Need some help here guys... Sorry for the long post... This is killing my brain. MK4 GTI. I have a Vehicle Speed Sensor (VSS) "Implausible Signal" error (P0501) and my cruise control isn't working. Clutch pedal switch, brake switch, and parking brake switch all replaced and verified working. VCDS sees the cruise control as being activated and shows the right values when it's turned on, etc... so I know the ECM sees the cruise control. The wiring harness for the VSS is getting 12V correctly as measured by my power probe. My temperature gauge is also now working intermittently. I have to shut the car off and crank it again for the sensor to start reading. I'm assuming this is because shutting the car off and back on is resetting a circuit or relay somewhere? Also, as a side note - my e-brake light on my instrument cluster doesn't illuminate when the e-brake is engaged (parking brake switch circuit tested good for 12v as well). I pulled out my Bentley manual and these are the wiring diagrams for both the cruise control and the VSS. If I'm reading this correctly, I see that the VSS connects to B163, the interior wiring harness that leads to my fuse box by the driver's door. It also leads to J285, which looks like is in the instrument cluster (J285 = "Control module w/ indicator unit in instrument panel insert"). All my fuses have been checked and are good (Thanks Power Probe ) The diagram for the Cruise Control switch appears to lead back to connection A20, which is "Wire connection in instrument panel wiring harness"....which I'm interpreting to mean it's the instrument cluster wiring harness? Here's where I need some advice. It seems like that my VSS issue is what's preventing my cruise control from engaging (from what I can gather). If the Cruise control can't ascertain the vehicle's speed via the signal generated by the VSS, it simply will not engage, even if the other components are good. From looking at these diagrams, and given both my temperature gauge is acting funky and my e-brake light not working in my instrument cluster... I'm leaning towards a possible issue with my instrument cluster that's messing everything up. Thoughts? Am I interpreting these wiring diagrams correctly?
If the speedometer works on the cluster then you know the speed sensor in the gearbox is all good so we can rule that out. the VSS signal the fault code refers to is another output from the cluster to the ECU pin 54, this also feeds a few other circuits including the radio. On that note this is a good place to start if you have an aftermarket radio, make sure nothing on the radio is connected to the blue/white wire on the radio plug. Also as you say the faults you're seeing on the cluster may also be the cause of the VSS error. Regardless the e-brake light is a problem as you cant pass an mot if this isnt working so put that on your list of things to check. Have a look at the switch on the handbrake lever it should short to ground when lever is lifted. with dash sensor check the wiring to it for starter and look for any obvious damage in the harness, the purple and brown/white are the wires you're interested in, and they go thru the 14 pin plug next to the battery so have a look at that too
Many thanks man. I do have an aftermarket radio. I've only owned the car for about 6-8 weeks, so I'm still working through a shit ton of problems on it. I've gotten all the mechanical things fixed at least. It sat on jack stands for a solid month while I fixed all kinds of stuff.... Anyways..... The radio this dingleberry had installed isn't even secured properly... It wiggles around. Last time I wiggled it with the car idling in my driveway the other day trying to get it out, I heard an audible "pop". I suspect the 25A fuse is blown. I will check the wiring in the back to see if he's connected it to the blue/white connection.... I wouldn't be surprised... The radio's harness is literally spliced together and wrapped in a crappy electrical tape job. I'm just glad I got the car for cheap. I replaced the parking brake switch a couple weeks ago when I installed my Diselgeeks short shifter. I will see if I can located this 14 pin connector. I'm assuming it's the one right near the starter wiring harness? I heard from others that coolant migration into the wiring harness was a MK4 issue. The VSS wiring runs right under the coolant flange. The coolant flange on this car is leaking a bit, as evidenced by the yellow antifreeze on the trans housing below (yay). The black plastic housing the wires is more or less crumbling/missing, meaning the wires are more exposed than I'd like. Is it possible that coolant may have gotten onto the wires and caused damage? If so, where in the world should I look? The more I think about this, the more it feels like the entire wiring harness needs replaced, especially if coolant has damaged the wires. And I can't find a wiring harness to save my life anywhere online.
the 14 pin plug is squished between the battery and the inner wing, can just about see it with battery in place if the wiring looks overall rough then you can just get another, good time to install a coilpack repair harness as well as they're usually all cracked by now: https://www.awesomegti.com/shop-by-...m-replacement-for-cars-with-oil-level-sensor/
My VSS error went away this weekend when I replaced my G47 sensor (left front ABS wheel speed sensor). However, replacing the sensor didn't fix the error. I had to dig deeper. I know that the brake pad level sensor was functioning ok via my multimeter and by plugging the pad level sensor into a spare, worn out brake pad I keep around to test the pad-wear sensor with (yeah I'm weird). Also, VCDS was showing no issues with the pad level sensor. So I knew that the wiring for the pad level sensor was good. The ABS wheel speed sensor and the pad level sensor share the same wiring harness - the one that goes up the strut. So I knew that at least 1/2 of that harness was working, most likely ruling out a bad harness or break in a wire internally. My old ABS wheel speed sensor tested deader than a door nail. My new sensor tested perfectly of course (~ 1.1K Ohms resistance). So with the old sensor verified as bad, the new sensor verified as good, and a presumably intact wiring harness, I narrowed it down further to a possible broken wire close to the ABS sensor itself. I pulled back the protective boot on the wiring plug that goes into the sensor. Yup. Sure enough, there was a break in the black wire. It was *very* subtle!! I only noticed it because the black had a slight white mark - the kind when you bend a small copper wire too far and the plastic around the wire stretches and leaves a white "stretch mark". I tested the cable with my multimeter and wire clips... yep. Bad wire. I went to a local junkyard, and cut off a left front ABS harness from another MK4 and verified the wires were good via my multimeter. Then I simply spliced the good wiring harness from the junkyard in. Keep in mind.. I'm only talking about the ~ 2 inches or so ABS wheel speed sensor side, not the full harness (pad level sensor + the ABS sensor) I checked VCDS after I was done splicing to verify the sensor was being seen and working. It was. I cleared the codes, measured the sensor in VCDS again to verify. I shut the car off and back on to triple verify (the ECM reads the ABS sensor upon turning the key). Yup - sensor was working. My VSS error went away as well!! As explained to me on another forum, the ECM is comparing input from both the VSS and ABS system to determine the vehicle's actual speed. It's constantly comparing the values so that if the wheels begin to turn faster than the engine, ABS will engage (that's what what it's designed to do after all). When the ECM cannot communicate with any particular ABS sensor, it will throw an "implausible signal" message - and prevent cruise control from engaging. (I've yet to test the cruise control as the car is still on jacks while I repair the oxygen sensor harness..... poor car was neglected as hell before I got it) I hope this helps someone else who happens to stumble upon this post one day. Here's a great video on replacing the ABS sensor and using VCDS to verify the sensor is reading correctly:
nice work finding that short. reminds me of the time I was troubleshooting an indicator fault. wires looked fine visually but nothing. decided to re-terminate and found the copper had corroded away to dust for a good couple inches under the insulation
