Brake booster woes

[Stonster]

So following my previous post about a sulky stop start 

I had the codes checked when the car was serviced yesterday.  I've got two error codes:

4649 & 4655, both brake booster pressure sensor

 

The guy at the garage said there are two brake booster sensors, one on the brake line and one in the ABS module.   The first can be replaced separately and the second needs a new ABS module.  Yikes!

 

He suggested probably just live with a non functional stop start, as you can't tell from the codes which sensor is at fault.  He also suggested that it doesn't affect the brakes, just the stop-start

 

Anyone had this issue and got anywhere with it?

[KenONeill]

No, but I know some people who think that an error code who's only practical effect is to disable "stop/start" is a good thing!

[Stonster]

Further on this, anyone know where the brake booster pressure sensor lives?  Part number 036906051G or 036906051C, I think...

[Breezy_Pete]

Item 4 here fella:

https://skoda.7zap.com/en/cz/citigo/cit/2012-727/6/611-611070/

In the brake servo vacuum hose.

[Stonster]

Thanks... Now just to work out where that corresponds to on the car! 🤔

[Breezy_Pete]

Brake servo is a big round black thing (usually) up against the back of the engine bay.  Just behind the brake fluid reservoir.

Edited 24 September, 2019 by Wino

[Breezy_Pete]

The pink arrow is pointing to the vac hose you're looking for:

 

[Stonster]

4 minutes ago, Wino said:

The pink arrow is pointing to the vac hose you're looking for:

 

Ah, that's great, thanks - so the pressure sensor is along that pipe.  Now to see if I can see it behind the air filter box and the heat shield!

[Breezy_Pete]

Please post pics if you can.

 

[Breezy_Pete]

By the way, while you're looking for/at the pipe, check the junctions where the hard plastic hose meets any of the barbed plastic fittings along its route.  These hoses are known for splitting at these points, and causing vacuum leaks.  It could be that there's nothing wrong with the pressure sensor, just a leak caused by such a split, making the pressure sensor give unexpected readings.

Here's an extreme example of the splitting I mean:

 

[Stonster]

Hmm.  I'm managed to find the brake booster sensor.  It's to the top right of the engine, next to the air filter box:

 

 

I have a part number on the sensor:  036906051G

 

Funny thing is, that part number throws up 'intake manifold pressure sensor'.  I think the correct part number for the brake booster pressure sensor is 036906051C.  But I don't know if there's any technical difference between them...

 

So, please citigo community could you help me out, if you've got a citigo with stop-start - take a quick look at this pressure sensor at the top right of your engine behind the air box.  Has it got 036906051G or 036906051C on it?

 

 

 

 

[Stonster]

Bet there's no way to test if it's faulty without replacing it either...

[SuperbTWM]

What are the two fault codes? are you sure you have sensor issues and not a genuine vacuum leak?

[Stonster]

12 minutes ago, SuperbTWM said:

What are the two fault codes? are you sure you have sensor issues and not a genuine vacuum leak?

Good question. 

4649 brake booster pressure sensor P0555 00 (032) electrical malfunction 

And

4655 brake booster pressure sensor P0556 00 (104) implausible signal

 

That to me suggests a sensor problem rather than a pressure leak...

[Breezy_Pete]

The brake servo booster hose is connected to the intake manifold at the engine end. So whether it's called brake booster sensor or intake manifold sensor is unimportant, it's just an air pressure sensor. It may well be used as a MAP (intake manifold pressure) sensor on other VAG cars.

The 7zap link I posted earlier says the G part is correct for your car.

What does it say on yours?

Edited 25 September, 2019 by Wino

[freemansteve]

I can't say for sure about this specific part, but it is common practise for the last character of part numbers to be a 'minor' revision code. In many cases this means it is interchangeable. Often it may indicate something like a change of manufacturer or country/factory of origin. My guess is that it is essentially the same part.

[Stonster]

So slight movement forward on this.  I've located the sensor and checked the wiring to it.  All looks fine.  There's good earth connection as well.

 

So two questions:

1) Is it seems to me that brake booster pressure sensor P0555 00 (032) electrical malfunction  &  brake booster pressure sensor P0556 00 (104) implausible signal could only be caused by an electrical fault in the sensor or the wiring to the sensor, right?  Otherwise I'd expect a 'pressure too low' or a 'pressure to high' fault  (for which there is a code - P0557 & P0558).  So I'd expect that the cause is most likely a faulty sensor.

 

BUT according the service invoices I've got from the previous owner, this sensor was changed about a year ago.  🤔 (presumably for the same fault, although I don't know.)

 

SO:  What are the chances of two faulty sensors in a row?  It seems unlikely to me.

