Making a 4 plug ECU Sub loom for 3 plug RX-T

discopotato03 · Post by **discopotato03** » Wed Feb 13, 2008 7:52 pm

Hi all , was brutally removing sections of harness from a 4 plug auto wagon today to try and patch together an engine control harness . What I'm trying to do is make a separate harness to run a 4 plug turbo engine in a 3 plug turbo car .

The later L is designed so that the fusible links are near the battery and coil/transistor - then it goes round the world (under the radiator to the drivers side . Then it ducks inside the inner gard and through into the cabin in front of the drivers door . There is another section that gets inside via a grommet just behind the drivers side strut tower . This also connects the fan relay and some sort of pressure sensing devise that I've never seen connected tube wise . Also the wiring from the transmission gets in via the same grommet .

Meanwhile behind the nearside strut tower there is another grommet that lets wiring in from the wiper motor/wiring from diff lock plug/a yellow wire from the coil which runs behind the heater and back to the drivers side . I believe this is the coil output signal to the tacho in the gauge cluster . I managed to separate this yellow wire back to the drivers side from the white plastic connector near the coil and somehow ended up at another identical connector and can't remember where it plugged into ! Will have to dig through the loom till I find it unless its patched directly into the clusters loom . Also behind the AC unit this yellow wire ducks into a block connector which I assume is to do with AC control ? Anyhow wire is doubled up on the plugs pin so I just cut the connector and removed it and the pin .

Because I'm a sadist I removed the sub loom with the diff-lock and wiper wiring so I could separate the lock wiring back to the first plug . The harness side of this plug runs along the inside of the dash to the drivers side so will be back to chase only the lock wiring to wherever .
BTW I removed the drivers seat and carpet and went looking for the elusive wiring to the diff-lock switch which is supposed to live under the drivers seat carpet . Found nothing except a black plastic hump and box section than runs down the side of the trans tunnel . Maybe auto cars don't have it . I've never been able to find this section of loom though people here reckon some cars have it . Virtually all late cars have the lock solenoids plug in the engine bay .

Also found out the hard way that I can't remove the rear strut mount bolts without a breaker bar . The manual reckons wagons have 196 in/lb rated rear springs where RX sedans have 159 in/lb and RX Coups 179 in/lbs .
My RX sedan sinks in the butt with a full tank of juice compared to near MT so this is not good . If anyone can tell me how an RX sedan goes with wagon springs I'm all ears . It didn't escape my attention that one car I looked at had 10 turns of spring wire where most had 9 . Will probably walk out with two sets of springs tomorrow but need to measure the wire diameter first - blows dust of very near caliper .

I don't know where my time goes , cheers A .

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Wed Feb 13, 2008 9:13 pm

You should only have to run a new AFM wire set and plug.

Another sub loom to the dizzy and ignitor under coil

And then add the 4 plug outlets for the ECU spliced in behind the other plugs if you want to keep the option of going back to 3 plug.

Thats about all i can think of off hand, i have done the conversions in both directions like that with no problem.

If you have to 2 looms then it would look neater if you just did a total loom swap you only have to do the front.

discopotato03 · Post by **discopotato03** » Thu Feb 14, 2008 6:45 am

Sounds logical though from looking at the EFI sections from early and late WSM's I can see a few variations . For instance the late one shows an injector resistor where the early one doesn't and the TPS differences . Fingers crossed that they used the same knock sensor in 3/4 plug systems .

My plan had been to strip the engine control stuff back to the CAS/dizzy plug and transistor/coil wiring . Also the loom side of the engine harness plugs back to that "tree" of wiring inside inc ign sw and the group of two and three relays thought the ingition system and fuel pump relays seem to be the important ones .
I think Paul from Subawreck mentioned the yellow wire off the coil is the same from 3/4 plug systems and goes to the tacho .

I don't suppose you made up a conversion wiring patch diagram by any chance ?
I would prefer to keep the original 3 plug wiring there because if I can't get the later stuff to work easily I don't have a dead car .

LOL if I ever had another L series it would have to be a 4 plug hot-wire car .
I noticed you mentioned converting one from 4 to 3 plug ?

Cheers A .

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Thu Feb 14, 2008 4:52 pm

Yeah have done the wiring in both direction but havent bothered to do a wiring diagram as most colours seemed to be the same

Got a feeling the 4 plugs have a single wire knock sensor if going from 4 to 3 you need to have the knock circuit which is a separate set of wires on the earlys going the other way you can us the early knock on the 4 plug by earthing one side of the sensor.

There is a plain yellow wire at the coil and it is the tacho wire.

discopotato03 · Post by **discopotato03** » Thu Feb 14, 2008 6:30 pm

I grabbed the MPFI harness off the engine today so will start stripping tomorrow .

Cheers A .

