Coax from O2 to the ECU

[Mathew Zelezen]

Hi all!
Question for you concerning the coax from the O2 to the brain...

86 Syncro

I had some issues with the coax grounding out about a year ago, so I ran my own ground from terminal #19 of the ECU and all was good in the world. (The post title was "Fuel delivery woes..." and thanks again to all those that helped).

I recently started getting a rough idle after the initial warm-up and thought it may be O2 related. When I unplugged the sensor wiring it stopped stumbling so I figured that was the culprit. One new O2 sensor later and its doing the same thing...

So if its not grounding out (which it can't) and the signal wire is broken somewhere, wouldn't it just loop to the ECU preset (as if it wasn't plugged in at all)?

I'm confused... is there anything else dependent or in cahoots with the wiring from the O2?

Cheers,
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[tencentlife]

Not sure what you meant about grounding the ECU.

The coax center wire should have a clean shot right to the ECU pin it feeds, not grounded , no other connections. But you know that.

The sheath should be grounded at one end only. It could be at either end but stock it's done right at the ECU.
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[Mathew Zelezen]

Check out this post Tencentlilfe..."Fuel delivery woes..." we spoke of this before.

So then why have a coax if it only grounds on one side? Why no just run a signal wire and a ground wire and not worry about pitting or grounding out? Does it serve some other purpose?

I have a ground running from pin 19 to the firewall and a new O2 and its smoother when its disconnected... go figure.
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[deprivation]

I have had this problem as well - I have a weird miss that goes away with the 02 disconnected. I dunno what the deal is and I have really spent some time and money on this problem.

Keep us posted.
_________________
1986 Westy 2WD auto a.k.a. "The Old Girl"
www.kittenfart.com

[morymob]

The ox sensor works with a small voltage change to control ecu fuel adj.Also it is sensitive to outside elect.inteference-plug wires, wiring etc so as the rule in ALL sensitive signal lines they are SHIELDED & the shield is grounded on ONE end ONLY & at end where the signal is input=ECU.Th grounds any induced /outside source noise. Shield on ends of these cables are famious for fraying & shorting to center signal wire,& all else working o.k. u end up running rich starting just after the temp sensor-2 changes ECU out of (bypass ox sig)when warm up temp obtained.

[tencentlife]

Yeah, what he said....but in longhand.
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Please don't PM here, I will not reply.

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[Mathew Zelezen]

Ahhhhh, so removing the ground doesn't eliminate the problem, it exposes that sensitive signal to an engine bay of interference...

Got it!

So where does one find a replacement coax? (and I'm betting on it being as elusive as unobtanium) so my next question is how do I make/adapt one?

I'm not big on default settings and would much rather try to come up with a solution.

Cheers,
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[tencentlife]

You could adapt a piece of regular TV coax to do the job. Nuthin' fancy, that. Just make sure to keep the signal line clean, solder conections, and ground the sheathing only at one location or you get ground looping, basically current will move along the sheathing due to ground V offsets at different locations. The signal on the center wire is only 0-1V and at microamperage so it's very sensitive to interference, and there's an abundance of it present with a high-energy ignition, alternator field oscillations, idle control valve pulsing, etc. etc.; basically the engine bay is a maelstrom of RF noise. You get the picture. I don't understand why they didn't RF shield the Hall gen wires, too.
_________________
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Please don't PM here, I will not reply.

Experience is death to doctrine.

[Terry Kay]

I'd try high end CB coaxial cable if your thinking about replacing your's.

It's a better grade of wire than the plain jane Television coax, and will offer better sheilding from outside electrical inertference.

Folks have been dealing with busted up Oxy sensor coax for years on Vanagon's and have always came up with a blank for the wiring.

I found CB coax to splice the stuff works real well--and there is coax wire unions available to make the job real simple, and clean looking.

Take a look at the CB wire.
_________________
T.K.

[morymob]

Do u have an electronics supply store around? Radio Shack may not carry but these places carry diff sizes coax 4 the electronic repair world.

[Mathew Zelezen]

OK I'm stuck guys...

I traced the whole system from the O2 plug to the ECU and here's what I found:

The coax to the O2 plug had some pitting right at the end so I cut that off, installed a new plug,
and made sure the ground wire (outer sheath) was nowhere close to the signal wire...

Then I followed it to the firewall where it seems to have a factory junction (everything was marked VW) and here is where I get confused.

Tencent, you said it should be a straight shot from the O2 plug to the ECU... but with this junction the ground is terminated and only the signal wire continues. Which means there is no shielding for the low voltage signal from the O2.

That being said, when I have everything connected the van runs great until its warmed up and switches over to read from the O2. Then it starts to run rough and then stalls out. A quick restart of the engine produces the same results. Its acting like the O2 signal is grounding out.
Now here's the interesting part... If I disconnect the O2 plug it responds exactly the same way. The ECU should see no signal from the O2 and go to its preset but it doesn't.

The only way i can get the ECU to think the O2 is disconnected is if I run a sepperate ground from the pin the O2 normally grounds to up and onto to the firewall... basically eliminating the coax altogether. The exposed wire below is to show that the coax ground has basically been eliminated (and yes I covered the end before I put everything back together...)

What the heck is going on here????
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[pushkick]

if you have a friend that has an oscilliscope look @ the signal that you are getting from the o2 sensor at the ecu then you will know if your o2 sensor is working within specs. you will also see any noise if it is there. to check ground just use a vom put on resistance and check between existing gound and coax shield should have no resistance. if you check between signal wire for (o2) and ground you shoul have megohms.

