Speed Signal: Unlikely or Suspicious?

"Because my speedometer only shows half of the actual speed I'm driving, the error memory always displays 'implausible speed signal'!"
Could it be that the motor has lost power due to this faulty signal?
A few months ago, this issue of insufficient engine power (starting at around 2000 RPM) was discussed several times in this forum.
Since the problem still hasn't been resolved, I might be able to make some progress by reaching out this way.
Just a reminder: The TDI PD (ATD) engine in my T3 bus has approximately 30,000 kilometers on it.
Best regards, Hoin.

This would then need to be installed in the ABS control unit, right?

So, the lamp in the instrument cluster should also turn on!

Do all four wheel speed sensors work?

Steffarn wrote:

This would then need to be placed in the ABS control unit, or?

Why?
There's also a speed sensor on the transmission in the TDI engine, and it's responsible for controlling the engine management system.

Quote:

So, the lamp in the instrument cluster should also turn on! Do all 4 wheel speed sensors work?

These sensors are only used for ABS and ESP systems, not for engine control. ABS sensors must be independent of all other systems, as required by regulations.

Hoin wrote:

Could it be that the motor has lost power due to this faulty signal?

It's certainly possible. As a result, the control unit (STG) can no longer control certain comfort functions, among other things.

I thought the speed sensor on the transmission was only for the speedometer in the instrument cluster and things like that!

What does the engine care about how fast the car is going?

It shouldn't have any limitations on its performance!

Steffarn wrote:

I thought the sensor on the transmission was only for the speed display in the instrument cluster and things like that.

The combination unit is powered by the engine control unit (ECU).

Quote:

What does the engine care about how fast the car is going?

e.g., for reducing jolts or limiting torque during acceleration.
What do you think would happen if you suddenly floored the accelerator at 1500 RPM and the engine control unit (ECU) didn't intervene to limit the power?
All TDIs are electronically dampened to avoid unnecessary strain on the powertrain.

Try this: Disconnect the sensor and go for a drive. You'll probably have a lot of fun, especially during changes in load... oh yes, the torque cutoff will also stop working! It's going to be a great "jerky" and "twitchy" experience .

Quote:

It shouldn't then have any limitations on its performance.

I'm not sure about that. Since the comfort functions are deeply integrated into the ECU software, it could cause a lot of problems.

Hi there,

Quote:

Since my speedometer only displays half of the actual speed I'm driving, the error log always shows "implausible speed signal"!

I assume you're not referring to your speedometer, but rather the measured value in the control unit. However, that conclusion is definitely incorrect. The control unit doesn't detect an implausible speed signal because the speed is too low. Otherwise, everyone who drives through a 30 km/h zone for just three days to get bread would have this kind of error. Conversely, there's no error either; my control unit once even "imagined" that it was driving at 260 km/h.
The system is very sensitive to fluctuations in the speed signal. Even jumps of +/- 2 km/h can cause the cruise control to disengage.

Where does your device actually get the signal from?

Greetings.
Sure, here is the translation of the text from German to English:

"Dieter"

Hello, I'm back.
Sure, here's the translation:

"Here's a little explanation again."
The engine is from a Volkswagen Gol 4 and it's installed in a VW T3 bus.
ABS and airbag control are deactivated, so there is no warning light illuminated on the dashboard.
The speedometer is also from a Golf.
The Hall sensor, also from the Golf, is installed between the speedometer cable and the speedometer itself.
So, the speedometer receives a digital signal, but the wheel rotation is only half of what it would be if the Hall sensor were normally installed in the transmission.
The odometer and the displayed speed are approximately halved.
The fuel consumption display, on the other hand, has been doubled.
I suspect that the control unit considers this to be implausible.
Yesterday, I traveled several hundred kilometers by train. The bus is running normally, but it's currently missing about 30 horsepower of its original 100 hp.
The error log consistently indicates an implausible speed signal.
Could this be the cause of the reduced performance?