PeeJay · Post by **PeeJay** » Thu Feb 14, 2008 8:01 pm

discopotato03 wrote:If anyone can tell me how an RX sedan goes with wagon springs I'm all ears

With new king spring, awful. It was hard as a rock and the damper was only about 20-30mm below full extension. I had to replace them.

discopotato03 · Post by **discopotato03** » Fri Feb 15, 2008 8:07 am

I'd say that was because the King springs would have been higher rate than std wagon and so the fitted length would be longer/higher .
The butt of a sedan is going to be lighter than a wagon so it very likely won't compress the springs as much , by going to an aftermarket spring designed to be higher rate for the wagons rear mass I'd say under a sedan would sit pretty high and be rather firm .

I'm going to try and use std wagon springs because they are 1) cheap and 2) higher enough in rate to act as a performance spring under a sedan weight car . We are probably going to have to make up something to sit the foot of the spring lower on the dampers body to correct the ride height issue .
From memory early Bluebirds had a system where the spring seat was suspended off a tubular section that located on the top of the strut body .
Some aftermarket rear wagon dampers sit a removable seat on a step pressed into the damper body , if we made a dimensionally similar seat that sat the spring lower that would work as well .

Cheers A .

discopotato03 · Post by **discopotato03** » Sun Feb 17, 2008 5:22 pm

I cut most of the tape and loom tube of that 4 plug loom yesterday .
The next step will be to remove the front light plugs and wires from the front loom . Then I suppose I can look at removing the heavy power wires from the link box back and hopefully all we should be left with is the ignition/CAS loom and that which connects the computer to the two large engine harness plugs .

I'm beginning to wonder if I should kill a four plug auto NA computer for its plug sockets , then I can splice the in between bits without touching my cars original 3 plug harness . This would make a short adapter taking the relivant wires from my loom , plus with the patch looms from the CAS/transister and hotwire AFM , and the injectors/dropping resistor .

One thing I do need to know is do I have to use late injectors from a 4 plug + dropping resister system ?
Since early cars have no resister does this mean they are saturated or high resistance injectors and the later ones lower peak and hold typoe injectors ?

Cheers , A .

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Sun Feb 17, 2008 7:43 pm

I am only guessing here but the dropping resistor may have had something to do with keeping current draw lower in later models as i have a set of turbo injectors (red Ones) from a late model and have been running them in my RX for about 5 years now with no ill effect

discopotato03 · Post by **discopotato03** » Mon Feb 18, 2008 7:37 am

My understanding of solenoid petrol injectors is that they are either low current high resistance (saturated) or high current low resistance (peak and hold) .

I believe the saturated ones don't always use a dropping resistor and the prob is that they can get warm/hot when they are open for longer time periods (for an injector) .
It seems that some turbo performance cars like RX7's/DR30's/GTR's have injectors with stronger internal springs to make them "snappier" which they need to be if bigger in relation to the cylinder size . To overcome the higher spring rate and the slightly heavier pintle valve these injectors are driven by higher current . They use the full current to open the injector and then switch to a lower current to hold them open - peak and hold .
I think the injector dropping resistor is placed in the earth return side of the injector circuit so that the injector drivers in the ECU (on/off switch) are not carrying the full current any longer than they have to .

Anyhow all this aside I haven't looked at at my Vortex injectors to see if they are different from my RX's early 3 plug ECU ones .

Loom update - have removed the wiring and plugs from headlights/Parker's/blinkers/washer motor and its looking a lot less daunting than it did before . At the far end now just have the CAS plug/coil and transistor plugs and the fusible link box and alternator wiring .
I'm in about the area just past where the engine loom plugs meet the main loom and about 2-3 feet past here half the loom is a sea of fine wires where all the body loom and fuse panel joins up .

I guess the next step is to segregate the wires from the section of the loom "tree" that run between the large engine loom plugs + dropping resistor and the four ECU plugs and any diagnostic/maintenance connectors .

Then I get to separate all the heavy white power supply wires from the link box/alt wiring and trace back the wires that power up the ECU ignition and fuel pump relays .

Then I'm going to need to "obtain" the 3 plug ecu sockets and see if I can solder the wires to it that patch into the 4 new plugs so that it can be plugged/unplugged .

The last two things should be making new knock sensor wiring (do all 4 plug harnesses have it ?) and the gearbox and boost pressure switch/es .

Time consuming , cheers A .

SUBYDAZZ · Post by **SUBYDAZZ** » Mon Feb 18, 2008 8:49 am

discopotato03 wrote:...The manual reckons wagons have 196 in/lb rated rear springs where RX sedans have 159 in/lb and RX Coups 179 in/lbs...

What manual is referred to here? And what is an in/lb?

discopotato03 · Post by **discopotato03** » Mon Feb 18, 2008 5:39 pm

1987 Subaru factory workshop manual , Aust edition .

The term in/lb means inch pounds or the force required to compress the spring one inch .

Cheers A .