Pesky ABS 4340 Error

2001 Carrerra 4, w/ Tip I've been chasing a P4340 "Brake Switch a" error for a long time now. As you can see from the picture, I have the interior ripped out so I can access and inspect all the wires. Both brake switch circuits appear to be working at the DME according to a scanner. The actual brake light circuit (Brake Switch 1) also correctly reads out at the ABS unit. But, on the scanner, the brake pedal shows as not actuated. The brake pressure signal changes when I press the pedal. With the ignition off, I removed the brake switch, and connected the normally-open side (Switch 1) and normally closed sides through a 10 ohm resistor and 20 Ohm resistor to ground, respectively. I removed connector X2/4, and measured 10 Ohms (Switch 1) to ground on pin 4. That's good, but just verifies what the DME was already telling me.. I removed connector X2/5, and measured 20 Ohms (Switch 2) to ground on pin 20. " " " I measured 10 Ohms at Pin 14 of the ABS connector (Switch 1), which verifies what the scanner says. Switch 2 doesn't appear to go to the ABs on the schematic. I believe the PSM connector is the white one near the ABS in the luggage compartment. I haven't had as much luck verifying the signals to the PSM and I'm not 100% sure I have the right connector. It's supposed to have both switch signals, but I only see the black and orange Stop 1 signal going to it. But, I'm also wondering if the brake pedal value int he ABS is derived from something other than the switch - like a pressure threshold or some such. Any ideas?

I believe there is indeed a brake pressure sensor that senses master cilinder pressure. Perhaps an AI-query for Bosch ESP 5.7 yields further insight.

I followed the procedure for the 5.7 ABS / PSM unit and it says the ABS / PSM controller is bad. But, on the first page, it says that the problem could be that the pressure sensor might be implausible. That'd be way cheaper and easier if that were the case. I just have to figure out the test procedure for that one.

The ABS is connected to both sides of the brake light switch. Have you tested the 2 pins on the ABS harness as the diag steps suggest? Also there's no PSM connector, the psm is integrated into the ABS module. There is a pressure switch, it has a 3 pin plug and is probably located on the ABS precharge pump. The ABS precharge pump is wired to the ABS with a heavy 2 conductor wire. The pressure switch has its own set of codes so is probably not the problem. The brake light switch is self adjusting, sort of. It has a ratcheting plunger. To readjust it push the brake pedal down, pull the plunger out all the way, release the brake pedal. I have also seen LED brake light bulbs mess up the ABS reading the brake light switch. You can read the two brake light switch inputs to the ABS with a PIWIS, possibly with a Durametric.

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It took me a while to realize that the PSM and ABS were integrated together. There were a lot of changes in 2001 and the C4 adds complications. A kind soul sent me the docs that covered 200 specifically informing the schematics. I was able to measure both signals at the ABS/PSM connector at 32 and 37, both with the connector disconnected and connected. I saw two signals that were inverted from each other as they should be. However, every scan tool I have days the stop light switch is working but the brake pedal isn't actuated. The brake pressure readout in the scanners shows negligible pressure until I press the pedal, then it increases depending on how hard I press. I have seen some weird reading come and go quickly, but I didn't know if that's the sensor or something squirrelt with the ODB2 system. Maybe I should just replace the sensor to see what happens? I did readjust the new brake sensor to see if that has any impact. TBD

If the signals at 32 & 37 don't invert - the one at 12V must go to 0V and vice versa - when you press the brake pedal then the brake light switch is bad or not adjusted properly.

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The first switch I put in wasn't OEM. It worked fine until it started sticking. Then I ordered an OEM and could NOT get it to latch in. So, I went back to the non-OEM, and currently am back to the OEM which I finally managed to get seated. I haven't driven it since I reinstalled it, so maybe a miracle will happen! I'm tempted to replace the pressure switch if not, as I did see some weird readings over ODB2. Those could have been false flags though One weird thing which probably doesn't affect it: I noticed that the model is listed as a 99 in the coding instead of a 2001. I tried to recode it, but didn't have the passcode. It is correctly reading the 5.7 controller, so I doubt the wrong year is messing it up. But I guess it could.

The ABS Pressure sensor is not available as a spare part. You need to buy a donor ABS unit or precharge pump and salvage the pressure sensor. The pressure sensor is very unlikely to be the problem as it is very reliable. If there are odd readings this can be fixed by unplugging and replugging the connector a few times to break up any slight corrosion on the contacts.

The Brake Pressure Sensor MUST NOT rise in value "before" the Brake Light Switch signal is received.. IF the Brake Pressure Sensor rises in value " after" the Brake Light Switch signal is received then it will not set code 4340.. This is #4 in the possible causes list,, but the test procedures in the OEM diagnostic Chart leave a lot to be desired on this one.....and it has caused many many people to unnecessarily relaced the ABS Unit...

Thanks for the help everyone! I measured the voltage output from the pressure sensor with the car running. Pedal up: 0.61 V Pedal down: up to 1.6 depending on pedal pressure. I didn't see any spurious readings on my meter, so either simply unplugging and replugging the connector scraped the contacts OR the fluctuating reading on the scanner was bogus. I'll be driving soon to try and gather live data on the bigger issue: the TipTronic going into Limp after warming up. Until it warms up, it drives normally. I've checked and rechecked the fluid level when running at the property temp range.

I'm hesitant to tempt fate, but I might have finally got that brake switch seated correctly. I'm not sure how many attempts I've made but I am NOT taking that switch out again!