sensor in when he reinstalled the distributor

This information is good to know, but what if the distributor is indexed improperly?   Experienced techs know how wise it is to mark the position of the distributor when we remove it and try our best to replace it in the same spot; both the rotor and the distributor body should always be properly indexed, particularly on engines with a cam sensor in the distributor.  But how can a conscientious tech be sure the cam sensor is properly set if some rude yahoo connected the battery cable and spun the engine over while the distributor was lying on the bench?  What does he or she do when the engine comes into the shop partially disassembled?   

Obviously, turning the body of the distributor can’t change the ignition timing, since the Crank Position Sensor is the chief player in that drama and reads directly off the crankshaft.  Be that as it may, the Chevy CSFI distributor can be indexed improperly and it can cause annoying driveability problems.  Whatever the facts may be, GM’s explanation concerning signal loss doesn’t address an out-of-time Cam Sensor, but Rotor alignment is an important factor to consider. 

At the risk of chasing a rabbit, let’s digress for a moment.  Rotor alignment on systems with a block-mounted Crank Sensor and a distributor-mounted Cam Sensor can cause interesting problems.  Those who became familiar with the early Ford EEC III system (it had a fixed crank sensor too) might remember how important it was to check distributor rotor alignment when we were fighting an annoying surge at road speed. 

Jeep Cherokees (4.0L) from 1987- 1999 had distributor-mounted cam sensors as well, and have the potential to experience the same phenomenon if the rotor alignment is too far off.  Since ignition timing can advance more than 50 degrees BTDC with high engine speeds and light loads, the spark can leave the tip of an improperly indexed distributor rotor, missing the distributor cap terminal it should be hitting and firing its spark at the terminal it has just passed.  Herein lies the reason for the road speed surge, and since ignition timing is almost as far advanced on a freshly started cold engine, problems may occur during that window as well.  For that reason, the old EEC III rotor was supposed to be indexed to center on #1 with the engine at 4 degrees AFTER top dead center.  On the 87-90 Jeeps, Jeep TSB 18-53-88 gave instructions to cut the locating tab off the distributor housing so the distributor could be adjusted. Then, using a modified distributor cap with a window cut in it to reveal the #1 terminal, set the distributor so that the rotor is “just departing” from the #1 spark plug wire post terminal when the crank is at 0 TDC on #1.  This centers the firing window where it belongs and prevents advanced timing related crossfires.

 When it comes to the CSFI Cam Sensor, GM published an interesting indexing procedure using the Tech 2 scan tool (you can also do it with aftermarket scan tools like the OTC Nemisys), but I wanted to know how to set the sensor up without the benefit of the scan tool.  The GM procedure is published in the shop manual under “Cam Retard Offset Adjustment.”   The Cam Retard PID should be accessed, and the engine speed should be increased above 1000 rpm (Cam Sensor Offset is inaccurate below that speed).  The distributor should be turned until the Cam Retard Offset reads zero (8-16 degrees on 96 models and minus 5 to 17 degrees on 1997 models, but unless you hacksaw them off, the distributor has ears around the hold down bolt that prevent the distributor from being turned). 

The stated reason for this adjustment is to prevent ignition crossfire; apparently, the cam sensor signal isn’t quite as important as rotor alignment, but is used to determine proper distributor indexing.  Jeeps and Dodge Trucks are indexed with a similar procedure using Daimler/Chrysler’s DRB III scan tool.

To develop a technique for aligning the Chevy CSFI Cam Sensor, I used a logic probe tapped into the PCM Cam Sensor input wire on a smooth-running truck to determine exactly when the Cam Sensor signal switched.  We first found #1 Top Dead Center compression stroke, then moved the crankshaft 43 millimeters past that point watching our logic probe to see when our signal switched.

NOTE:  Don't shove the logic probe tip hard into the female PCM/VCM terminal, else you'll damage it!!!

  The measurement we got corresponds to 24 degrees after top dead center, which appears to be a magic number for the cam sensor signal; Ford has been using that same point for cam indexing since the late ‘80s, probably because that’s when all the fire has gone out in the cylinder and it's okay for the injector to deliver fuel after that.  Finding the Cam sensor wire at the GM VCM (or PCM) is relatively simple, and while measuring the voltage with the high-impedance tool of your choice, work the distributor back and forth until you find the Cam Sensor switch point with the crankshaft at 43 millimeters past TDC.  A Cam Sensor Harness connector, a project box and an LED can be used to build a tool for this if you don’t want to find the wire at the VCM/PCM.  R.W.M.