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MG TD TF 1500 - Negative- positive-ground fuel pumps?

Hello,

I received so good advices to my last question, that I come again with the TD SU fuel theme.

My TD has been converted to negative ground and I know that the easy way to go is to install a negative-ground pump.

But, to make things more difficult, what are your recommendations for installing an original positive-ground pump?

More specifically:

Is it easy to install a positive-ground pump in a negative-ground electrical installation?
Is the metallic pump body isolated from the internal circuits, or is the body grounded?
Where is the ground connection in the pump? In the metallic body?

I assume that it would be possible to install such a pump by just isolating the pump body from the chassis. Although if the metallic body is live, a short-circuit could be easily produced by accident if some metallic piece (like a spanner) touches the pump body and the chassis.

As always, I will appreciate and also enjoy your inputs.

Regards.

Jesus
Jesus Benajes

Jesus,

Original fuel pumps are non-polarized. Some later ones have internal noise suppression circuits (diodes).
Recent replacement pumps have solid-state driver circuitry and are definitely polarity sensitive.
Bud Krueger

Jesus,

Bud is right original pumps were not polarized the later ones had a diode added across the points, making it polarized. If you want to change polarity all you need to do is change the diode, (it has to be reversed biased) the Burlen part numbers for these are: CZX 1004, Negative earth and CZX 1005, Positive earth. If you have the soild
state pump, this will be more difficult to modify.


John

John Scragg

I'm just thinking out loud, so I may be way off base. If the polarity of the maganetic coil is reversed, won't the pump plunger move the opposite direction when voltage is applied? If yes, the points won't open and release the plunger. You would just get one stroke of the pump then a hot magnetic coil.

If this is the case, all you would have to do is swap the coil's leads. They are located under the points' cover.

I don't have time today, but I try and test this in a couple of days.

Evan
Evan Ford - TD 27621

Jesus - I would strongly recomment NOT trying to mount a positive ground pump in a negative ground car. If you were able to isolate the pump from ground, you would then have a great lump of metal exposed on the car and it would have an voltage (with an extremely high current capability) sitting out there ready to suprise the first unwary or careless person with a wrench or screwdriver in hand that comes near it - an isolated pump would be a disaster waiting to happen. Isolating the pump from ground is more than just insulating the mounting bracket from ground, there are also the coper fuel line from the tank and the braded fuel line to the carburetors. As Bud Krueger points out, the original pumps are not polarity sensitive. I don't know when they started putting diodes in the pumps, but a check under the cap will reveal if there is a capacitor (axial leads) or a diode (red and black leads at the same end). All of the later pumps have diodes, but as John Scragg points out, that can be changed to match the polarity of the car very easily.
Evan - a selonoid (which is all the coil/diaphragm arrangement in the fuel pumps is) doesn't care what the polarity supplied to it is (or even if it is DC or AC current). When there is current through the coil the diaphragm is pulled toward it, regardless of polarity. Cheers - Dave
David DuBois

asking myself for what reason you would have a positive grounded pump on a negative grounded car? Like David says, you almost must put on an enormous warning on the car sayng that the pump is isolated and no one can touch it with screwdriver without loosen the groundcable. Why taking so many riscs as there are negative grounded pumps that look the same as positive grounded ones? Would not take the change of getting the car on fire just to try to make things original.

regards,

Jan
Jan Mekes

David,

For some reason I was thinking the diaphram disk was a magnet too. You are right, it is steel. It will always be pulled toward the magnet. The pump doesn't care whether its positive or negative ground. (diode models excluded...)
Evan Ford - TD 27621

There is a capacitor across the points to reduce arcing in the early pumps (diode in later). The capacitor is electrolytic and therefore polarity sensitive. You must reverse the leads to change pump polarity as you do with the diode supressor.
Don Harmer

In answer to original Q:- Pump base *is* grounded.

Even if you isolate the pump, that metal fuelline from tank to pump might be a problem.
Will

This thread was discussed between 29/11/2002 and 02/12/2002

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