1.6 to 1.8 Coil Swap
Conceived by Ray Ayala (Pictures: Clive Wong))
This only works with the '94 - early '95 4-wire coils, not the later 3-wire coils. The 3-wire coils don't have tach drivers in them. If you use 3-wire coils, you need to make your own tach driver (see below).
1994 to early 1995 coil
4 wires on each connector, not 3!
Make sure that you get the coil harness too as the 1.6 harness doesn't fit. You only need the connectors of the coils, the other ones (green and gray in the picture) are not needed.
Remove the old coil
pack, igniter, and the wires connecting the igniter to
the old coil pack. Connect the two igniter trigger wires from
the ECU to the two 1.8 coil trigger inputs (brown, brown/yellow).
Connect +12v (the old coil/igniter power source) to
both 1.8 coil power inputs (lavender). Connect the
chassis tach wire to both 1.8 coil tach outputs
(black/white). Connect a 1K 1/2w resistor between the power input and the
chassis tach wire (because the 1.8 tach drivers are open-collector).
Make sure that the new coil ground wires (black) are
grounded to the coil pack, bracket and the head.
The diagram shows a 470ohm resistor, it is advised to use a 1K instead!
One more thing about the resistor: it IS required for
a Link 1.6 ECU but not for a Link 1.8 ECU, and not for
an OEM ECU (according to others that have tried it).
The Link 1.8 ECU contains the resistor internally and its value is 680 ohms, and AFAIK no Link 1.8 owners have any problem with it. But others who have done the coil swap into a 1.6 have had better results with 470 ohms, while still others have had better luck with 1K. It's entirely possible that cars equipped with AT and/or cruise control will need a different resistor value if those devices (which also are connected to the tach wire) contain
resistors of their own. But I'm beginning to think that it's more likely that some junkyard coils may contain damaged (weak but not entirely broken) tach drivers in them that respond better with a different resistor value.
The only other thing I can think of is that even though the coil-to-block ground may be good, the block to chassis ground connection may be weak.
You'll need to reuse the old coil bracket and adapt it a bit because the 1.8 coils are a bit bigger.
modified 1.6 bracket
1.6 coils left - 1.8 coils right
1.8 coils left - 1.6 coils right
1.8 coils in 1.6 bracket
1.8 coils in 1.6 bracket
The 1.8 coils are a bit bigger and closer to the firewall. It probably isn't necessay, but just to make sure they didn't hit the firewall, I bent the ridge on the firewall (pretty though stuff BTW) to allow for some more room.
The 1.8 coils fire in a different order than the 1.6 coils, so you need to swap
the spark plug wires around a bit. Put the wires in a 2-3-4-1 sequence
instead of the 1-4-3-2 sequence on the 1.6.
If you really want to keep the wires in the same position, you could of course swap the left and right coils in the bracket.
Using 3-wire coils
The 4-wire coils are rather hard to find so if you can only find 3-wire coils, no all is lost. It's possible to make a tach driver with one single transistor.
Any logic level N-channel Mosfet with 100V rating capable of more than 100mA should do (for example a RFP10N15L). Put the CAS SGT signal on the gate, tie the drain to +12V by a 1K resistor and feed it to the tacho, and ground the source. Be careful in the handling of MOSFETs; even the big brutes can suffer damage from electrostatic discharge. I put a paperclip on mine, while they're still new in the bag. I only remove it prior to energising the circuit so the device remains shorted out during handling and assembly.
You should disconnect the wire from the ECU in OBD-II ECUs (which try to drive the tacho) or from both coil packs on OBD-I cars (which may or may not be shorting to ground depending of how the output came to be damaged). The IG- wire is B/W and appears at the diagnostic box as well as the ECU, coils, and tacho.