Fuel and temperature gauge; worse after repair :-(

Hi!

My Passat has been annoying me for a while because the fuel gauge doesn't always go all the way up when the tank is full, and also the coolant temperature gauge seems to be reading too low. I've discovered that the voltage regulator on the instrument cluster is very likely the culprit. Since the car was already going to the workshop for a few things, I asked them to replace the voltage regulator; I didn't want to do it myself because of the airbag.

The car just came from the workshop, and now the indicators are even lower! The fuel gauge is stuck somewhere around the 30 mark, and the temperature gauge needle barely moves from its resting position.
The workshop says, according to information from VW, that there is only one voltage regulator available for the KI (not the MFA). The workshop would be willing to install a new instrument cluster for me at the purchase price, which in my case would mean a new circuit board for a whopping EUR 275. They couldn't explain why the fault was even worse after replacing the voltage regulator than it was before. The voltage regulator was not designed to be plugged in and screwed on; instead, it had to be soldered in.

I also noticed that when the lights were first turned on after replacing the tachometer, it briefly dropped to zero and then displayed the normal reading again.

Did they perhaps replace the wrong part?

Sure, no problem.
Best regards.
Florian.

Kanute wrote:

Did they perhaps replace the wrong part?

Hi,

Since the error symptoms have changed, they were probably in the right place, but they may have installed the wrong "replacement part" or made another type of mistake.

I'm actually surprised that a car repair shop does electronic soldering work .

Hello and thank you in advance for your reply.
By the way, the workshop says that the voltage regulator wasn't just a matter of plugging it in; it had to be soldered. Otherwise, I'm reading here and in forums like the 35i forum that it's just a plug-in component. I once asked at the VW parts service around the corner, and according to them, there is only one instrument cluster voltage regulator available for a Passat of this year's production for EUR 8.87.
Oh my, what a lot of effort again for such a small part.
I am always grateful for knowledge and advice.

Florian.

Did you ever even glance at the screen? Whether it's plugged in or soldered should be visible on the drawing.

wolfi_b wrote:

Didn't you even take a look at the screen? Whether it is plugged in or soldered should be visible even on the drawing.

Ahoy! The one with the VW logo in the catalog has removable feet and is designed to be plugged in. In my car, the feet were bent and it was soldered in place. The parts specialist at VW took the old voltage regulator (with "M63AF LM2940T 10.0 P+" printed on it) from me, took it to his electrician, came back, and said that the new part with the straight pins could also be used; you just need to bend the pins and solder the part in place.

At the electronics store around the corner, where the voltage regulator is unfortunately currently out of stock, the old part was once tested, and its output voltage was 5V.

"I could say, 'Workshop, you've made the problem worse, now make sure it works properly, and do it quickly.' I need the car on Friday, and I need the temperature gauge to be working, because I have a longer trip planned. But I'm thinking maybe they'll fix it faster if I investigate the cause myself. However, my knowledge is limited; where could the problem actually be?"

Best regards.
Florian.

Kanute wrote:

The parts specialist at VW took the old voltage regulator (which has "M63AF LM2940T 10.0 P+" printed on it) from me, went to his electrician, came back, and said that the new part with the straight pins could also be used; you just need to bend the pins and solder the part in. At the electronics store around the corner, where the voltage regulator is unfortunately currently out of stock, the old part was once tested, and its output voltage was 5V.

Um... So, there's something that surprises me. According to the part number, it seems you had one with a 10V output voltage installed. It seems strange that it would output 5V in case of a defect (as measured by the electronics store).

What does the new regulator say?

Can you measure the new regulator? Datasheet, including pinout, is available here: [url]http://www.national.com/ds.cgi/LM/LM2940.pdf [/url]

Regards,
Guste.

Kanute wrote:

(M63AF LM2940T 10.0 P+ is printed on it). . . At the electronics store around the corner, where the voltage regulator is unfortunately currently out of stock, the old part was once tested, and the output voltage was 5V. . . I think maybe they can fix it faster if I investigate the cause myself.

According to the model number, it should output 10V... but even in electronics stores, there is no guarantee of correct measurements (just like in auto repair shops).

There can be many reasons why your AI is no longer working correctly, and unfortunately, some of these causes may lie outside the voltage regulator and can only be identified by electronics technicians.

