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I'm trying to figure out what's involved with removing and replacing the engine wiring harness on our '02 diesel Jetta. The harness appears to be available as a spare part and you can find them on Ebay, so they clearly can be removed by a (semi) sane person.

However, looking at the harnesses on Ebay, and following the path of the harness from the ECM in the center of the air plenum below the windshield through to the engine compartment, makes me think that some finesse is required. The Bentley manual is silent on the subject and while YouTube has lots of videos on replacing and repairing the glow plug harness I haven't found anything on the rest of the engine harness.

Any suggestions for better sources of documentation (is this covered in the real factory service manual) or general tips on tackling this project?

  • Not sure this question makes sense. VW Jetta ? or was that a Bentley or Glow Plug Harness? Huh? What, exactly what is your goal here? What is wrong, and what are you trying to fix? – zipzit Jan 15 '17 at 5:54
  • @zipzit, fair enough. Car is an '02 Jetta, service manual is published by Bentley Publishers, in "our fair city, Cambridge, MA." My goal is to remove the entire engine control wiring harness from the ECM out to the various components on the engine. The problem is a changing cast of DTCs all of which seem to point to a wiring problem (mostly intermittent opens or shorts to ground). Not blowing any fuses… – dlu Jan 15 '17 at 6:00
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I'm biased based on my experience, but perhaps it might be helpful. I spent time in the Army troubleshooting (and teaching mechanics to troubleshoot) vehicle electrical systems. After that job I worked for a US automotive manufacturer, first as a design engineer and then as a new model launch person, including countless hours spent troubleshooting automotive product problems.

My focus on repair is: 1) understand the root cause of what is wrong. 2) Fix the thing permanently, to a high quality standard. 3) spend as little money out of pocket as possible. Note: I didn't say perform the repair quickly so I can get on the road tomorrow, because I need the car to get to work. That isn't the type of work I do.

The biggest causes for concern on wiring:

  • Pinouts (this occurs when a single electrical connector isn't fully seated within its plastic hardshell connector.)
  • Poor crimps at connectors.
  • Electrical connectors plugged into the wrong hole within a plastic hardshell connector. Generally this is pretty difficult to do, but possible if someone made a repair to a harness and swapped two circuits by mistake. Modern harnesses are tested pretty carefully for this at point of manufacture.
  • Burnt wiring (generally related to proximity to engine exhaust)
  • Abraded wiring (where things move across a sharp metal edge...) Remember your engine is mounted on isolation mounts, and it has to move. Also, there is a lot of vibration involved. This too can cause wiring insulation to get cut.
  • I've also seen corrosion within a hardshell joint, generally caused by water.
  • I've also seen Transistor to Transistor Logic (TTL) circuits disrupted by water on a connector during a rainstorm. (This was way difficult to troubleshoot!) Remember not all portions of your vehicle may be designed to be dry.
  • Burnt out internal fuse within a wire assembly.
  • In one case I heard of somebody sabotaging a wire harness by placing a pin thru a number of circuits in the middle of the harness. I didn't see this personally but heard about it from a mechanic I was working with.
  • Connections to ground can be problematic. NEVER use simple sheet metal screw to thin sheet metal for ground. You need multiple threads of engagement to ensure reliable ground over time.

I have seen two issues on Volkswagen products personally that were disappointing. In one case a wiring harness to a ignition controller was located in a wet area of the car. This caused corrosion within the male/female electrical connector joint. In another case I saw a riveted connection (buss bar to buss bar) within a fuse block assembly that didn't pass current. I was quite surprised by this one. The fix was to toss the entire fuse box. Note: these were a long time ago.. nothing recent.

With that said, I would probably NEVER purchase an entirely new harness as a replacement for a frustrating problem. Instead what I would do is this.

  • purchase a factory wiring diagram and / or repair manual. You absolutely will need to know each pin callout to verify the harness is built correctly. This may cost $300 of so, depending on your manufacturer.

  • Carefully remove the wire harness from the vehicle.

