[DML] FUEL PUMP AND VISUAL INDICATIONS

[mike clemens <rmclemns@xxxxxxxxx>]

Group,

Having watched the thread on John's new pump and the questions that have been raised, I thought it time to chime in, since I have repaired 3-4 fuel pumps and no less that half a dozen fuel quantity indicators.

First, there are three (3) separate electrical circuits going to the fuel  tank.  One circuit is the fuel pump, with a positive and and negative set of leads.  This is a very simple setup, which I assume everyone understands.

Where everyone has problems is understanding the fuel quantity  transmitter.  There are three wires going to the "screw on cap".  Two of those wires are positive and one is a common ground for both the positive wires.  One of the positive wires is connected to the winding of resistive wire that goes down the center pole in the transmitter.  This one is connected to the fuel quantity gage on the dash.  The other positive is connected to the single long wire that runs down the side from top to bottom inside the transmitter.  This is the circuit that turns on the low fuel light on the dash.  Both of these circuits are independent of each other and that is why there are so many variations with fuel quantity and low fuel light in all the different Deloreans.  The fuel quantity resistive winding varies in every transmitter I've had apart.  It was not well designed and the wire has a tendency to slide to the bottom of the cylinder since it was not affixed to
 the plastic center pole by anything but wire tension.

The low fuel light only comes on (as long as there is no corrosion) when the float 'almost' reaches it's lowest point inside the sending unit.  The light should come on when the tank drops to 1.3 gallons, but I have seen them come on at 4 gallons and some never come on at all, due to a bad connection.

The fuel pump working, not working, or sucking air has nothing to do with what you see on the fuel gage or the low level light.  If it has fuel at the pickup, it will provide it to the engine, if not, then it will suck air.

Now, on a side note, one of the biggest problems with the tank itself, is the fact there are no internal baffles molded inside the tank.  It was designed this way to cut costs since the tank is a one piece extrusion.  Every car I've worked on has baffles built in, even my little MG's.  That way, when you go uphill, downhill or whip around a long turn, the gas in the tank can't slosh to one part of the tank or the other and starve the pump, unless of course your angle of bank is held for enough time for all the fuel to drain through the baffles. 

Lastly, for those of you who are wondering and want some proof of all this, take an empty fuel tank that you just cleaned out on your car.  Add two gallons of fuel to the tank.  Start it up and make sure everything is working as advertised.  Now, run the front of the car up on a set of 5 inch ramps and let it continue to idle.  You'll be totally shocked to see that the car will fuel starve itself in less than a minute, since both those gallons are now in the back of the tank and the pickup for the pump (whether the old design or John's new one) is in the center front portion of the tank.  I hope this answers some of the questions concerning fuel, fuel pumps, and fuel indicators on the Delorean.  If you want, you may contact me offline with any questions.

Oh, I got Josh's car running again and that will be another post.  I still don't know why it wants to bog at 2500 rpm, but will keep working on it.

Mike   TPS   1630


      

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