"Average consumers" can, at best,
-> Use a magnifying glass to search for any metallic particles (or other potentially conductive debris) on the circuit board that shouldn't be there.
-> Look for poor or cracked solder joints, or for breaks/cracks in the circuit board.

guste100 wrote:

Um... So, there's something that surprises me. According to the part designation, you had one with a 10V output voltage in it. It seems strange that it would output 5V in case of a defect (as measured by the electronics store). What does the new regulator say? Can you measure the new regulator? Datasheet, including pinout, is available here: [url]http://www.national.com/ds.cgi/LM/LM2940.pdf [/url] Regards, Guste

Hi and thanks for the replies!
That the broken one outputs 5V doesn't surprise me, since it's broken .

The new regulator is still in the car at the workshop. Now things get interesting. The workshop just called me and said they've taken some measurements, and the new regulator is outputting 4V when installed (measured directly at the regulator) and 10V when removed. Conclusion: Something is broken on the circuit board. Ouch. It sounds like it's going to be a real deep dive into our pockets.

Greetings.
Florian.

Quote:

...the new regulator outputs a voltage of 4V when installed (measured directly at the regulator) and 10V when removed. Conclusion: Something is broken on the circuit board.

Even if the diagnosis seems likely: Did they also measure the input voltage while the regulator is installed? It's possible that there's a bad connection on the input side, causing the 12V input voltage to drop. In that case, the output voltage would also be too low. Additionally, it might be useful to measure the initial current draw by supplying 10V using a different (current-limited) power source.

Kanute wrote:

Something is broken on the circuit board. Ouch. It sounds like it's going to be a real

deep dive into our pockets.
Or, if possible, someone with knowledge of circuit design could take a look. This could be someone from your network or, for example, a local electronics store. It can't possibly be that wild, because the old speedometers didn't contain anything so mysterious.


Regards,
Guste.

guste100 wrote:

Even if the diagnosis is likely: Did they also measure the input voltage while the regulator is installed? It's possible that there's a bad connection on the input side, and the 12V input voltage is collapsing. Then, of course,

not enough would come out at the exit.

Thank you for the tip. The input voltage has also been checked and is at 12V. If I had more time right now, I would move that thing somewhere else.

Best regards,
Florian.

When installed, the new regulator outputs a voltage of 4V (measured directly at the regulator), and 10V when removed.

The fact that it outputs 10V when fully extended might be due to the lack of any load connected to it.

I once measured the output voltage of a voltage converter (from 24V to 12V) using a voltmeter. Result: 16V.
Back then (when I was still in training), I also thought it was broken.
Then I added a small load (a brine solution) and it immediately settled into the correct operating point at 13V, as it should have.

It's not just that there's a measurement error somewhere.

Martin R wrote:

The fact that it outputs 10V when not under load could be because there is no load connected.

This seems unlikely, because it's a 10V model, which should maintain 10 volts even under load (up to 1A).

Hi everyone,

The LM2940T is a highly integrated regulator series. Whether the input is loaded or unloaded, it regulates the output voltage within a few milliseconds.
If the new regulator is set to 5V, they might have sold you an LM2940T-5.0. You will need an LM2940T-10.

If you then have an LM2940T-10 and the output voltage is still dropping, check the capacitors at the input and output of the regulator on the circuit board. Replace any defective capacitors with a 100nF ceramic capacitor and a 100uF electrolytic capacitor in parallel. Alternatively: Come by and we can take a look together, or send it to me.

Edit: I just realized that you're from Hamburg. Well, that's probably a bit too far from Stuttgart...

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

"Micha"

Mephisto wrote:

Hi everyone, The LM2940T is a highly integrated regulator series. Whether the input is loaded or unloaded, it regulates the output voltage within a few milliseconds. If the new regulator is set to 5V, they might have sold you an LM2940T-5.0. You will need an LM2940T-10.

The new part is regulated to 5V when installed and 10V when removed. And it's not an LM2940T-10 from National Semiconductors, but rather an "ITT G 8932 TCA 700 Y". I once tried to find datasheets for it, but I wasn't successful.

And thank you for the offer, Micha, to take a look at it. Is that really a little too far ?

Best regards,
Florian.

Kanute wrote:

The new part regulates to 5V when installed and 10V when removed.

It would be interesting to see the current draw "when installed" sometime... IMO, the voltage is being randomly reduced to 5V to prevent the regulator from being damaged by the power loss caused by a suspected short circuit somewhere in the output circuit.

Hi,

I'm just wondering where the workshop plans to get the TCA 700 Y from. As far as I know, it's no longer in production. I would have chosen the standard 08/15 78S10 model, which is also used in Audi combination instrument panels.


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

"Micha"