TDI Brake Pedal Signal

[trentjim]

Thought I should perhaps start a new thread as I seem to have a specific problem to solve.
van starts - ignores accelerator pedal - idles at around 1200 rpm - glowplug light starts flashing after 20 or so seconds
VCDS of group 6 block 2 shows brake sensor stuck on? and curiously, each time the brake pedal is pressed (watch block 2 as 3rd digit changes to a 1) the coolant temp sensor and fuel temp sensor values change. video below...
https://goo.gl/photos/Yb1LKoq3rboL61Ji8
can anyone help me interpret this, I presuming that this might help me narrow down the zone of failure...

[trentjim]

Bit of an update... took a while but I eventually tracked down the yellow/white wire for the brake switch to an anonymous relay in the engine compartment. Looks like both this wire and the red/white of the clutch sensor are disconnected from the blue/brown sensor ground wire when the relay sees 12v from the brake circuit. Ok so far...
Now for the question....from the weird behaviour of the sensors noted above and the lack of response from the switches I conclude that my sensor ground is insufficiently grounded. Find 1090 ohms resistance between sensor ground and chassis. Just about to run a supplementary earth from the sensor ground at the relay to the chassis and I had a moment of doubt. Is 'sensor ground' synonymous with 'vehicle ground' or to put it another way, does the earthing have to happen in a controlled way through the ecu or is it ok to ground sensors elsewhere if convenient?
Cheers in advance for any opinions on this,
Jim

[syncroandy]

Sender earth is 'floating', not connected to chassis earth. On a 1Z harness:

ECU F36 clutch switch input is normally connected to sender earth then when the clutch pedal is pressed it goes open circuit.

ECU F brake signal input is normally earthed (via the lamp filaments) then goes to 12v when the brake pedal is pressed.

ECU F47 brake signal input is normally connected to sender earth then when the brake pedal is pressed it goes open circuit.

(Note to reader: I DID NOT do this guys harness !)

[trentjim]

Thanks Andy,
So in terms of faultfinding the ground cable what should its voltage be relative to the neg battery terminal. I'm getting over 5 volts when the ecu is on. Which intuitively seems high for a ground voltage. I presume it is the f47 connection to a sender earth of 5v+ means the ecu sees it not connected to ground.
I've traced the ecu earth lug to the chassis, where another 5 brown earth wires share continuity and the same earth point on the chassis. Have detached, cleaned and reconnected these.
I have a feeling that I'm going to need to pick the loom and connector plug apart.
Does anyone know a source listing all the expected voltages at the sensors?
Cheers,
Jim

[ELVIS]

Pretty sure I PMd you the other day chap.

Just in case......

When I bought the van the clutch and brake peddle switched were hanging in the engine bay!

Can put you in contact with the chap who put the relay etc in if needs be. Quite bizarrely the reason I didn't sort the wiring was because I fitted two of Andys looms back to back and didn't have the time. Lol

[syncroandy]

Forget about voltages, as I said, sender earth is 'floating'. If you wish to verify the connections, unplug the ECU and use a continuity tester to check the connection from pin T68/33. Take care not to damage the plug contact with the tester probe. Senders using sender earth:

G28 crank sender
G70 MAF
G72 intake air temp
F36 clutch pedal switch
F47 brake pedal switch
G81 fuel temp sender (in pump)
G79 gaspedal
G62 engine temp

If using a relay to provide the F47 signal, wire it as below, with the brake signal 'F' also going to ECU pin T68/44.

Thanks Andy,
So in terms of faultfinding the ground cable what should its voltage be relative to the neg battery terminal. I'm getting over 5 volts when the ecu is on. Which intuitively seems high for a ground voltage. I presume it is the f47 connection to a sender earth of 5v+ means the ecu sees it not connected to ground.
I've traced the ecu earth lug to the chassis, where another 5 brown earth wires share continuity and the same earth point on the chassis. Have detached, cleaned and reconnected these.
I have a feeling that I'm going to need to pick the loom and connector plug apart.
Does anyone know a source listing all the expected voltages at the sensors?
Cheers,
Jim

[trentjim]

