Petrol gauge dial/arm flickers

[Merc_Girl]

To my eminent members

Not sure if the question should be here or under fuel, but here goes.

So, the fuel gauge on Bessy is perfectly well behaved if the fuel level is below the half way mark.

However, when driving, and fuel levels are over the half way mark and more towards the full end, the arm swings considerably, as much as 1/4 tanks worth. Obviously this is somewhat disconcerting when you are trying to figure out when you need to estimate when you need to top up!

The sensor has been replaced, but I guess I shouldn’t assume that that isn’t the problem.

I’m sure there is a whiff of fuel as well when she is filled up. Not sure if that may be part of the reason of the wayward actions of the fuel gauge or totally unrelated.

As always, your thoughts/suggestions would be most appreciated

[lpeterssen]

Dear Merc_Girl

The fuel sender unit requires a good ground source to work properly.  The grounding spot for it is on the trunk, on the left rear fender next to the wheel arch.

Take a piece of sanding paper and polish the o-ring shaped terminal the best you can.  Also sand the body to which the oring terminal is screwed to, in order to have perfect continuity to ground.

On the other side, take out the Bakelite connector that is attached to the fuel sender unit and with a small roll of sanding paper polish those contacts the best you can.

Open the Bakelite connector and check the cable integrity soldered to the terminals. 

I have found in so many wiring harness that I have refurbished, that cables there have a tendency to corrode and break.  If that is the case, cut each cable end, remove insulation and sumerge the cable ends in a solution of citric acid overnight. Once cable ends are shiny in copper color you can solder them with confidence. Do not forget to neutralize the acid solution action (soft action do not worry) and summerge  cable in baking soda solution.

Best regards
Leonardo Peterssen

[Pawel66]

I think that, indeed, erratic ground connection is probably the culprit for erratic hand operation in the gauge, so what lpetersen is advising is the first step.

Next steps, if this does not work, from my experience would be:
1. Take out the sensor from the tank and manually move the float (I think you should have the early type of sensor). See how the hand behaves and see if there are no damages on the wire along which the connector is sliding.

2. If not obvious result here, what I saw on the new gauge was erratic hand movement caused by too tight fastening of the gauge back plate - easing pressure on the tiny screws helped. Maybe something changed there over years...

[Merc_Girl]

Thanks chaps

Much appreciated. I’ll start investigating

[Benz Dr.]

If the sender is similar to a 190SL unit this is a common problem. Things wear over the years and fail to make solid contact so grounding would be on my list of things to check but not the only thing.

[Merc_Girl]

Thanks Doc 😁