Brake signal implausible

After 1500 miles I'm finally getting around to hooking up cruise in my MK1 ALH swap.

It's had a brake switch trouble code accompanied with a flashing glow plug light forever. Brake lights functioned properly, and I could find no issues caused by the code, so I just ignored it.

16955
P0571 - 35-00* - Implausible Signal

Today I discovered the cause, or rather a symptom, probably not the root cause. The ECU thinks the brake pedal is depressed. I confirmed with VCDS that the cruise stalk is functioning properly, but cruise cannot be activated due to the phantom brake that the ECU sees.

T1 constant 12v
T2 dead with ignition off, constant 12 with ignition on
T3 dead with ignition off, 12v with brake pedal resting, dead with brake pedal depressed
T4 powered with brake pedal depressed

Wire 1 is obviously the power supply. Wire 4 is obviously brake light controller. Wire 3 seems to interact with the ECU, and it's behaviour would makee sense. The behaviour of wire 2 seems wrong. It powers up with the ignition, and seemingly has no interaction with the brake switch.

Can anyone explain a little more about the function and final destination of wires 2 and 3?

I should be able to look at a Bentley in a few days, but don't have access at this moment.

Had a chance to peek at a Chilton manual. Wire 1 comes from brake light fuse, connects internally through the switch to power wire 4, connected to the brake lights. Wire 2 should see power when the ECU does, and it indeed does. Wire 3 should connect to ECU pin 65, and it does.

The only issue I'm seeing on the wiring side is 5 ohm resistance between wire 3 at the brake light switch and pin 65 at the ECU. Could the 5 ohms be throwing the ECU off by sending improper voltage readings?

Wire 3 loses voltage when the brake pedal is pressed. Is this proper operation, or indicative of a faulty switch? Is the brake switch normally open or normally closed?

Laserface said:

Had a chance to peek at a Chilton manual. Wire 1 comes from brake light fuse, connects internally through the switch to power wire 4, connected to the brake lights. Wire 2 should see power when the ECU does, and it indeed does. Wire 3 should connect to ECU pin 65, and it does.
The only issue I'm seeing on the wiring side is 5 ohm resistance between wire 3 at the brake light switch and pin 65 at the ECU. Could the 5 ohms be throwing the ECU off by sending improper voltage readings?
Wire 3 loses voltage when the brake pedal is pressed. Is this proper operation, or indicative of a faulty switch? Is the brake switch normally open or normally closed?

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Use a relay or a MOSFET to switch the brake pedal and brake light wires at the same time. I have it written up here in a different thread on exactly how to do that.

Hasenwerk said:

Use a relay or a MOSFET to switch the brake pedal and brake light wires at the same time. I have it written up here in a different thread on exactly how to do that.

Click to expand...

David I did a search and caint find your write up. Im hooking my brake switch for cruise using the relay method and caint seem to get it correct. I think maby my relay is to slow.

Motohead1 said:

David I did a search and caint find your write up. Im hooking my brake switch for cruise using the relay method and caint seem to get it correct. I think maby my relay is to slow.

Click to expand...

http://fastforward.ca/signalprocessors/ has both the brake pedal synchronizer and the full CCS logic swapper if you have a older vehicle that is backwards to the 2000+ logic that VW / Audi uses.

This is the documentation for the brake pedal sync https://docs.google.com/document/d/1AAEOqZc0bF3BRmuuRA63q6g0h6m_k3P6W_xbvFo3X0g

Thanks!