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MG TD TF 1500 - Electric fuel pump, positive ground
|Does anyone know of a small electric fuel pump which is 12 volt positive ground ? I have a spare SU pump for the TF but this one is for my Model A and the SU is a bit bulky. ( I have fitted a downdraft carb and thus can't use the gravity feed). Thanks, Richard.|
|Well, since there's fuel involved, I am hesitant to say this. But If you are SURE you have the pump isolated from chassis ground, you can hook it up with 2 wires. Maybe the pumps don't get their continuity from ground and you don't need to isolate, but make sure.|
|DO NOT, UNDER ANY CIRCUMSTANCE, TRY TO HOOK UP A PUMP THAT GETS ITS GROUND THROUGH THE PUMPS BODY, BY ISOLATING THE BODY FROM GROUND. To do so is just an inventation for disaster. There are a number of pumps available on the market that use a two wire system and do not depend on the body of the pump for one side of the circuit. The little square, run all the time, make a lot of noise, Facet pumps is one that will work and is readily available. For other brands, see my article on permanently installed back up pump at: http://www.mgexperience.net/article/backup-fuel-pump.html|
for a list of various pumps that will work. Cheers - Dave
|Thanks Tom and Dave, Tom, I thought about doing it the way you describe, but realised that it would end up with potential across the fuel line between the engine and the pump, which could create a bomb!|
Dave, I can get a little Facet pump but was not sure whether it would run on a positive ground system. I assume from your info that I can ground the pump's body and run the positive wire to ground also, switching the pump through the negative wire. Is that correct? Regards, Richard.
I am confused by your warning. Maybe I do not understand your point but the pump is attached with a rubber mounting. So, electrically there is no connection between pumpbody and carground. The workshop manual also shows an extra wire between pumpbody and firewall.
|The fuel line makes contact all along the chassis and at the tank...carbs!|
|gblawson - TD#27667|
|Gordon, I am not so sure since I have fiberrings all around in the connections (banjo's) of those fuel lines.|
|Richard - The small, low pressure Facet pumps have two wires, red and black, coming out for connection to power and ground. Both wires are isolated from the pumps mounting tabs, so the red wire can be attached to the positive point of the electrical system and the black wire to the negative point. This will work for either positive or negative ground, since there is no connection through the pump from either wire to the outer metal case of the pump (later Facet pumps have an epoxy body, which is even better).|
Huib - Regardless of the fact that the SU fuel pump may be isolated from ground by sound mounting rubber and the fuel lines are non-conductive, the body of the pump is still at the potential of the wire connected to the ground screw on the side of the coil housing. This leaves a big chunk of metal connected to 12 volts that is just waiting for an errant piece of conductive metal (screwdriver, wrench, the carburetor you are installing, etc) to come in contact between it and ground. At which point there is a shower of sparks, an extremely hot piece of metal in your hands and all the smoke being released from the car's wiring harness. This has the potential of a trip to the emergency room for burn treatment of the hand, replacement of the item that touched the pump and ground and replacement of a wiring harness. As I remember, Gordon had a first hand experience with the hazards of mounting a negative ground item into a positive ground car and trying to keep everything isolated. Trying to make such an installation, is just an exercise in potential disaster and should not be done. Cheers - Dave
|Related to this discussion: the fuel line may not ground out the fuel pump but it does form the circuit for the fuel gage.|
|OK, skip that last one. I thought I was on the MGA BBS.|
|Does fuel itself conduct electricity?|
|"Does fuel itself conduct electricity?" No.|
This thread was discussed between 04/07/2007 and 06/07/2007
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