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I've been having a lot of strange boost problems recently. I've gotten really good at pulling the UIM and rats nest. I've tested everything multiple times, replaced a lot of things, and I'm starting to think the crispy old harness may be at fault.
Besides the tremendous cost to purchase an OEM engine harness (the Wiring Specialties harness looks like possibly a good alternative to OEM), how hard is it to replace this thing?
It looks as though the crank sensors have their own sub-harness, so taking the front accessories off (trying to avoid the PS pump...) shouldn't be an issue. I'm also running IGN-1A's so the ignition sub-harness isn't an issue, either. What I'm most worried about is a) getting my hands behind the ABS unit to get the new harness boot into the firewall and b) getting the old harness out/new harness in from under the dash -- is it held in with any clips or ties that are impossible to reach without removing the dash?
The CAS connectors are part of the main harness, not a separate, sub harness. As is the OMP connector. The only harness that's separate is the starter harness, and it's joined to the main harness by one wire. The coil pack has it's own harness too, but it's very small, just the coils and grounds, and it joins to the starter harness. All that being said, it's not that difficult to remove the harness, and before you install the new one, wrap it in heat barrier. You'll be amazed how much it helps.
It's a substantial job but not too bad all things considered.
- Remove the passenger kick panel to access the ECU. Unbolt the ECU from the car, make sure the ground wire is unbolted, unplug the connectors to the ECU. Follow the harness up the firewall and remove the L-shaped metal cover over the harness. Find the blue and the white connectors and unplug them, they are usually up a bit on the firewall behind the blower motor box.
- The harness is typically bolted to the firewall lip in the engine bay. Unbolt that.
- Spray some WD-40 down onto the rubber boot that is around the harness as it goes through the firewall. Get a long flathead screwdriver or pry bar and pop that grommet free. It's best to have someone inside the car feeding the harness out as another person gathers it up on the outside.
- Remove upper intake manifold and alternator
- Start removing the harness. If you have the rat's nest, many times it's easier to just remove the rat's nest and wiring harness as an assembly. When I break down an engine I typically remove the rat's nest, harness, and fuel rails all as 1 big component. The rat's nest is just bolted to the rotor housings with 12mm bolts.
- The connector to the metering oil pump may be a little tricky with the engine in the car. That harness runs under the water pump housing and there are metal tabs that hold the harness in place.
That's the 1000 foot view. I don't think I've pulled a whole harness in the car, I have always done it with the engine out and on a stand. The CAS and OMP connectors will be the tricky ones.
So Dale when you pull the motor do you disconnect the ECU side of the harness and pull it out the front of the firewall with the motor on the way out? Is the boot easy to get back in the firewall on the way in when it’s all lubed up?
Yep I always pull the engine harness with the engine. My philosophy is to disconnect what holds it in the car, get it on the engine stand, then I can comfortably work on it sitting on a stool next to the engine instead of hunched over an engine bay.
Yep, the ECU harness isn't bad to get back in place. Again, it helps to have 2 people - one inside to grab the connectors as they come through the firewall to guide it down and the other to feed in the harness. Juice up the boot with some WD-40 and use a pry bar/screwdriver to pop it in place. Also helps if the person in the car pulls some on the harness as you pop the boot in.
All things considered the firewall part of the harness is like 2 minutes of work, it's nothing. Huge trick is using some WD-40, that makes all the difference.
Dale's writeup is very extensive and pretty much all-encompassing. The only thing I'd add is blocking off the top of the LIM when you pull the UIM. Otherwise your job could potentially get much more complicated lol.
Not sure if it’s mentioned, but if you remove the bolts holding the ABS pump in place, it will flex enough so you can get your hands in there and it’s a million times easier to get the old stuck harness out. At least in my case. Good luck.
also, I recommend buying the oem harness instead of aftermarket. It’s priced competitively if you use Mazdacomp.
What gives? If it were me, i'd do a little more troubleshooting before ***** nilly replacing the harness. Turn off your water injection to take out a variable (unless it it tuned on water). Check your ignition system. Bench test the coils. Any loose connectors. Looks like you have good ground, but you may want to check it from the coils (are they grounded through the frame?). Are your plugs fouled. Etc.
Not sure if it’s mentioned, but if you remove the bolts holding the ABS pump in place, it will flex enough so you can get your hands in there and it’s a million times easier to get the old stuck harness out. At least in my case. Good luck.
also, I recommend buying the oem harness instead of aftermarket. It’s priced competitively if you use Mazdacomp.
Thanks, I was thinking of trying that -- good to know it's easy and works.
What gives? If it were me, i'd do a little more troubleshooting before ***** nilly replacing the harness. Turn off your water injection to take out a variable (unless it it tuned on water). Check your ignition system. Bench test the coils. Any loose connectors. Looks like you have good ground, but you may want to check it from the coils (are they grounded through the frame?). Are your plugs fouled. Etc.
Or could it be a fuel issue (@22 psi).
At the time of posting the referenced link above, the car was running well other than a hot-start issue that I narrowed down to a leaking secondary injector. I subsequently pulled the UIM & rats nest and replaced the injector, which fixed the hot-start problem. In the process I introduced a new boost issue -- I could no longer make more than 3-4psi, and the turbo flapper door was apparently staying open by the sound of it.
Yesterday I pulled the UIM and found that the turbo control and charge control solenoid electrical connectors were swapped. After fixing that the car now boosts properly again. Very glad it wasn't a harness issue.
I asked about the harness here because as part of the troubleshooting process one inevitably weights both the cost and time commitment to perusing a given avenue. I knew the cost component of replacing the harness, and created this thread to ask for advise on the time component, to better help me plan and weight whether/when to pursue the harness route. Sorry I offended you.
This car never ceases to surprise, however -- today on my 200-mile long drive home the car started running worse and worse, like a 4-cylinder running on only two or three cylinders. Lots of vibration, very rough idle and very, very slow to rev under 3k RPM. It seems to smooth out after 4k but is lacking full power. I'd been noticing it idling a little rough the last few times I started it up. I'm hoping it's just a melted coil. Will compression test and check the coils another day...
I just checked one of the logs from the trip home and the fuel pressure is ~33-34psi at idle, and rises as expected under boost. I assume that's good enough evidence to eliminate the fuel filter as a cause.
IGN-1A with an "old" style SBG harness that I added an OEM Sumitomo connector to. SBG "RHD" (framerail) mount. I used 8awg wire to ground the rear rotor housing to the firewall, the front housing and rear housing to each other, the front housing to the left strut tower, and the strut tower to the battery negative. The grounding + 10.5 plugs fixed some ignition breakup I had previously.
BUT, the coils have less than 5k miles on them. They did not come with any branding stickers on them, but I ordered from a reputable rotary shop.
I'm going to have to test them all anyway, but I assume it's not normal for IGN-1As to fail so quickly. I admit to do enjoy driving the car at high RPMs fairly frequently (talking like sitting on the highway at 6k for 20-30 seconds at a time)
and I know you've had some ignition problems in the past. F1BlueRX7 has run into a lot of problems with the AEM coils wired the Sakebomb way, I know you've added grounds but I wonder if you have the whole thing sorted.
Also I believe DriftinJim sells just his harness which is wired properly. But you'll want to test coils to make sure they aren't dying.
04-15-21, 11:19 PM
