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I am assuming by the way you said "Electric Shift assist" (which OMC uses) versus shift interupt (which Mercruiser uses) that you have an OMC drive.
This was written on the internet
"ESA" stands for "Electronic Shift Assist." This is a sealed, transistorized gadget bolted to your engine and wired into the primary side of the ignition. A microswitch in the shift linkage enables the ESA, and it intentionally messes up the engine spark. While the ESA is operating, the engine idles slower, rougher, and with less torque; this makes shifting easier.
The ESA seems to be most critical when disengaging a gear and returning to neutral. The interference "bite" on the clutch dog combined with the torque of the turning engine and resisting prop make the dog bite harder to stay engaged. The ESA defeats this by slowing and weakening the engine momentarily.
Pursuant to the point above, if your Cobra engages gear easily, but disengages with difficulty, your ESA is suspect. Test it by running the engine at idle in neutral, and clicking the ESA microswitch in the shift linkage (starboard side of carburetor). The ESA microswitch is the one with the metal arm on it, but you won't hurt anything if you click them both. If your ESA is working, the engine will noticably slow down. New-style ESAs have a built-in timer, and will only munge the ignition for a few seconds.
If your ESA tests OK when engaged by hand (above), see if it engages when you shift. Run the engine at idle and study the linkage while somebody else engages/disengages the drive. Does the ESA microswitch get clicked on?
OMC had a batch of defective ESA springs; they were too stiff, and prevented the ESA from engaging. Diagnose this by watching the shift mechanism while the engine is running (as above). See "Wrong Shift Interrupter Spring" (120k JPEG), and/or try "crunching" your spring to weaken it. I bought my Cobra when it was nine years old, and it still had the wrong spring, and my ESA never engaged while shifting!
Bad ignition points can cause the ESA to malfunction. Points are cheap, and ESAs are expensive; if there is any question about the points or condenser, replace them first.
If your Cobra has points in the distributor, OMC makes an upgrade kit that replaces the points with electronic ignition, and a new ESA. Ask your local OMC dealer; when I last checked, this kit was US$200 for my 4.3L Cobra. This is an excellent value; the ESA itself is about $200, and a Pertronix (non-OMC aftermarket) electronic ignition kit (no ESA) is US$70-80. Alas, my wife doesn't see this as essential (yet).
The ESA may have an RPM limiter in it. I have no idea what the limit might be, and very little interest in pushing my engine that hard. If any reader discovers the limit, I'd be interested to know.
Basically it says the ESA is on the primary circuit -- which to me is the primary side to the coil ---do you still use a coil on the Chevy system???
you ESA has to cutout the coil primary somehow.
good luck
_________________ DaleG -- West Palm Beach FL 2003 240Horizon -- 5.7Gi -Duoprop 
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