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Electrical guides and mods

Forfour Fuel Gauge Fault

Modification Details




What Are The Symptoms?

If your fuel gauge on your 454 forfour is flashing, or the fuel level moves up and
down for no reason, there is a good chance that it's a problem in the speedo.

The issue is the same as we have seen in the dash pods on the 450 fortwo.
Wide thermal changes, vibration and lead free solder equal problems.



Thanks to Nanak for the video showing the problem (and it strangely fixing itself temporarily).

What's The Problem?

The issue is on the printed circuit board inside the speedo. So, take the speedo apart.
Spin the PCB over and look at the back. At the top of the right hand side of the speedo
you will see 2 buttons, just left of both of these buttons is a (relatively) large SMD resistor.



These resistors can be identified by the number 331 on the top. The first two numbers are the resistance
and the final number is how many zeros. So 33 with 1 zero is 330. These resistors are 330ohms each.



We all know that lead free solder suffers from cracking, so if you look at how little amount of solder is used,
you can understand why their position in the top of the speedo can cause problems.

In this position they get the radiant heat from the sun through the windscreen.
Things expand when they get hot and shrink when they cool down.
This constant movement causes tiny stress fractures in the solder joints.

Eventually, a crack makes its way all the way across and you have a connection problem.



If you take your multimeter and measure the ohm reading from both sides of either resistor you should
get a reading of approximately 165ohms. If you get this reading on both resistors, your speedo is OK.



However, if 1 of the resistors has a poor connection, you'll get a reading of approximately 330ohms.



I'm Confused, Why 165ohms?

People mistakenly see the numbers and assume that the reading they should get is the reading on the resistor.
However, they aren't taking into account the way that these 2 resistors are connected.

Look closely at the board and you'll see that the resistors are in a parallel circuit.
This means that the resistance is halved. Half of 330 is 165.



So, if you are getting a 330ohm reading, only 1 of the resistors is connected correctly.

Fixing The Problem

The resistors will be ok, you won't need to replace them. They just need resoldering to the PCB.
Fire up the soldering iron and remelt the existing solder and maybe add some lead solder extra.

I Don't Have A Multimeter Or A Soldering Iron

That's not my fault, go and buy them, they are cheap. Or find someone who does have them and ask if they'll help.

Credits

Thanks to Frédéric for photo 5 showing the 330ohm reading.

Thanks to Nanak for the video.

This procedure was found on 44forum.de but I don't know who originally posted it.
If you know who it was and can provide a link, I will add the person's name here.

I'm not like a certain other website who takes all the credit for stuff they didn't come up with.


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