Unfortunately the rubber used for sealing the contacts to the harness is prone to crack after years of use in the hot environment of the engine bay. When it cracks, the moisture from the air can enter the contact and cause it to corrode. Of course the best thing is to replace the old harness altogether but it’s not the cheapest.
Use a contact cleaner for the job. Cotton pads and a small gentle file or a small screwdriver can come in handy as well.
The old contacts are usually brittle so be careful disconnecting them! Use a small screwdriver to pry away the wire/spring that locks them in place.
Clean the contacts thoroughly. Be careful with the file if the contact is in good shape, you don’t want to destroy the plating if it’s still there. Bend the contact plates with the screwdriver so that the hole where the pin will be inserted gets narrower.
Fill the contact with grease. Use a high temp resistant (~200C or 390F) dielectric grease. This will prevent future corrosion. I use Electrolube SGB, contact treatment grease.
Use some insulation tape to wrap around the cracked rubber as well.
If the contacts are in a really bad shape or the plastic is broken then you can order new pigtails (the connector with a bit of harness attached).
The ones that I know of that can be ordered are:
Injector connectors and alike: PN 24079-25P26 (~$20), they will fit except for a plastic track on the injector connector that you have to cut away. They come in 3 per set.
Coil connectors: PN 24079-85M00 – They come from a Pulsar and come in 4 per set (~$50).
Since the new connectors come with a bit of the harness, cut the old ones away. Expose the harness and solder (or crimp) and insulate. Now, crimping is the best way if done right but I’ve found soldering to work just as well. It is easier and gives smaller connections.
WARNING! Note exactly how the old harness was connected so you can connect the new contact likewise.
Remember to apply the contact grease, just in case. You can also get new connectors from Nissan if you feel like repairing the harness. I found it a bit tricky to do right though. Ask your Nissan dealer for them if you want to give it a try.
The harness that most likely needs to be replaced (on a 300ZX Z32) is the EGI-harness (called EFI in the service manual). It is the harness between
the ECU and the engine. Do not confuse it with the Engine Room Harness which is not the same.
This requires the engine to be taken out. So if you have your engine out for some reason and the harness is in a bad shape, this is the time to do it (remember to replace the knock sensor with its harness as well).
Since the engine is out anyway, there’s not much left to unhook so just remove the brackets holding the harness as well as the ECU.
The hole through the fire wall is very tight, especially for the ECU connector. Remove the plastic cover from the connector and try to pull it through at different angles (turn it around). I modified the new harness at the same time and let the AVC-R wires run along with it.
Apply dielectric grease to all of the contacts to prevent corrosion.
The part numbers for the EGI harness can be found below on page 4-G-1 (Harness Assy EGI) for cars made until July 1990, on page 4-H-2 for cars made until September 93 and on page 4-N-3 for others.