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Spoon's ECU Code Update and WARNING or turn a $70 fix into $700

1992 Turbo Legacy with faulty knock sensor code flashes

Well the last I wrote about, I was having ECU code flashes related to my knock sensor, cam angle sensor, and all four injectors after a knock sensor replacement.

There is a little 2 mm diameter wire that plugs into the knock sensor and terminates at the ECU.

When I replaced my knock sensor I had the big idea to plug in the wire before installing the sensor, because the mounting location on top of the block is hard to reach under the intake manifold, and I didn't want the sensor to slip off into some inaccessible cavity.

Well, that wire was already hardened and fragile, and I was not as careful as I had promised myself I would be, and I ended up breaking the wire off right at the connecter so there was no way to repair it.

I did learn from the matching wires I pulled from two cars I found at Pic-Ur-Part / Ecology Auto Wrecking, that the wire did truly have an outer wire that didn't go anywhere...it terminated short of the connector and was covered by electrical tape, confirming that mine was in original condition before I broke it, and was not some micky mouse repair.

I tried two methods to clear the codes after the sensor replacement with no luck.

I gave up and took it to my Subaru dealer near work to diagnose and possibly re-repair, for a 100 bucks.

I learned some stuff my repair manual does not tell me. That little wire that plugs into the knock sensor is a tiny coax and the outer wire is the "shield" which protects the inner wire from radio frequency interferrence.

The voltage to the ECU carried by the little inner wire is so small, there is extreme sensitivity to any condition other than the OEM installation, such that it is considered NON-REPAIRABLE!

My two hour long Siamese contortionist solder job was for nothing!!!

Now of course this wire is not available as a seperate dealer item, it is part of the ENGINE HARNESS!

Even if one could obtain the wire, there is an excellent chance that it passes thru one or two connectors at the firewall and under the dash before finding its' way to the ECU, and even with special tools, removing the little wire from the multipin connectors and the ECU terminus is a job for a Leprachaun.

So with the option of replacing the wire essentially not available, there is the next step up of replacing the engine harness.

Well, the parts department guy went out to look at my car, and checked his computer and came up with a part number. I am reluctant to burden any of you with the boring details of the dealer price, which is at this point academic anyway, since the computer came up with zero (0) in stock nationwide!

Unlike good ol' American iron, Japan, or at least Subaru does not always support the spare part requirements for older cars with continued availability, and chances of having one made via factory order are less than excellent.

Did I care to try for a special order?

I took a pass since...oh you have a perverse curiosity? If the harness were available it would cost $440, and probably cost about that much to install

Of course the turbo harness is not the same as the N/A part, so if I'm going to try and locate one on the grey market, I have to be careful that is well understood by any potential provider.

So that is my story to this point my friends on how to turn a $70 repair into a $700 repair, and I havent' even gotten out of the starting blocks.

I want to club somebody who designed that wire to be so small yet so sensitive to damage.

Any ideas? Anybody know of a 91-94 Turbo Legacy totaled from the rear with an intact engine harness up for grabs?

The silver lining? I learned from the dealer that I can clear the engine codes by removing the positive battery cable and jumping it to the negative battery terminal, which will drain the ECU capacitors and reset the computer.

Well here's what I, and other board members with the knock sensor wire problem have done. You are correct about that wire being a two signal coax line.

What I did was simply cut the wire right at the ECU. The coax line separates at the ECU and becomes a ground and a signal wire. It goes into the second smallest ECU plug. ECU Pinout Diagram

Then I simply spliced into the signal wire and ran the new wire back through the firewall and to the KS. I spliced it back into the ECU side of the old knock sensor harness so it could easily plug into the sensor side of the knock sensors harness.

You do not need to do anything with the ground. It is a ground being sent by the ECU, not received by the ECU, so there will not be any ill effects if you leave it disconnected.

i spliced the one in my turbo wagon as well. no problems 20,000 miles after the splice job...

Hey I'm really LIKING what you guys are telling me

Thanks a million ... or at least about several hundred !

So since I already have the junkyard wire spliced at the knock sensor end, are you saying what I can do is get to the wire at the ECU end and cut the ground / shield wire where it seperates from the coax, leaving essentially just the signal wire connected to the ECU ?

The diff in that as far as I can see it from what you did is my splice is at the knock sensor end instead of the ECU end

Are you guys saying to leave the ground wire disconnected? Its sheilded for a reason. You triggers and knock sensor outputs are faint signals compared to other stuff so they are prone EMF interference.


When I wired in my Link is was anal about making sure all the shielding was hooked up as well.

Larry Witherspoon wrote: So since I already have the junkyard wire spliced at the knock sensor end, are you saying what I can do is get to the wire at the ECU end and cut the ground / shield wire where it seperates from the coax, leaving essentially just the signal wire connected to the ECU ?

Exactly.

93forestpearl wrote:Are you guys saying to leave the ground wire disconnected? Its sheilded for a reason. You triggers and knock sensor outputs are faint signals compared to other stuff so they are prone EMF interference.


When I wired in my Link is was anal about making sure all the shielding was hooked up as well.

Hmmm I suppose you bring up a good point. I did mine the way I described and haven't had any problems, and I don't think anyone else has, but I do see how that could be a potential problem.

How do you know you don't have a problem? The sensor might not be working, but hasn't needed to yet. Just because a house hasn't burnt down doesn't mean it's fire-proof.

No I know it works because it pulled timing when my dad put in the wrong gas and it began to knock/ping.

You really should make sure the shielding is on.

You can go by a coax cable or some other shielded wire at an electronics store.

i know this is an old post sorta, but do you think regular coax wire would help... meaining center wire being 1 solid strand of copper

cj91legss wrote:i know this is an old post sorta, but do you think regular coax wire would help... meaining center wire being 1 solid strand of copper

Probably, but it's overkill and silly. Go get standard shielded COM cable at any electronics place.

Personally, I just cut out the damaged (brittle) section of the wire and replaced it with a 10" section of standard 20 gauge wire - I was careful of course to strip back the shielding and heat-shrink it so there would be no short with the sensor conductor. I replaced the knock sensor with the new one that doesn't have the cracking problem and my code went away and hasn't come back. That about about 6 months ago. The shielding wire is important for where the wire runs through the harness alongside other wires to protect from inductive interference, but if you notice that the pigtail on the sensor itself (beyond the connector) has no shielding.... a few more inches without shielding where the cable isn't bundled alongside the rest of the harness shouldn't affect it to speak of.

GD

At the very top of this thread is a short story with subject; "Spoon's ECU Code Update and WARNING or turn a $70 fix into $700". It ends with me in a jam. The continuation to a conclusion and solution has as subject; " Witherspoon Wages War With Worlds Wildest Wire" and can be found here; http://bbs.legacycentral.org/viewtopic. ... highlight= in the electrical forum
It is about a 15 minute read, and is intended to provide humorous insight into what is involved with replacing the wire from the knock sensor to the ECU and the problems that you may encounter in between.

I had a simular story with my car but the dealer wanted 1700.00 for that harness. If I had caught this thread before I had cut into my spare bulkhead harness, I could have shipped it good luck