Had a couple of failures on my GM TB and was curious about the construction of the COP ignition coils they used.
I was somewhat surprised at the level of integration (hybrid circuit board) and the subassembly steps used in the construction.
It was interesting and perhaps you think so too-
First pic is top of unadulterated coil. Notice the whitish colored hard epoxy used to encapsulate the heatsink. That is some tough stuff! I’ve dug out a lot of epoxy in my years but that stuff was some of the hardest yet. This heatsink turned out to be a separate enclosure that was plastic box molded around the heatsink as you’ll see later.
Second pic is just business end of the COP assembly. Notice the robust EMI shield around the shaft, that encloses the coil and prevent electrical noise from eminating from the assembly. A welded tang goes to the mounting bolt and the heatsink.
I used a heat gun to soften the epoxy as much as possible before digging it out. This exposed the connections made to the coil. I could now measure the coil which is a type of autotransformer. The secondary was open…so the failure mode is established.
Now what is under that heatsink? A bit of digging, cutting proved futile so I resorted to more medieval methods and used a hammer to bash it off.
Check it out, a hybrid circuit board!
The power device is Chip On Board. This is what they call a metal core PCB and is effective at dissipating heat from the bonded components to the heatsink to which it is adhered. You can see the MOSFET wire bonded under the conformal coating. That coating is like whale snot. When cured, it is still quite sticky and jello like in consistency.
See that brown smokey discolored area above the cap? That is evidence of corona.
With some of the whale snot removed, you can see the film resistors deposited directly onto the PCB. Notice the white slits in the black carbon areas? That’s the laser trimming done to fine tune the resistance on each assembly.
The “box” with the heatsink is built first. Calibrated by laser etching and tested. Then that box drops down over the coil which has been inserted into the tube of the COP. The wires are soldered from the coil onto the leads from the box and the whole thing potted with a hard epoxy…
I seen Ford, Honda, and Toyota COP’s but I’ve not seen any GM ones. That is very interesting, and I wonder what is inside other COP’s? This is the first one I’ve seen with a heat sink, which I assume is to keep the circuit board components from getting fried by the heat. What does GM get for each of these COP’s?
Dang, no WONder they get $80 and up for the little buggers, Never seen a heat sink on one before, either.
My Saab has a COP cassette that contains all 4 COP assemblies as well as spark knock detection. The whole shebang is about $350 for aftermarket and almost $500 for new-old-stock Saab parts.
Great post OP! Very informative. I always thought the COP was just a different place to put the coil, but it worked the same way, a current pulse to the primary turning on, then off, from the computer fires up the coil. But from your photos, I guess the way it works, the COP receives a low voltage low current signal from the computer. That pulse tells the COP circuit it’s time to fire up the spark plug. So it turns on the MOSFET which draws as much current as it can through the coil primary, then at some predetermined time the coil primary current is turned off, which causes a big voltage pulse to occur on the secondary, firing the spark plug.
Or maybe it is even more complicated, maybe the COP does the timing advance retard function too? I wonder …
Edit: The other interesting thing about your photos is the importance of the heat sink function. There’s no diy’er mickey-mousing recommended when installing COPs, for the heat sink to work, everything has to be solidly attached.
Uncle Turbo, they’re typically around $80 just like Mustangman mentioned.
George, you had it right-
I guess the way it works, the COP receives a low voltage low current signal from the computer. That pulse tells the COP circuit it's time to fire up the spark plug. So it turns on the MOSFET which draws as much current as it can through the coil primary, then at some predetermined time the coil primary current is turned off, which causes a big voltage pulse to occur on the secondary, firing the spark plug.
The only pick to nit is that the low voltage pulse from the ECM drives the MOSFET directly. The MOSFET is driving an autotransformer which is essentially a tapped single coil serving both functions as primary and secondary.
The design has a high impedance path for thermal dissipation from the MOSFET. So they have two paths for heat dissipation; convection across the heatsink and conduction through the metal mounting structure.