Startproblem PD | Posts 16+

Jan6K wrote:

... It would be interesting to know where this limit lies in the engine control unit. ...

Hi,

For car radios and navigation systems, the specification is 9-16V. The devices must function without any limitations within this voltage range.
I can imagine that this area might be defined more broadly, especially when it comes to engine or ABS control units.

Hello,

ABS and MSG functions operate without limitations within a voltage range of 10V to 15V. With limitations, they operate within a voltage range of 8V to 16V.
'Up to approximately 7V, an error can be stored in the error memory, but the function is blocked. An undervoltage error is stored in the control unit if the voltage drops below 8V for 5 seconds.'

Thomas K. wrote:

The ABS and MSG systems function without restriction within a voltage range of 10V-15V. They function with limitations within a voltage range of 8V-16V. Up to approximately 7V, it is possible to store an error in the error memory, but the function is blocked. An undervoltage error is stored in the control unit if the voltage is below 8V for 5 seconds.

Ah, yes, thank you for the data.

"In my opinion, 10 volts are often dropped quite quickly during starting, especially if the battery is not brand new or has only been charged for short distances."

Speculation ON:
If a TDI engine starts better when cold than when warm, it's possible that the engine control unit (ECU) is already in a "voltage emergency mode" and therefore adjusts the fuel injection amount differently – being more generous with the fuel when the engine is cold and has sufficient voltage, compared to when the engine is warm.
Speculation OFF.

What I would also be interested in knowing:

Did you derive these values from corresponding specifications, or from your own experiments?

And what specific limitations arise between 8 and 10 volts, and between 15 and 16 volts, respectively?

ulf wrote:

Did you derive these values from corresponding specifications, or from your own experiments? And what specific limitations arise between 8 and 10 V, and between 15 and 16 V, respectively?

Hi Ulf,

The car manufacturers provide the suppliers with a specification.
For example, it states: "Full function" for a voltage range of 10-15V.
For certain specific applications, there may not be a precise specification available, but the filter manufacturer often accepts certain limitations. However, these limitations often relate only to temperature stability.
A power supply unit (PSU) must function flawlessly within a temperature range of -40 to +85°C (and within the specified voltage range). Outside of this voltage range (i.e., 8-10V and 15-16V), the functionality is no longer guaranteed across the entire temperature range.

"What surprises me, however, is Thomas' statement about the voltage range for ABS and engine control units. It's actually lower than for car radios! And this is for a safety-critical component! In my opinion, 15V as an upper limit is quite tight, since the alternator already provides a charging voltage of 14.4V (and in some Ford models, even 15...V due to a specific battery)."

Hello,

These are general specifications from the automotive manufacturers' requirements documents to the suppliers.

Basically, it almost doesn't matter which control unit it is; the information is almost the same for every project, whether it's a motor control unit, a lighting control unit, a comfort control unit, etc.

I'm afraid I don't know the exact limitations in the case of MSG.

Hi,

I was just looking at the BMW specifications. They require "9-16V full spec" for all electronic devices in the car (including the control units).

Hello,

I have here a DC specification document where the areas mentioned above are specified for the E-Class.

Tests with ABS regulators have shown that they still operate safely at 18V, but the undervoltage range, starting at 8V and below, is more critical.

Okay, I have a specification from BMW regarding the airbag control unit, which also mentions a voltage range of 10-15V.

I suspect the different specifications are likely model-specific, especially the newly released models, which are quite 'different' due to the increased amount of electronic components.