VAG COM: Check Battery Voltage - Easy Guide

Hello!

Since not everyone (including me ) has a compression tester at home, and the 'normal' leak-down test doesn't work with automatic transmissions, I came up with the following idea: Due to the internal resistance of the battery, the on-board voltage drops by a certain amount with each compression event (which is particularly noticeable when starting with a weak battery and the interior lights on).

It would be advisable to disconnect the connector to the ESP (Electronic Stability Program) or the PDE (Piezo Direct Injector) and log the on-board voltage during startup using VAG COM. Then, you can use VAG Scope to view the voltage drops and infer information about the compression (differences between individual cylinders).

What do you think of this idea?

mfg Nebelwerfer_SDI (due to a burst intercooler hose )

Mhh... I think that's only partially informative, because the battery's performance also decreases. Can you then infer which cylinder you are currently testing, or is it just about seeing if they are all the same? What do you have then, certainty that they are all equally good or equally bad?

Perhaps it could be helpful as a starting point, but I don't think it will achieve much with VAGCOM and an already weak battery... maybe with a "better" measuring device for determining the on-board voltage.

Nebelwerfer_TDI wrote:

It would be advisable to disconnect the connector to the ESP or the PDE, and to log the on-board voltage during starting using VAG COM. Then, you can view the voltage drops using VAG Scope and infer the compression (differences between individual cylinders). What do you think of this idea?

That's thoughtful, but I think the "low" sampling rate (compared to the compression sequence) isn't sufficient to obtain a meaningful result.
-> It will definitely be better with an oscilloscope .

since the battery's performance also decreases,

It gradually and evenly decreases in pressure... but any lack of tension (e.g., a leaky valve) should be noticeable.


So, what do you have then, the certainty that everyone is equally good or equally bad

Basically, it's the same as after the 'Hoppeltest' - but without the ramp section, and also for automatic transmissions .



That's a nice thought, but I think that the 'low' sampling rate (compared to the compression sequence) is not sufficient to obtain a meaningful result.

'To address this, a large resistor with 0.1 - 0.5 Ohms might help, at least as long as it doesn't overheat - or a very weak battery, although the control unit still needs a reasonable voltage to function .'

-> It will definitely be better with an oscilloscope .

Okay, I'd like something like that too; I once saw it in a used car test - at the ÖAMTC (the same as the ADAC in Germany) - they also measured the starting current using a Hall sensor and then displayed it graphically - that's why I had the idea with VAG COM .

Hi,

You won't be able to detect anything with VAG-COM and "slow" control units with an approximate sampling rate of 3 Hz. You will, however, be able to detect it with a decent oscilloscope.

Best regards, Rainer.

@Rainer: What do you mean by 'slow' control units? Are you referring to the old 8-bit ECUs from the 1Z, AAT, or AEL? They aren't really that slow - I even managed to represent the quantity fluctuations of a jerky 1Z engine with them .

If I replaced the burst intercooler hose this weekend, I'll try it .

Hi, think about what signals can be sampled with a 3Hz rate, and how quickly your starter motor spins the engine...

I finally tried it out - with the predicted result. There was fluctuating voltage, a few sporadic glitches here and there, but no clear, periodic oscillations . It would have been too good if that were so easy; still, thank you for your answers!

Best regards,