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Sorry for my earlier post, I was kind of tired and not looking at things correctly. The FCU is relay G16, and takes the pulse signal and converts it to analog voltage for the speedo. Not sure about how things are labeled, but I was wondering if the TDM/TOM? is the speedo in the cluster.
No worries, my friend, all brainstorming appreciated.
Any idea what kind of signal I'm looking for going in and out of the FCU? The sig coming out of the ABS sensor and going into the ABS control unit is an AC voltage; I've detected that signal on the sensor itself with a VOM. I'm presuming that it's still AC going out of the ABS CU.
The FCU relay itself is labeled as follows:
Pin 2 = 15. Traditionally, the 15 circuit in VAG cars is keyed power
Pin 5 = Ausg. "Aus" is the German word for "out" so I'm guessing that's the signal out. What am I measuring for here, with the wheel turning and the key on?
Pin 6 = Eing. "Ein" is the German word for "in" so I'm guessing that's the signal in, and I'm presuming it's the same VAC as from the sensor itself
Pin 7 = 31. Traditionally, the 31 circuit in VAG cars is chassis ground
I may just go straight to the FCU and see what's going on there.
An interesting, possibly related, sidebar is that when I was troubleshooting this problem I noted that the ABS relay was not cycling on with the key. Troubleshooting noted that the relay was not getting keyed power. Per my fav diagrams, the power sides of the brake light switch and ABS relay use the same source (brake lights were working fine). I pulled up the relay and its wiring mass and saw everything going into a big factory-taped-up harness and said "eff that" and ran a bypass wire from the power side brake light switch to the 15 wire at the bottom of the ABS relay and then it worked fine.
So that gives me worry. I'll first check for keyed power at the FCU and if I get none then I may have found my culprit. And if so, it implies a bigger problem with the relay and wiring harness. Which is not good.
Think good thoughts for me today... - GA
Last edited by GregAmy; 05-27-2024 at 12:11 PM.
Reason: typo
Additional note: I bought the Durametric tool and the ECU is not seeing a speed signal. I've yet to dive into my favorite diagrams to troubleshoot that.
I think I'm going to shoot myself...at a minimum this is a "eff me and the PO" moment...
I'm digging under the dash, passenger side, checking wires and I see something else odd...a cut wire harness. Um, wut? I look closer, note the wire colors, and go to my fav diagrams...and note the colors correlate exactly to T40, the plug between the ABS CU and pretty much all the stuff that's not working right now (location F96 in the diagram for y'all following along at home). You know, like the ABS relay power wire that I bypassed, as well as the ABS CU signal to the FCU? Those wires?
On top of that, the harness was cut so close that I'm struggling to try and get to it to repair it. Eff me.
Say a prayer to your favorite deity for my knuckles and patience.
Durametric shows the ECU is now seeing a speed signal.
Snipping my ABS relay jumper wire showed that was the prob to being with.
Cruise doesn't work. That's a job for another day.
Odometer doesn't work, but we know what that probably is (ditto, for another day. I've got some LEDs onbound to make the dash brighter so we'll do those together). Where's the best place to get replacement gears?
"Why"? I texted the PO and he said he was trying to clip some wires under there to extinguish the airbag light (it has a Momo wheel). Something he read on the Internet. I'd already fixed that light with a resistor and a Durametric reset.
This process was painful. Worst part is this thread will be helpful to exactly no one going forward. But thanks for listening to my angst, as putting this in writing really helps with my sanity.
Yuuuup. Probably the smallest $13 part that FCP Euro sells, and they had it in stock.
We now have odo-age.
Now, about that cruise control...
In case you haven’t heard, NEVER reset your trip ODO while the car is in motion. I don’t know if it affects our Speedo’s like it does older VDOs, but it almost always breaks those.
In case you haven’t heard, NEVER reset your trip ODO while the car is in motion. I don’t know if it affects our Speedo’s like it does older VDOs, but it almost always breaks those.
That's a myth, resetting can cause a dying gear to fail regardless of moving or stopped.
05-27-2024, 12:05 PM
