Coolant Temp Gauge Changes Above 1000 RPM

Hello fellow enthusiasts,

I'm having a small issue with the coolant temperature gauge in my A4 (8D5) AFN.

The temperature gauge changes (showing a lower temperature) when the engine speed is above idle speed!
This effect is only visible when the coolant is still quite cold (65°C). The warmer the coolant, the less pronounced this effect .

Example of the advertisement:
Idle: 65°C
Engine speed surge at 2500 RPM: 60°C.
Idle: 65°C
etc.
This change is not slow, but it 'flickers' back and forth.

I have already read several posts describing faulty temperature readings, caused by things like a G62 defect or voltage regulator issues, but in this case, none of those problems were present.

What I've tried so far without success:
- G62 connector, checked and cleaned -> no improvement.
- My partner fiddled with the cable connected to the G62 and watched the display, but there was no effect.
- Unplugged the connector from the G62, previously measured the two resistors and replaced them with fixed resistors (the 'upper' one: 475 ohms, the 'lower' one: 1000 ohms). --> The display is still flickering!!!
- all other gauges in the instrument cluster (oil temperature, voltmeter, fuel gauge) are stable.
- The battery voltage remains stable (of course, at approximately 14.2 V).

Does anyone have any ideas (speed-dependent, cold solder joints in the instrument cluster )?

I look forward to your responses.

Hello,

I suspect the problem lies in the AI, as it has a "damping function" that prevents the needles from flickering, even with loose contacts on the sensor.

Best regards, Rainer.

Hello Rainer,

I disassembled the AI unit and now I have the bare circuit board in front of me.
If you carefully move the temperature gauge pointer with your hand and then release it, it will slowly return to its leftmost position.
The damping is already present within the pointer instrument itself, so there is likely no significant additional electronic damping required.

This movement of the unpowered instrument feels significantly slower than the 'jitters' observed during operation, which suggests that in the event of a fault, the instrument is being driven back and forth by the control system.

I did not find any cold solder joints or similar issues on the PCB.

Do you have any recommendations for someone who can repair an AI with such an error?
They can all suffer from pixel errors...

And every day, the marmot greets us: When you reinstall the AI, at least one light bulb in the lighting system is always defective - this has always been the case .

Best regards, Kassen.

It could still be due to a voltage regulator issue.
hg
Herbert.

Hi Oldi!

Using VCDS, you can perform an actuator test for the instrument cluster (excluding oil temperature and voltage, as these are purely analog and not diagnosable) via address 17.
This can also happen with the engine running, and potentially at higher RPMs, which is where you're experiencing the complaints.

How does the actuator diagnosis look in the various operating states of the engine? Does anything "twitch" (back and forth), or do all the indicator elements move cleanly from stop to stop and then settle cleanly in the center position?

The next tip would be to read or log the coolant temperature (still address 17, ECU!!) and monitor the fuel level when the engine speed changes.
Are the logged input values remaining constant here?

I believe the fault lies in the instrument cluster or its power supply. However, if the voltage regulator were faulty, the fuel gauge should also exhibit the same flickering. As mentioned before, both display units are heavily damped, so a sudden coolant temperature change from 90°C to 0°C would only be reflected on the instrument cluster after nearly a minute, and very slowly at that.

Try the above advice and let me know .

Best regards from Munich!

The stepper motor for the pointer could also be defective. It's replaceable. It needs to be desoldered, and it's also clipped onto the circuit board. I've replaced one before.
Were they sold individually at the auction house?

Hello fellow enthusiasts,

The problem has been fixed.

First, I temporarily connected the AI unit by connecting pins 15, 30, and 31 and generating a speed signal. By the way, you can use standard 26-pin, double-row, 2.54 mm electronic connectors (available from suppliers like Segor, etc.) for the yellow and blue connectors.
After that, I examined the signals related to the affected 'measuring instruments,' specifically the coolant temperature gauge (KMTA) and the tachometer (DRZM) .

The following conclusions can be drawn from this:
- The DRZM (Digital Rotary Encoder with Zero Memory) and the speedometer are stepper motors. They have two coils, each of which is driven by rectangular pulses.
- The KMTA and fuel gauge are standard moving-coil instruments, but they are also driven by square wave signals (pulse width modulation, PWM).
- All rectangular signals for the instruments are generated by the IC 'ITT CDVH-E597'.

And: All the signals look like complete garbage . They have very strong interference (the bright yellow, thick bars).
These thick bars change their width and frequency depending on the speed. This is the reason for the strange effect (the temperature reading changes with speed).

After that, I traced and tested the signal lines from each instrument to the integrated circuit.

Pin: Signal.
75, 76: DRZM coil 1.
76, 77: DRZM coil 2.
78: PWM for KMTA.
79: GND of the IC.
80: +5 V from the IC.
1: PWM for fuel gauge.
2, 3: Speedometer coil 1.
4, 5: Speedometer coil 2.

While creating this mapping, I noticed that the +5 volt pin of the IC was also subjected to very strong interference .
Therefore, I temporarily connected a capacitor (10 nF, ceramic) here. This restored the clean square wave signals, in addition to the supply voltage.
The pictures speak volumes .

Now, everything is working again like it was before.
I have not investigated the causes of these severe disturbances in detail, perhaps a blocking capacitor, which is almost 20 years old and located somewhere, is no longer as good as it once was...

Best regards, Cash Registers[/list][/list]

Congratulations and thank you for the feedback!

Hello fellow enthusiasts,

Based on these measurements and the successful repair, I believe that a significant portion of the defects in the analog displays can be attributed to this particular issue.

If the sensors are working correctly (!!!), but the needles are no longer doing what they should, you can try to fix it with the capacitor. You don't need an oscilloscope for this; you just need to be able to solder a little, or know someone who can .
You definitely couldn't do anything wrong with this capacitor.

Therefore, here's some more content for the search engine:
Combination instrument, speedometer, coolant temperature gauge, fuel gauge, analog display, part numbers: 8D0 919 034 E, 8D0919033, 8D0919034, 8D0919035.

LG Cash Registers.