I have asked the same thing in the 5.4 modular section with no anwser so thought I would ask here also,
So my question is What are the perameters for testing and how to test a coil from my 97' expy, so I want to find out if it's bad or not.
There was a member on this forum who a few years ago detailed exactly the test procedure for the COPS. I have the posting in my office and I will steer you toward it when I get to to work tommorrow unless someone else knows where to find it in the meantime.
COP's can be checked with an OHM meter but you can only check resistance of primary winding(low voltage side) and secondary winding(high voltage side) with a standard DVM or DMM, to check for insulation break down a special meter is required ( Megger ) one that can simulate the high voltage generated by the COP. What I have found ( now sold 97 expy) is that the carbon resistor ( in the spring ) increased in value which reduces the amount of voltage to the plug.
This was the reply that I was referrring to from the thread "97 Expedition misses @60mph":
New User Join Date: Jul 2006
You lay down chest first on the engine and expect to be there awhile...
Glad you got it fixed. Note that the 'misfires' that are detected by the computer are done by sensing current during the spark interval. What that really means is that if the computer says a misfire occurred it is because the spark plug didn't draw enough current to satisfy the software. This does NOT mean the computer will detect ALL misfires. If the miss occurs due to arcing to the head instead of the plug (which occurs with bad boots often) then the computer may not detect it if the current was within the acceptable range. That's why you didn't get any codes and also why it usually has to be severe to get the misfire code. With time, the arcing may increase in resistance and lower the measured current and the computer may then start to indicate misfires.
Note also that most COP failures that are reported are usually boot or corrosion related and the COP is actually OK. Cleaning the green copper oxide off of the bottom COP terminal and replacing the $5 boot usually fixes it. For some reason (bad one in my opinion) the only contact point between the boot spring and the COP terminal is along the edge of the terminal. Moisture causes the corrosion and worsens the connection that is already to small and area to begin with.
The difficulty comes from finding which one needs cleaning or boot replacement without having to check all 8.
My Expy started missing so I went to work on it. I had two cracked boots that I couldn't tell were cracked until I squeezed them between my fingers. Checking the plug wells with a light and inspection mirror showed arcing marks. I replaced all the boots and it ran great.
Reports of a lot of COP failures make me curious. I'm a test engineer so I wanted to know what was really happening and get to the bottom of the supposed COP failures. My experience told me something else was going on here.
I proved COPs don't really fail very often to myself with several 'bad' COPs that I received from a Ford mechanic. I measured the inductance and resistance of the primary and secondary of some brand new COPs I bought and compared the readings to my known good ones in my Expy and to the bad ones. In all cases they read the same. I believe that changing the boots and plugs are what really fixed the vehicles. I'm not saying you can't get a bad COP, but it is my experience (and FORD's) that most are changed for nothing. FYI, good COPs should read as approximately 0.5 Ohms and 1.25 milliHenrys on the primary and 1.5 KOhms and 10.3 Henrys on the secondary. You need and LCR bridge or LC meter to measure the inductance.
Hope this info helps someone else.
Sounds like your mechanic knew exactly what to do. I'd keep him
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