Mk5 R32 2007 wiring woes

[iooroiidvd]

Hi. I'm a total amateur when it comes to vehicle wiring and have pushed through my limits a couple of times already, hopefully not breaking anything so far, fingers crossed anyway

MAF stopped working, figured out there was no 12v feed, traced an issue back to (I thought) a relay, new relay arrives, same problem

Tested the relay "terminals?" and found that 85 "port?" which I believe should be earth was live with approx 3.5v

Pulled the fuse box off and tested ports to pins on the back for continuity and everything checks (everything triggering as expected and nothing triggering that shouldn't) out so I think the fuse box is OK but now I am well stuck because my ability to interpret the wiring diagrams has reached it's limit.

Not sure how to progress, any help, guidance or advice would be very appreciated

[iooroiidvd]

Actually after a bit more study of the wiring diagrams I don't understand the bits in red.

That might of course be my not understanding the diagrams or something that's actually wrong?

Fuse box relay slot	Voltage	Expected pin continuity	Tested pin continuity
85 control ground	3.5v	26/2	26/2
86 control positive	12v	26/5	26/5, 26/7
30 supply positive	12v	Fuse 51 on fuse holder B?	40/18, 40/23, 40/27, 40/31, 40/35
87 output	0v	26/6, 26/14, 26/19, 40/33	26/6, 26/14, 26/19, 40/33

[iooroiidvd]

So on further reflection my thinking is to ignore the unknown, if the problem was in the fusebox there would be some continuity between port 85 and one of the other pins?

[Crasher]

I need to be at work to study the wiring diagrams, over the years I have seen a lot of BUB engines with degraded wiring harnesses, especially in the section from box to chassis leg. Which fault codes are you getting?

[iooroiidvd]

Oh, sorry Crasher, I came back to put an update and just saw your post, I didn't get a notification

After pulling half the car apart and testing everything all the way back to the ECU connector PIN I had a quick chat with someone who actually knows what they are talking about and had a full on DOH! moment.

The Mk5 R32 only powers the MAF for a couple of seconds after the ignition is switched on because it's the same relay that routes power to the fuel pump, tested that, it worked OK. Put everything back together and everything is working again

I'll post my learnings here in case any of it is useful to anyone else in a similar spot the future.

I'll strongly caveat this with the fact that if it's not entirely obvious I am not qualified or an expert in any way so treat all of this with the caution it deserves and do your own research to validate and verify.

Fuse 1 in the dashboard fuse panel is ignition switched power to the coil side (86) relay (167) in the engine fusebox.

That relay is earthed (85) by an ecu switched pin 65, the ecu being located in the plenum under the windscreen.

In Mk5 R32’s this relay will only power initially for a couple of seconds on door open or ignition being turned on because it also provides power to the fuel pump.

The switch side of the relay goes out through pin 87 to fuse ten (10A) in the same engine fusebox and should provide ~12v to socket 2 in the MAF connector.