Thanks Andy/Elvis
I dont really think there is anything wrong with the wiring or the relay, it all checks out fine

To clarify where I am...

verified that the relay is correctly wired
verified that the relay operates properly (bench test)
verified that the relay operates properly (back probed)
verified continuity from pin 33 on sensor multiplug to relay spade of paired brown/black wires (0.5 ohm)
verified continuity from pin 17 & pin 20 of sensor multiplug to relay spade shared by white/red & white/yellow(0.5 ohm)
removed, cleaned & reattached loom earths to chassis.
note that with relay removed (brake/clutch sensor open) temp sensors read -29.7 and with relay inserted (brake/clutch connected to sender earth) fuel temp sensor reads -5.4. press pedal (break connection) & it returns to -29.7.

Scratch my head as there doesnt seem to be a problem with the relay or connections to the multiplug. less certain of bad earth, wonder about voltage leaking from live side?

remove the blue/brown spade and the white/red & white yellow spade from the relay holder
measure 5.5v between blue/brown pair & neg battery post.
measure 13.8v between white/red/yellow pair & neg battery post
measure 6.4 v between the spades
wonder what these voltages actually should be?
connect the spades and measure 6.5v between connected spades and neg battery post.
ecu still shows 1,1,0 in measuring block.

what i would like to know is
how to tell if the multiplug socket 33 is actually making a good connection to its ecu pin
how to tell if the ecu is providing a suitable earth to pin 33
the wiring diagram shows all the sensor earths connecting to connector 220 - "earth connector (sender earths) in engine wiring harness", but does connector 220 have its own earth? with pin33 reading the voltage, or does the earthing only happen via this pin of the ecu?

or am i barking up the wrong tree & the 13.8 on the sensor wires is telling me I am short to a switched live somewhere in the loom?

Bit stuck at the minute...
cheers,
James

[syncroandy]

Your F36 clutch switch input is wired to your brake signal relay and you think there's not a problem ?

I'll get my coat..

[davegsm82]

If I may chime in...

My switches are dangling about in the back of the van, but I've taped both of them down to simulate both clutch and brake not being pressed.

From what I understand, they are only really used for the cruise control and also a warning light (just leave the brake sensor wire to the ECU disconnected) to indicate when all brake light bulbs have blown or been removed. Also if I remember correctly the ECU will rein in the power if the clutch is pressed to prevent tearing the clutch to bits.

There is a minor fault with the throttle pedal on my bus, it's nothing major but does cause a lack of throttle response until you jab the throttle a couple of times after starting. Doesn't flag a MIL but does log a 'improbable signal' code in the ECU.

Dave.

E D I T: Also, regarding the sensor earth, I had trouble with mine but can't remember the outcome, but I seem to remember they are completely separate, however there should be good continuity from sensor earth to battery -ve, i.e. in the region of less than a couple of ohms when the plug is connected to the ECU.

[syncroandy]

WHAT ????

I wouldn't consider driving a TDI with non-functioning brake inputs to the ECU, they are part of the safety-related design features of the TDI system.

Also any fault on the pedal sender should not be tolerated, this is drive-by-wire folks, if the computer thinks you've got your foot to the metal when you're actually approching a red light, and there no functioning brake input, you are going to have a very bad day !

I would strongly advise anyone else reading these posts not to cut any corners in regard to the pedal ECU inputs.

[davegsm82]

Don't worry, it's not permanent. I just haven't got round to mounting the switches yet, I'm not going down the relay-route. As for the throttle problem, I've scoped it out and it's a signal drop-out from the wiper, no chance of mistaken foot-down full throttle action. That's also what the switches incorporated in the pot are for.

I trust my ignition wiring to make sure that bad day never happens too

Dave.

[trentjim]

Sorry Andy, the clutch sensor isn't connected to the relay...
As things stand now the clutch sensor &brake sensor are both directly connected to the signal ground.
This should give me 0,0,0 in the measurement blocks, and 0,0,1 when the brake pedal depressed.
0,0,0, should mean my throttle gets listened to
I get 1,1,0 and 1,1,1 when pedal depressed...


Dave... Thanks for that... dig deep man... remember... lol