  • Lay the harness out on a clean workbench, and using the wiring schematic, verify the function of each and every circuit in the harness. Inspect for burn marks, abrasion marks and full seated pins / sockets in each and every hardshell connector.

  • Verify the function of each circuit with a Volt Ohm Meter. Verify continuity and verify there are no grounds between circuits (remember the pin placed in the harness issue?) Verify pin A on one end goes to wire A on the other end, then verify that Pin A continuity doesn't go to pin B, pin C, pin D... pin z. (and repeat)

  • Plan on spending a considerable amount of time on the task.

The good news here is that when you find the defect, you will absolutely know it. You will know EXACTLY what went wrong and how you can prevent it from occurring again in the future.

Good luck with it. Let us know results.

One thing, when I mention "Plastic Hardshell" I'm talking about things:

plastic hardshell connectors

Additional Edit: One thing to consider. I suspect that your car was fully functional when new. It obviously is having issues now. So that means something changed between new and now. Electrical systems in a perfect environment don't normally "wear out". What makes things go bad over time is:

  • Moving parts (anode/cathode contacts in a circuit breaker wear out. Wire gets worn on sharp edge during engine vibration. Things that move wearing out other parts of the system.)
  • Corrosion. (Water and all things electric is just bad. Normally when things are dry they won't corrode. Look for water getting into hardshell connectors.)
  • Heat.
    • Proximity to exhaust pipe?
    • High voltage discharge? -- we're talking spark plug wires here... these wires can 'short' out over time if the insulation gets burnt.
    • High temperature? I've seen component failure due to very high temperatures... think Las Vegas in the summer, and then running engine while low on coolant. I worked on my son's Mustang that nearly drove me crazy. The car just wouldn't get spark. Turns out the insulated-gate bipolar transistor (IGBT) chips in the Powertrain Control Module (PCM) were dead. The PCM module would send messages just fine, but no spark. Took me way too long to figure out that one.

Focus your investigation on these causes for troubles. And based on what you've said in your comments, I'd start first looking for corrosion issues related to water leaks.

  • Interesting. I've been pouring over the factory (I think) wiring diagram in the Bentley manual and another from @Ben looking for common elements – so far I've found that most of the DTCs (for intermittent opens or shorts to ground) share a common hot wire and that the they terminate on the ECM in a cluster of 3 adjacent pins. I'm getting optimistic. – dlu Jan 15 '17 at 6:46
  • Say, would you happen to know how the ECM diagnoses an "intermittent open or short to ground?" The elements involved are all the coils of solenoids, so when the control pin on the ECM was inactive the ECM firmware could be checking for something like +12 V on the pin, so the DTC could mean that the relay supplying power was flakey (or the harness was bad). – dlu Jan 15 '17 at 6:51
  • With that description, why are you limiting yourself to just the engine harness? I would think the power supply loom (which surrounds left and right fenders and front grill area, all the way back to the Engine Control Module (ECM) are suspect as well. Note: remember that inside the A-Pillar area is a drainage area for water.. Does this car have a sunroof? Guess where the front sunroof drain is routed? Does this car have its inner plastic fenders mounted correctly? – zipzit Jan 15 '17 at 7:07
  • Yes it has a sunroof and has had leaks at the driver's side A-pillar (top end was loose). I think the inner fender liners are correctly mounted (they are there and have all of their screws - I pulled and replaced them when I put in a lift kit). I'd been focusing on the engine harness because all of the affected signals were in it and I'd found damage on the portion running along the firewall. But seeing how power is supplied to the affected components I think you're right about the power supply loom. – dlu Jan 15 '17 at 7:30
  • When you say top end was loose are you talking about the sunroof corner drain hose being loose? Fender liners matter because of what happens when driving in wet weather. There is a huge amount of water thrown off the tires. If that water gets into the electrical stuff on top of the fender or within the A-pillar area, that is bad. Note: On your car, I'd start my journey with anything in the path of that leaky sun roof, pull the hardshell connectors apart, and inspect carefully for corrosion. Let us know what you find! see my answer edit.... – zipzit Jan 15 '17 at 19:15

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