 

2) Also, it ought to be easy to change the sensor, because it's easy to reach it.  But rather than bolts, it's held on by plastic clips (argh.  Anyone unclipped clips like this before?  It looks like you need to squeeze all four lugs together at once.  How on earth do I do that?

 

 

[Breezy_Pete]

Might be an idea to check the wiring end to end, I can help you probably by telling you where they go. Telling me your engine code will help me find the relevant diagrams.

It is improbable that 2 sensors will fail in a short time, I think.

 

Those clips look like the type where you press the central pin downward, allowing the four quandrants to move inwards to free the sensor. Once the pin is out the way, they'll probably move inwards quite easily. It's hard to make out whether there really are such pins from your picture. The rear one looks like it has one, but the front one not? May have been lost during previous change?

Edited 26 September, 2019 by Wino

[Stonster]

Yes, the sensor is the easiest fix, but least likely...

 

Engine code is CHYA.

 

There are 3 wires to the sensor.  A brown one which I've checked to be earth, and two others. One should be 5v supply, I think, and the other the signal. But if you know where they end up that would be amazing!

[Stonster]

It could be a vacuum or servo problem, I suppose. I've checked the hoses, but I don't think there's any way to check the brake servo

[Breezy_Pete]

Ok, Pin 1 at pressure sensor is a brown wire, ground/earth, connects to a common junction in the loom somewhere and then goes to pin 4 of a 56-way connector at the engine ECU.

Pin 3 is a yellow wire, +5V, from engine ECU pin 28; also supplies the gearbox neutral sensor.

Pin 4, grey/yellow is the signal going to engine ECU pin 45.

If you can see 5V between yellow and brown when ignition is on*, that probably proves the supply and ground connections. The signal wire probably needs continuity testing connector to connector.

 

Other thing to do is look for corroded contacts or water ingress etc in the electrical connector at the sensor end, if not ECU end too.

 

* assuming that that the 5V is present when just ignition is on, not engine running, which seems like a safe assumption to me, but it is an assumption!

[Stonster]

On 26/09/2019 at 15:25, Wino said:

Ok, Pin 1 at pressure sensor is a brown wire, ground/earth, connects to a common junction in the loom somewhere and then goes to pin 4 of a 56-way connector at the engine ECU.

Pin 3 is a yellow wire, +5V, from engine ECU pin 28; also supplies the gearbox neutral sensor.

Pin 4, grey/yellow is the signal going to engine ECU pin 45.

If you can see 5V between yellow and brown when ignition is on*, that probably proves the supply and ground connections. The signal wire probably needs continuity testing connector to connector.

 

Other thing to do is look for corroded contacts or water ingress etc in the electrical connector at the sensor end, if not ECU end too.

 

* assuming that that the 5V is present when just ignition is on, not engine running, which seems like a safe assumption to me, but it is an assumption!

 

So this gets more and more weird... or I get more and more stupid!  

 

I've been into the plug and sensor and measured the voltages both with and without the engine running.  

Pin 1 is indeed a ground sensor

Pin 3 is the supply voltage - stays at 4.9v volts regardless of what the engine is doing

 

Pin 4 is the weird one.  I would expect this to be some voltage less than the 5v input voltage, and to vary with pressure. If the sensor is not connected, I would expect 0v here.  

Instead its stays around 5.55 volts regardless of whether the engine is running or not, or even whether the sensor is connected or not.  This puzzles me.  Is there a reference voltage applied at the ECU?  I'm just wondering if there could be a short at the ECU between pin 45 and (presumably) pin 44 or 46, causing pin 45 to be at 5.55v and not to vary with pressure?  I feel like I might be missing something...

 

So - does anyone have an ECU pin diagram?  The ecu is behind the battery, right?  Anyone heard of pins shorting before?  It seems it bit far fetched to me but I'm struggling to understand the symptoms.

[Breezy_Pete]

Could be as simple as the output from the sensor being a something like a PWM waveform, and the ECU having a pull-up resistor on its digital input for this line. (So it would rest 'high' and get pulled down to 0V with each pulse). The extra 0.5V is odd though, maybe just a mismatch of earth references.

You'd expect different 'effective' voltages with different pressure levels though, when connected, even measured with a simple voltmeter, I think.

 

[Stonster]

ah, yes, that would make sense.  It might be that the ecu has a pull-up resistor and is set at 5.5v, but pulled down by a signal from the sensor. Then without a lower voltage from the sensor it remains at 5.5v which is what I measured it at.  Sounds like the sensor might be faulty.  More investigating needed, I think...

Edited 18 October, 2019 by Stonster

[Stonster]

I did have to really dredge my memory / wikipedia to remember pull-up and pull-down resistors!