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There are a few kilo-ohms of resistance on the pump terminals in the tank, though I know that's not indicative that the motor works.
Gargravarr,
I don't have a lot of experience with Mk2 or Mk3 fuel pumps, but from what I've read, the pump resistance should be between 0.1 and 3.0 ohms.
If the resistance across the terminals at the tank are in the kilo-ohms range, I strongly suspect your fuel pump is shot.
If you have other fuel pumps I would measure the resistance on them to compare. If they are under 3 ohms, then I'm pretty comfortable predicting a bad pump.
Dropping a gas tank scares me so I would double check for comparison's sake.

If you are running 14V to the pump and assuming no in-series resistor, the current should be 4.66 Amps which seems reasonable for a fuel pump.
If you are running 14V to the pump and the pump internal resistance is 2 kilo-ohms, the pump will only draw 3 milli-amps.
I'm pretty sure 3 mA isn't going to give much pump pressure.

Again, I would compare what the other fuel pumps measure.

Good luck getting it going

Dale
 

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An interesting topic.

I suspect that the resistance of the motor doesn't change dramatically. The windings of the armature are copper and while the resistance will increase with temperature and the contact resistance of the graphite brush against the commutator will bounce around with movement, the dramatic change in current draw is due to back emf.

When a coil of copper wire has current running through it, it creates a magnetic field around the wire. When the current through the wire stops, the magnetic field collapses and as the magnetic field crosses through the wire, it induces current flow in the opposite direction. The faster an electric motor spins, the more often the magnetic field is created and collapses. This is called back electromotive force, or emf.

When measuring the resistance of a motor with an Ohm Meter, the motor is not spinning so no back emf is being generated. What you are measuring is the true resistance of the circuit. This will probably measure a reasonably low resistance. As a result when a battery is attached, the inrush current into the motor is quite high, probably in the range of several amps. This is the stall resistance.

When the motor is spinning at speed, the induced back emf effectively creates current flow backwards through the winding. The battery current and the current generated by the back emf are in opposite directions, effectively cancelling each other out. You may have have 5 amps of battery generated current flow and you may have 4 amps of back emf generated current flow with the resultant measured current flow only 1 amp. This appears to be due to a change in resistance, but it is not really. The current draw drop that you measure through the motor is due to back emf, not a change in resistance.

Youtube video on back emf

I don't know how you would measure the resistance of the motor while it is running as a result. You can measure the resistance with the motor not running. You are absolutely correct in that the current draw of the motor will change, but it will drop at speed. For diagnostic purposes I want to measure the static resistance to ensure the brushes and the commutator contacts.

If you measure several kilo ohms resistance with the motor stopped, the inrush current would only be several milli amps. I would expect the resistance with the motor stopped will be 3 ohms or less. If it is not, I suspect the brushes on the motor are worn out.

Now, being a magnificent god of service, I would compare the static resistance of your other motors and compare them to the one in the vehicle. They shouldn't be wildly different. Other things like bad bearings can fail on a pump, but I suspect it will be a brush issue.

Reading what I just wrote reminds me of conversations I used to have with my salesman. I have a diploma of technology and he had a PhD in Chemical Physics. I would ask him about a problem I was having and he would go heavily into the theoretical aspects of the problem, commonly filling a black board with calculations. He would usually say what he thought it was, but then there would be 2 or 3 other things which could cause it as well. Then he would usually say 'well, it really could be anything!'

I go with Dr. Dave. It could be anything! The video above really does remind me of my peddler.

Dale

Oh, and if I'm wrong, oh well.
 

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Rob,

How did you clean your tank?
I had lean-out problems several years ago. Under heavy load at 100 km/hr, the motor would start to surge and lose power. The check engine light would come on. When I slowed down, the light turned off, but it stored an error code indicating an O2 sensor/Lean issue. The problem got more and more noticeable.

Rather than just changing the in-tank fuel filter sock and main fuel filter, I took the vehicle to a radiator shop and had them coat the fuel tank with a product called 'Red-Kote', which seems to have stood up for several years.

Damon Industries - Red-Kote

There seem to be several different vendors who sell similar products, but my shop uses this so I went with their recommendation.

I thought that the inside of my fuel tank would be disgustingly rusty, but in fact, it looked almost perfect before coating.

Fuel tanks used to be steel and were coated with a coating of 'Terne, an alloy of lead usually with 2–25% tin and small amounts of zinc, nickel, and magnesium. . The amount of lead continually decreased due to environmental concerns. I believe that in the 1980's, the amount of zinc had increased dramatically.

Terne is supposed to be quite resilient, but that pesky Ethanol is hygroscopic, and draws moisture into the gasoline. This is especially an issue with a vehicle that is parked a lot, and not driven much.
There are several coating that are used to coat the inside of fuel tanks to remove. Terne should be water resisitant, but if the pH is low or high, it will tend to strip.

Gosh knows what would give the fuel a green tinge, but I suspect it is some of the Terne stripping off.

Dale
 
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