Frank gets tangled up in a Skoda Fabia wiring conundrum, and he finds that when one door closes, a window opens…

Most of the problems I encounter in my current role are of a simple nature and are often solved with a scan tool or a digital multi meter (DMM). I should point out that the DMM I use is an extremely high-spec example. However, most of the tests I rely on can be conducted with a much more basic tool.
    
An interesting problem arrived in the guise of a Skoda Fabia 1.2 tsi. The vehicle was known to me due to a previous repair to the driver’s window module (J386). On that occasion, a broken power supply in the A post loom was responsible. I should add the passenger window was also faulty, but the owner did not authorise further work. More on this story later.
    
On this occasion the vehicle was towed in as a non-runner. As often happens, preliminary tests and swap outs had been carried out on my two days off. So, Tuesday AM at the handover it was explained that there was no fuel pump operation, and a new genuine pump had been fitted, without effect.

Plan
I attached our TOPDON scan tool and conducted a global system scan. My attention was drawn to an absence of the PCM on the HC CAN network. This directed me to scope the network and check at the PCM and power control module. The network was good, with correct voltage 1.5/2.5 and 2.5/3.5 with clean transition (rise time).
    
So, this was my plan; Simplistically a PCM requires, power, ground, triggers, and outputs to ignition, injectors, and fuel pump to stand a chance of running the engine. Now reverse that logic - Why? It quickly establishes what is missing. We knew the fuel pump was not working, and due to excellent engine access, we quickly established no outputs. It was starting to look like terminal PCM failure.
    
At this point we informed the owner of cost. Despite the vehicle being in excellent condition, he indicated his wish to scrap the vehicle. We offered to purchase the vehicle at scrap valuation of £250 with an online purchase valuation of £2,500. So, we now own the vehicle with no concern for diagnostic or repair cost.

Simple path
Following my logic plan, I progressed on to check the power and grounds to the PCM. Taking a simple path, which I guarantee will get more complicated later, I began checking the fuse panel in the offside dash panel. Using the fuse guide sticker, I quickly established no voltage at several fuses. Locations; SB7 and SB8; SB28 through SB31. SB7/SB8 are both powered from the Simos (PCM) current supply relay J363, which in turn is supplied via J17 fuel pump relay. The J17 relay is controlled via an ignition pin, 15 positive, with ground via Simos pin 26. Please refer to Fig.1. So far, so good. We knew the PCM was not powered up and we knew the original suspect, the fuel pump, was also not working.
    
My attention was then drawn to the correct function of the J17 / J363 relays. Anyone out there with experience of the relay panel on this 2011 model will know it leaves almost no access for relay removal let alone testing in situ. If you take a close look at Fig.2, which shows the power control module, you will notice I have cut out a large section of the lower dash panel which is covered from sight by the fuse panel cover. Game on! Both relays were fully functional when tested off-car.

Steady on
I now focused on the fuel pump relay control. The ignition supply pin 15 was present, but was also present on relay pin 31, which should be at ground to latch the relay. The ground was latched via Simos PCM pin 26. I stripped out the PCM and exposed the wiring armed with my trusty DMM check pin 26 which was positive. Please refer to Fig.3. So, I provided a temporary permanent ground. Please do not do this unless your confident in your knowledge of current flow schematics. Returning to the fuse panel, sb7/8/28/29/30/31 were now all live. The vehicle cranked and started promptly.
  
 So, the fault was a switched ground failure on pin 26. My time extended to five hours including gaining component access and modifying the dash panel, with a new PCM probably costing £900. You may recall I explained the fuel pump relay is latched from an ignition switched supply. If I provide a permanent ground to pin 26 the fault will be fixed, and the battery will not go flat as the relay is switched off with the ignition. I love jobs like this.
    
I was getting almost giddy with the excitement. “Steady on” I said to myself, “there is still the passenger power window to fix!”
    
Removing the door card and exposing the loom through the door aperture confirmed the wiring was in good condition, unlike what I found in the previous window repair. Obtaining the current schematics for the power control module, which was still exposed from the previous PCM repair, I quickly established no power supply to the window control module j387, theoretically via fuse 51/52, here we go again!
    
Fuse 52 is N/S front but was not wired in the panel. Not fancying several hours more loom searching, I took the entire current flow schematics home, and sat quietly studying them. Sad really, however I did notice my vodka glass was always full!
    
My new plan for the following morning involved strong coffee, and reversing the problem, conducting a continuity test from the window module power supply, working through all fuses rated at 25 amps. To my surprise there was a missing fuse location 25 with continuity to j387. When fused up at 25a, both windows now operated.
    
The real lesson here may be this: Do not rely on what you expect without testing and, where necessary, go fishing. Of course, I will never find out the answer to the ultimate question from this job: Who took the fuse out and why?