How to repair/clean faulty sender unit?

My 93' Cabriolet is not sending fuel to the engine (no cough or sputter, just nothing getting there). I tested with a multimeter and there is power getting to the top of the lifter pump housing, but not the pump. Usually they are quite noisy, but there was nothing. I extracted the lifter pump and confirmed it was working by connecting it directly to the battery for a moment.

The problem appears to be the sender unit. It visually appears a bit old, discoloured and scaly, bit otherwise intact. I tested again with the unit outside of the car and it seems that whilst the top of the unit receives power, it does not reach the pump.

A friend suggested submerging the unit in Coke overnight, but I am unsure of this.

I can't find a replacement online. Would anyone know of what I could do to clean or repair this? Anyone ever had a similar issue?

The Fuel pump out of the car acting differently than in the car suggests to me that you have a faulty connector.

If you take the fuel pump off and jumper it to Earth and 12v and it runs, then you reattach it to the connector, and apply power to the sender, (3 wires) one power one earth and one for the sending unit that powers your gauge, then you either have a bad connection at the pump to power, or a cold solder joint on the connections which can be quite common for a pump that has overheated.

Everything I know about the in-tanks pumps from how to remove to the sender is in the following two links.

Oh, on the power plug, the violet/black wire is for the fuel level sense. Red/yellow is B+ and Brown is ground or earth.

1984.5-1993 mk1 How to change your in-tank fuel pump | Volkswagen Owners Club Forum

Everything  I know about fuel senders, and fuel gauges.
Repairing your Fuel Sending Unit | Volkswagen Owners Club Forum

Ifn't it twere me, I would replace the in-tank pump, as ones that have gone iffy or you don't know the age of it is better to be safe than sorry, as in Manhandleing it you can free it up for a bit…..

Check your solder connections between the feed through and the pump connection, specifically the ground.
 

Thanks mate.

After reading this I cleaned the sunder unit's connections with some WD40 and a small file and brush. I haven't tried it back in the car yet but I was able to make the pump spin by touching a 9v battery to the top of the sender unit, so that's encouraging.

I quite possible I fried or damaged the pump, too. I've had loads of issues with the relays and in-tank pump ever since taking ownership of the car. A few days ago the fuel pump relay died on me so I swapped it with the load reduction relay just to get me home. I've replaced the intank pump once already with a Topran part - maybe I'll splurge out this time and buy a Bosch component.

 

Update:

I tested the sender unit and motor outside of the car after a clean up - worked fine, so I was hopeful that solved my problem.

I reinstalled the pump and gave it a go - still nothing.

I got out the multi-meter again and saw that when the ignition was turned to acc I would get 10v. However, this would drop to 0 on cranking.

So I'm guessing the problem is back at the fuse board. Fuses are fine, of course. I could feel the fuel pump relay and load redirection relay clicking away as I cranked the engine. Opening them up too they looked fine on the inside.

So I'm stumped…out of ideas.

Well, if it was me and I was getting less than battery voltage to the thing 10V I would suspect bad grounds or earths, and or a power drop on the b+ line that indicates corrosion.

Now the funny thing is that the Lift pump and the Main pump have the same power inlet….or source, so what is the main pump doing (the external one).

Inquiring minds want to know?

When you place the Key in, and turn it to run, do you hear the main pump prime for 4 seconds?
If not then look to the Source or bad grounds.

If it were me, and I got 0 on the start side but the run side has voltage, then I would suspect that the ignition switch has issues.

The ignition switch when in the Start position also enables the Run (although the load reduction relay or "X" circuit / relay is opened so that hi-load accessories aren't running and directing all the power to the starter.)
So it could be just as simple as a bad ignition switch.

Just my two thoughts on the Subject…..

Have you replaced your Battery to frame and frame to engine grounds yet?
They are such a good thing to do as Earthing is everything on Digifants.

Read the riddle in my signature.

Now if you have the "Air Bag" getting to the switch is a little bit trickier so let me know….

If you look at pin 15 and pin 50 The both are the Start side of the switch, and of you look to pin 15, it is tied on the run side when you have the starter engaged, and the X circuit only powers the load reduction relay when in the run position.

Thanks Briano,

Nope, nothing from the external pump when I turn the key or crank.

I'll check the grounds (and recharge the battery - it died after I spent a while testing and cranking). I know I've had an issue in the past with the ground on top of the rocker cover covered in grime.

I'm also going to inspect the rear of the fuse box. Maybe there's a wire loose or damaged back there. I'll maybe try bridging the fuel pump relay to eliminate that as an issue, too.

I'm toying with the idea of buying a new fuse box/board and rewiring it. There is some damage where the last owner melted part of it (where the fuel pump relay is). Though the job doesn't look that encouraging for an ametuer.

No airbag on this car. I'll post updates on how I go.