[Mathew Zelezen]

Thanks Pushkick, I did check the continuity of the wires and its a brand new Bosch O2 sensor.

My question still remains on the grounding of the coax. If its spliced (as can be seen in the picture of the wire at the rear firewall) then there is actually no shielding around the wires in the engine compartment...
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[Mathew Zelezen]

So am I the only one out there with the splice at the firewall?????
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[tencentlife]

Matt, in the diagram it's a straight shot from sensor to pin 2, no intermediate junctions. Coax shield is attached to pin 19, which also has a branch straight to ground. That's what it says, all years, and that's how it would have to be to work properly. Sorry, I'm too busy to go digging into my vans to check the actual construction.

I'm not real clear on what's in your pics, but I see the green coax coming right back to the ECU plug. Are you saying that in your second pic there is a double-male connector midway, and the sheathing on only the ECU side of that circuit is grounded? That would cause problems, I think. The coax goes nowhere else, it should run from sensor, including sheathing, one piece of cable all the way to the ECU, and the shield grounded there and only there. Whatever you got, if it's different, it should be made right. Any evidence of a PO having altered that? That's a Triumph-Adler ECU; makes me think a PO had problems and replaced that too, or did you do that one?
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Please don't PM here, I will not reply.

Experience is death to doctrine.

[pushkick]

my guess is that there is no way the signal from the o2 would be affected by shielding problems in reference to engine warming up . if the signal had noise on it, it probably would be present during engine idle and warm up and startup . if you have a good o2 signal when engine is cold and when the engine warms up the signal goes to crap, my guess is the 02 sensor is not working properly. but first you must make sure your signal is good. do you have access to a graphing volt/ohm meter. if not just go get a piece of coax and temporairly wire it in and see if it solves your problem. i looked at my wiring and i seem to have coax from sensor plug to ecu. and the thing to check is do you have a good ground before the engine warms up. then look at the o2 sensor signal it should look something like a sawtooth wave. on a digital vom it should climb slowly up and then fall back to a low signal. i forget the period (lenght or freq) of the wave but you get the geneal idea. i had a sensor that was only a month old go bad.

[tencentlife]

Yeah, I think someone chopped Matt's coax because it was damaged, and from a wreck just cut out what was easy to get at in the engine bay, and used the VW male-male connector to patch them together, not knowing about the shielding issue. That's what I get from the pics, anyway.

I'm not clear on the white wire that's cut in the ECU conector pic. Is that the grounding leg coming off 19? I thought it was brown; maybe not.

But pushkick, noise on the line wouldn't have any effect on performance during warmup. Nothing on that line has any effect during warmup, because the ECU completely ignores that line until Temp2 crosses the warmed-up threshhold. It also ignores it during a predetermined restart interval (which is based on coolant temp). After the temp threshhold or the restart interval, its finally looking for the o2 signal. If the signal is noisy, who knows what trouble that will cause in the lambda response, but I feel it's a safe bet that there would be some erratic running involved. The RF is going to be most intense in the engine bay, since there are so many RF sources there, so the absense of shielding there is going to produce the most noise on the line.

I think Matt also said he's replaced the o2 sensor a couple times already. And testing it's output is one of the easiest things to do in any case. If they put out signal at all, they are never wrong so far as I know, only slower and slower to respond as they age. The freq of the sawtooth wave isn't predetermined, it's an artifact of the mixture response and the sensitivity (age-based) of the electron pump in the sensor. An old, slow sensor will have a lower and lower freq.

I think the simplest solution is just to take any old piece of coax, and set it up like it was intended, bypassing the existing coax at least temporarily, and see what it does.
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Please don't PM here, I will not reply.

Experience is death to doctrine.

[Mathew Zelezen]

First Picture= O2 sensor plug to coax
Second Picture= The splice at the firewall
Third Picture= The white wire (ground) that used to go to terminal 19
Fourth Picture= The ground I ran from terminal 19 to the body

So the splice is another PO "alteration" And I thought I found all of those already...

The PO had an idle problem and his mechanic replaced the ECU, Mass Air Flow Sensor, Idle Stabilizer Valve and apparently the coax to the O2 as well and still didn't solve the problem.

When he sold it me I bought a Bentley and was busy trying to locate the Idle Stabilizer Control Unit behind the taillight when I found that the little space was PACKED with acorns and mouse turds! He had a packrat problem and the little sucker chewed all the insulation off the wiring...

Thank you so much for taking the time to help me with this guys...

So can you buy coax that breaks into a signal and ground at the ends?

Cheers!
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.

[tencentlife]

Any piece of common coax cable has a signal core and a braided sheath under the outer insulation. The sheaths normally make contact with the outer ferrule of, say, an F-type TV connector, or whatever, the part that crimps over the cable. What you do is strip it down in layers, stepping back from the tip, so you have exposed the core, a short bit of the intermediate insulation, then the braid exposed for a short length, an inch or two is enough, and finally the outer covering. Take the braid, pull it off to one side and gather it up so it can be crimped or soldered into another conector or a piece of wire so you can reach a grounding point with it. Mind the stray braid wires, trim them off as needed to keep it from fraying and possibly contacting the core. It just has to be isolated from the core and connected to ground, at one point only.
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Shop for unique and useful Vanagon accessories at the Vanistan shop:
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also available at VanCafe.com!

Please don't PM here, I will not reply.

Experience is death to doctrine.

[Mathew Zelezen]

Thanks Tencent... I owe you some brownies.
_________________
87 Syncro Tintop
~Rocky J Motor
~Steedle Trans


Of all sad words of tongue or pen, the saddest are these: It might have been.