Hi,

Does the intercooler have enough fresh air supply?
A missing tachometer signal as a cause for power loss... that's rather unlikely. In my case (with my AHF system), it would just mean the anti-vibration function is disabled, but otherwise it would still run normally.
But who knows what secrets are hidden within the depths of the STG software?
Take a look at the intake air temperature while you're driving. If it increases too much, the engine's performance will likely be limited (and it's not exactly good for the engine's lifespan in the long run).

Best regards, Rainer.

or it might be due to the adaptation of the tachometer wave to a digital/analog signal.

Okay, here's the translation:

'Edit: What I mean is that the speedometer sensor might already be providing an incorrect speed reading.'

Hi,

What exactly is the argument against the trivial solution of attaching a second magnetic sensor to the drive shaft (directly opposite the existing one)?

Then there are 2 pulses per wheel revolution, and that's it.

Best regards,

January.

Hello Jan,
The tachometer wave is not the drive shaft.
In my case, the tachometer shaft drives the Hall sensor.
I know how to obtain an exact signal (using a frequency doubler).
Hello Joergs.
The speed of the tachometer shaft depends on the wheel rotation; therefore, your reasoning may not be correct.
Hello Rainer,
The intercooler is optimally positioned under the bus, in front of the drive shaft, so I can rule out overheating.
I actually think the lack of power can't be caused by the speedometer. I just wanted to rule out that factor.
"I've mentioned this before, but when accelerating, I feel like the engine wants to go, but it's being held back. Up to 2000 RPM, the following happens: the boost pressure on the LDA (Lambda Dynamic Air) rises above 1.2 bar and then drops back to 0.7 bar even with full throttle. If I briefly release the accelerator and then press it again, the pressure spikes up above 1 bar for a second. This happens every time. Above 2000 RPM, the pressure is always significantly below 1 bar."
This can likely only come from the valve for the boost pressure control system, but that valve is controlled by the engine control unit (ECU), and I suspect that the ECU is reducing the power output.
The question is why.
No further error entries in memory.
LMM ist geprüft, Dieselfilter ist org. VW and new [cars/models].
No leaks in the turbocharger system.
In my opinion, VTG is also acceptable.
Maybe something else will come to mind later?
Best regards, Hoin.
[/img][/list][/quote]

Just take a look at the mass airflow sensor (MAF) values while you're driving.
Are they still in good working order when there's a power shortage?

Hi,

Have you checked with VAGCOM what pressure the control unit is reading? To me, it looks like a basic setting process, but at the "wrong" value (I assume your LDA is working correctly).

If you know how to get a proper "V" signal, why not just try it out experimentally?

Best regards,

January.

"Hi, I also have a T2, T3 with a TDI/AFN engine. Regarding the sensor: VW offers one that provides 4.8 pulses instead of the usual value. About the lack of power: Have you checked the catalytic converter, or is it even installed? The EGR valve is probably okay, but it's basically useless anyway, so you might as well remove it. Are the hoses for the boost pressure control swapped? Or have you only recently noticed a loss of power? Oh yes, the engine control unit has a fixed voltage-to-needle lift threshold, which is likely causing the error message. What happens if you briefly turn the ignition off and on while driving? Does the power return then? I'm waiting for your reply. Best regards, Bernd."

what happens if you briefly turn the ignition off and on while driving

Be careful with such experiments.

We once had a customer who drove into a ditch because the steering wheel lock 'engaged' even though the key wasn't removed.
(at least that's what he claimed).

However, it still happens from time to time that the steering wheel lock 'engages' even though the key is still in the ignition - I've experienced this several times. There's also an internal instruction from the manufacturer (though not VW) to check this during a workshop visit.

"Yes, what you're saying is correct, but you shouldn't remove the key in a curve while traveling at 140 km/h. Instead, do it when accelerating, for example, in first or second gear, if you notice that the engine power is failing. Turn off the ignition and then turn it back on. On the other hand, one would expect someone who assembles a VW T2 with a TDI PD engine to have some technical knowledge."